If I remember correctly, during Vietnam, troops would add a few drops of insecticide to the small can of peanut butter that came in C-rations and light it off. The oils in peanut butter will burn but they need to be heated first; thus, the few drops of insecticide which are more easily ignited. Then they would put their steel helmet (minus liner) over the burning mixture. The helmet became a radiant heater as they squatted down under their poncho. I see that 0311 already mentioned this.