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please clarify this debate for non-chemist…
Does excess lye neutralize in soap suspension over time?
There’s a discussion in the handmade soap making community with little science to back the claim that if we make a lye heavy soap, PH greater than 10, that in time the lye will neutralize. Some old time soap makers say that after months the lye in the soap is safe. Some like me want evidence and to understand how this could be especially since whatever lye needed to convert the oils, butters and fats in a given recipe is utilized and the rest is then suspended in the soap. as soap cures water evaporates that lye crystallizes and appears on the surface as ash. Is it actually neutralized? Thank you.
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5 Replies
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Soap is a crystal matrix, how could free NaOH migrate to the surface from inside? I don’t believe it.
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Hmmm, I don’t believe it migrates to the surface as much as it is already there and the evaporation of water/moisture allows more of it to accumulate and be visible on the surface.
The question is does it neutralize over time
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NaOH is converted to NaCO3 on exposure to air. Excess NaOH in soap converts to NaCO3 and may appear as a “bloom” on the surface of soap after long standing. It can sometimes look like mould (mold) growth.
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Using the correct SAP values for your oil or fatty acid as well as allowing a certain percentage of your oil phase to remain unsaponified should prevent this from happening. Getting the correct SAP value from your supplier is the best thing you can do under these circumstances and using that in your calculations is most wise.
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if you neutralise lye (or any other inorganic hydroxide), you get water; however, as you point out, soaps lose water rather than gain it as they cure
you also need an acid to neutralise it - it won’t happen spontaneously
in short: no, it doesn’t neutralise over time
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