Interestingly enough I thought about that very subject about 3/4ths of the way through the test. It occured to me that there's an awful lot of surface area for it to radiate heat off quickly. I did end up putting the lid on it (slightly off center). The esbit was too far gone for it to boil after that but it may have gotten quite a bit hotter had I had the lid resting on it from the beginning.