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Does soda ash appearing on soap mean that oxidation of oils is about to occur?

  1. Dec 17, 2009 #1
    On the German soapmaking forum, there are several posts noting that the presence of ash and the oxidation of oils(rancidity) often go together.

    When the ash appears in small amounts, I have not noticed any oxidation. When the ash develops over much of the surface of the soap bars, the soap has gone rancid, in two instances in my own soapmaking.

    If it is indeed the same ash, is the rancidity due to a lack of strong alkali in the soap, to finish the saponification process, because the sodium hydroxide has been turning to sodium carbonate?

    Are there other questions to ask to discover the cause of oxidation of oils?

    We use distilled water so that the minerals don't get in the way of saponification. We use oils as fresh and pure and we can get. We buy lye as pure and fresh as we can. We use utensils as clean as we can. We weigh the lye, water, and oils, and check them twice and thrice. We calculate lye amounts very carefully according to the values of the oils.

    It is costly and very disheartening to have to throw out a batch of soap due to oxidation.

    Any help in pinpointing the cause of oxidation would be most appreciated.

    Thank you very much!
  2. jcsd
  3. Dec 17, 2009 #2


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    Are you using unsaturated oils?

    The carbonate can occur during the saponification process if CO2 from the air is allowed to contact the lye (either as a solid or dissolved in the batch). You might be getting that to happen early in the process of the saponification if you are using very pure oils. Try adding some free fatty acids at the start or use a bit of soap to help in early emulsification of the batch.

    In saponification there isn't any oxidation going on really. If oxidation is occuring, it is likely due to oxygen attacking the double bond in any unsaturated fats you might be using. That process usually only occurs at high temperature and produces additional low molecular wt. acids. That could throw off your measurements.

    Rancidity is due to free fatty acids being present rather than the sodium salt of that free fatty acid. The best treatment for that is to add additional base until all of the free fatty acids are converted to the soap. You never need to throw out a batch of soap if you just add some attitional lye or sodium carbonate.
  4. Dec 19, 2009 #3
    Chemisttree, we use both saturated and unsaturated, depending on the recipe. Most of the time all goes well. But, the odd batch that goes bad is quite disheartening and expensive.

    We choose the oils according to their properties, also trying to keep them high in oleic acid to prevent the soap from going rancid.


    I have never had a batch of soap go bad that didn't have ash, which is what prompted my question.

    I am going to switch to a taller, more narrow pot, and will cover the molds longer, to try to keep air from getting into the mixture.

    Do I understand you correctly, that if a batch has turned rancid, even if there are spots of discoloration, that it could be rebatched with lye or washing soda? That would eliminate all superfatting, which would make it laundry soap or for cleaning.

    Thanks very much for your help. I really appreciate it.
    Last edited: Dec 19, 2009
  5. Dec 21, 2009 #4


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    I believe you can superfat at the end of the soapmaking process or at least at the end of the trace step. At that point the lye should be almost used up unless you have used too much.

    You can rebatch with lye or with washing soda. Washing soda is better because it doesn't combine with CO2 and throw off your calculations like some old lye might. The rancid character is free fatty acids so you are already past the saponification step. All you need to do is add a base capable of deprotonating a fatty acid. Very mild bases are enough for that... even bicarb will do it.
  6. Dec 21, 2009 #5
    Chemisttree, thank you very much. Next time I need to rebatch, I will enjoy experimenting.
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