40d:Ash

From Dwarf Fortress Wiki

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.

Potash is created from ash or lye* at an ashery by a dwarf with the potash making labor enabled. Potash has a value of 15 and does not have quality modifiers. Potash is used for fertilizer on farm plots. Fertilized farm plots have larger yields, as measured by the larger stacks of the plant when harvested.

(*Yes, you could turn ash to lye and then that lye to potash, or just turn ash directly into potash - same end result, your choice.)

Pearlash is baked from potash at a kiln or magma kiln by a dwarf with the furnace Operator labor enabled. Pearlash has a value of 20 and has no quality modifiers. Its sole use is in the manufacture of higher-quality types of glass and glass items.

Ash, potash and pearlash are bars, stored in a bar stockpile and viewable in the stocks screen under "bars".

Ash, potash and pearlash are all fire-safe materials, 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".