Ash

From Dwarf Fortress Wiki

A dwarf with the wood burning skill enabled and a wood furnace can make ash from a wooden log.

Ash is an intermediate good only used to make potash or lye at an Ashery.

Ash is listed under "bar/block" on the stockpile menu.

A bar of ash is also left behind when a fire man is killed.

Ash is a fireproof material, and can therefore be used to build a smelter and forge(with anvil) in places without easy access to stone.