As a fairly late-game material, you won’t be able to obtain the
Charcoal Ash until you unlock it from the Stump BURG.L Chip which is found at the end of the Stump Lab Outpost. Once you’ve unlocked the recipes, you’ll need to purchase the “Advanced Building:
Ash Cement” upgrade for 1,500 Raw Science. This will unlock both Charcoal Ash, and Ash Cement.