See that white powdery looking substance on the tops of this soap? That's soda ash! But what is it, and why is it on the tops of the soap?
Soda ash is actually a chemical compound called sodium carbonate (Na2CO3). It's a white, somewhat fuzzy powder like substance that sometimes appears on top of cold process soap as it goes through the saponification process in the mold. Chemically speaking, it's a type of salt, and it absorbs moisture from the air. It's actually used in some commercial applications, such as a water softener and dying fabrics.
So then, where did it come from when it appears on soaps? As we learned in the Basic Soap Chemistry series, the lye (NaOH) reacts with the various fatty acids of the oils to produce soap. However, sometimes those sodium atoms don't fully connect with the oils, leaving some floating free in the soap. This sodium then reacts with the carbon dioxide in the air, combining to produce sodium carbonate.
The good news is that soda ash is completely harmless. It can be removed in several different ways if desired, but it has no affect on the finished soap. It is solely an aesthetic issue. There are times when I decide to remove the ash, and time when I leave it intact. For example, on this soap I left it because I thought it looked interesting with the textured tops. But on the soap below, I planed the tops of the soap to remove the ash so that you can see the pretty blue and yellows of the funnel pour technique I used.
Soda ash does not form on every soap. This can be do to lots of different factors, and there is no real predicting if it will appear or not. But you can rest assured that it's not something that should bother you!