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Why and how does soda ash appear while soap is curing?

  1. Dec 16, 2009 #1
    Ash (sodium carbonate) sometimes appears on soap while it is in the mold, or while it is curing. It is supposed that the alkali in the soap absorbs the carbon dioxide in the air, forming sodiaum carbonate/soda ash on the soap.

    The ash appears inconsistently. It can appear on only part of the surface of the soap in the mold. It can sometimes appear after a soap is unmolded, and then only on some surfaces of the bars.

    The carbon dioxide in the air is constant. The alkali and the oils in the soap have already begun saponification.

    Many live in highly humid areas and do not get ash on their soaps.

    What does the appearance of ash depend on?

    If someone can help with this, it would be greatly appreciated by many who make soap.

    Thanks very much?
  2. jcsd
  3. Dec 16, 2009 #2
    Maybe humidity should be at the right value: Na2CO3 can crystallize with a variable number of water molecules and if humidity is too high, it dissolves in the same water it absorbs from air; if humidity it's too low, it doesn't form big crystals with water and you don't see the ash. Just an opinion, however.
  4. Dec 16, 2009 #3
    Lightarrow, thanks for your post. If I understand your post correctly, it is the water in the air drawing out the soda ash? If the humidity is low enough, the soda ash which might form, stays inside the soap?

    Some soapmakers find that if the soap stays covered, for example, with plastic wrap, until it is unmolded, that the soap is much less likely to develop ash.
  5. Dec 16, 2009 #4
    Water and CO2. The soap's bar have to absorb CO2 from air, to form Na2CO3, but you don't see it because it's dissolved in the soap; but when you let it dry a little, the carbonate dissolved crystallizes and its volume increases a lot, so you see the ash, even if the carbonate is in little amount.
    Hope to have well expressed my conjecture.
  6. Dec 17, 2009 #5
    Light arrow, thanks very much. I am going to make notes of temperature and humidity, to see if I can find a combination which precludes the ash forming.
  7. Dec 18, 2009 #6


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    I know of the white surface on soaps that you're talking about. But are you sure it's sodium carbonate? The amount of carbonate coming from the air is miniscule, on the order of parts-per-million. (and that's in relation to the water, which is also only a small part of the soap)

    I'm not sure air humidity or other factors would make much a difference if it were the case. Also, you'd really expect to see it being formed continuously (since I don't think the sodium carbonate would migrate to the surface).

    I suspect rather that it's the formation of rough soap crystals on the surface. That would be affected by humidity, and be a bit more 'random'.
  8. Dec 18, 2009 #7


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    You are using the cold process, right? Right after you have trace you pour into the molds but the stuff isn't soap yet. You cure for much longer for the saponification process to complete. During that time you leave your molds exposed to dry air and the water part of the incomplete reaction can wick to the surface and dry leaving a whitish residue. This residue (likely a mixture of lye and sodium carbonate) will combine with CO2 from the air to produce sodium carbonate.

    Leaving your uncured soap exposed to dry air will encourage the water phase to wick to the surface and dry out, leaving a solid at the surface. Moist air and covering the mold with saran will minimize the wicking/drying.
  9. Dec 19, 2009 #8
    Thank you very much for your posts and help.

    Alxm, I don't know enough to know if the ash is always, indeed, the sodium carbonate formed from the lye soaking up carbon dioxide, or something else. We have all found that making soap in low humidity contributes to a successful soap.

    Chemisttree, yes, it is cold process. I will be experimenting with leaving the mold covered longer. The batch I had that got lots of ash, was one I experimented with by not covering the mold. It developed a lot of ash, and ended up oxidizing.

    Those who live in areas where the humidity is very low have less trouble with ash developing.

    Someone at thedishforum posted this:

    Ash is efflorescence of salts from within the soap, due to water migration to the surface of the bars. This is why it seems to appear gradually, and can even show up on already cut bars during cure. It happens in other similar situations, like concrete or masonry, where it can occur seemingly just as randomly as it does in soap (in fact, it isn't random--it's well studied and documented).

    In the soap pot, there are many different ions present. The lye we use is usually only 92-99% sodium hydroxide, and the rest is a mixture of other salts, especially sodium carbonate, because sodium hydroxide slowly reacts with carbon dioxide in the atmosphere during manufacture and storage. All these salty impurities are present in our soaps. When ash forms, it's because these dissolved salts get carried to the surface of the bars with water that is migrating out of the soap too rapidly.

    Now, some clues about why this happens. For salt-bearing water to migrate rapidly to the surface, there must be space between the soap molecules to allow this process to occur. So, density plays a very large role in ash formation. In very dense soaps, water with dissolved salts will not be able to squeeze between the soap molecules to get to the surface. In soaps that are less dense, water can more easily migrate, and then evaporate once it reaches the surface, leaving behind that ugly deposit of salts known in the industry as efflorescence. Certain additives will contribute to this phenomenon--they can decrease the density of the soap. Lavender essential oil, for example, will decrease the bar density measurably, which can allow water migration to occur more readily. Certain base oil choices can contribute to this also, because the fatty acid composition can affect how tightly packed the molecules are in the finished bar.

    So, density of the soap is important, and also the humidity of the environment is important. In low humidity, the salt-bearing moisture evaporates *before* it reaches the outer surface of the soap, which prevents ash from forming on the surface. In high humidity, the water reaches the surface before it evaporates, leaving behind a layer of deposited salts when it finally does evaporate on the surface.

    The addition of additives that contribute to available salts for deposition can also factor into this. If you add clays or other sources of mineral salts, it might cause a heavier layer of ash if all the other conditions are present.

    As to split batches--well, if all other things are constant (additives, stirring, etc.), I'd look at whether the ashed portion was from the top of the soap pot or the bottom, and the relative humidity of that specific mold's environment.

    You can read about efflorescence in masonry here:


    and you can read about efflorescence in bar soaps (a patent to prevent it in industry) here:

    http://www.freshpatents.com/Process-for-ma...20060128580.php [Broken]

    and you can read records about preventing efflorescence in soap manufacture dating back to 1908, when this book was published (see p. 59):

    Last edited by a moderator: May 4, 2017
  10. Dec 19, 2009 #9


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    Interesting. You have two processes going on here. One is the saponification reaction and one is the drying mechanism. Both are important in making soap but the timing and the rate of the two processes can conspire to screw up your batches.

    Saponification rate is a function of the concentration of the ingredients (lye or soda ash and fats), temperature, the degree to which they are mixed (emulsified) and of course, time. There may be some other factors I haven't considered such as the nature of the fat itself and any impurities present. 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. I would recommend that you place the molded soaps somewhere warm (75oF) and tightly covered to minimize drying during the crucial saponification step. After a time (left for you to determine), perhaps 48 hours, uncover and allow them to dry as usual. I believe that should minimize the ashing problem

    Superfatted soaps are going to be the most prone to rancidity of course. I have seen that rancidity is most prone in oils high in oleic acid like olive oil and that makes sense to me. Oxygen attacks the unsaturated part of the fat molecule and produces additional low molecular wt. acids. These low molecular wt. acids are more volatile and can be more apparent by nose at a lower concentration than even higher concentrations of free fatty acids such as palmitic or stearic. Polyunsaturates are the absolute worst in this regard. If you anticipate rancidity issues, I would avoid oilstocks high in polyunsaturates like soy (56% polyunsaturated), corn (58% polyunsaturated), safflower (72% polyunsaturated), flaxseed (70% polyunsaturated), or sunflower oils (70% polyunsaturated). That leaves palm (10% polyunsaturated), coconut (2% polyunsaturated), canola (32% polyunsaturated), peanut (32% polyunsaturated) and olive oil (11% polyunsaturated). And I'm a little leery about the canola and peanut.
  11. Dec 21, 2009 #10
    Chemisttree, thank you very much for your interesting and helpful post. I appreciate your taking the time to write this.

    There is much to think about.
  12. Nov 10, 2010 #11
    Chemisttree, I have enjoyed experimenting with various ways of covering the soap, and have added bits of soap to the new batches. Your recommendation have helped, and my soap are turning out very nicely.

    I opted for simple and make mostly 100% olive or mostly olive with castor oil for grooming as well as 100% lard soaps for laundry and cleaning.

    Thank you very, very much for your help. It's been a great year of making soaps.
  13. Nov 11, 2010 #12


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    Cool! Now make some money doing it and you're golden!
  14. Nov 12, 2010 #13
    Chemisttree, thanks very much for your kind thoughts. It would be a great bridge to cross to go from my wee batches, to having consistency in large batches, and a very different realm.

    I'm happy in my frugal soaping for my own needs and to give to those near and dear.

    Your posts have been of great help to me, for my own soapmaking, and in answering the concerns of other soapmakers.

    I have sent you many thankful thoughts in my soaping adventures. :)

    ETA: I've just noticed that I didn't proofread yesterday's post very well. I apologize for the mistakes.
    Last edited: Nov 12, 2010
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