Ash

Ash is an item that can be used in a variety of crafting recipes and in potions.

Block drop
Ash piles will drop 1 ash when mined with a shovel or broken by water.

Various foliage (grass, fern, nether roots, dead bush, any leaves) will drop 1 ash when broken with a molten sword.

Incineration
Incinerators can output ash.

Brewing
Ash can be placed into a cauldron.