Here is how to build from scrap material, a very workable and durable solar cooker.
If you take care of it it will last your lifetime.
It uses the double-jar cooking method, and the reflectors have been redesigned from the original.
The original was made with a carboard winebox, but now I am using those old campaign-style plastic yard signs, covered with aluminum foil. These are essentially waterproof, unlike cardboard, and will last far longer. Either works.

This unit, according to Billy and his mom, is the "very best" cooker for potatos, and sweet potatos. They are thrilled with its performance, and say that they never have to worry about overcooking, either. They have forgotten them overnight, and when finally eaten were perfect.
I have cooked some batches of rice with it, and find that it is possible, however, to overcook rice. This can be corrected by positioning the leading edge to get into shadow sooner.

A sealed jar holds sunlight in the form of heat. It will be hotter inside a glass jar than outside of it. And, the sunwaves are partially trapped inside the glass, so, it can keep getting hotter and hotter.
Now if you have a large jar, and put another smaller jar inside of it, this is then double insulation. What happens is the inside of the inner jar gets hotter than the inside of the larger jar. So we are amplifying the quality of the heat.
That's the secret of double jar cooking.

So get yourself a large gallon glass jar with a wide mouth (drop by your local bar, tip the bartender, and ask them to save you one of those olive or cherry glass jars.)
You need a smaller jar also. I use those old glass spaghetti sauce jars, as they are sometimes Ball jar types, and do have measuring marks on them. Make sure the inner jar is not too big to fit inside the larger jar!!!
Also you need a reflector (in this case a winebox) which is made with foil, spray glue (or tape) and a backing.

Before I paint my inner jar black, I like to put a piece of masking tape lenthwise just over the measuring marks. This allows me to see my measured quantities, and to check on the food while it's cooking. I use cheapo spray on flat black paint, and after drying I put it inside the jarge jar, on the reflector, and let the paint bake-on for a day or two. The black paint is also important in that it helps keep your foods from bleaching out and changing color. Carrots get very dull looking when exposed to concentated light. Celery becomes ghastly pale looking. The world is unsightly enough without having to look at odd colored food (no matter how good it is)

One advantage of cooking in a jar is that it helps hold in moisture. Dry food, ick. Moist food, good.
To cook a quantity of rice is very simple. Measure (using the jar's scale) 8 ounces of rice into the jar. Next fill it up with water to the 16 ounce level, and add just a touch more water. This will give you 20 oz. of cooked rice, so make sure your jar is at least 20 oz's. Now screw on the lid, and back it off just a little bit. You don't want it too tight, I think, but just don't know if tightly sealed it will break or not. (If not, it may be possible to pressure cook, which is another VERY nice angle to investigate....)

Point the reflector at the sun, place the small jar in it, and carefully cover that with the larger jar upside down.
Now you can forget about it for an hour and a half, to two hours. If you go away and forget about it, don't worry, it's probably done. And it will be unburnt. I've never had a jar cooker ever burn food. Solar won't do that unless it is very highly focussed as in a parabolic.

When your rice is done (or to check it), make sure you do not place your HOT glass jars on a cold surface. They may shatter. Set them on wood or something insulated. Also, as you will learn quickly, you should use a potholder or gloves to pick up the cooking jar. I have measure its temperature in excess of 220 degrees.

We know that jar cooking will do rice and potatos nicely.
It might even do small breads, but that requires further study and experimenting.
I suspect it might be able to cook pre-soaked soybeans if left in all day and tracked several times.
Soybeans take a lot of cooking to get well done, and don't ever eat undercooked soybeans. ick.
They must squash between tongue and pallet, else they're not done...!
The cooker will bake onions, carrots and other veggies.
I don't know if it will do meats, tho' it will reach boiling temperatures, so I don't know why not.
And, having made soymilk in a standard box solar oven, I bet this cooker can do that too.
Again, needs experimenting with.

Since it takes only about an hour to heat water to near-boiling, I can see the cooker as a nice device to purify water for drinking. Pathogens are killed at above 160 degrees, apparently, and this cooker will easily reach that. With a good 8 hours of sunlight, you should be able to disinfect about 80 ounces of water, more than enough to supply one or two people with 'safe' drinking water. That's assuming no chemicals in it, etc.
A possible survival tool?

I hope that you will build a simple jar cooker, and that you will share your experiences.
One thing to consider is the angle of your reflector. The first cooker was angled less than 90 degrees. The latest one is at 90. I suspect that the smaller the angle is, the hotter the jar will get. This does cut down the cooking time, which may be to advantage in lesser cook times via the sun moving out of focus.
Please share your experiences with this.

I really do like the possible idea of pressure cooking by tightening down firmly the jar lid. It may present some dangers if the glass were to shatter, but if one uses good canning jars, that might not be a problem. But don't even think about trying it unless you show due caution. I would not move the jars at all until the sun has been off of them for some tens of minutes. And even then with great caution;.
Pressure jar cooking holds a lot of excitement for me, and I wouldn't be surprised if the temperature/pressure in this system got over 300 degrees or more. That's enough heat to cook just about anything.

This is a cheap, easy to use, small, portable, low-tracking, long lived solar cooking system, which can be built easily within a couple of hours. It will give you 20 ounces of cooked food in one use, and if used twice in a day, can double that or more....
With three of these units in use, you can in theory cook 60 to 180 ounces of food in a good full day of sunlight.

For free.

Please, keep us abreast of your progress.
Chef Craig

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