How is development of ash and oxidation of oils in cold process soap related?

In summary, this post discusses how ash can be a sign of soap going bad, how oxidation and crystal formation can also occur, and how grains in butters can contribute to crystal formation in soap.
  • #1
Vintageliving
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I posted this on thedishforum:

On the German soapmaking forum, there are several posts noting that there seems to be a correlation between ash and oxidation of oils. Can you speak to the presence of ash and soap going rancid?

And got this reply:

Yes, I could see this being an accurate observation. Rancidity is an oxidative process. When fatty acids oxidize, some of the products of those reactions are aldehydes and ketones. Aldehydes with fewer than 6 carbons, in particular, will form soluble carboxylic acid salts when treated with sodium hydroxide (lye). Because of their solubility, those salts are probably some of the precipitates that form when soap ashes. This might explain an observed correlation between rancidity and ash.

Additionally, saturated fats, in particular, can form crystals under certain conditions (grains in butters, anyone?). Because they lack the reactive double bonds needed for tight crosslinking during polymerization into the soap colloid, they sometimes precipitate out, and because of their solubility when converted to soap, can migrate out of the soap as water moves to the surface. This happens in in old oil paintings, for example. http://cool.conservation-us.org/waac/wn/wn...1/wn20-108.html [Broken] The trick is to identify what those conditions are, that cause crystal formation.

I might dig into the chemistry of this further, after New Year's, when I have time to go peruse the chemistry library. I have limited access to articles from home. I'm quite sure that commercial soap manufactures know *exactly* what causes efflorescence, and how to prevent it.


Could someone spell out what happens to the oils in the soap, and the soap itself, when soda ash develops? If soda ash is possibly a sign that the soap is going bad, then preventing soda ash is more important than many who make soap believe.

Thanks very much for your help!
 
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  • #2
Drying can interrupt the process by moving the water phase and the lye dissolved in it to the surface. Once on the surface, that fraction of the lye doesn't react with fat or free fatty acids elsewhere within the soap. In that case, I would expect the soap to be more prone to rancidity since there is an imbalance between the free fatty acids and the lye.

Oxidation can occur as well, especially in fats high in polyunsaturates. Perhaps there is a correlation between the rate of saponification in polyunsaturates and the rate of saponification but I don't know of any.

Additionally, saturated fats, in particular, can form crystals under certain conditions (grains in butters, anyone?). Because they lack the reactive double bonds needed for tight crosslinking during polymerization into the soap colloid, they sometimes precipitate out, and because of their solubility when converted to soap, can migrate out of the soap as water moves to the surface. This happens in in old oil paintings, for example. http://cool.conservation-us.org/waac.../wn20-108.html [Broken] The trick is to identify what those conditions are, that cause crystal formation.
This is very suspect. I don't believe any of it. I don't believe that unsaturated oils crosslink to stabilize colloids in soap. If it were true, soaps would behave very differently than they do.
 
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  • #3
Chemisttree, thanks very much. I had a hunch that the lye becoming soda ash on the surface made it unavailable to become soap.

Thank you, too, for your comments about the section of the post on the soaping forum about the crystals. It didn't sit right and I didn't know what to look up to question it.
 

1. What causes the development of ash in cold process soap?

The development of ash in cold process soap is caused by the reaction between the oils and lye during the saponification process. This reaction produces a chemical compound called potassium carbonate, which appears as a white powdery substance on the surface of the soap.

2. How does ash affect the appearance and usability of cold process soap?

Ash can affect the appearance of cold process soap by creating a dull, grayish layer on the surface. It can also make the soap feel rough and less smooth to the touch. However, ash does not affect the usability or cleaning properties of the soap.

3. Can ash be prevented in cold process soap?

While it is difficult to completely prevent ash in cold process soap, there are some techniques that can help minimize its development. These include using lower temperatures, covering the soap during the curing process, and using certain additives like sodium lactate.

4. How is oxidation of oils related to ash in cold process soap?

Oxidation of oils is a chemical process that occurs when oils are exposed to air and moisture. This process can contribute to the development of ash in cold process soap. The more unsaturated fatty acids present in the oils, the higher the likelihood of oxidation and ash formation.

5. Can ash be removed from cold process soap?

Ash can be removed from cold process soap by gently scraping or washing the surface of the soap. However, this may affect the appearance of the soap and it may be difficult to remove all the ash. It is usually best to embrace the natural occurrence of ash in cold process soap rather than trying to remove it.

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