I’ve received quite a few questions about the Wonderbox (or Wonder Oven) thermal cookers I have made. If you don’t know what a Wonderbox is, it is a fabric “box” in two pieces (bottom and lid) that acts as a heat retention cooker–like a big thermos. They are a great way to save on fuel, and keep your kitchen cool during the hot months! Get a pot of soup to boiling and then take it off your fuel source, put it in your Wonder Oven and it continues cooking without using any more fuel. Check this post to see how they work. Even with all the available online instructions, there are still a lot of unanswered Wonder Oven questions, so today I’ve got some answers for some of the most frequently asked questions about Wonder Ovens.
How much fabric do I need to make a Wonder Oven?
It depends on how you cut your pieces out. If you cut laying out as I did in this post, and are nice and tight with your piece layout, you can get a Wonder Oven cut out of as little as 2 1/2 yards of standard quilting-type 45″ width cotton fabric. I like to start with 3 yards just to be safe. If you’re going to lay your pieces any other way, you may need 3 1/2 or more yards.
Do I need 100% cotton fabric? What about all cotton thread?
The instructions say to use all cotton fabric and all cotton thread, presumably because a hot pot could melt a polyester fabric. Yes, use 100% cotton fabric to build your wonder oven–I have melted a polyester stool cover with a pan out of the oven, although your boiling pan on the stove won’t be quite that hot. I sew my Wonder Ovens with a variety of “all purpose” thread. It is either cotton or cotton covered polyester. I tried to melt a piece of the cotton/poly type on the bottom of a boiling pot and couldn’t. I have never had a problem using my Wonder Oven sewn without 100% cotton thread. Of course, you’re free to make your own decisions on what to use if you sew your own.
How much filling do I need to fill a Wonder Oven?
This depends on how tightly you fill your Wonder Oven, but you’ll need about 2 cubic feet of filling to fill both the pieces.
These foam beads I got online are really big, will they still work?
The beads you can order most places online like walmart.com, amazon, or ebay, that say “slightly larger than a pea” are approximately 3/8″ balls. They are not small like the old style beanbag chair fill. They will still work to fill your Wonder Oven, however a smaller bead retains heat better. See this post for a chart on heat retention of different fill types. I use small 1/8″ beads in the Wonder Ovens I sell.
Where can I get the small beanbag fill?
I order mine from Foamerica. There may be other sources as well. You’re looking for beads that are approximately 1/8 inch in diameter.
Is there any way to keep all those beads from going all over the place when I’m filling my Wonder Oven?
Not really. But you can help yourself out by taping your funnel to the fill opening, using paper and metal for scooping and avoiding plastics, using a spray bottle of water to knock down some of the static, and filling the pieces outdoors (if it’s not too windy).
How big is a finished Wonder Oven?
Mine fit nicely in a 16″ square box.
The Wonder Ovens you show on your site don’t look like they’re stuffed as full as others I’ve seen online, why not?
A Wonder Oven does not need to be stuffed to capacity to work well. In fact, if it is slightly under-stuffed it allows for more variety in the pot shapes and sizes that it can accommodate.
Why are your Wonder Ovens in a box?
Because it is not stuffed tight, the Wonder Ovens work best in some type of container like a box, bin, or tote. This gives the oven better structure and keeps the insulation tight to the pot you are cooking in no matter what size or shape that pot is. It also makes your Wonder Oven easier to move and keeps it cleaner. My personal Wonder Oven (pictured in this post) is in a round, rope handled tub.
Please, if you have any other questions, feel free to contact me and we’ll get an answer for you! And of course, if you don’t want to sew your own, go ahead and order one of my Wonder Ovens for sale. :)
Keep preparing! Angela
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I’ve been making some rocket stoves and the insulation I am using is vermiculite. Could this be used in the Wonderbox for insulation?
Polystyrene beads have an R-value of about R-2.5 per inch. Vermiculite has an R-value of about R-2 per inch. Vermiculite should work well as insulation for a thermal cooker. (I use fiberglass in my rocket stove and shredded foam egg cartons in my thermal cooker.)
This is fantastic!
Vermiculite from certain mines has been found by EPA to contain asbestos. I would be wary of using it inside the house.
Look into perlite, instead? It is essentially a natural glass: fire resistant, melts only above 200 degrees F, is food safe, chemically inert, not soluble. However, it does allow water retention in pore spaces. So I assume, without experience to back it up, a wonderbag would be washable and dryable with this as filler. Users comment in the Home Depot website that the horticultural perlite, though labeled “coarse” contains a lot of dust-size particles, smaller than the 1/8″ EPS beads. Perlite use in heat-retention cooker bags might be worth exploring.
I have been considering using a wonder box, to help lower cooking costs as well as learning the ins and outs of using one. Are they really as efficient as they seem to be?
I had milk spilled on my wonderbox. What are my washing options?
Hand wash and air dry, but there’s no reason you couldn’t use a gentle hand sprayer to help wash it. The foam beads are close celled, so won’t soak up water even if they are submerged.