What soda ash looks like
Soda ash is a white, powdery or flaky film that sometimes forms on the surface of cold process soap, usually on the top of the loaf where it was exposed to air. It can look like a light dusting of flour or a layer of ash, or it can appear as a crusty, slightly rough coating. On dark or brightly colored soap, it stands out more.
It often shows up in the first day or two after pouring, though it can also show up later on exposed surfaces during the cure.
What it is
The name is a little misleading, because the substance is not “ash” in the sense of burned material. Soda ash is generally understood to be sodium carbonate. It forms when unreacted lye on the surface of fresh soap reacts with carbon dioxide in the air. In other words, it is a surface reaction between a bit of remaining lye and the air, not a sign that the inside of the bar is full of lye.
Because the reaction needs air, soda ash tends to form where the soap meets air: the top of the mold, the edges, or any exposed face. Sides that sit against the mold usually stay clear.
Why it is cosmetic
On its own, soda ash does not make a bar harsh or unsafe. It is a thin layer on the surface and does not generally indicate a problem with the recipe. Most makers treat it as a look issue. It can be washed off, shaved, steamed or simply left alone, and the bar underneath is typically unaffected.
There is one caution. Fresh soap is still caustic during its first day or so, and the surface may be more so than a cured bar, so handle uncured soap with gloves. Also, soda ash is not the same thing as other white spots or patches. A white or powdery appearance that comes with an unpleasantly harsh feel, a crumbling bar or pockets of material may point to something else. If you are unsure, read about lye heavy soap, and when in doubt, do not use the bar.
What makes it more likely
Soda ash is often linked to several factors, though the effect varies from batch to batch.
- Exposure to air. The more air the surface sees early on, the more likely it is.
- Cooler setting. A batch that does not gel and stays cool seems to be more prone to ash on top, which is one reason the gel phase is connected. A soap that stays warm often shows less.
- Lower water. Recipes with less water, which relates to water discount choices, seem to show more.
- Recipe differences. Some recipes are more prone than others, and the reasons are not always clear.
- Fragrances and additives. Certain scents or additives are said to encourage ash, though the evidence is mostly anecdotal.
- Drafty or dry rooms. Moving, dry air during the early hours can encourage it.
Ways makers reduce it
None of these is guaranteed, and results vary with the recipe and your conditions.
- Cover the surface. Pressing a piece of plastic wrap or a sheet of paper lightly on top of the poured soap limits contact with air. Some makers simply cover the mold with a lid or board.
- Insulate the mold. Keeping the batch warm so it gels is often reported to help. Gel and warmth also change other characteristics of the bar, so choose according to your design.
- Spritz the top. A light spray of rubbing alcohol on the surface soon after pouring is a common practice. Use it carefully in a ventilated space, away from flames and heat, and do not breathe the fumes. Alcohol is flammable.
- Review the recipe. Adjusting the recipe is possible, but any recipe change should be recalculated with a reliable tool and not be done by guesswork.
- Plan around it. Since soap ash mostly affects tops, some makers pick designs where the top is trimmed away or covered with decoration.
How to remove it
Once soda ash has formed, several options exist.
- Leave it. It is harmless, and it will wash off in use.
- Wash it. Rinse the bar under running water and gently rub the surface. Dry it afterward.
- Steam it. Brief exposure to steam may smooth the surface.
- Trim or shave it. A soap cutter, vegetable peeler or plane can remove the top layer. Wear gloves if the soap is still fresh. The article on cutting soap discusses trimming bars neatly.
- Stamp or decorate over it. If you plan to decorate, you can mask a rough top. Stamping and decorating finished soap is covered separately.
Avoid harsh abrasives or chemicals, and do not use anything on soap you intend to put on skin unless you are confident it is appropriate.
Keep perspective
Soda ash is among the most common soap blemishes, and most makers meet it early. A bar with soda ash is not a failed batch. If it bothers you, try one change at a time on your next batch, such as covering the top or insulating the mold, and write down what happened. That kind of steady comparison is the point of learning from failed batches, and over time you will find what works in your own space.