I have made some adjustments to the instructions for the way I made my medium-sized Solar Oven. The original instructions for this oven can be found online at:
http://www.backwoodshome.com/articles/radabaugh30.html
Gathering materials
The first step involves a search for cardboard boxes in your local grocery and department stores. For the oven box, you are looking for two particular boxes. These can be rectangular (easier to find) or square (their collectors work better). The smaller of the two boxes becomes the inner box, so it defines the cooking area and the power of the cooker. For a medium size cooker, the area of opening of the inner box (length times width) should equal 120 to 160 square inches, and it should be 9 to 12 inches deep. The larger box, the outer one, must be two to three inches larger in all directions. For the model I made, I used a square box 12 ½” by 12 ½” by 10” deep for the inner box. The outer box was a rectangle 15” by 16” by 12” deep.
The oven box
For collectors, find four flat pieces of regular (not double strength) cardboard from appliance or bicycle stores. These should be about two feet by three feet. And gather five to seven more boxes which you will cut up for insulation.
The top of the inner box must rest one inch lower than the top rim of the outer box
Other things to gather:
- Eight ounces of white paper glue (such as Elmer’s™).
- One tube of Shoo Goo or E-6000 glue.
- 4-5 sheets of aluminum plates used to print newspapers. (I found them in Snyder, TX for $2.00 a sheet - you may be able to find them for less elsewhere.)
- Alternatively, you can use one small roll of 18" wide heavy duty aluminum kitchen foil instead of the aluminum plates.
- A piece of double strength glass ½" larger than the length and width of the inner box (I used two layers of glass that I had in some old picture frames - 16“ X 20“. The local hardware store cut them to size for me for free). Buff the edges of the glass by rubbing a rock or metal over them, so they’re not dangerously sharp. Or you can cover the edges with heavy duty silver tape.
- A small amount of flat black paint.
- Some cotton cloth from recycled clothes.
- Some string.
Take the outer box and cut up scrap cardboard pieces to fit in the bottom (cut the cardboard with a mat knife, being careful not to cut yourself). Make these layers thick enough so that, when the inner box is placed in the outer box, the top rim of the inner box is one inch lower than the top rim of the outer box.
The outer box must have two opposite flaps left sticking out. Tuck the other two between the inner and outer boxes. The inner box must have all of its top flaps bent out and all the way back so that they fit between the inner and outer boxes.
Cut more pieces of cardboard to stuff between the inner and outer boxes until the inner box is wedged tightly. Doubled-over pieces look nicer at the top edge. The tops of these filler insulation pieces must be arranged so that, when the glass rests on the top rim of the inner box, it makes a good seal. (You don’t want gaps where the heated air will escape.) Ideally, the top edges of the filler cardboard will be slightly below the top level of the inner box. Also, it must be easy to slip a finger under the glass for removal. In use, the cooker will be tilted toward the sun. Therefore, the sidewall, which will be lower when it’s tilted, must be arranged so it will support the glass in position.
Arrange the filler pieces so the g lass rests on the top rim of the inner box, and so you can slip a finger under the glass to remove it.
Note for later: The box will smoke slightly during the first couple of times it is heated up, but this is just a curing process. Also, cardboard shrinks slightly when heated, so you will have to repack the cardboard later to keep the inner box tight.
Here is the pattern for the collectors. [Please see the link above for the pattern.]
The dimensions for the width of the collectors will depend on the dimensions of your glass top.
Draw the collectors, as shown, on the four flat pieces of cardboard. (I prepared a pattern from a piece of the paper that was between each aluminum plate. I placed the pattern on the cardboard and traced around it, then cut out the cardboard collectors.) A square cooker will have all four collectors the same size, while a rectangular cooker will have two sizes, based on the length and width of the glass. The 67° angle can be found using a protractor, or by folding a piece of paper as shown in the diagram below. [Please see the link above for the pattern.]
You can find the 67° angle by folding a piece of paper as shown here. [Please see the link above for the pattern.]
Cut out all four collectors. Then take a tool with a blunt point and crease a line along the dotted lines. (Let me make another note here: I chose to make fabric hinges for three of the connecting edges of the collectors. You will need to cut off the side tabs on both edges of two collector pieces. You will also need to cut one side tab off the other two pieces, but be sure to cut the tabs on opposite sides. This is very important. You should have a tab on the left side of one collector and a tab on the right side of the second collector.) Bend in on the crease lines. All folds go to the same side. Next, bend the upper and lower flaps only all the way over and glue them down. Press with weights until the glue dries.
Gluing aluminum to the collectors
Take your paper pattern and fold all tabs to the inside. You will cut the aluminum plate so that it fits just inside the large main area of the collector. Cut the aluminum slightly smaller than your pattern for the cardboard. Do not cover the side tabs. Glue aluminum to the smooth side of the collector (the one without the folded tabs) with the shiny side out using Shoo Goo or E-6000. I found that it was advisable to glue the top edge only with the glue. The aluminum plates expand when they heat up in the sun and become very warped. By leaving the sides and lower edges free, they could expand and contract and be more effective in reflecting the light into your oven.
Alternatively, roll aluminum foil over the collectors, and rub your finger over the side flap bends to show where to cut. Cut the foil so that it does not quite reach these bends; it will be easier to center later. Do not cover the side flaps.
Leave two of the flaps on the outer box out, so you can tie the collectors to them when it's windy.
Smear a glue mixture (two parts water, one part white glue) over the dull side of the foil, using a piece of cloth and two to three tablespoons of glue mix. Line up the collector and lower it onto the foil, tap it lightly, and turn it over. Apply the foil to the side of the collector that is not glued to the bent-over upper and lower flaps. Being a little off center is okay, but if it’s off too much, peel off the foil and try again. Press on the edges of the foil and pull out large wrinkles. Take a clean cloth and rub outward on the foil to smooth it.
Connecting the collectors
I made a square cooker and I decided to make fabric hinges for my collectors. I cut my fabric 6” wide and the length of the side minus the folds at top and bottom. It wound up being about 12” long for the one I made. Remember, you have two collectors with one tab each. These tabs stay free. Place your collectors face down (shiny side down). Leave a little space, about ¼”, between the collectors so they will lay flat when folded. Glue the cloth hinges at the three side pairs without tabs. You will now have four collector panels connected by three cloth hinges and two tabbed sides that are free. Make holes in the tabs close to the fold and use string or a long shoe lace to tie the tabs together.
Alternatively, for a square cooker, glue two opposite sets of side flaps together, as below. Then lay them out as they will fit on the oven box. Cut off the side flaps from one of the unattached corners. Connect this corner by laying these two collectors next to each other (about ¼" apart), with the foil sides down. Now cut a cloth about 18" x 4" and glue it over this corner, as shown. When the glue is dry, fold inward on this cloth hinge and arrange the collectors so that the unattached side flaps are on the outside and line up with each other. Poke holes near the bends of the side flaps, and tie the two segments tightly together with elastic material. This will allow these corners to separate slightly when the collectors are folded flat, but pulls the corners together when unfolded.
To make a cloth hinge for the collectors: First, cut off the side flaps from one corner. Next, glue the cloth to the panels
Connect the collector panels with glue and with laces.
A slip-in piece made from cardboard and cloth is attached to the upper collector. This slips between the cardboard filler pieces of the upper sidewall for quick attachment of the collectors to the oven box.
[Please see the link above for the pattern.]
To make it, cut a piece of cardboard 16" one way and the length of the glass the other way. Crease two lines and bend as shown. Then cut a piece of cloth six inches one way and the length of the glass the other way, and glue three inches of the cloth inside the folded cardboard (leave three inches outside), as shown. Next, glue the cloth that was left out to the upper collector (glue it to the doubled-over lower flap of the collector). On square collectors, this would be to the right of the cloth hinge. I made two slip-in pieces and attached them to opposing collector panels. As you look at my solar oven, there is a slip-in piece on the right and on the left of my cooker box.
Finishing up, setting up
The drip pan: A dark baking tin is used for a drip pan to hold food and catch boil-over. It’s nice if the drip pan puts a slight pressure on the sidewalls, for stability. If the baking tin has handholds, these may be bent for a better fit. You can also build a drip pan from wood, or by bending sheet metal. I used an extra piece of aluminum plate to make my own drip pan. I measured the inside of my cooking box and made the drip pan about 1” smaller so it would fit easily inside. I added 2” all around to fold up and make the sides (this would be 4” extra in both directions). Place the aluminum plate, cut to the correct size, shiny side up. Measure in 2” and score well with a blunt instrument. I used a dull pencil. Do this on all four sides. Turn the aluminum over. You will see a small square in each corner. Score from the inside corner to the outside cut corner. Do this in all four corners. These scores will all radiate out from the center (but they do not go all the way to the center - they are just across those little squares in the corners). Cut off the corners just below the score lines (see drawing). Score about ¼” down along one side. I used a metal ruler to make my score marks. Keep the ruler in place and fold the edge up. Remove the ruler and fold the edge all the way down so it is flat. Make a second fold the same way you did the first. You will have smooth edges by doing this and it will prevent anyone getting cut by the raw metal edges. Repeat this on all four sides. Turn the aluminum over again so the shiny side is up. Fold up each side on the score lines. At the corners, fold them carefully to the inside. Press the excess fold flat against one side - it doesn’t matter which one you choose.
The glass: Check again to see that it rests on the top rim of the inner box with no large air gap. The glass will become hot, so handle it by the edges or use a cloth. Keep the glass clean. Remember to buff or cover the edges with tape so they’re not sharp.
Propping the oven toward the sun: Use rocks or other objects in front and back. If you use only one prop, the wind can easily tip the cooker. I made a special holder from scrap wood that can keep the oven tipped at two different angles. One angle is better during winter and the other is for other times when the sun is more directly overhead.
Aiming: Use shadows created by the cooker to orient it; don’t look at the sun. Look into the oven and adjust it so there are no shadows top or bottom, left or right. Aim the cooker slightly in front of the sun’s path so you don‘t have to adjust it so often..
Wind: Poke holes in the flaps left out of the oven box and also in the collectors on the top and bottom. Tie collectors to flaps with cotton cord. The scrap wood support I mentioned really helps to keep the wind under control without other tie-downs.
Cooking
Jars: Boil and steam food in recycled jars, half-gallon or smaller in size. Painting them black will reduce cooking time, but leave a clear strip to see food by using tape when painting.
When cooking, don’t overfill jars with beans and grains, as these foods expand. You can poke a hole in the lid or leave it on loose to avoid pressure buildup. If food is actively boiling, open jars slowly to release any built-up pressure. I’ve never broken a jar because of pressure buildup. If a lid sticks, tap around the edge, or pry up under it, to release any vacuum pressure. It should then open easily.
Learn to cook with more than one jar at a time. Start with the longest-cooking food, and when it is boiling, add more jars. Learn which foods (grains, potatoes, squash, lentils, etc.)—and what quantities of them— you can cook in one pass, so you can cook when you are gone all day. Learn cooking patterns that fit your lifestyle.
- Beans: Use 2¼ cups of water for 1 cup of beans. If you boil them actively over a long time, add more water. Pintos take the longest.
- Grains: Most grains cook better if you preheat the water in the solar cooker for an hour or so. Use a two-to-one water-to-grains ratio. Long grain rice can be put in the cooker in cold water.
- Vegetables: These cook in jars with little or no water, or they can be added to beans and grains. Potatoes and sweet potatoes cook well on the rack, if lightly oiled. Otherwise, put them in jars. Winter squash cooks in its own skin. Corn on the cob steam-bakes in its own sheath.
- Bread: Bread cooks best in dark bread pans. Oil the pans. Let dough rise in the cooker without collectors attached, then add the collectors when you’re ready to bake. I have found that it takes a bit longer for bread to bake in my oven, but it tastes great!
- Pizza and pies: Bake the crust first.
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