Why did my soap develop a white coating?
Short answer
That's soda ash: a harmless, powdery film of sodium carbonate formed by carbon dioxide in the air. Spray fresh soap with alcohol to prevent it.
Explanation
Soda ash doesn't affect how the soap performs; it's purely cosmetic. To prevent it, spray the freshly poured surface generously with 91–99% isopropyl alcohol.
What to do
- Spray immediately after pouring and texturing the top.
Source: The Big Book of Homemade Soap — Recipe steps and FAQs, e.g. Rosemary & Citrus Luxury Bar