Don't fret, it's not mold. It's not a badly formulated soap or a soap that's been left sitting around way too long. Soda ash (sodium carbonate) is a natural product of the soap-making process that is more prominently visible in some bars than others (for a variety of reasons).
For those not super familiar with the soap-making process, here are the basics. When you mix Sodium Hydroxide (Lye) which is a strong base with fats (oils, butters) a reaction occurs called saponification. Saponification is the essence of soap-making - if your ratios are correct, all of the lye will be saponified with the oils (soapmakers generally “superfat” or add at least a bit more oil than technically needed to make sure) and the result will be soap crystals suspended in water phase with the bit of oil left over in there to nourish your skin after you wash. Sometimes makers superfat quite a bit but that is a topic for another blog post . . .anyhow, the soap needs a good cure time to allow the crystals to develop fully and the excess water to evaporate out (generally at least 4-6 weeks). During the initial part of the saponification process (about the first 48-72 hours), a lot of chemical changes are taking place. The soap batter heats up, the pH changes up and down numerous times, some colors will morph, and so on. It's quite amazing to watch and learn about!
Now for the soda ash part – during the first part of the initial saponification process (the first few minutes and hours after the soap batter is mixed), some of the unsaponified lye reacts with carbon dioxide in the air where the soap is exposed, creating sodium carbonate. That sodium carbonate appears as a white-ish, almost “ashy” looking substance on parts of the bars that were exposed to air while saponification was still occurring. It can appear as a thin or thick layer, or even a smattering of dots here and there. In fact, if a maker cuts a loaf of soap very early while initial saponification is still underway, they may even get a little ash on the backs, sides and bottoms of the soap!
Depending on formulation, whether or not the soap went through the gel phase, amount of water in the soap batter, temperature the soap batter was poured at, whether the maker covered the soap after pouring and a number of other factors, you may or may not see visible soda ash. And that's just fine. It's not necessary – but if it happens, it's simply an aesthetic feature.
Some soap makers choose to embrace soda ash on all of their designs where it happens. Others choose to change up major design features/formulations just to avoid it, and then there are those that steam, plane, or wash soaps fastidiously to get rid of it when it happens. Whatever the approach, soda ash is simply a natural part of the process.
- Lisa Jenkins, Owner, Heart's Path Designs.