In my article on Making Natural Liquid Soap, I state, "If you run most recipes through a lye calculator you'll see that there seems to be way too much lye! Indeed, liquid soap recipes are usually formulated with about a 10% lye EXCESS. This is to ensure that all of the oils are saponified." This is true.
But there's more to the story.
Catherine Failor's book Making Natural Liquid Soaps has been, and still is the "Bible" of liquid soap making. Even though I've been making liquid soap for a while, I still refer back to it every time. The information and the recipes in the book are great.
But...I always like to run all my recipes, even ones out of famous books through lye calculators. I was a bit dismayed to find out that Failor's recipes have about a 10-12% lye EXCESS in them. She goes through a neutralization process...but still...10-12% always seemed like a lot of excess lye. The recipes worked...and didn't burn my skin off...so I didn't think much about it...until recently.
Though, by his description, not "an established liquid soap maker" but rather a soapmaker with "a fairly good knack for practical chemistry and a propensity to experiment with processes until I feel I get it right", Steve Mushynsky (and his wife Gina, too) of Summer Bee Meadow has finally shed some light on the subject.
According to Mushynsky, "Potassium hydroxide flakes are NOT 100% potassium hydroxide. The crystal structure of KOH actually intrinsically contains about 10 to 11% water bound up in its structure, along with about 1% other impurities (mostly potassium carbonate). If Failor's recipes were using theoretical 100% pure potassium hydroxide, they would figure out at 12.5 % excess KOH. After accounting for the water and impurities in the flakes, her recipes work out to about 1.5 to 0.5 % excess hydroxide. This correlates with her practice of making her base pastes a little excess in hydroxide to avoid free, unsaponified oil fatty acids that would cloud and possibly separate later and then "neutralizing" to remove excess hydroxides and reduce PH."
Make sense? What he's saying is that the potassium hydroxide (KOH) we use is only 90% KOH - the rest is water and other impurities. So the 10-12% lye excess is only a 0 - 2% excess. Now that I can live with...and understand how a bit of borax or boric acid could neutralize!
If you're going to make liquid soap, please visit Steve and Gina's site at Summer Bee Meadow and click on "SBM Soap Calculator." Steve has recalibrated the lye calculator to reflect the 90%/10% purity issue. To duplicate the "complete saponification" that you want for really clear liquid soap, use their lye calculators for your liquid soap with a 0% superfat.
So now...(depending on whether you use one of Failor's recipes...or use the lye calculator at Summer Bee...) you can use either the "Failor" method to neutralize your soap...or the "Summer Bee" method.
I outline both methods in How to Neutralize Your Liquid Soap Paste.