Log

In Slavic Power, a wooden log is a primary resource, which has a flammability of exactly 100%, meaning that it can keep the flame in the furnace at a steady level when used as a fuel. It does not, however, add any more energy.

It is a good idea to keep the flame at a low level when using a log as fuel, because it prevents it from going down any further, but when there is more fire, the logs burn much faster. You could put a log in the fuel slot, and then light a fire with a match, which will keep it at 5%, and the logs will not burn out very fast - but you will still be able to heat items in the furnace.