The wonderful museum at Tangwick had an explanation for the many burnt mounds I kept seeing on maps. Apparently they date from the age before the existence of metal pans. I have seen Ray Mears in Canada use this technique to heat water in pre-metal birch bark pots that would burn up in a fire. Rocks are heated up in a fire and then placed in the water. The thermal energy from the rocks transfers to the water, raising the temperature of the water until the two reach thermal equilibrium. Repeated application of this technique broke the rocks and the broken bits were piled up - a burnt mound. I decided to try some data. Rock has a density of about 3 grams per cubic cm. A 10cm cube of rock would thus have a mass of 3kg. Granite has a specific heat capacity of about 800 J/kgK. A fire could heat the rock up to 500oC. Say that 5 litres of water, with a mass of 5kg, started at 10oC. Give thermal equilibrium as temperature T. Then 5 x 4200 x (T-10) = 3 x 800 x (500-T). I get T as 60 oC which sounds about right.
