A friend of mine, Roberto, recently purchased a Solar Oven. It is a plastic gizmo that uses mirrors to collect heat from the sun.
Roberto invited me to try baking some bread. He warned me that the oven was designed primarily to work with 'surface heat' rather than store heat or create large air temperatures.
Nevertheless, we managed to get the air temperature up to 135 C on an Autumn day in South East Queensland (Australia) with intermittent sun.
In Summer, with full sun, we predicted that 180 C would be an achievable air temperature. That's still at the cool end of baking temperatures. For those not familiar with Celsius or bread temperatures, 240 C is a typical starting temperature for bread (temperatures are often reduced as the bake progresses).
This was my first bake from a recently developed starter/leaven, as featured in the first few weeks of CultureCam. And for Roberto it was the first time that bread had been attempted in his 3 day old oven. So it was quite exciting.
I decided to bake a small (about 300 g) loaf of pumpkin bread. It was a well hydrated (wet) dough, and in retrospect a drier dough would probably have baked better over what turned out to be 90 minutes of baking. That is a long time for a small piece of dough, but was not unexpected as the oven instructions said that you can expect to multiply most standard cooking times by a factor of 3.
It took 20 minutes for a small amount of condensation to appear on the perspex lid. After 40 minutes, condensation was heavy and dripping. The oven is a tightly sealed unit...so I was relieved when lots of moisture appeared (I didn't see how the bread could be baking without moisture leaving the dough).
The bread did bake eventually, after about 1.5 hours. At such low temperatures, we decided that the bread cooked more than it baked. However cooking sourdough under the power of the sun did give a sense of independence.
We joked that more solar ovens need to be made before oil is depleted and there is less plastic available to make them. At least there will still be plenty of hot rocks to bake on.