Actually, Sterno stoves were used. They were so valued, that they were passed down through the generations and is why we can not find any evidence of them in archeological digs.
http://www.sterno.com/retail/pages/stove.html
;^D
funny funny, but seriously, 16-brick stoves only take a few twigs to boil water and the heat is focused up not on the ground, leaving no scorched rocks