Solar oven from glass, mirrors and stone we had lying around!

I can't claim credit for this, as it was my husband's idea and he did most of the work. But, with stuff we had lying around, some free mirrors we found on the side of the road and in dumpsters, and two mirrors we bought at thrift stores, we have a solar oven that gets up to 300º F in full sun!

So many mirrors, so much glass!

The oven itself is made from double-paned windows that were getting tossed out at my husband's old job. He held them together with metal tape and wood. The lid is made of glass, too. He wanted to make it all out of glass so that more light/heat would be able to enter. He did put mirrors on the back side of the oven, too, as well as a mirror on the bottom, to reflect light back in.

At the bottom are some black granite slabs a local homesteader was trying to get rid of. They help absorb heat and also do a good job of adding thermal mass to store the heat.

The lid is made out of glass windows taped together with clear exterior repair tape.

To reflect more light/heat into the oven, we propped up mirrors all around it, angled to reflect the sunlight into. The mirrors that had frames got attached to wooden stands that hold them at the correct angle. The smaller mirrors are just propped up with bricks and buckets.

It's not pretty, but it works!

the surface tempurature of the rocks is hotter than this water, but we don't have a better way to measure these hot temps!

I threw some eggs, honey, raisins and nut-flour into a cast iron pan and put it into the solar oven.

Batter in the skillet. Will it bake?

I left it there for an hour and half while I wove a basket, and came back to find it perfectly baked!

Looks done!

Mmmm! Perfectly baked!

My husband and son also melted a plastic Hot Wheels car in there, and water is now sizzling on those slabs of granite. This solar oven is cookin'!

Poor little solar-melted car!

Nice !!!

I'm guessing that black stone is an important part of this build. It would be particularly helpful if you were having an intermittently cloudy day that was still "mostly sunny".
Great job!

Jay Angler wrote:I'm guessing that black stone is an important part of this build. It would be particularly helpful if you were having an intermittently cloudy day that was still "mostly sunny".
Great job!

I think so, too! My husband wanted to just have it glass all the way around and mirrors below, to get the most light inside as possible. But I kept saying we should put in something dark, preferably with thermal mass, to absorb the heat. The stones work really well. When we baked the batter in the pan, we totally ignored the oven for the whole hour and a half. We didn't fiddle with the mirrors or orientation to optimize the light going in. When we took the pan out, a lot of the mirrors were no longer directing light into the oven. Regardless, not only was the batter baked, but the stones were so hot water sizzled on them!

By having the sides of the oven be double-paned glass, a lot more light and heat energy is able to come in, but we also sacrifice insulation (a lot of solar ovens are insulated on all sized except for the top). But the black granite kind of takes the place of that insulation, but absorbing and holding the heat. We need to get a better surface thermometer--ours only goes to 200, and the rocks get hotter than that. I wish I knew just how hot they get!

Come to think of it, the black granite is kind of like the rocket mass heater inside the teepee at Wheaton Labs. The teepee doesn't hold much eat, but the rocket mass heater does, and that heat keeps everything cozy, even without good insulation!

I have always put unglazed clay tile into mine for thermal mass, same idea, tile was just what I happened to have around.

Wonder if the black slab would get hot enough to burn the bottom of what you're baking. It might, if it was directly inside the glass box, not underneath it?

Should be easy enough to paint any rock or brick black.  
In a pinch, I wonder how well silver paint could provide reflectivity if you didn't happen to have mirrors or highly-polished metal available.

I am not sure what paints are food safe other than milk paint.

Best purchase I ever made was my All Seasons Solar oven. My husband calls it my easy bake oven. I bought it used / missing a panel but easy enough to make the replacement.  My goal was to recreate it double the size. Unsure if it will be double the heat, but pretty sure it will make it much hotter. Right now I can certainly bake cookies (do this too often) and roast veggies and potatoes but it takes all day. I can almost boil water. Trick is to use an all cast iron pot and lid inside a completely enclosed glass bowl - 2 large round pyrex bowls work but I had an old convection oven that was all glass so I use that topped with a large glass plate from an old microwave. I decided not to buy the internal parts new so I'm now always looking around like a hawk for the just right cast iron pots and pans to fit it.

Anne Miller wrote:I am not sure what paints are food safe other than milk paint.

It shouldn't be too hard to make your own food safe black paint. Paint is made of two elements: pigment and binder.

Thankfully, black is one of the easy pigments to find or make, as you can get a very nice black from ground charcoal. For thousands of years, charcoal was what artists used to make black, and it's non-toxic. You can grind up some charcoal from any fire in a mortar and pestal (my history students love grinding up charcoal I got from my woodstove.)

The next step is finding a binder, and there's quite a few non-toxic binders--some of which might be at home. Casein (from curds) works well, and it's what's in milk paint. But, you could also buy gum arabic to make a watercolor or ink (that's what watercolors are made of). Another binder is a drying oil, like linseed--this makes an oil paint--, but it'll take a long time to "dry"/polymerize. Chinese black ink is made with gelatin, so you might be able to make a black paint with gelatin (I haven't tried it yet).

The main questions is, which paint will stick to a smooth surface. I think casein (milk) paint will likely flake off the stone if it doesn't have lime to help it bind to the stone, and the lime would make the paint lighter. It'd be a fun experiment to try the various paints and see which one sticks!