"Ash" is any unexpected white crystals that form on soap, usually shortly after the soap is made. Ash typically forms on soap made with a cold process method.
Some soapers hate it; others "embrace the ash" and tolerate it when it appears. I do not know of anyone who has a surefire remedy to prevent ash.
Sodium carbonate is probably the most common type of ash
Most people assume "ash" is sodium carbonate (Na2CO3), also called soda ash or washing soda. I would say most of the time they are usually right.
Freshly-made cold-process soap may contain tiny amounts of free sodium hydroxide (NaOH) for a short time after the soap is made. Any free NaOH at the surface of the soap will quickly react with carbon dioxide (CO2) in the air to form white crystals of sodium carbonate.
Soda ash often forms a dull white coating on the top and sides of a soap bar. It will feel slightly rough and powdery on the fingertips. Occasionally soda ash will grow into a thicker layer that looks like white "fur" or it will form sparkly white crystals large enough to see with the naked eye.
Soap crystals are a second type of ash
We talk of "stearic spots" in soap, which are visible white crystals of soap embedded in smaller invisible crystals of the surrounding "normal" soap. Large, visible soap crystals can just as easily form on the surface of soap as well as within the soap.
In my experience, soap crystal ash only forms on the top of the bar, but other soap makers may find it forms elsewhere on the bar. It forms a layer that looks like a layer of white or ivory paint. It feels smooth and slightly waxy, just like soap (which makes sense!)
Ideas to PREVENT ash before it forms
Use less water to make the soap. Try using a 33% to 40% lye concentration
Pour the soap batter into the mold when the batter is at thicker trace rather than at emulsion or very thin trace.
Ensure the soap stays warm enough so it can gel while saponifying. Soap a little warmer and/or use a CPOP (oven warming) method, a heating pad, or an insulated cover.
Cover the top of the mold with plastic wrap or other air-tight cover and leave the cover in place until the soap has cooled to room temperature -- or even as long as 2 to 3 days after the soap is done saponifying.
Ash that is prevented by a cover is more likely to be soda ash. The cover reduces the amount of carbon dioxide gas that can reach the saponifying soap, so less sodium carbonate can form.
Spray the top with isopropyl or ethyl alcohol shortly after the soap is poured into the mold.
Ash that is prevented by alcohol is more likely to be soap crystals. Alcohol may prevent soap-crystal ash if sprayed on the top of freshly soap batter by preventing large crystals of soap from forming. The original formulation of Pears transparent soap was made transparent by the use of ethyl alcohol.
Ideas to REMOVE ash after it has formed
Spray ashy areas with isopropyl or ethyl alcohol.
Soda ash is not very soluble in alcohol, so high-proof alcohol is not likely to remove soda ash. Alcohol that has a moderate to high water content, such as vodka (40% alcohol, 60% water) or 70% isopropyl alcohol (70% alcohol, 30% water), will dissolve soda ash due to the water content. Steam or plain water (see below) is effective, not flammable, and much less expensive, compared with alcohol.
Soap crystals are soluble in alcohol, but dissolving soap in alcohol is a slow process. Alcohol will work better to prevent soap crystal ash than to remove it.
Steam the soap bars with a clothes steamer or steam iron or over a pan of simmering water.
Soda ash is is very soluble in water, so water is a good choice for removing soda ash.
Steaming dissolves the sodium carbonate, so it can spread in a thin layer over the entire surface of the soap. The sodium carbonate crystals that form after steaming are likely be small and all but invisible.
Once soap-crystal ash has formed, it will be impractical to remove by steaming.
Wash the soap bars with water either when the soap is in the mold or after the bars are cut.
Another good solution for soda ash. Rinsing with water dissolves and physically removes the sodium carbonate.
Once soap-crystal ash has formed, it could be removed by thoroughly washing the affected areas with water. This may give the bar a "used" appearance. If tap water is used to wash the bar, the minerals dissolved in the water may trigger DOS (spots of rancidity) on the soap.
Remove a thin shaving of soap with a planer.
A surefire solution for removing either type of ash, although planing is wasteful and changes the overall appearance of the bar.
Do not do anything -- "embrace" the ash when it happens.
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