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When we decided to build one I searched the web high and low for an efficient and doable solar oven project.
After trying this and trying that, taking what I felt were the easiest and best of the various suggestions for solar oven construction, we ended up deciding that a plain old cardboard box with a tight fitting lid was the best base to work with. Actually, two boxes, the larger with a tight fitting lid and a smaller box, without a lid, that can fit inside the bigger box. The smaller box is the inside of your oven so make sure it is big enough to hold the containers you will be using!
To start out, the bottom of the smaller box needs to be painted/colored black, using a dull paint (wanna absorb them light rays rather than reflect them like glossy would do), tinfoil is then taped up on the insides of the box. All four sides.
Prepare your bigger box by wadding up newspaper and lining the bottom of the box. After putting a good layer of paper down then set your smaller box inside and proceed to fill up *around* the smaller box with newspaper, wads of it pushed down between the two boxes to fill up the area between the two, this is "insulation" to help hold in the heat.
Then, if your big box top doesn't already have a plastic view screen, you need to cut out the top of the box, leaving a good inch around the rim, and then thoroughly tape cellophane (or heavier plastic) over the top, trying your best to get it as airtight as possible! Put the lid in place, on the big box which now holds the wadded up newspaper and the smaller black painted and tinfoil covered box. Wrap some tape around the edge of the lid if it doesn't seal quite tight enough.
Once you have done this, you have a fairly basic yet totally functional solar oven! Make it even more efficient by adding a few tinfoil panels with which to prop up around the box to direct more rays inside the box. This can be done easily with another box, split open, laid out w/ tinfoil taped onto one side. Prop your panels up around the box to bounce more light rays into the box, you will be able to see the reflection as you are setting everything up, just aim them into the box as much as possible.
Oh ya, and the oven needs to be slightly tilted, one end up a little higher than the other (use a brick or rock under one end) to direct the opening toward the sun rather than just straight up to the sky.
I would suggest you use glass containers and you need to paint them dull black also, we used quart jars spray painted black.
We made cookies and boiled eggs, among other things, using Portland Oregon sun :-)
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