(If looking for the tree, see Ash (tree))
Ash is created from a wood log at a wood furnace by a dwarf with the wood burning labor enabled. Ash has a value of 5 and does not have quality modifiers. Ash is an intermediate good only used to make potash or lye*.
A single, ordinary bar of ashes is left behind when a fire man is killed.
Ash comes in bars, stored in a bar stockpile and viewable in the stocks screen under "bars". It is also a fire-safe material, and can therefore be used where that is needed or desired.
"Ash" is also the name of one of the types of trees that grow in some biomes. The adjective attached to items made from ash wood is "ashen", in the same way that items of oak can be termed "oaken".