Ash is a white, powdery substance that mysteriously appears on soap as it is curing - usually on the surface of the soap as it is sitting in the mold hardening overnight after it is first poured. It will usually only form on the sides of the soap that are exposed to the air while hardening.
What is it?
Soap makers have traditionally believed that it is sodium carbonate formed by the free sodium from the sodium hydroxide (lye) and carbon dioxide in the air. Other soap makers have theorized that it's actually microcrystalline powdered soap residue.
Soap making guru Kevin Dunn, author of Scientific Soapmaking recently did some extensive tests on ash and has confirmed that it is indeed sodium carbonate. Among the clues he used to test his hypothesis were that it was soluble in water and that it only occurred where the soap came in contact with air.
What do I do about it?
It's not harmful, just a nuisance. Some soapers even prefer the "rustic" look. The easiest thing to do is just trim it off with your soap beveler or planer, or just wash it off under running water. There are also ways to help prevent it:
- After you've poured the raw soap into your mold, cover it with a layer of plastic wrap to minimize the contact with the air.
- Make sure that your recipe is properly balanced. A lye-heavy soap will be more prone to ash - in addition to being too harsh to use.
- Don't unmold the soap until it's (as Kevin Dunn says) "tongue neutral" - which means you have to do a tongue test - or just wait until you're sure the soap is completely done saponifying.
- Add a bit of bees or soy wax to your soap recipe. This seems to eliminate it as well - most folks use the wax at about 1-2% of the amount of their total oils. (e.g. 1.5 oz. of soy wax in a batch of soap with 100 oz. of oils)
- Just get used to it - it seems to happen most (to me at least) in recipes that have a lot of lavender essential oil in them, like the Lavender-Mint Layered Soap