From Ars Magica
Arcane Ash is obtained by burning an Arcane Compound in a furnace. It is a core component of all Essences and is used in several advanced crafting recipes, such as the Greater Focus.
Certain mobs, such as the Wisp also have a chance to drop arcane ash. It can also be really rarely found in Mage Towers and especially Mage Archives.
Max Stack Size