Copyright © 1991 by Barbara Prosser Kerr

To order a printed copy including photos and diagrams send $15 US to this address:

Kerr-Cole Solar Box Cookers
PO Box 576
Taylor, AZ 85939

Preface
Introduction
Chapter 1 HISTORY
Chapter 2 BASIC THEORY
Chapter 3 PATTERNS OF USAGE and THE SUSTAINABLE EMERGENCY KITCHEN
Chapter 4 INDIVIDUAL DESIGN ELEMENTS
Chapter 5 VARIATIONS
Chapter 6 SBCs—A GLOBAL SOLAR FORM
Chapter 7 NON-COOKING USES
Chapter 8 THE EXPANDING WORLD OF SOLAR BOX COOKERS
Chapter 9 A SAMPLING OF WAYS TO BUILD CARDBOARD SBCs
Chapter 10 USING A SOLAR BOX COOKER

The goal of being in harmony with nature led me to what has become a life’s dedication of research in Solar Box Cookers. The publishing of The Expanding World of Solar Box Cookers represents the development and growth of Solar Box Cookers (SBCs) along with the practical knowledge and the potentials of SBCs generated during the past 17 years. Hopefully, this book will fill the need for background information so designers may utilize our fund of knowledge as they work to design locally appropriate SBCs.

I am greatly indebted to Sherry Cole, my good friend and business partner in solar box cookers. She had the initial vision and has held it firmly over the years. The quiet mainstay of the work, she has been unstinting with her financial and emotional support. She has devoted herself to personally handling both the office from her home in Tempe, Arizona and the production warehouse for the EcoCookers and kits in South Phoenix. She has been irreplaceable in critiquing, proofing and editing my designs and writings. Most of the time, she has appeared to have had the time of her life, radiating joy, carrying us through the highs and lows of years of effort. But, it seems to me that I got the best, most fun, part of SBC pioneering work in tinkering with designs and eventually establishing the Kerr-Cole Experimental Home in Taylor, Arizona.

Sherry and I are particularly grateful to Renz Jennings who made the "worm barn" on his ranch in south Phoenix available to us for storage and also for use as the production base for all our solar box cookers and kits. Without this gift, which has continued over the years, we could never have produced our lines of wooden Patio SBCs and cardboard EcoCookers.

We are also indebted to Dr. Robert Metcalf, his wife Mary Beth and sons Tom and Jim. Bob has deep practical experience with SBCs through cooking main meals for his family during every summer season since 1978. He also has a firm commitment to seeing solar box cookers used to reduce human suffering and environmental devastation and has inspired many hundreds of people in the Sacramento area and elsewhere to adopt the convenience of solar boxes. Since 1978 he has supported and guided students in the study of various aspects of solar box cooker technology; and has been a national and international advocate/ambassador for the use of SBCs. At California State University, Sacramento, where he is a professor of Biological Sciences, he was asked to present the Livingston Annual Faculty Lecture in part because of his work with SBCs. He devoted his 1990 Sabbatical year solely to solar box cookers and is current president of SBCI.

Further, we are indebted to Bev Blum who had the original vision of a nonprofit educational organization for SBCs. The group of deeply dedicated volunteers organized Solar Box Cookers International. Under her guidance this volunteer organization has disseminated knowledge of solar box cookers, allowing for the technology to be adapted to a variety of forms suitable for meeting local needs and customs world wide. She has nurtured SBCI through its tender initial development and is now faithfully shepherding it through its "awkward age" of rapidly expanding growth.

I deeply appreciate the generous permissions received from Heidi Kirschner, MD, SBCI, Mark Aalfs and Tom Sponheim to republish material from their work, including a representative selection of plans.

Others too numerous to mention have contributed money, time, energy and wisdom. With their enthusiastic input, the solar box cooker effort has soared. Integrating solar box cookers into our homes has established the validity of the vision. This book is offered to them with thanks.

In particularly I want to thank Georgianna Borgens, Rick Blodgett, Germaine Cain, Don Coan, Joe Cook, Yvonne Freeman, Mary Lou Krause, Dave Martin, Chris Roth, Gwynne Smith, Bill Sperber, and all the other friends for their joint work with me, their criticisms and for their help with the preparation of this manuscript. Their support, generous sharing of time and information have made it possible to present the work in this form.

I have special thanks for Tom Sponheim who shepherded the effort for the greater part of a year, encouraging me when completion seemed impossible, then editing and preparing the manuscript for the printer. He has been a real friend in deed. Without him, this book would never have seen the light of day.

The Expanding World of Solar Box Cookers is targeted toward people who are familiar with solar box cookers and the literature distributed by Solar Box Cookers International and Solar Box Cookers Northwest. Probably most readers will have extensive experience in daily solar cooking as well as with building their own SBC. Nevertheless, I hope it will equally serve the interested SBC neophyte. The first eight chapters, by far the largest section, will focus on the rich detail surrounding this emerging technology; the last two chapters will give several fairly simple cardboard plans and instructions for using a solar box cooker along with beginning recipes.

Responsibility for the contents of this book and the form it has taken rests solely with me. I ask that my readers inform me as to any errors identified and new information obtained by their personal experience so corrections and updates can be incorporated into future material.

I assume some sections of this book may be read without reference to the whole. Therefore, I ask also for tolerance of sections where safety precautions have been repeated in different contexts. Nevertheless, safety in the use of techniques in this book is the responsibility of the SBC user.

Solar box cookers (SBCs) are the earliest form of solar cooking in Western culture. An early European record of cooking in a solar box was made by Horace de Saussure, a Swiss naturalist experimenting as early as 1767. He reported successfully cooking fruits at that time with initial temperatures of 189.5 F (87.5 C). Over the years, de Saussure and others focused their solar box cooker design work on variations of shape, size, sidings, glazings, insulations, reflectors, and the composition and reflectance of the internal surfaces.

These components have been varied endlessly in an effort to increase solar box temperatures. Our solar boxes now successfully reach temperatures between 275 -300 F (135 -149 C), depending on size and location of use. Since many foods will cook with an internal temperature of 190 F (88 C) and water boils at 212 F (100 C) at sea level, it is clear that solar box cookers have reached adequate cooking temperatures for a long time.

Meanwhile, the higher temperatures of cooking over fire have continued to force us to stir foods and protect them from scorching from excessive bottom heat, while needless amounts of fuel and human labor have been unnecessarily used. Now, we can do better than that by using readily available, free solar energy whenever the sun shines.

At the time I was first exploring solar cooking, my home was dedicated to developing Earth-Conscious Homemaking. I was exploring all the ways that could be used in an urban home to reduce consumption of earth resources. As a part of this effort, I made and used a lot of solar cookers. At one time I had seven different types of solar cookers and some duplicates in my back yard. Most were homemade. There were various parabolic and non-parabolic troughs, one five foot dish parabolic concentrator, and several four-reflector slant-face ovens—one made from the Halacy plans. There was the beautiful octagonal Solar Chef designed by Sam Erwin. Almost daily, neighbors and friends would join me for potluck dinners using the solar cookers.

When I put together my first solar box cooker in March of 1976, I had simply heard that an Indian designer, Ghosh, had made a solar box that cooked, but I had not been able to obtain plans or illustrations of it. Unaware that it "should" be made of metal or wood, I used cardboard boxes and thus opened a new line of SBC development. Had I been more knowledgeable, I would not have been so surprised that it cooked very well, if somewhat slowly, at its initial temperature of 240 F (115.5 C). With improvement in the thermal dynamics of the box, 300 F (149 C) was reached, a more than adequate temperature for boiling, roasting, baking, and gentle frying...surely a solar kitchen range.

During June and July of that year, Sherry Cole, my good friend and neighbor, and I faced a definitive test for solar box cooking. We had a large social action group in our homes. In addition to office and sleeping space, we offered to provide meals for the volunteers, planning to use the backyard solar equipment as much as possible.

Meeting the daily food needs of 35 to 40 hungry adults presented a perfect opportunity to compare the practicality of the various solar cookers. It provided fine tests of comparative function under stress. Our choice of cooking apparatus gradually shifted away from the multiple reflector and parabolic cookers to solar box cookers. While we initially had been aware of some of the special qualities of solar box cooking, it was through the routine serving of large quantities of food that the major strengths of the solar box technology emerged. They reliably cooked large quantities of food but required minimal attention. With a hectic schedule, it was not possible to give focusing cookers the attention they required. When attention lapsed, the focusing collectors would go out of focus and the food would not be done when needed. In addition, little gusts of summer wind would close or topple the multiple reflector cookers—again resulting in partially cooked food. But if food was placed in a solar box cooker early in the day, we were assured cooked food at mealtime.

Our parabolic and multiple reflector cookers came to be used only for foods requiring shorter cooking or warming periods, preferably while someone was close at hand to tend them. They ended up being used mainly for optional dishes, while solar boxes became the workhorses for cooking the main dishes. An additional solar box cooker or two would have done as well or better at meeting our needs than my yard full of the more intricately designed cookers. When using solar box cookers, once the food was in I could give full attention to other chores. In addition, on the few semi-cloudy days, food in the SBCs cooked much better than food in the cookers that depended more heavily on reflectors.

Although solar boxes have been used for home heating, water heating, and distilling, the box design has not yet been widely used for cooking. Perhaps this is because solar boxes are so unassuming; it is hard to jazz up a large, simple box to appeal well in advertising. It needs to be seen in action to be believed. But solar box cookers have clearly demonstrated their effectiveness when it comes to meeting food service needs whether it be for individuals or groups, households, schools, businesses, or other organizations.

Sherry Cole identified the widespread value of such a device. We joined forces to tell as many people as possible about solar box cookers. As the effort began to attract media attention, we had a string of visitors, including several from the engineering department of the nearby university. Earnestly and with all good intent, I was advised to drop the solar box as it was an inherently inefficient design. One good-hearted engineer sat next to a solar box that was quietly and cheerfully cooking a large pot of beans and told me that "solar boxes just won’t cook," and proved his point by referencing a scale in a technical book he had brought along!

This experience made it clear just how different our design goals are from engineering approaches. Engineering approaches and available literature often emphasize maximum solar collection at the expense of simplicity, functionality, appropriate materials and ease of use. Such designs do not lend themselves to being used as routine home appliances, nor can they be built without special tools and skills. Our design goals emphasized simplicity in design and construction easy for nonprofessional builders, the use of appropriate and available materials, ease of operation, and dependability under a wide variety of conditions. Our cardboard designs assumed the use of recycled materials and minimal additional costs.

After it became known that Sherry and I had experience with inexpensive solar box cookers, we were approached by two elementary school teachers. They wanted us to produce a lightweight teaching model made of cardboard that could be taken apart and reassembled repeatedly by third grade students in a special gifted program. They wanted a teaching device and a good cooking unit combined. This work led to the development of the patented Kerr-Cole Solar Box EcoCooker and the production of kits and complete stoves. Although designed for maximum simplicity, the EcoCooker design proved to be a useful teaching tool for all age and educational levels. We received both national and state energy innovation awards on this work.

While we did make wooden models, and still sell plans for a wooden design, the cardboard designs have been most popular. To allow viewers to see the construction details, the teaching model boxes can be completely disassembled. They can then be reassembled in a few minutes to start cooking.

In 1978 Dr. Robert Metcalf, an early SBC user, identified the SBC as a potentially powerful tool for public health efforts to reduce microbiological illness, and as a component for many environmental and social efforts from nutrition projects to tree planting efforts and more. With the full support of his Department Head, he has used his degree in microbiology and his position as Professor of Biological Sciences at California State University Sacramento to lay the scientific base for major expansion of SBC usage. Studies done with his students established the safety, microbiologically, of solar box cooking and the feasibility of water pasteurization by means of SBCs. With the safety of cooking and pasteurization scientifically established, Dr. Metcalf has spoken to thousands and has given hundreds of demonstrations to the academic and environmental community. He has spent private funds to travel to Washington, DC and elsewhere to speak to and demonstrate the solar box cooker for key people in government and private volunteer organizations. He and his wife have opened their home to travelers from all over the world coming to see the incredible cooking box.

Dr. Metcalf has donated hundreds of hours of his private time to give speeches and demonstrations. He interested Dr. Fred Barrett of the United States Department of Agriculture and together, with funds from US-AID, they prepared a video, "Four Square Feet of Sunshine," which introduces the concept of solar box cooking and its potential benefits. He also had a major role in preparation of the video, "A Bright Future." He recruited numerous friends, including Dr. William Sperber, from the Pillsbury Company, a company which provided funding for an SBC project in Bolivia as a part of an Applied Nutrition Program with the Freedom From Hunger Foundation. Together with an anthropologist, Dr. Aaron Zazueta, and local Aymarans, in March, 1987, they walked out beside potato fields on the altiplano of Bolivia and held the first international solar box cooker demonstration focused on technology transfer through the use of a functional cardboard model.

Dr. Metcalf also interested Leland Brenneman of PLAN International, formerly Foster Parents Plan, and worked closely with him as well as Dr. Sperber during a second Pillsbury-sponsored international project which was held in El Progreso, Guatemala in January, 1988.

Although the work by Sherry Cole and myself was progressing, it became clear that many more resources were needed to spread the knowledge and practice of solar box cooking. SOLAR BOX COOKERS INTERNATIONAL (SBCI) was formed in 1987 as a non-profit volunteer organization to respond to inquiries and provide central coordination of volunteer donations of time and money, develop educational materials, and promote knowledge of the technology and use of SBCs worldwide. Our goal came to be to make solar box cooking as widely known as cooking with fire. SBCI publishes a newsletter covering the broad aspects of what is happening in the SBC field. Their work is closely coordinated with SOLAR BOX COOKERS NORTHWEST (SBCN). Formed in 1989 in Seattle, SBCN publishes a fine quarterly newsletter specifically on solar box cooking, with reports on SBC activities around the globe. SBCN also coordinates an on-going international computer conference on EcoNet dealing with appropriate technology in general and SBCs in particular. Through these organizations, numerous volunteers with a broad spectrum of skills and professional qualifications have donated hundreds of hours of work. Through their efforts in providing workshops and literature, information on SBCs is now moving out to people all over the world.

The educational materials that SBCI has produced, which are available from their office, have greatly multiplied the effects of individual volunteers in the spread of knowledge about solar box cooking. But perhaps their most important contribution to date has been development of a training system and a workshop outline with materials to assist individuals in teaching others how to build and use SBCs to cook with the sun. These organizations deserve all possible support.

Information on other solar cooking efforts in many locations and with diverse designs has been gathered and published by the German Appropriate Technology Exchange. It appears at this time our effort is the one most heavily focused on technology transfer.

In practice, all one needs to produce good solar cooked meals is a common sense understanding of a solar box cooker. Solar cooking a few familiar recipes soon makes using an SBC easy. Since it helps for solar box cooker designers and teachers to have a more detailed understanding of solar cooking, solar radiation, solar heat production, heat traps and heat transfer, these items will be covered at length in this chapter.

"Solar box cooker" is used in this book as a generic term which covers any of many designs of solar cooker characterized by a large, insulated, horizontal cooking chamber with a glazed window on the top to allow sunlight into the insulated box. Usually there is one reflector stabilized by an adjustable prop to reflect additional sunlight through the window. Customarily the reflector is part of the lid. Solar box cookers may be constructed of any serviceable, nontoxic materials. The inner box and glazing must also be heat resistant.

Three BASIC NATURAL LAWS are employed in making and using solar box cookers. One natural law is that when solar radiation (sunlight) strikes a dark surface it changes to infrared radiation (heat). A second natural law is that when light falls on light-colored or shiny surfaces it reflects and so can be directed to where it is needed. A final natural law is that solar radiation (sunlight) passes through a transparent window easily, but infrared radiation (heat) does not, so heat can be trapped. "Sunlight, the energy resource that powers photosynthesis and drives the earth’s weather, is the foundation for all life on earth. Energy resources such as coal, oil, and gas, formed from solar grown organic matter, are not renewable and will be consumed in a tiny fraction of the millions of years required for their formation. Although solar energy is a diffuse resource, its effective use has been documented since the time of the Greeks, the Romans, and the ancient cliff dwellers of the North American Southwest. As we deplete our non-renewable energy resources, we must continue to develop ways to use solar energy directly.

"Throughout history, humans have applied solar energy principles to provide for a variety of needs....As we approach the next millennium, the problems of world hunger, contaminated drinking water, deforestation, and the fuelwood crisis, are becoming more acute. The invention and use of simple, inexpensive, and low-tech solar box cookers, have resulted in the emergence of solar cooking as a solution to this variety of problems.

"With an understanding of basic principles of heat flow and access to simple materials such as cardboard, aluminum foil, and glass, one can build an effective solar cooking device...."8

The basic purpose of a solar box cooker is to heat things up—cook food, purify water, and sterilize instruments, to mention a few....

"The basic heating dynamics are:

A solar box cooker cooks because the interior of the box is heated by the energy of the sun. Given this heat input, the temperature inside of a solar box cooker will continue to rise until the heat loss of the cooker is equal to the solar heat gain.

Given two boxes that have equal heat loss, the one that has more gain, from stronger sunlight or additional sunlight via a reflector, will be hotter inside.

Given two boxes that have equal heat gain, the one that has less heat loss—better insulated walls, bottom and top—will reach a higher interior temperature.

The most efficient use of solar radiation by SBCs is determined by the location of heat production both directly and through the use of reflection and by the competence of the insulation retaining the heat thus produced. Both direct and reflected sunlight passing through the glazing onto a dark pot produce heat on the pot sides which flows directly into food since heat naturally seeks to equalize temperatures by moving from hotter areas into colder areas. Sunlight also falls on the reflective oven sides and the dark bottom of the oven. Reflective sides in an SBC throw additional light onto the dark pots in the center as well as adding to the sunlight on the dark drip tray.

A reflective bottom will reflect sunlight up the sides and back out through the glass without it ever becoming heat. For this reason the bottom of solar box cookers is customarily dark. Then even if there is no dark pot in the oven, heat from the dark tray rises to preheat the oven air. A totally reflective oven will not get very hot unless there is a dark object inside.

Cold air is excluded and the heat produced is trapped by insulation in the sides and bottom of the SBC as well as by the glazing and the close fitting lid. The hot oven air transfers additional heat to the pots. The temperature range an SBC will achieve is determined by a combination of all these factors, as well as by others discussed in Chapter 4.

Don’t get overwhelmed with all these details which are provided to give background for trouble-shooters, educators, researchers and designers. Actual cooking consists of putting the food in a dark, lidded pot, pointing the SBC at the sun and giving it sufficient time to cook. That is all most people need to consider when using their SBC.

The MAXIMUM TEMPERATURES for empty SBCs focussed in full sunlight range from 220 F (104 C) to 300 F (149 C) and occasionally above. Temperatures always temporarily drop when the ovens are opened and particularly when cold food is put inside. They immediately begin to heat the food which keeps the temperatures naturally low until the food is hot. Cooking occurs at any point above 190 F (88 C); boiling temperature is 212 F (100 C) at sea level, lower at higher altitudes. Any SBC which reaches 250 F (121 C) or above is considered a good model. Ovens peaking below that level take longer to cook and do not brown food but may be very serviceable.

COOKING TIMES are effected by a number of other factors in addition to the caliber of SBC. An SBC’s function varies with sun angles, sun conditions, location including latitude and altitude, weather and mass. Temperatures are a result of a combination of all these varying conditions so cooking times in written materials are always approximations.

SUN ANGLES are effected by daily changes and seasonal changes as well as differences in sun angles dependent on the latitude. During the early morning and late afternoon with their lower sun angles, foods clearly cook more slowly than during mid-day. When the shadows are long, cooking times will be longer and it will not be possible to finish hard to cook foods, or large quantities of food without special care. It is not necessary to wait for optimum sun angles to utilize an SBC. Preheating of food can be done by the low angles of the morning sunlight, while food that is already hot and cooking will continue to cook in late afternoon at low sun angles.

The most frequent special approach to SBC cooking with a low functioning SBC and/or low sun angles is to heat food briefly before putting it into the SBC for the remainder of cooking time.

Angles also change slowly as the daily arc of the sun swings higher or lower across the sky over the seasons, with best cooking capacity in summer and reduced cooking capacity in winter. With regard to seasonal changes in sun angles, we are revising our expectations as we hear from SBC users. Individuals can use SBCs for single servings when family quantities might be impossible. As winter comes on, the solar cook begins to feel when solar cooking of the foods and amounts needed can no longer be done.

Areas of 20 LATITUDE or less have year-round solar box cooking angles and proportionally less as one goes further from the equator. By about 30 latitude, there is a noticeable drop in sun angles in the winter but easy to cook dishes can be prepared any sunny day. At about 40 latitude, cooking is easily possible 7 to 8 months of the year in Sacramento (Sea level 38.5 N. Latitude). In Seattle (Sea level, 47.5 N. Latitude), they have found that reduced quantities of food can be cooked even in January. This is at a far higher latitude than our early experience led us to expect winter cooking. A 5 pound (2.25 kg) chicken was cooked in Seattle on November 2, 1989, which was a windless, sunny day. They were using a homemade SBC of the SBCI Eco-design with double oven cooking bag in double glazing. There is also a report of cooking a full meal in mid-February in Seattle. Neither of these cooks was using any special cooking methods.

Thus, for well over half the year, assuming solar cooking in the south latitudes is comparable, the area of potentially effective solar box cooking reaches from lower Canada in the northern hemisphere almost to the tip of the South American continent in the southern hemisphere. In brief, geographically this covers all of Afghanistan, all of the African continent, all of the Arabian peninsula, all of Australia, all of Central and South America except the very most southern parts of Chili and Argentina, most of China, southern portions of France and Italy, all of the India peninsula, the Hawaiian Islands, all of Japan, Indonesia, Mexico, most of the Middle Eastern countries north to the level of Bucharest, parts of New Zealand, Pakistan, the Philippines, southern portions of the Soviet Union, all of Spain, Sri Lanka, and Thailand, as well as all of the United States. This list is only representative of the areas of the globe for reference by people from various locations and does not represent all the territory where SBCs could be used. Also, this list does not consider local factors such as persistent cloud cover, severe air pollution, etc., which will eliminate some areas.

ALTITUDE, as opposed to latitude, has little effect except perhaps to provide somewhat better sun conditions as the atmosphere and pollution thins at higher altitudes. Yet, because water boils at lower temperatures, cooking some foods may be slower at higher altitudes. It has been reported that these factors seem to balance each other out and solar cooking proceeds at high altitudes much like anywhere else. Solar box cooking definitely has been accomplished at 12,000’ altitude, 15 latitude in March. On the Bolivian altiplano, rice was reported to cook more rapidly than it would in Sacramento with the same sun angle. Their small potatoes as well as their fish were delicious.

SUN CONDITIONS also effect cooking time. Bright, clear sunlight is obviously the most powerful. With the solar box design, cooking can continue but more slowly when there is some reduction in sunlight due to cloud cover or overcast conditions, high humidity, and air pollution including smog and dust. Sometimes cooking can be improved by simply moving the SBC away from the side of a road, to where it is sheltered from traffic pollution by a building or a tree. During harmattan, cooking was successful in Sierra Leone in January.

MARGINAL WEATHER is not a great problem. A full sunny day is not required for cooking in a solar box oven, in contrast to primarily reflective solar cookers. Sunlight must be strong enough for a fairly distinct shadow. SBCs use indirect as well as direct radiation and so can cook, although more slowly, under somewhat overcast conditions. Likewise, when there are intermittent clouds, but with full sunlight for roughly 30% or more of the time, cooking will proceed in a solar box, although more slowly. This wide range of radiation used by solar box cookers is one reason the designs are particularly suitable for serious solar cooking. They can be depended on to produce cooked food for a large portion of the year in most of the world’s temperate and tropical zones. On a completely cloudy day or one with less than 30% sunshine, traditional methods can be used.

On a semi-cloudy day, easy to cook foods may be chosen or food may cut into small pieces, or divided into several pots. If necessary, foods can be started with hot water, or even brought to a boil before placing in the SBC. If miscalculation or changing conditions leave the food not quite done, a little conventional fuel can finish cooking the meal.

MONSOON solar cooking may be possible simply by identifying the bright hours and adapting to the length of cooking time available. For instance, during the monsoon season in Arizona, there are usually about two hours of bright early morning sun before clouds close in. Easy to cook foods started early can be well begun or often completely cooked in this limited sunny time. Moderately hard or hard to cook foods may brought to a boil by other fuels before being placed in the SBC. Or just as the clouds arrive, pots which are fully heated and cooking in an SBC can be converted to retained heat techniques by packing insulated cushions around the pots inside the cooker. See the Retained Heat section in this Chapter.

When it is WINDY AND COLD but sunny, SBC cooking is slowed somewhat by chilling as wind moves across the glazing. On a bright, very cold, still day, one need only allow extra time for cooking. When it is cold AND windy, a second layer of glass or other glazing ¼ to ½ inch (6 to 12 mm) above the first glazing is essential. It creates a layer of still hot air to insulate the window. Sometimes an SBC lid and prop must be tied on, or the whole oven braced with rocks to withstand gusting winds and yet it will still cook. Water condensed on the inside of an SBC window may simply be wiped off periodically with a soft cloth. Condensation on the window is more often due to cooking without a tight lid than to weather conditions. Thin glazings that flutter in the wind are less effective than rigid glazing, either glass or plastic. For more about windows and glazings, see the glazing section in Chapter 4.

The MASS within the oven insulation—walls, tray, pots, and food—all make a difference in cooking time. The well-insulated oven, lined with foil-covered cardboard, having a light metal, dark colored tray has very little mass as compared to a wooden or metal oven lined with sheet metal or having a heavy metal tray. Both will heat, but the thick sheet metal will require longer to heat initially and will cool correspondingly more slowly when sunshine is reduced. The same considerations apply to pots. Thin-sided, darkened stainless steel or graniteware pots start to cook quicker than heavier ceramic, tinted or painted glass, earthenware or cast iron containers. However, due to heat stored in the thermal mass, once the food is cooking in the heavier cast iron and ceramic pots, it will continue to cook through intermittent clouds better than thinner pots. Once the mass is hot, all pots cook similar foods in about the same time.

These construction and equipment details cause some of the variations in the cooking ability of various stoves. They also make it clear why under most circumstances adobe, brick or ceramic walls do not make a good interior siding for solar box cookers...too much heat goes into the walls of the stove rather than into food.

In addition to the mass of the oven and the pots, another important consideration is how much food mass is being cooked and the shape of the mass. Large mass, say 10 pounds (4.5 kg), in one pot will cook more slowly than the same mass divided into two or three pots. Whole potatoes or meat in a large solid piece cook more slowly than the same foods cut into smaller pieces. For cooking regularly under good solar conditions, it does not matter much how heavy the pot or how the mass is distributed—heavier and thicker simply takes more time. But if cooking conditions are marginal, a thin pot with food cut into small pieces may cook when more massive configurations do not. Although aluminum pots transfer heat quickly and are available around the world, I hesitate to recommend them because as yet scientific studies have not been done to establish if significant amounts of aluminum compounds enter the food when cooking for the lengths of time required by SBCs.

Extra mass is sometimes temporarily added to an oven to store heat for a rapid start, as when cooking breads. Additional mass also provides stabilizing heat during days with variable clouds and stores additional heat for holding foods after sundown. Bottles of water may appear to be a better choice than rocks, adobe bricks, railroad spikes or cast iron pans for heat storage because water can store the most BTUs. However, the maximum temperature unpressured water can reach is the boiling point, 212 F (100 C) at sea level. Although it takes about 5 pounds (2.25 kg) of bricks to equal the heat storage capacity of 1 pound (0.45 kg) of water, bricks can store heat up to the maximum temperature of the SBC. This provides a higher temperature and greater quantity of stored heat which is available for breads or other foods when they are first placed in the SBC.

The strategy of using mass should be varied depending on the needs in a given situation. If there is ample sun and ample time, putting food and extra mass in all at once is a labor saving combination of steps. However, heat forming in the oven will go equally into all available mass. If food is put in alone first, all available heat will flow into it, which starts the food cooking more quickly. Mass added after food is up to temperature utilizes excess heat not going into the food and so does not delay cooking.

RETAINED HEAT cooking goes hand in hand with solar box cooking. In traditional retained heat cooking, a fairly large amount of food in a pot with a tight lid is brought to a boil and simmered for 5 to 30 minutes depending on the type of food being cooked. (See "Simmering Times," page 13). After simmering, the food is quickly moved to a well insulated box and packed with light, clean pillows of insulation. Care is taken not to disturb the lid so steam does not escape. Cooking levels of heat will be retained for up to four hours. In two to three times the usual stove time, food is cooked by the retained heat. This technique works well with 3 pints (1.4 liters) or more of food mass, but small recipes may not retain heat for sufficiently long periods to complete cooking and may have to be reheated after an hour or so.

When combining retained heat and solar cooking, if food has gotten thoroughly hot in an SBC, but clouds arrive before the food is finished cooking, a switch from solar to retained heat cooking should be made before the oven temperature drops below the boiling point. For large recipes this may be accomplished by simply closing the reflective lid on the pots of cooking foods. For smaller recipes, the solar oven is opened,taking care not to allow steam to escape from under the lids, pots are pushed close together along with any heated additional mass. Insulating pads or soft cushions are tucked closely around the pots and well heated mass. The SBC lid is then closed. This effectively makes the transition from solar to retained heat cooking for small quantities. The cooker lid remains closed until shortly before serving time, when the food is tested. If not completely done, a very little conventional fuel will usually finish the job. As a caution, if the mass is not well heated to the core, it may absorb some of the heat from the food and so quickly lower the overall temperature and delay cooking.

Usually solar/retained heat cooking is done right where the SBC is located. However, a lightweight portable SBC can be moved temporarily indoors for its retained heat cooking time if the sun clouds over or if it rains. It may also be brought inside more or less permanently during the off season or at night and function as an insulated box for retained heat cooking. Used in this way the SBC continues to save fuel rather than simply being stored until conditions are right for solar cooking.

Some of the ADVANTAGES OF RETAINED HEAT cooking over conventional cooking are that it uses reduced amounts of fuel, reduces the amount of attention directed toward cooking, and food can be prepared any time, day or night under any weather conditions. This procedure is well worth exploring in different areas of the globe and works very well in conjunction with solar cooking.

FOOD SAFETY with regard to retained heat cooking has been studied by Dr Kirschner. She states, "Cooking with retained heat in a fireless cooker means that the food cooks while the temperature drops very slowly. I have checked innumerable times, using many different amounts and types of foods and sizes of pots, and found that for up to 4 hours—and often much longer—food stays steaming hot in the cooker, and the temperature is at or above 145 F (63 C) .... Should you want to leave the food in the cooker for longer periods ...bring it back to the boiling point and simmer it for 5 minutes to be absolutely sure it won’t spoil."

Food safety for food cooked by any method requires meeting specific rigid conditions. Cooked food at temperatures between 125 F (52 C) and 50 F (10 C) can grow harmful bacteria. This temperature range is known as the danger zone. To protect against food poisoning, microbiologists and home economists strongly recommend that food be kept either above or below these temperatures. These precautions are the same whether food is cooked with gas, electricity, microwaves, wood fire, or solar heat as well as foods cooked by retained heat, crock pot, barbecue pit or any other method. In cooked food held at room temperature, there is a chance of Bacillus cereus food poisoning, a major intestinal illness. Worse, if the food is not thoroughly reheated before consumption, there is a chance of deadly botulism poisoning or salmonella. Even if it is reheated, when cooked food has been in the danger zone for three to four hours, there remains a risk of food poisoning3 in solar cooked food as in food cooked by any other method.

It has been carefully documented with regard to solar box cookers that it is safe to place raw refrigerated or frozen food, even chicken or other meat, in an SBC in the morning several hours before the sun begins to cook it. Refrigerated food placed in an SBC remains sufficiently cold until the sun starts to heat the SBC. Once the full sun is on the oven, the heating of food proceeds quickly enough so that there is no danger of food poisoning.2 Uncooked grains, beans and other dried raw foods can also be placed in an SBC in advance. Both of these methods facilitate absentee cooking.

There are three main points at which caution is required: it is dangerous to keep cooked food more than three or four hours in an unheated or cooling SBC unless both the SBC and food have been cooled rather quickly to below 50 F (10 C) in which case the SBC is serving as a cool box; it is dangerous to let cooked food remain overnight in an SBC unless it is likewise cooled; and it is dangerous for food to partially cook and then remain warm in the SBC when temperatures are not sustained as might occur on a poor solar cooking day, at the end of the day or when clouds move in. Cooked or partially cooked food should either be cooled to below 50 F (10 C) or cooking should be finished with an alternate fuel. If food has remained in the temperature danger zone for 3 to 4 hours it should be considered spoiled and should be discarded. Reheating the food does not correct the problem as heat does not inactivate all toxins.

Food does not have to be visibly spoiled in order to be toxic and cause illness evidenced by nausea, vomiting and diarrhea. Even if food has not been at the incubating temperatures of the danger zone for the full 3 to 4 hours, absolutely discard food that is bubbling, foaming, has a bad smell, is becoming discolored, or gives any other indication of spoilage. Discard it out of reach of animals and children and thoroughly wash the pot. Discard it without tasting it as even small amounts can make an adult very sick.

If temperatures below 50 F (10 C) cannot be obtained, it is still valuable to drop food temperatures as low as possible and as quickly as possible rather than allowing food to remain warm since bacteria grow more slowly at lower temperatures.

An alternative method of holding cooked food is to reliably maintain the temperature of the entire food mass above 125 F (53 C). This can be achieved by first heating the food to boiling, simmering for a few minutes to allow heat to penetrate to the center of each particle and for a pocket of steam to collect under the lid. Then proceed as for retained heat cooking. This provides the level of temperature needed throughout the food, whereas leaving a pot of food on a very small flame may allow food at the edges to remain in the danger zone. Where neither of these methods can be used, it is best to cook amounts of food that will be consumed in one meal relatively soon after being cooked.

ADVANTAGES OF SOLAR BOX COOKING over both conventional cooking methods and retained heat cooking are that it uses only sunlight most days. SBCs can cook large or small amounts of mass. A wider range of pots can be used in an SBC, since an absolutely tight lid is not required, although recommended. And SBCs can produce delicious boiled, simmered, baked, or roasted and lightly browned food with a more varied texture.

FUEL SAVINGS with either solar cooking or retained heat cooking as well as with the solar/retained heat combination can be significant. In India, families using SBCs regularly reduce use of conventional fuels or firewood by about half. In Arizona the fuel saving is more than that. It has not been formally calculated for many places but families using solar box cookers as their major stoves become adept at using what sunlight is available to reduce their use of other conventional fuels such as firewood, gas or charcoal to a minimum.

SBCs have been well received in a great many areas. The solar box cooked food often duplicates or at least closely approaches the traditional flavors, but some regional foods may take careful trials to match customary textures and flavors. Acceptance of SBCs sometimes depends on discovering the best solar box cooking method for favorite local foods. A very small group, or even one individual, can pioneer solar cooking in an area. This can be a social pleasure as well as test a lot of recipes.

In addition to cooking traditional foods, the SBC offers opportunities for cooking foods in different ways in more versatility, so that new recipes may be used. Such recipes may or may not be traditional, but certainly can be very good. Baking breads, cakes and cookies can be a particular delight where baking is not customarily possible. Experimenting can be fun. Most Americans enjoy Chinese, Japanese, Mexican, Middle Eastern and other different foods as well as any of the foods encountered while they grew up. The popularity being experienced by American fast food chains as they expand into other countries highlights the ability of traditionally habituated palates to accept new tastes.

Flavoring is a delicate process. Some of the differences in taste can come from cooking methods. Browning onions, garlic and other spices prior to adding the remainder of the ingredients induces a different flavor, subtle but important in some stews and casseroles. Rice and other grains must have the familiar texture as well as the right flavoring to satisfy—that is, to taste "right." Since people in each locality can taste the subtle differences in their foods, local pioneer solar cooks are in the best position to test different methods. Experimental cooking can be done, using any of at least seven different approaches and the results compared for the best flavor.

1. Put all ingredients into pot at the beginning...get it on early...don’t worry about overcooking. This is the SBC standard, trouble-free method and usually works well.
2. Heat the pan perhaps with a little oil, then add the uncooked ingredients... Good for steaming fresh, moistened leafy greens according to some; others like their greens only when in a bath of delicious cooking water. Preheating the SBC and pans is also good for cooking most breads and starting meats.
3. Bring water to simmering, then add air temperature food—raw grain or meal or heavy vegetables. Stir mushes, grits and porridges well initially and stir again after a half an hour or so. Stirred at those two points, then they can cook unattended as long as necessary without burning or sticking. Vegetables like broccoli, cauliflower or green beans cook well dropped into steaming hot water for 5 to 10 minutes, then drained and kept in a pot in the SBC for another half an hour or so. Soups or drinks may be made with the cooking water to preserve nutrients.
4. Bring water to simmering while separately heating the dry ingredients in the SBC. When both are hot, combine them and cook for reduced amounts of time. The consistency of pasta is greatly improved by this method, particularly if a little oil is stirred into the pasta before heating. Sauces can be prepared in this way for dishes which are simply not the same if all cooked in a single pot. Sadza, the staple made from ground white corn in Zimbabwe, may be made this way. Hot corn meal (Mealy meal) is added to near boiling water in proper proportions, and vigorously mixed for about a minute to achieve the proper texture. Some rice that becomes mushy if cooked from the beginning in water, comes out with separate grains when heated separately and combined to cook for a shorter period.
5. Briefly precook selected ingredients over a small fire...Saute garlic and other spices and herbs before combining with the rest of the dish and putting it in the SBC. If a difference in taste is due to the absence of browned flavor or of smoke, this may provide just the needed touch.
6. Add some ingredients near the end of cooking...grated cheese, some spices or herbs, butter, vinegar, etc.
7. Prepare new recipes for solar cooking using traditional ingredients.

If certain traditional foods cannot be duplicated, reserve those recipes for cloudy days when former cooking methods must be used.

Integration of SBCs into the lifestyles of individuals takes many forms. None of the uses are mutually exclusive and, in fact, most people enjoy the flexibility they present.

Briefly listed, here are some of patterns we see:

Such vignettes only hint at the variety of uses made of SBCs. The final example, The Sustainable Emergency Kitchen, will be covered in more detail since it can be worked in a number of different ways and can serve as a pattern for recreational or vacation cooking, or it can meet serious emergency needs on a long term basis. Savory food can be created from either fresh or stored food supplies for months or years while requiring a very minimum of fuel by using an SBC in combination with retained heat cooking augmented by small wood fires and with flint and steel on dry cotton fluff or some other method for quickly starting flames.

Equipment for an emergency must be as simple and labor-saving as possible, sparing energy and time for other critical activities. An emergency kitchen must be versatile and adequate to function one way or another any day or at night if needed. Rain, blizzard, wind, or any emergency must not interrupt the flow of nutritious cooked food for a family.

Many people plan to cook on a wood fire in an emergency. Yet there are problems with this. Many traditional cooking fires take large quantities of wood—an unrealistic demand at this time in history. As stores are depleted, wood may need to be gathered locally and carried home. Manual gathering and transportation of cooking fuel is strenuous work, exhausting and time-consuming. Utilizing mechanized methods of firewood gathering requires gasoline, lubricants and replacements to keep a truck and chainsaw functioning. All of these are products of a smoothly functioning industrialized economy and may not be available in certain areas or even globally during a longstanding emergency. Even with the help of a pack animal, moving fuelwood is heavy, bulky work. With exhaustion of the supply, fuel gathering must be done at greater and greater distances, and at increasing expense of time and human energy.

Furthermore, the numbers of people in our present populations can denude an area very quickly, leaving the earth stripped of the protection of trees and shrubs. Widening circles of devastation are growing today around many population centers where wood and charcoal are heavily used for cooking. It will take generations for the land to recover from such overuse if it is ever possible under the changing circumstances.

It is essential to plan to protect our remaining fuelwood trees by holding fuelwood cutting and gathering below the level of natural replacement so that our natural tree cover can grow back and the land return to a healthy condition. It also makes good sense to plant especially fast-growing fuelwood trees around the home in advance of any actual need since they take several years to grow. Mature fuelwood trees and orchard trimmings, if used sparingly, can do a wonderful amount of cooking, particularly if the wood is burned in one of the many varieties of improved woodstoves.

Solar energy is a non-consumptive and non-polluting fuel which is delivered freely and even in excessive amounts in the areas where most people live. The solar box cooker can be the heart of a sustainable emergency kitchen. The SBC would be supplemented at night or on cloudy days with retained heat and small wood fires. In addition, the Sustainable Emergency Kitchen may include a variety of solar water heaters, a solar water and milk pasteurizer, and/or a solar food dryer. There might also be an organic food-producing garden and food producing solar greenhouse.

Small wood fires differ from regular campfires and from most traditional wood stoves in that a minimum of wood is utilized. Gathered wood is often ½ inch to 1 ½ inches (1.25 to 3.75 cm) in diameter...a common branch size and a common size for kindling. In a good stove, four or five small sticks at a time are sufficient. The flame is concentrated in one spot directly underneath the pot. Pans are placed directly over the flame which carries more heat than embers and is available immediately. A variety of fuels can be used—branches and twigs, pine cones, scrap lumber, twists of papers, strips of cardboard, dry corn cobs, dung—all in the same type small wood stove.

In recent years a great deal of effort has gone into designing efficient small wood stoves and there are many different designs. There are also some efficient home made designs that have been used over generations.

When using a single pot fire for multiple dishes, work through the menu, cooking the food that will take longest first, bringing to a boil and then simmered according to the schedule prepared by Dr. Kirschner. As each pot is removed from the fire, it is wrapped in a soot cloth, sack or paper bag before placing it in the retained heat cooker. When all foods are sufficiently heated and packed away, smother the fire to conserve the unburned ends for next time.

A new flame can be made almost as quickly as with a match by striking a spark onto a small fluff of very dry loose cotton or other very good, dry tinder. The initial flame can be used to ignite a roll of paper or very thin piece of wood to use as a taper.

Learning to cook using sustainable emergency kitchen equipment and techniques is not really very complicated. Practice helps make this a simple process. The value of SBCs as emergency cookstoves and as retained heat boxes does not detract from their use for recreational cooking, for fire safety, for keeping the home cool in the summer time, for reducing utility costs, etc. All these and more are legitimate reasons to incorporate SBCs into a homemaking routine. Using solar box cookers for fun or convenience when there is no emergency can organize the necessary equipment and can produce a very useful skill. Having an SBC in regular use makes it easy to cook through times of emergency with a minimum of disruption.

This chapter on Design Elements will detail as fully as possible our experience to date. It gives an informed base on which additional information can accumulate as the expanding field of solar box cookers continues to grow.

The Kerr-Cole EcoCooker cardboard design, patented in 1980, for years has been a standard. Assembled it measures 29" x 25" x 11" (72.5 x 62.5 x 27.5 cm). With 3 inches (7.5 cm) of foil and crushed newspaper insulation, it is fitted with one piece of single strength window glass 24" x 20" (60 x 50 cm). The base is built with two open-topped boxes, insulated, then covered with "toppers" that stretch over the outside, across the top and down the inside of the assembly. This makes a very substantial cardboard SBC which still remains a peak performing model. In the Temperate Zone it reaches temperatures between 275 to 300 F (135 to 149 C) and performs even better in equatorial locations. This design has been widely popularized by SBCI through excellent educational materials, home building plans, and workshops. (See plans in Chapter 9, Part A).

In 1990, Mr. Joe On of Sacramento suggested a design requiring less materials. The JoeOn design is simplicity itself: after tracing the outline of a smaller box onto the closed top of a larger box, one simply cuts out the traced hole and inserts the former into the latter. (See plans in Chapter 9, Part D). The dimensions vary depending on boxes available, but often they are much smaller than the Eco. Generally the wall thickness is reduced to 1 to 2 inches (2.5 to 5 cm) and plastic glazing is used making a smaller, lighter model. Maximum temperatures range from 220 F to 275 F (104 to 135 C) and occasionally above. While the design does not have the years of service behind it and is considerably less substantial than the EcoCooker, it is quicker to build, preheats rapidly and given adequate time, normally cook very well. There are a number of designs developed following this general pattern and very satisfactory SBCs are produced.

A third line of designing involves two strips of panels and formulas for adapting to a wide range of dimensions. (See plans in Chapter 9, Part B). This relatively new development is included here since it conceptually may serve to stimulate thought toward many different models: homecrafted from scraps glued together or panels of different materials as well as possibly being adaptable for mass production particularly to meet emergency needs. The dimensions can be set to the size of glazing available, or to the customary pot height for an area, or to a predetermined width of cardboard or other material. Given a particularly good available outer box, the strip pattern can be used to build a fitted inner box. The designs of the lids are similar for all three SBCs except for the variety in props.

These three lines of designing do not exhaust the field. Many efforts have been made toward designing a collapsible model and one, which performs very well, is ready for general marketing. Another design, a cylindrical form, is also in production. Neither of these lends itself to handcrafting.

Designers should use these illustrations only as background information and freely combine solar box components using whatever materials are available in a way that meets the needs of their own people.

The SIZE is important. For serious solar cooking for a family or group, SBCs need about 3 square feet (0.27 square meters) of window glazing to function adequately, cooking a maximum of about 10 pounds (4.5 kg) of food on good sunny days. Four square feet (0.36 sq. m.) is preferable for a really good, multiple-purpose solar box cooker as it will cook about 12 pounds (5.4 kg) of food. Large cookers will continue to cook although lesser amounts on semi-overcast days.

Smaller SBCs having smaller glazed areas collect less solar radiation, have limited pan size and often lower maximum temperatures. They can not heat as much mass as larger SBCs. While they may cook amounts suitable for one or two people, they usually can not handle sufficient food for a family or small group, unless several small SBCs are utilized. Small ones have generally been more of a recreational oven rather than a major appliance for food service. They have their place in the family of solar box cookers perhaps for easy transportability, for demonstrations, for school projects or in cooking for a single person but just don’t expect a pony to do the work of a horse. Small versions clearly have wide potential for use by isolated individuals, the homeless, or transients who cannot deal with the bulkier standard size and have no need for the quantities of cooked food they can produce.

For special times, SBCs of any size can be gathered into one location to produce food for a crowd. Sixty to 80 people on many occasions have been served from SBCs gathered in Dr. Metcalf’s backyard in Sacramento for his gala, delicious "Solarques."

Collapsible models of SBCs being designed are opening an entirely new field of development as they combine larger cooking capacities with smaller storage needs and much easier transporting.

The DEPTH is determined by the height of the tallest container to be used, plus 1 inch (2.5 cm). Broad, shallow ovens take less material and heat more quickly. They have a higher solar gain collector area to heat loss surface area ratio. With greater heat gain per heat loss, the effective heat will be greater and temperatures will be higher as well.

For maximum cooking area, the sides are perpendicular. Slanted interior walls provide slightly higher temperatures in the center but restrict the area on which pots may stand sometimes making the use of cookie trays, large roasters, or more than one pot impossible. Slanted sides are also much more difficult to build than perpendicular ones..

The SHAPE of an SBC can be rectangular, square, round or oval. Rectangular and oval SBCs have a broad side facing the sun for maximum collection of sunlight as the sun passes overhead. This makes the best use of reflected light, as well as direct sun, and reduces the need for changing focus. Square SBCs are efficient ovens and have the added advantage that pieces for all sides are similar. Rectangular ones seem to pack and transport easier than square, particularly as they can be picked up easier by small-framed people.

Round and oval SBCs are intriguing and skilled craftworkers have produced them. So far no written plans for an effective homecrafted version have been seen. One of the problems is securing a reflector to the curve of the oven without losing too much sunlight on the top surface. There are also problems making a round frame and perhaps obtaining round pieces of glass for the glazing. Round and oval forms without reflectors could be a good choice near the equator where reflected light is less essential.

Corrugated CARDBOARD is widely available in some parts of the world for recycling. It makes a fairly durable, biodegradable unit. If new, it is often inexpensive, especially in urban areas. Cardboard is rigid, lightweight, and has good insulation qualities. It cuts easily, glues easily and yet has sufficient structural integrity for the purpose. Like heavy cloth, leather, and some basket-making fibers, it will form its own hinges without hardware. It is not expected that all homemade SBCs will be made of cardboard, only that it meets the needs of the SBCs in which it is used. Pressed paper board has also been used but is more difficult to work.

The exterior siding is often different from the interior. Wood exterior and cardboard interior makes a very functional combination. Wood is biodegradable, yet weather resistant and durable. Wood/cardboard combination SBCs have been cooking since 1978 with minimal upkeep. Completely cardboard ones have been cooking for eight seasons so far with one cardboard SBC still cooking after eleven years! These have, of course, been protected from rain. Often these units were untreated, but some had a commercial white kaolin coating. Water-resistance of cardboard is increased by coating the SBC with oil, fat, or petroleum jelly and placing it in the sun until the coating penetrates. Other exterior weather resistant coverings are contact paper and house paint.

The most appropriate SIDING will depend on the materials and skills in an area and are based on availability and/or cultural norm. The complete range of siding materials has not been established and is an important area for local investigations. It is crucial that the material for the inner box not give off toxic fumes or strong odors either cool or at cooking temperatures. The exterior of an SBC may be made from any paneling available.

For good stewardship of Earth resources, biodegradable sidings such as cardboard, wood, natural fibers, earth and adobe are encouraged.

Cardboard ovens appeal to many, particularly environmentalists. Ultimately they are almost completely biodegradable. When the design is homemade, cardboard, often recycled from bins behind hardware and grocery stores, can be transformed into a major home appliance—the cook stove. If cardboard is not available, many other lightweight materials may be substituted. And free fuel, the sunshine, is available most of the year for most of the people on the planet.

Adobe, dung or mud/dung mixtures that have been formed into thick slabs or bricks are suitable for exterior sidings. Dung is needed primarily for returning to earth as fertilizer; however, if it is being gathered and burned, its use to make SBC sidings to reduce the loss from continued burning seems justified. Other vegetable fibers mixed with a nontoxic adhesive, could be pressed into a forming mold to make slabs. When earth or adobe are used in a siding, it is important to insulate the inner box from the earth mass otherwise the heavy earth will act as a heat sink keeping the oven below cooking temperatures. There are reports from Zimbabwe of people placing a boiling pot in an uninsulated adobe box, covering it with a glass and thus continuing to cook by solar energy for the remainder of the time needed. Zimbabwe extends from approximately 22 to 15.5 South Latitude...a Sunbelt land. We do not have any reports of placing cold, raw food in uninsulated adobe and successfully solar cooking it.

Thick quilts or cloth pillows may be shaped into the oven form, allowing a large part of the SBC to be spun and woven. Filled with carded wool, cotton or other fiber and sewn together, care must be taken that there are no heat leaks at the seams, perhaps by overlapping to eliminate cracks or thin areas. It might be necessary in Equatorial locations to shade the outer cloth in some way to prevent deterioration by ultra-violet light.

Woven cloth dipped in oil or grease was historically called oiled-cloth and used for rainwear (known as "slickers") and for containers to store perishable materials on shipboard. Similar oiled cloth could be used for exterior sidings of SBCs. Dried animal skins have made containers for many people over the years and probably could be adapted to solar box cookers. When such materials are used, stitching the shape together and sealing the punctures with natural resin might work better than gluing.

Basketry outer forms work where skill and materials to produce them are available. A paper mash siding could be made of recycled fibers as for egg cartons. Some egg cartons are vacuum-formed from large vats of paper fibers. This process might be adapted to production of SBCs locally.

The pit solar oven simply utilizes a hole dug in the earth as exterior siding, along with good insulation, such as pine needles or hay, and a reflective inner box. The lid and glazing may be standard, although in equatorial areas, the reflector may not be needed.

INSULATION in SBCs with the inner box/outer box construction is usually accomplished by lining the space between boxes with foil. Foil insulation may have patches but the foil should make a complete cover. Wrinkles are no problem if pressed flat. Air bubbles behind the foil make it easier to tear but work alright for insulation. Priorities on the use of limited amounts of foil are as follows: 1) cover the inner oven space including the inner top of the lid around the glazing and the reflector if one is essential, 2) the inside of the outer box and 3) the oven side of baffles. Foil alone is sufficient in many SBCs.

For additional insulation, lightly pack the foiled space with any light-weight, clean-smelling, non-toxic substance. Such bulk insulation is packed loosely as it insulates best if quite airy. Yet it is packed sufficiently tight so that it will not settle over time, leaving an empty, uninsulated space at the top. Also, with loose packing the sides of the oven are not forced out. A baffle, foiled or not, helps insulate, in part by preventing settling and in part by blocking the convective flow of air. Baffles, which are sheets of cardboard or other material used to partition the insulation space within each wall, are secured approximately in the middle of the space held in place by insulating filler on both sides. A slanted baffle adds to the structural strength but is not significantly better for insulation than a perpendicular one according to work done by Dr. Ed Pejack.

The Pejack study ranks bulk insulation materials from poor to best as crumpled styrofoam cups, crumpled newspapers, straw, wool, rice hulls, and feathers. Field tests on this ranking are not complete, however, styrofoam in any form is not recommended because it is fabricated with chlorofluorocarbons which destroy atmospheric ozone. The newer gases substituted in making styrofoam are untested for use around food and should be avoided unless it is firmly established they are safe around food at SBC oven temperatures. When styrofoam was used prior to understanding its role in atmospheric ozone depletion, it was considered unreliable as an insulator as it sometimes melted within the SBC wall resulting in a poorly functioning oven. Where styrofoam was successful, the inner oven wall was relatively thick and sturdy so reduced amounts of heat penetrated as far as the insulation.

Crumpled newspaper has insulated hundreds of successful SBCs and is the preferred insulation in many areas. Field work with feathers is limited but seems promising. Loosely carded wool was outstanding in two field uses. Those tests were run with sheep wool, but the texture of some dog wool is indistinguishable. Probably any animal wool would insulate well. Shredded paper, hay and pine needles also have been satisfactory.

Pressed fiberglass insulation serves well. Loose fiberglass batts also insulate well but both present health hazards. Fibers are released while the material is being worked and penetrate the eyes, lungs and perhaps are swallowed. Foam insulation is made with toxic gases which continue to off gas for long periods. For these reasons both fiberglass insulation and foam insulation are not recommended.

Where foil and insulating fill are not available, multiple layers of cardboard either with or without spaces between the layers have produced useful solar ovens. If there are spaces between the layers of cardboard, they may be simply left as insulating air cavities or may be filled with crumpled newspaper or other material insulation. Unless using foil, baffles and lots of air space, a large quantity of insulation is usually needed and can make an SBC very heavy.

To test the effectiveness of oven insulation, heat the oven in the sun until it is quite hot. Then place a hand underneath and on the side beneath the reflector. Shadowed areas do not feel warm in a well insulated oven. If insulation is inadequate, has melted, or has shifted, this can be felt through the exterior wall. If there is question about a side or front area, place something to keep that area in the shade until the test.

INNER OVEN LININGS of foil are our primary choice for the following reasons: it reflects a good proportion of the sunlight onto the pots and it provides reflective insulation around the inner oven holding in the heat.

Some experience indicates dark sides for inner ovens may function as well as shiny sides under semi-overcast conditions but in most areas, reflective sides work best.

Sheet metal oven linings are quite durable and have more mass so make heavier SBCs that are somewhat slower to heat initially. They retain more heat than foil to continue cooking longer through intermittent clouds and at the end of the day than ovens with lighter weight linings. However, if sheet metal extends from the inside to outside the glazing, it will function as a radiator, draining heat from the oven. A thermal break is needed. One useful configuration brings the sheet metal about ½ inch (1.2 cm) over the upper edge of the oven where it may be nailed or otherwise secured (Page 26, Fig. 4). The glazing frame then completely covers this inner metal edge. Also, a metal oven lining should not touch the glass or heat will be conducted out of the oven through the glass.

Galvanized sheet metal may be used inside the oven if desired. SBC temperatures are not high enough to disperse zinc, which melts at 419 F (215 C). Temperatures near or above the melting point cause some molecules to migrate and could pose a health hazard. If making any design of solar cooker that reaches near to the melting temperature avoid galvanized metal inside an oven.

LIDS having either a plug (Page 24, Fig. 1a) or a cap (Page 24, Fig. 1b) work best as they reduce heat loss out of the crack between the oven top and the glazing. Reflectors on the lids are made either the size of the glazed window opening (Page 24, Fig. 2a) or the size of the total top.(Page 24, Fig. 2b) Lids covering the full top have advantages in that as the sun passes, more of the reflected light will fall on the glazing even when the sun is far to the side. Lids the exact size of the glazing begin to lose reflection as soon as optimum focus is passed and so reduce somewhat the non-focused performance. In addition, the smaller lids which can press directly on the glazing do not protect as well as full top sizes.

Lids that hinge at the back edge of an SBC (Page 25, Fig. 3a) reflect some light onto the oven top thus losing potential heat. Lids that hinge at the edge of the oven opening (Page 25, Fig. 3b) throw a greater proportion of reflected light into the SBC resulting in better cooking than those hinged at the back surface.

PROPS need to allow a reflective lid to swing from far forward to far back from the perpendicular. The prop must then firmly brace the reflector in the chosen position against both forward and backward movement. This can be done with strings, sticks, wires or a combination. Props have customarily been secured from the upper corner of the lid to the forward corner of the outer box. This requires a stick longer than the box and is difficult to store. Props that are secured from midway up the lid to midway along the side of the SBC give sufficient stability and are short enough to store within the inner box when the SBC is being transported. Whichever length is used, the methods of adjusting the angle of the lid and fastening it in position remain the same. For some prop designs see Page 18.

Not included in the box on Props, is a simple firm wire bent at the ends to poke into holes in corrugated cardboard or drilled in wood. Small pads of cardboard squares may be glued to the lid to receive the wire ends and these pads replaced as needed, or the wire may go into the cardboard of the box and lid themselves. The wire alone is often sufficient, but in areas of persistent wind, a small cord with tightening device may need to be used as well. Or it may be necessary to use a stick form of prop as the wire acts as a spring in gusty wind.

The prop should be on the "morning sun" side of the oven. This, of course, changes from the northern to southern hemisphere. The point is that as the sun moves, the prop shadow can either fall onto the ground, or across the glass. The shadow spreads with the dropping sun, and if it falls on the glass it reduces the oven function unnecessarily. This reduction takes place as the oven goes out of focus and therefore is particularly crucial. While SBCs can cook slowly on as little as 1/3 of the full sunlight, if the collection area is further reduced by shadow, cooking power is reduced. This is important when doing absentee cooking, cooking under less than full sun, or with low angled sun as in winter cooking or cooking late in the day. Serious solar box cookers, built to provide meals under whatever available sun conditions, should have all possible solar collection area. Place the prop so, as the sun moves, the shadow migrates toward the edge of the oven, not across the glass—on the east side in the northern hemisphere and on the west side in the southern hemisphere.

REFLECTORS for maximum usefulness should be the full size of the top and the hinges should bend at the edge of the inner oven, as discussed under the section on Lids. (See Fig. 2 on Page 24 and Fig. 3 on Page 25). Typical thin kitchen-weight aluminum or tin foil, not heavy duty, is our first choice for reflectors. When foiling the reflector, some wrinkles are inevitable. Flattened wrinkles and smooth patches do not significantly reduce the effectiveness of a reflector. Air bubbles behind foil are fragile on a reflector and change the direction of reflected light as well, thus reducing the effectiveness of a reflector. Bubbles can be reduced by pricking with a sharp object and pressing air out before glue dries. Any glue on the surface of the foil should be carefully and gently washed off as it will block reflection.

To insure stability, the combined weight of the lid and reflector are designed to be much lighter than the empty oven box. Glass mirror reflectors have proved to be top heavy and fragile as well as producing a blinding reflection. In addition, mirrors function through back surface reflection which reduces the sunlight available for cooking. Sunlight falling on a glass mirror must first penetrate the glass to the reflective surface on the back and then pass through the glass again on the way out. A fraction of the sunlight is absorbed on each pass reducing the amount of solar energy a back surface reflector can deliver to the oven. Front surface reflectors, such as foil or polished metal, reflect a greater proportion of sunlight and are a better choice as they deliver more sunlight to the oven. Galvanized metal is durable and produces a useful amount of reflection without the blinding quality of mirror-finished metal but will dull in time when exposed to acid gases and other air pollutants.

GLAZING is a key component to a successful SBC. All glazing blocks some sunlight. Thicker glazings generally block more sunlight than thin ones. Glazings may be regular window glass or certain synthetic glass-like substances.

Two layers of glazing work better than one as the heated air trapped in the small space between layers acts as an insulator over the face of the oven. Two layers of thinner glass or other glazing with ¼ to ½ inch (6 to 12 mm) air space between provide better insulation than the same thickness combined into one pane. Since the oven face is the point of most heat loss, double glazing conserves significant amounts of heat that otherwise would dissipate. A second glazing also protects the inner glazing from chilly winds. Although it blocks a little more of the light, in addition to conserving more heat double glazing by decreasing the moisture condensing inside the window may eventually allow more sunlight through. The net result is a hotter SBC if double glazing is used, particularly on cold and windy days.

Clear window glass, standard single thickness (2.5 mm), has proved very satisfactory. Thinner glass has less tendency to crack from heat than thicker panes, probably because it heats more quickly and evenly, reducing thermal strains. Plate glass is very heavy and has been satisfactory only when tempered. Some SBC designers choose single glazing for portable models with temporary second glazing as an option for poor weather. Often permanent double glazing is chosen for all-weather SBCs and this is probably ideal where materials are available.

Window glass, although fragile, is usually available in urban areas, while tempered glass and synthetic glazings are specialty materials requiring unique supply lines. Window glass is available around the world and therefore in that respect is preferable. Glass does not degrade in sunlight and if protected from thermal strains and impacts, is more durable than most plastic glazings, even those that are treated against degradation by ultra-violet rays.

Low iron glass allows more radiation to penetrate into the box and makes a hotter oven but is not essential. One way glazing has been found to reduce the function of an SBC according to an engineering study done through Solar Box Cookers Northwest. Recycled automobile glass, particularly flat pieces from the side windows of vans, has been used successfully but even pieces that appear clear may be tinted to some degree to cut down on heat in the vehicle or have added materials to make safety glass. Both tints and all added materials block some solar radiation.

Glass may be tempered if desired although the cost of tempering is so high that occasionally replacing glass may be cheaper in the long run for homecrafted SBCs. Glass that has simple heat cracks may be held in place by silicon sealant or nontoxic glue or a narrow strip of tape. Mass produced SBCs customarily use tempered glass.

The first glazing being nearest the food must be nontoxic to prevent poisonous fumes from entering the food. Glass, of course, is non-toxic. At this time, among the available plastics, the common alternatives to glass are heat-resistant nylon (such as Reynolds Oven Cooking Bags), and polyester or polyethylene plastic films available near the foil section of grocery stores in the United States. Many grocery store plastic films have a polyvinyl base and are not suitable for SBC cooking, but some are clearly labeled made of polyesters or polyethylene. Polyvinyl may release a vinyl-related gas when heated to SBC temperatures for the periods of time needed by SBC cooking. For this reason it is not recommended for use in SBCs unless further research establishes its safety for this specific purpose. Translucent fiberglass sheeting is made with a toxic resin and off-gases for a long time. It is not recommended for inner glazing, however, it is frequently used as outer glazing because of its impact resistance, lightweight and durability.

A large size cooking bag (heat-resistant nylon) can be cut open and glued on a frame to lay on top of the oven under the lid, or may be glued directly onto the lid like glass.

To work a plastic glazing, lay glue around the window opening, place the plastic, pull it tight and hold with masking tape until the glue sets. If desired, make a second glazing on the other side. Masking tape alone will not hold in the heat.

Pull it very tightly to reduce flutter on windy days as that will fan heat out of the oven. Such heat resistant glazings make very light ovens and are moderately durable but in time kitchen-grade plastic glazings become brittle. Thicker, oven-safe polyester glazings, some of which are treated to resist degradation by ultra-violet light, may be available as special order items.

Single strength window pane glass overlapping the window opening by ¾" attached with silicon sealant or glue may be used if preferred.

An oven having a permanent, double glazed window, and a side door, resists wind chill best. Where the top is used as the door, the lid may be built with either a plug fitting down into the oven, or with a cap folding over the edges to help prevent wind from blowing heat out. (See Figures 2a and 2b in the section on Lids.)

Most SBC builders simply glue their glass or plastic glazing onto the cardboard of the lid. The cardboard is slightly flexible, reducing the incidence of thermal cracks in glass, yet providing good protection during handling. Some ovens have been made with a narrow strip of cardboard sandwiched on the outer edge between two pieces of cardboard frame thus providing space for glass. Such frames are either glued or stitched on the outer edge a fingerwidth from the edge of the glass. Glass is allowed to move slightly within this casing and sealant is not essential.

Flexible glazing may be wrapped around a cardboard frame to give the two layers. Cardboard frames are simply laid on top of the oven under the lid.

Alternatively, FRAMES for the glazing may be made of wood. For single strength glass in a wooden frame, a small (1/8 inch or .3 cm) clearance around the edge of the glass as it lays in the frame reduces breakage from thermal expansion. Glass is secured with a flexible sealant. The inner glazing must be well sealed to prevent heat leaking from the oven. Second glazings need a maximum of half an inch (2.5 cm) separation from the inner pane. Many home builders do not permanently seal two glazings together, but simply lay the second glazing, either glass or plastic, above a well sealed inner pane. This arrangement allows easy access for cleaning. Double glass panes if permanently sealed, require at least one small opening from the space between panes to the outside to relieve pressure which otherwise might break the glass.

Frames will be true if the pieces from opposite ends are squarely cut and equal. With equal lengths for opposite pieces, a frame can be squared by using the corner of any book, magazine or commercially cut piece of paper as a guide.

An insulating strip between the glazing frame and top of oven is not essential if the top surface is smooth. For irregular tops, solid commercial insulation strips work well if heat resistant. To make a crafted gasket, lay a bead of silicon sealant, paper mash or similar material on the top edge of the oven, cover it with waxed paper and close the lid. Remove the waxed paper when the sealant has hardened. The Dhauladar design by Didi Contractor uses a double fold of woolen fabric glued to the surface beneath the edge of the glass with a final fold brought up to rest on the top edge of the glass thus thoroughly wrapping the edge of the glass and binding it to the frame. It is a permanent seal as access in this design is through a door in the side.

It is essential that INTERIOR PAINTS used for pots, trays or walls be nontoxic and without persistent odor. Some house paint contains mercury or other substances as a fungicide and should definitely not be used inside an oven. Such paint gives off toxic fumes even at room temperatures and would give off even more at cooking temperatures. Nontoxic white paint has reportedly been used for inner sides in place of foil. It is important that the white be as shiny and white as possible. Such ovens have generally cooked less well than similar foiled models. Ceramic paints so far have not performed well and give off odor for very long periods of time as well as performing poorly.

For trays and the outsides of pots, any dark color will produce heat better than any light color. Light-colored or bright pots reflect solar radiation away and so are slower to heat, or heat only from the hot air with little or no heat produced on the surface of the pot. Dark red, dark green, dark blue, and dark brown colors may be used. Paints on pots are best kept thin since thick paints insulate. On surfaces darkened by handicraft methods even "nontoxic" paints still may not be safe for ingesting so food should not be laid directly on a home-painted tray but needs to be cooked inside a pot or on a commercially dark tray. Paints that contain toxic solvents may be used ONLY IF THE TOXIC SOLVENT WILL BE COMPLETELY DRIVEN OFF BY PREHEATING. Determine this by using only commercial paints recommended for barbecue equipment or other specific food-related uses. Nontoxic homemade paints include children’s non-toxic black paint powder, such as Tempera, soot from clean wood mixed with nontoxic water-based glue or soot mixed with cooking oil that smokes at low temperatures. High temperature cooking oils will remain sticky indefinitely but low temperature oils will become baked on.

Any new SBC is best heated initially for several hours without food to clear out any residual vapors. Some engine and conventional black paints never get over releasing heavy odors and will ruin an oven. If in doubt, run a preliminary test before risking any questionable paint.

Any EXTERIOR PAINTS or varnishes may be used to weather-proof the outside of the oven. The color of the outside does not affect oven temperatures if insulation is adequate.

Dark POTS with tight dark POT LIDS greatly speed solar cooking. There is a wide variety of potential equipment. Pots and lids of thin material heat faster than thick; metal faster than ceramic or earthenware except cast iron which is good but slow to heat initially; metal generally faster than glass; glass faster than gourds. All will make cooking equipment depending on availability. When darkening pots and lids, treat the outside only.

Occasionally it is fun to see the cooking. Lids may be left off of dark cakes, some cookies, corn chips with melting cheese, and a few other foods if desired. More moisture will condense on the oven glazing and the food will heat more slowly. Moisture condensing on the inside of the glazing may be wiped off intermittently with a clean soft cloth. If steaming persists and there is good sunlight, a small pebble or stick temporarily may be placed under the glazing frame or around the access door to open it slightly and allow moisture to vent. To reduce the condensation problem while still allowing full visibility, a small square of glass can be substituted for a lid. This is not recommended for routine cooking due to color changes which may indicate vitamin loss.

Foil is not generally recommended to wrap food for solar cooking, however, temporary pots or lids may be formed out of one layer of darkened foil in absence of any other equipment. Conventionally foil-wrapped food cooks very slowly if at all because shiny foil, particularly in multiple layers, insulates by reflecting sunlight and heat away.

Glass jars make good pots although they cook better if darkened rather than left clear. Also, darkening the outside of food containers will protect some of the B Vitamins. When painting the jars, a strip of masking tape placed from top to bottom before painting can be removed when the paint is dry to leave a tidy strip of clear glass for visual inspection of the inside.

Shiny pots or bread pans may be placed into brown paper bags or dark cloth bags to successfully cook without painting them. If using paper bags or cloths, heat them alone for several hours before using them with food in order to drive off any unwanted vapors. Alternatively, a number of separate foods may be cooked in small, clear jars all under one dark tray or dark lid of a roasting pan.

When using jars for cooking, make a hole in the lid of any non-canning jar, such as mayonnaise jars, peanut butter jars, etc., to prevent steam buildup. Dome and ring canning jar lids that are designed for food preservation automatically release excess steam pressure yet are safe only when used on canning strength, food preservation jars. The thickness and strength of non-canning glass jars is not intended to take the strain of steam under pressure and could break explosively unless vented.

Some low-fired earthenware pots do not initially cook well although dark colored, hard-fired earthenware pots with glazing work very well. Perhaps the poor performance of some earthenware is due to liquid soaking through and evaporating on the outside, or perhaps it is due to the thickness and porous nature of low-fired clay pot sides. Experimentally, in the efforts to use low-fired, unglazed earthenware, the goal has been to approximate the hard-fired pots by filling the pores and to providing a form of glazing. On a homecraft basis, this has been done by saturating the pot with food-type oil, fat or natural resin which both closes the pores and changes the surface. Oil also will conduct heat rather well and this may be part of what improves cooking in low-fired, earthenware pots following oil treatment. Light colored earthenware needs to be darkened on the outside only, perhaps by rubbing a dark food, nontoxic dark powder or soot from clean wood into the oil coating. Even so, there may be forms of low fired earthenware that are difficult to use for SBCs.

Gourds to be used as pots need to be fully ripened, scraped and cleaned to a thin hard shell and, if of a light-skin variety, need to be darkened on the outside. Gourds almost completely filled with food cooked better than gourds with a little food in the bottom. Experimentally, like with low-fired earthenware, treating inside and outside surfaces with vegetable oil or animal fat seems to improve the cooking times. Low temperature oils which scorch to a useful dark brown are olive oil, used lard, and peanut oil. The point at which oils will darken can be lowered by utilizing old oil that has been used for cooking, perhaps which still has food particle in it, or which has an added emulsifier.

The Santa Domingo gourds tested were kept sitting upright while they grew. After harvest, they were dried outdoors in the shade for some months until the seeds begin to rattle. Then the tops were cut off and the seeds and inner pulp removed. They were scraped to a hard surface inside and out. Our test gourds held between 1 and 1.5 quarts (.95 and 1.4 liters) and after preparing as pots were almost spherical with 3 inch (7.5 cm) openings. The sides were approximately 1/8 inch thick (.3 cm).They held liquids, did not flavor the food and produced cooked food over a period of many weeks. Heat seemed to penetrate slowly. Gourds cooked faster with lids made of a small squares of clear glass rather than dark lids, which sacrificed some level of B Vitamins in some foods. This was judged to be a minor effect compared to the convenience of being able to grow your own solar pots in some cases.

On an experimental basis, where very large gourds are available their shells might be evaluated for use as inner boxes. If they proved successful, used in conjunction with gourd pots in an insulated pit or other outer insulation, the materials needs for low temperature SBCs in equatorial regions might be reduced to simply foil for the inner surface of the large gourd and the glazing...or perhaps to only the glazing? This is highly speculative and cannot be appropriately evaluated at 34.4 N. Latitude where our work is done.

POT LIDS can be HOMEMADE by turning the pot upside down on a flattened piece of metal and drawing an outline. Cut the metal about ¼ inch (.6 cm) outside the line. File or sand the sharp edges to dull the cut surfaces and protect the fingers. This may be done simply by rubbing a rock along the cuts. Working regularly around the lid piece with pliers, bend the edge down at the line. Sometimes the fluting looks nice. If necessary, cut from the edge into the line to allow the pieces of edge to overlap. Darken the outside of the lid and heat it for several hours before using it with food. Temporary lids made be made by the same method using a brown paper bag and creasing the paper at the line. These also need to be heated alone for several hours before being used with food. Sometimes small holes are punched in a lid to allow moisture to escape and produce a drier crust on foods such as breads or breadcrumb toppings, cakes and cookies.

Nutritional research in great detail has been done by Professor George Hammons and others at Philander Smith College in Little Rock, Arkansas, USA. After analysis of many different nutrients in food cooked equal times by conventional methods and by solar box cookers using dark pots with lids they concluded that solar cooked foods retained nutrition as well as or better than foods cooked by conventional methods in spite of the longer cooking times required.

Small pieces of glass may serve as lids on many different pots, as mentioned previously. The question as to how much vitamin loss occurs when food is cooked without a pot lid or under clear glass needs to be researched. Judging from color changes, there may be some vitamin or nutrient change if food is exposed to bright sunlight while cooking. This may be more important when cooking liquids than when cooking solids, since liquids will circulate by convective flow exposing a greater proportion of the food to the sunlit edges. Riboflavin, a major B vitamin destroyed by light, is found in milk and milk products, eggs, liver, kidney, grass, fruits, leafy vegetables, yeast, etc. These foods with light-sensitive vitamins definitely should be cooked in dark containers.

Research is still needed comparing the effects of solar cooking with food exposed to the sunlight after it has passed through various layers of glazing with solar cooked food using lids. Research is also needed on whether or not there are significant amounts of aluminum compounds absorbed into the food from aluminum pots when foods, particularly acid foods, are cooked at the relatively low temperatures for extended periods of time typical of solar box cooking. Until the facts are established, foods should be cooked with dark covers and aluminum pots not be used unless no other pots are available. Also, there are unresolved questions at this time about the possible uptake of metal compounds into food cooked in tin cans.

TRAYS used for the heat absorber on the bottom of the solar oven need to be dark, almost cover the bottom, and support the weight of pots and food. Ideally, they should contain any liquids that might spill and be removable for cleaning. Within this range there is great variation and all the criteria are sometimes not met. Lightweight, darkened or enameled metal trays are usual and may be made of steel or aluminum. Recycled, dark-enamel conventional oven trays have been utilized successfully. Dark cookie sheets, if the proper size, will work well. Moderately well-functioning SBC trays have been made with soot-blackened cardboard trays.

Crafted trays can be formed by hand with the techniques used for making pot lids. Roofers’ metal flashing or printers’ "mistake" sheets make fine trays. After starting the bends with pliers, the rims may be finished by hammering over a piece of wood with a square edge.

All trays need to be elevated slightly off the bottom of the oven. A University of Washington engineering study states that a thick tray will transfer heat to the pot more effectively than a thin one. This is particularly true when the tray is elevated above the foiled bottom.18 Field tests confirm these effects. If the hot metal tray is in direct contact with the foil on the bottom, heat will be conducted through the foil and into the bottom insulation. Thin wood strips, little pillars of cardboard, or narrow strips of cardboard glued into circles may be used to elevate the metal tray a bit (1/4 in., approx. 0.6 cm) above the bottom foil. With the tray floating in space on these small supports, there is a minimum of tray-to-oven contact and heat flows most readily into the cooler pots. Many successful solar cooks over many years have never done this, but it is one proven way to work toward higher oven temperatures where they are desirable.

GLUES and pastes must be nontoxic and odor-free when heated. Originally Elmer’s White or Carpenter’s Wood Glue was most used. Water-based and nontoxic, it is widely available in the U.S.A. Any nontoxic glue that continues to hold in the oven temperatures works. Most rubber glues are toxic; heat-melt glues will not hold.

Homemade paste or glue can be made from some grains or flours, such as wheat, rice or oats, and work particularly for attaching foil to cardboard. Laboratory tests have established that wheat paste has adhesive strength equal to or exceeding Elmer’s White Glue. 18

Use a grain that makes thick sticky water when cooked. In the pastes tested the addition of about 1/8 part sugar or honey made some of them stick better. In Arizona, food glues which have dried are not edible to insects.

Cartilage from animal legs (the portion that ties muscle to bone, not hoof) can be boiled a long time and when hot spread for a durable hot glue useful on outer parts of the SBC. There are old references to fish glues but no record has yet come to me how that was done. Animal glues kept in a glass jar or tin can become soft enough to work when heated in an SBC. At the end of the work session, heating any of the organic glues or pastes in a container with a lid pasteurizes them so they are slower to spoil, particularly if they are then kept at low temperatures, like food. If using a non-canning jar, be sure to leave the lid loose until removing the jar from the SBC to relieve steam pressure.

If nontoxic, water-based glues or pastes are hard to obtain, consider taping, tying with spun fiber or bands of woven cloth, or stitching to hold the major pieces together.

This chapter, based on our experience to date, contains information to assist local designers. The creative mind of the local designer is by far the most important ingredient. To produce a large, insulated, glazed solar box cooker many different materials may be used in a new design, or any component may be changed according to the needs of the area and the designer. It is essential that new designs retain the fine cooking capabilities of the introductory models. In many, many areas around the globe through a friendly, harmonious relationship with natural forces and creative use of the materials readily available at hand, ordinary people familiar the background provided here can produce a major household appliance—a cookstove with free fuel delivered every sunny day.

In designing solar box cookers for a variety of uses in different locations, the primary changes will be in the application of different materials and skills, rather than major changes in the time-tested basic design components. Cardboard, the preferred material for our portable cooking and teaching model, may not be suitable for many locations. The major Kerr-Cole designs have been based on use of cardboard, foil and crushed newspapers because they are easily available where we have done our work, environmentally sustainable and because they work well. In other areas, other materials may be more appropriate.

Each material has its own characteristics and design challenges. For instance, in using metal sidings, care must be taken to assure heat is not drawn out of the oven by a thermal bridge forming a radiator. In using basketry, care must be taken either to weave tightly or line the inner and outer units so air cannot penetrate or heat escape. This is part of the challenge for experimental designers...to use what is locally available to produce an SBC design that will function as needed.

The broad, shallow, well-insulted cavity with a dark bottom, the approximate size, the relatively large, glazed window, and in most areas of the world, the adjustable reflector must be maintained.

These solar box cooker components can be fabricated from any nontoxic material the craftworker chooses. In addition to being nontoxic, other material considerations include availability, minimal expense, ease of working, water resistance, cost, durability and weight. Different SBCs that are beautiful, functional and convenient can be produced all over the world using traditional construction methods with locally available materials.

Temperatures alone are deceptive because an SBC may or may not trap heat above, for example, 250 F (121 C) to 300 F (149 C). For some people, this sounds like a low temperature.

However, such SBCs can cook well. Under less than ideal conditions they are superior to designs based more exclusively on direct reflection since they use both direct and indirect solar radiation. A well-designed solar box oven will produce a larger, more even flow of heat and therefore deliver more cooked food than designs that rely more exclusively on reflection. Furthermore, many SBC cooks prefer 250 F (121 C) to 275 F (135 C) over 350 F (177 C) or above because the lower temperature ovens cook the food more gently and need less attention than higher temperature ovens.

Multiple reflector ovens that can reach 350 F (177 C) or higher and parabolic cookers which can even set fires utilize primarily direct solar radiation so their function drops more quickly than SBCs when clouds, haze, or pollution scatter the sunlight. Multiple reflector solar ovens and parabolic cookers present wind scoops that makes them liable to turn over in the wind. Also, a parabolic cooker may produce a point of high heat sufficient to fry and char food. Yet because the point of heat changes position so frequently as the sun moves, it requires a tracking device or frequent attention to refocus. The point focus has limited practical ability to produce multiple pots of cooked food. For most low temperature uses and particularly for cooking, the temperatures of a small focal point are not needed. Furthermore, the focal spot is a hazard as it can set materials on fire, or damage human tissue. Most parabolic designs are unstable in the wind and pots chill quickly in even moderate breezes. Few have the ability to retain heat through episodes of passing clouds. The most telling point perhaps is the extreme difficulty in producing an adequately curved parabolic reflector and mount with simply techniques.

A good approach to designing a solar box cooker to meet local conditions is to become familiar with a standard SBC which is known to cook well at that location. Then, change those design elements that need to be modified to meet special needs or local conditions. And finally to run parallel tests between the new design and the standard to evaluated the comparative ability to cook.

All new designs need comparative tests. Cooking actual food in the quantities usually served is the best ultimate test of a solar box cooker. Cooking is the primary purpose of an SBC and cooking is a direct test of an oven’s function. How long cooking takes is important. It may be equally important to evaluate how easy an oven is to use, can it be easily maintained or repaired, is it stable in the wind, can the children effectively and safely work with it, etc.?

PARALLEL COOKING TESTS can include the following:

Place identical small amounts (1/4 cup; 60 cc.) of a quick cooking grain, such as rice, with the usual proportion of water in similar small jars or darkened food tins covered with a small piece of glass, one container for each SBC in the test. Start cooking in the usual manner. Without opening the ovens, watch for grains to emerge from the water cooked, noting which SBC cooks the grain fastest, second, and so forth. Give each SBC an initial ranking.

Other parallel tests could include ability to handle large mass, such as considerable amounts of grain in earthenware or cast iron, and the ability to handle the variety of pots or trays of food needed for a complete meal. Some standard tests rank the ability to heat a specified quantity of water, such as 2 liters in a standardized pot.Tests should be run under full sun conditions, and also under semi-overcast and very weak sun to identify the designs’ ability to utilize indirect solar radiation. Some tests should be run on windy and cold days; and some at very low sun angles, such as early or late in the day to simulate winter sun conditions.

Solar box cookers are a very "forgiving" technology. They do not have to be perfect, nor optimized in every respect to serve well. A practical, useful goal is to make designs that will cook absolutely the most conveniently possible, given what the designer has available and needs of the users and the types of cooking to be done.

The evolution of the basic SBC concepts to many different designs serving a variety of needs and cultural norms will dramatically increase the application of this simple technology to pressing food and environmental problems.

All of these were successful cookers. There are many potential variations. This flexibility is one major reason the solar box is a global solar design for solar cooking.

The basic SBC—a large, flat-topped, insulated, glazed space with or without the single reflector—is so simple people often think, "So what’s the big deal? Anybody could make one of those." That is music to my ears. The big deal is that these designs work! They cook! And they cook on free, widely distributed, abundant sunshine delivered automatically to the site. The SBC can quickly address a high-priority need in many locations—access to heat for cooking. These factors arouse enthusiasm and are the driving force of the spread of SBC technology. The simplicity of basic solar boxes leads to confidence and easy comprehension and is ideal for transfer of passive, low technology, solar energy usage across cultural and language barriers around the globe.

To maximize ease of such transfer and incidentally minimize the use of resources, the introductory design goals have been carefully refined. Following are the criteria of design that we have used:

The design is to be simple to comprehend and construct by people without extensive building skills.

Materials are to be widely available, as inexpensive as possible and easily substituted.

The cooker should use minimal Earth resources per unit and be maximally biodegradable ultimately, yet durable while in use.

All components within the cooking space are to be nontoxic at cooking temperatures following a brief initial seasoning by solar heat.

Construction is to require only minimal tools usually available in or around a home.

The cooker is to be capable of cooking a full meal for a large household or group within a reasonable length of time, such as between sunrise and the midday meal, or between noon and sundown meanwhile requiring minimal attention.

The cooker is to be safe for people, including children—without danger of setting fire, producing blinding reflection, or causing a blister when touched on the outside. Safe for food—with little danger of upsetting or losing food.

The cooker is to cook quickly enough that there is no danger of food poisoning when correctly used.

These design goals have been met by the basic SBC designs developed by Kerr-Cole Solar Box Cookers and others working in the group of volunteers associate with that work.

Our basic design goals do not contain the targets of maximum collection of solar radiation, maximum production of heat, maximum thermal efficiency or maximum durability. These are incorporated to the degree possible as secondary design goals. Our basic introductory SBCs are tailored to be obtainable independently, individually, and under adverse conditions such as lack of money, lack of market outlet, lack of advanced knowledge, lack of government or other organizational support or in emergencies.

Herein lie some of the special characteristics that have made current solar box cooking efforts acceptable on a wide scale, cross-culturally, where previous solar cooking efforts have not been self-sustaining. It is delightfully empowering to construct a major labor-saving, home appliance by one’s own initiative and effort. It is within the grasp of vast numbers of people who, independently, can handicraft, use and maintain their own SBC. If it is to be mass-produced, ideally manufacturing would be done as local industry, in small businesses close to the point of sale and still using locally available labor and biodegradable materials.

Reasons for DESIGN CHOICES include the following:

The BOX FORM is familiar, lowering the emotional challenge of putting food into an SBC as opposed to more unusual designs. It looks simple, often homey—yet it can be made in a beautifully artistic way by skilled workers through careful detail work and personal decorating. This is an extremely important factor.

The BOX FORM can easily be adapted to many other uses for low tech passive solar heating such as greenhouses, water heaters, pasteurizers, or incubators, and so it is a good form for basic solar education.

The SINGLE REFLECTOR/LID, in addition to requiring little material, is simpler to build and requires less reflective material than the multiple reflectors or curved reflectors required by some cookers. Yet it provides