I'm late to the conversation, so I'm just now catching up on the discussion this morning. I think all of you are making some good points. Here are some things that I have personally observed or have been observed by reputable soapers and I have concluded to be true about handcrafted soap in general:
A modest amount of excess lye will disappear with time to skin-safe levels in a "normal" KOH or NaOH soap, meaning one made with water typical for these types lye soaps. (Kevin Dunn's experiments with -5% superfat, my experience with soaps with a similar lye excess).
An abundance of excess lye in a "normal" soap recipe is not likely to disappear to skin-safe levels. (cmzaha-Carolyn's experience with her "normal" soap, others' experience with "superlye" soaps with normal water content)
An excess lye in a "high water" soap will disappear with time to skin-safe levels. (my and others' experience with "superlye" soaps with a high water content)
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"... So, how can a soap have no excess lye, and a pH of 12.5 or 13 and not be caustic?..."
The short answer is that the pH measurement and being "caustic" (in the sense of excess alkali) are not the same thing. With the fatty acids typically used for soap making, the pH of a typical well made soap with no excess lye -- a "not caustic" soap -- will range from about 9 to about 11.
I know soaps can have even higher pH values and still be skin safe.
In one of my posts cited above, data from a reputable study shows a few commercial soaps with pH values ranging up to 12.4. The irritation index of these pH 12+ soaps was low enough that they would not bother normal skin, so obviously skin-safe soaps with pH above 12 do exist. I do not know why the pH is so unusually high in these particular soaps, but I have no reason to dispute the authors' findings.
That said, the pH of most soaps used in this study ranged from 9 to 11, so I think it's safe to say the pH for most handcrafted soaps is more likely to be in the range of 9 to 11, give or take a bit.
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Okay, so why can we use something that's clearly alkaline or acidic and not get our hide blistered off?
When let loose in the normal world, strong acids and alkalis want to react with anything and everything they contact. That's just their nature. Anyone who has gotten a lye burn knows straight lye -- a strong alkali -- damages the skin quickly and deeply. In this circumstance, the only thing the hydroxide (OH-) ions from the lye can react with are the fats and proteins these ions encounter in the skin. Anyone who has gotten a burn from a strong acid such as nitric, hydrochloric, or sulfuric acid (raising my hand) knows the same is true for the hydrogen ions (H+) in the strong acid.
The answer to moderating the harsh action of these strong alkalis and acids is the use of buffers, chemicals that acts as sponges, so to speak, for absorbing and releasing excess H+ and OH- ions. Buffers are able to placate hungry acids and alkalis so they don't harm our skin.
The first sketch below is a rough example of the difference between a non-buffered mixture versus a buffered mixture. Don't hold me to any pH numbers here, folks, because the sketch isn't based on real data -- I'm just using this to give you a visual clue of what I'm talking about.
In the unbuffered mixture, as you add acid, the pH drops immediately as a direct reaction to the added acid. The reverse happens as you add a base (alkali) -- the pH rises immediately in reaction to the added alkali.
Soap and the fatty acids from which the soap was made together form a buffered solution, so let's pretend the buffered example is a mixture of soap and fatty acid.
The "S" curve for the buffered solution shows that the pH doesn't change a lot between points 1 and 2. This is the region in which the soap and fatty acids are working well to buffer the pH of the mixture.
As an acid is added to the soap-fatty acid solution, the soap breaks down into fatty acid. This causes OH- ions to be released to neutralize the extra acid. As a base is added to the solution, the fatty acid saponifies -- turns into soap -- and this causes H+ ions to be released to neutralize the extra alkali. This is the region in which the soap would be skin safe -- we're counting on the buffering action of the soap and fatty acids to protect our skin.
Above point 2, all of the fatty acid has been converted into soap. As more alkali is added past this point, there are no more fatty acids to buffer the solution and "eat up" the excess OH-, so the soap becomes lye heavy and pH begins to rise sharply as more and more alkali is added. Above point 2, the mixture starts to behave more like an unbuffered solution. If you would wash with this lye heavy soap, your skin would end up being attacked by the alkali because there is no buffering action to protect your skin.
Below point 1, all of the soap has been converted into fatty acid. As more acid is added past this point, there are only fatty acids in the mixture -- there is no more soap to eat up the excess H+. The mixture starts to act like an unbuffered solution, and the pH begins to fall sharply.
When we make soap, our soap should ideally be mostly soap. We want it that way, because fatty acids (or fats) don't clean well. But we're playing close to Point 2 (the upper end) of that important buffered region. That means there is not a lot of extra fatty acid (or fats) in the soap to help buffer against extra alkali. But that's okay -- our soap will be skin safe -- as long as the soap mixture is still inside that buffered region.
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My second sketch is an attempt to show that not all fatty acid and soap mixtures have the same relationship between pH and buffering. Some soap and fatty acid mixtures are properly buffered when the pH is a bit higher, others when the pH is a bit lower.
The variability in the fatty acids in the fats we use for soaping AND the variability in handcrafted soap recipes AND the difficulty in accurately and easily measuring pH of soap all make it tough for a handcrafted soap maker to absolutely know "my soap is skin safe at X pH and unsafe at Y pH." Even in industry, where recipes and ingredients are more consistent and pH equipment and pH testing procedures are top notch, a direct pH measurement is not used for determining whether a soap is lye heavy or not.
Soap 1 might be a mixture of oleic and stearic acids and oleic and stearic soaps (say a lard soap). Looking at the pH at Point 2, I hope you can see Soap 1 will be properly buffered at a higher pH than Soap 2, which might be a mixture of lauric and myristic acids and lauric and myristic soaps (say a coconut oil soap). The pH at Point 2 for this soap is slightly less alkaline.
That is why the pH is not really a sure-fire test for skin safety. You really want to know the answer to this question: "Is my soap safely within buffered region?" For an oleic-stearic soap, it will be safely buffered at a slightly higher pH. A lauric-myristic soap might not be buffered properly at that pH -- it might have a bit of excess alkali that would be harsh to the skin.
This whole thing about buffering and dealing with excess alkali is all helped by the cure period. It takes time for this complicated mixture of soap, fatty acid/fat, water, etc. to all sort itself out. It makes a lot of sense to allow a reasonable time for the soap to mature -- the cure period is not just about evaporating moisture.
