Ashes are a pretty useful resource in Graveyard Keeper. Not only is it used to create an ingredient that is required for fertilization, ash also has several other uses, such as making black dye and flavor enhancer.
Ashes can be obtained by cremating corpses, but in the early stages the game does not really tell about it. To cremate a corpse, you first need to unlock the Cremation skill in the Theology tab in the skill tree. After that, you will have the option to create a "Place for the incineration of corpses." Once you've done that, head south of the morgue where you'll find a blueprint table surrounded by trees.
Using this table, you can build a corpse incineration site in one of four different slots. But keep in mind that for this you need to make eight pieces of wood - using a regular or circular saw.
After that, you can put a dead body and burn it. Return to the site after a while to collect some ash and some salt.
