2013-11(Nov)-04: On the Importance of pH

Tweet from earlier today:

For multiple batches of soap, I've seen weird stuff happen when honey is added. Just confirmed my suspicions that honey is acidic (pH ~3-6).

— Open Source Soap (@OpenSourceSoap) November 4, 2013

This photo shows a jar of very concentrated lye, or sodium hydroxide:

For my standard recipe of soap, I use ~175 grams of sodium hydroxide added to 450 mL water (450 g, at 1 g per mL) which works out to 389 g/L. Converting to molarity, we just divide by the molecular weight of sodium hydroxide (NaOH). I usually end up adding atomic weights or consulting Google. Sodium (23) plus oxygen (16) plus hydrogen (1) equals 40. Dividing 389 by 40 gives 9.72, which is the molarity. 9.72 molar lye is some dangerous stuff, so I have to take special precautions when using it and storing it.

Step One: Dissolving the lye.
All the textbooks tell you to slowly add the solid lye pellets to a stirring solution of water. And that's how I would do it, ideally. But lye is incredibly hygroscopic (that's hygro, with a "G") meaning it readily absorbs water from the atmosphere. Before I have a chance to add all the lye pellets to the water, some of the pellets melt into a caustic goo that sticks to whatever container the lye was in. So I defy conventional wisdom and add water to the lye pellets, causing a rapid and violent heating of the water. I try to add the water quickly, then immediately stir with a silicone-coated paddle to disperse the lye and avoid any hot spots where there are chunks of solid lye. Almost immediately, the water heats up to ~160F, meaning the jar gets too hot to touch.

Step Two: Storing the lye.
As I consume lots and lots of cashews, I have lots and lots of empty cashew containers from Costco (each contained 2.5 lbs. of nuts). These containers happen to be almost exactly the same size as a 1 quart Mason jar, so after mixing the lye I immediately screw on the lid and put the jar into the plastic cashew container. This is my form of "secondary containment," so I'm not completely screwed if the jar breaks.

Step Three: Reheating the lye.
This is another potentially dangerous step--particularly the way I USED to reheat the lye. First on the "why" part. For cold-process soap, both the oils need to be about the same temperature before they are mixed together. That temperature is a hair above body temperature. I generally adjust the temperatures to ~95-105F (~35-40C). After it cools down, the lye is ambient temperature of my garage, which can be pretty cold in winter. I have a heated tank for the vegetable oils, which are sometimes as hot as 135F when they come out of tank, so have to cool down a bit before I can make soap. To heat the lye, I used to put the jar directly in the microwave (minus metal lid, of course). But this was a very scary affair. The jar would make strange buzzing / vibrating noises, lye crystals would build up around the air/lye interface, and that region would also heat up a LOT more than the bulk lye solution. I would microwave the jar for 15-30 seconds at a time, then stir the solution, measure the temperature, and repeat. Once, realizing I had several minutes of heating left, I microwaved the jar for longer, perhaps a minute. At some point, the disturbing buzzing sound was replaced by a sharp "pop" that made my heart race. The jar had cracked--more like severed, actually--in a clean break right above the air/lye interface. I very carefully, and wearing a nitrile glove, picked up the jar and transfered the solution to a new, unbroken jar. So now, I heat up a bath of water in the microwave, then use this hot water bath to heat the jar of lye to the proper temperature, as in the photo.

Lye may be scary, but it is critical for making soap the right consistency. Too much lye will result in a very caustic soap, since all the oils are saponified and there's still leftover lye that hasn't reacted. But too little lye (too much oil) results in a mess, too. I've seen this when I use additives that are acidic, such as coffee or honey. I should have known better about using espresso without neutralizing it, but the acidity of honey did take me by surprise, a bit. I'll have to do a titration soon to figure out how much lye I need to add to compensate for the part that honey neutralizes.

In the last post I mentioned a new fragrance in the works based on Bois 1920 "Come La Luna." Well, the first version is out, and I'll be posting the recipe soon. As with my mimic of Eau Sauvage, I'll be Anglicizing the name for convenience of monoglots, so Come La Luna will be dubbed "Moon Dust" until / unless I come up with a better name. Moon Dust isn't such a bad name, if it ends up sticking.

Last Modified:
10 November 2013

Contact: open.source.soap at gmail