- For the tree, see Ash (tree)
Ash is an intermediate good used to make potash, lye, or to glaze ceramics. Ash is produced by burning wood at a wood furnace, and comes in bars, stored in a bar/block stockpile and viewable in the stocks screen under "bars". Ash bars are a surprisingly-sound structural material, usable for building constructions, workshops and furnaces of equal parity to, say, granite. Although the precise material properties of ash are unknown, it is fire-safe, (somehow) magma-safe, and can even resist the heat of dragonfire, making it a usable material when working with intense heat.
Procedurally generated creatures (such as titans and forgotten beasts) may occasionally be composed of ash, though killing them will just result in a useless pile of ash for your dwarves to clean up. A killed fire man will leave behind a usable bar of ash, though these creatures are rare.
Step 1 - Just add fire to anything.
[MATERIAL:ASH] - reconstructed from data extracted from memory