Soda ash
Updated
On this page5
A common cosmetic complaint in cold process soap, and entirely harmless.
A white powdery film
A chalky white bloom on the exposed surface, usually the top of the loaf, sometimes on the cut faces.
It wipes or washes off and the soap underneath is perfect.
Customers read it as mold, which is why it costs sales even though it costs nothing else.
What it actually is
Sodium carbonate, formed when unsaponified lye at the surface meets carbon dioxide in the air.
Which is why it forms on exposed surfaces and not inside the bar, and why it appears in the first day or two rather than later.
Why it forms
Air contact on a soaping surface before saponification has finished.
Ungelled soap is more prone, because it stays reactive at the surface for longer.
Pouring at a thin trace exposes more liquid surface for longer.
Cool, dry, drafty conditions make it worse.
Removing and preventing it
Steam it briefly, wash the bar and let it dry, or plane the surface. All three work and the last is what most production makers do.
Prevent it by covering the mold with film in direct contact with the soap surface, or by spritzing the top with alcohol immediately after pouring, which is the standard trick and is genuinely effective.
Pouring at a slightly thicker trace helps, and so does gelling.
And where it still appears, say so on a card: explaining that the white bloom is a natural surface reaction and not mold turns a return into a conversation.