Firewood is made by cutting up tree trunks or logs into blocks of wood and finally into firewood, preferably with a splitting axe. Each block of wood gives between 20 and 25 units of firewood. In c3.18stable splitting firewood require skill: too low and you get 5 piece per block, for example.
To light a fire on an open tile, there must be at least 2 pieces of firewood on the ground. When lighting a fire in a fireplace, there must be at least 4 pieces of firewood (6 with version 3.17) in the fireplace. Burning firewood in fireplace is the most economical way of using wood for fires if you have enough skill in 3.18stable. If not, chopping slender trunks then make stakes, as it use Common skill.
Fires can spread to adjacent tiles that have firewood on them. This tendency can be exploited to prepare many fields for farming, but can also potentially burn down a player's cabin. Burning fireplace wont do so, however.
Firewood can be carved (not in 3.18stable)