Glossary
Soda ash
A powdery white film that can form on cold process soap, usually just a cosmetic issue.
Soda ash is a fine, white, powdery coating that sometimes forms on the surface of cold process soap. It happens when free, unreacted lye near the surface reacts with carbon dioxide in the air rather than with oil, forming sodium carbonate instead of soap.
It appears mostly on soap that was left uncovered while setting, or that cured in a drafty or humid spot, and it is a cosmetic issue rather than a sign the whole bar failed. Bars with more texture or fine detail on top tend to show it more noticeably than smooth-topped bars.
To reduce it, many makers spritz the top of the poured soap with rubbing alcohol shortly after pouring, or cover the mold loosely while it sets. Once ash has formed, a soap planer or a light steam from a kettle can remove or smooth it away. A common mistake is trying to scrub it off with water, which can leave the surface uneven.