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Building a Solar Oven, Part 2

Well, now that the kids are feeling better, maybe I can continue on about the solar oven.  :)

After I foiled the insides of both of the boxes, it was time to put them together.  I am using dry lawn clippings from the yard as insulation.  Apparently, dried grass makes a rather good insulation.  I did some poking around online, and found a number of sites that mentioned using it as insulation in solar ovens, as an ingredient in walls, as emergency survival stuffing in clothing, and even as a normal part of going outside to play in Alaska (until recently) — it would be stuffed into shoes, and wrapped around the feet.

The first step was putting some sort of spacers in the bottom of the outer box to help hold the oven box up off of the bottom.  I could just put an inch or two of grass in the bottom and set the oven box on top of it, but I figure the grass would compact over time and become less of an insulator (due to reduced trapped air), and, if the grass compacted, then the oven box would sink below the top of the outer box, causing problems its seal with the lid.

So I considered several ways of making spacers, and finally decided to simply cut small squares of cardboard and stack them.  I sprayed adhesive on one side of the square and affixed it to the bottom of the outer box, then did the same for three more squares, figuring that four stacks should give good support.  I added a second layer, then a third, and so on until I had about eight or so layers (a little over an inch high).  I rotated the squares so that the corrugation crossed the corrugation of the previous layer, hoping that would help further prevent compression of the squares.  The stacks of squares are roughly 2″x3″, but they vary a bit, as I didn’t worry about getting them all exactly the same size.

If I had had my brain plugged in, I probably would have tried to put a fifth stack in the middle, since that is where the weight will be concentrated. It may not have worked out, though, because the flaps of the box don't meet in the middle, making the middle lower than the places where the other stacks are. I think it will be alright, though, because my oven box is double-walled, meaning I will have four layers of corrugated cardboard making the floor of the oven, and spreading the weight more evenly across the bottom.

Once I had the stacks done, and as much of the adhesive removed from my hands as possible (that’s sticky stuff!!), I started putting the dried grass in.  This is mostly St. Augustine grass, with some weeds interspersed.  I feed this same dried grass to the rabbits, though it isn’t their favorite, and is only of mediocre forage quality… but it’s FREE and right here in my yard!  Anyway, I continued to add grass, lightly pressing on it to see where it was too thin, until I had a nice, firm (but not packed) layer of grass that just barely covered the support stacks.

I tried to make sure I had a nice, even layer that would give good support to the oven box.

I then concentrated on the oven box, using Duck tape to seal all the seams and also to reinforce the corner supports.

I'll never have the chance again, so I pretty much taped it to death.

Finally, I had reached the point at which it was time to put the oven box into the outer box.  Once that was accomplished (a little harder than it sounds, because of the foiled flaps), I began stuffing grass around the sides.

Again, I went for a nice, firm packing, but not hard pressed. It was a bit difficult to keep the oven box perfectly centered, but that isn't really as important as making sure that there is some space between the boxes on all sides. The walls of both boxes ended up bulging a little, but that would be remedied as time went on.

Once it was suitably stuffed, it was time to fold the flaps down.

The extra height of the oven box will be used to secure it to the outer box.

Once I had the flaps taped down, I cut the oven box flaps apart just to the top of the outer box.  Some of my corner supports extended above the top of the outer box (oops… shoulda measured that), so I had to cut through them down to the top of the outer box.  Then I sealed the gap along each side of the oven box to the outer box with Duck tape.  Using the outside of a pair of scissors, I made a dent along the outside of the oven box flap so that I could fold it over neatly.  The idea was to fold it over the top of the outer box, and then fold it again down the side, and then tape it down.

I stopped and thought for a while, though, looking at it and thinking about how thick that was going to be, and how it was going to be more difficult to make a lid that would fit well.  I finally sprayed one flap with adhesive and folded it down, but it wasn’t working well because it was so thick.  Then my mom, who had joined me outside at this point, suggested peeling some of the thickness away.

Since I’m going to have plenty of layers of cardboard at the top of the box anyway, I thought that was brilliant!  So I cut the Duck tape seals I had just made, and Mom helped me peel a layer of cardboard from the flaps, including the little bits of the corner supports that were too high.  I had to be very careful not to cut all the way through the flaps.  I then cut the flaps down so they wouldn’t go over the edge of the outer box.  This would make a tight-fitting lid much easier to make.

Peeling away the second layer of cardboard, since the double-walled construction of the oven box wasn't advantageous here.

This side has had the second layer of corrugated cardboard peeled away, and has been cut down.

I then sprayed the surfaces to be glued — the top of the outer box, and the peeled side of the oven box flap — using a piece of scrap cardboard to catch overspray.  I don’t want to taste adhesive in my food!  Maybe I’m just picky.

I waited 30 seconds for the adhesive to set, and then I pulled the top of the outer box and the top of the oven box together tightly, and folded the flap down and pressed it into place.  I held it to be sure the two surfaces adhered well.  Then I scored the foil on the flap and peeled most of it off, since I don’t need foil on the flaps, and I need the flaps to stick well.  Then I taped the flap down.  This is where the bulging was mostly rectified.

It is important to not have any Duck tape on the inside of the oven, because it fumes a bit when heated. I kept the tape about 1/4" away from the edge where it drops off into the oven.

Once I had all of the flaps peeled, cut, glued, and taped, I taped diagonally across the corners.

Since I had peeled the oven box flaps, I didn't have large spaces in the corners that needed filling.

The base of the oven is now complete!  :)


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6 Responses to “Building a Solar Oven, Part 2”

  1. ladysown says:

    what will you be doing with this???

    • Miss M says:

      I’ll be using it to cook various things, to help lower the electric bill a bit. The first thing I will try in it is bread. I love baking bread. :)

  2. ladysown says:

    you can use this to cook bread????

    I have GOT To see the final product! :)

  3. eco2pia says:

    I’ve been watching this with interest and a bit of envy, because for a solar oven to work I believe you need…SUN! North of Seattle, that just isn’t gonna happen 9 months of the year. I wish I could make one big enough to crawl into right about now…