Wednesday, March 31, 2010

Scientific soap making part II: curing soap and water discounting

First, let's be clear that we're not talking about "curing" in the sense of helping to get over an illness or disease.  We're talking about curing in the sense of aging and mellowing: storing or keeping something under controlled conditions for a period of time long enough for it to develop certain desirable or unique qualities -- like complexity of flavor in an aged cheese or fine scotch.

With cold process soap making, the time spent actually making the soap is minimal.  What takes time is curing the soap once it's made, and that's four to six weeks depending on the soap.  Or is it?

Curing soap does two things.  It ensures that the soap is fully saponified, and it allows moisture to evaporate so the bars become firmer.  If the soap goes through a full gel stage, saponification will be nearly complete within 48 hours.  If it doesn't, it may take longer.  But in no case should full saponification take longer than two weeks, so after that it's just about letting it dry out and become nice and firm. 

With that in mind, it stands to reason that if you use less water to make it, there's less moisture to evaporate.  And that should shorten the drying time, right?

To test this, I tracked the weight loss of three different soaps over a six-week period, and found that the soaps made with a lower percentage of water (a solution of about 60 percent water, 40 percent lye) reached a stable weight in about 3 weeks.  Those made with a higher percentage of water (about 66 percent water, 33 percent lye) took about 4 weeks to reach a stable weight.  Clearly, using less water shortens the drying time.

Some soapmakers maintain that drying time is bascially immaterial, and that curing soap for six weeks or longer somehow makes it even milder and better.  One even suggested that a soap that has cured for a year is about as close to the ultimate as you can get. 

But what happens to soap in a year that doesn't happen within 3-4 weeks of having made it?  Soap isn't cheese, and the chemcial and biological processes that are key in aging cheese or wine are irrelevant when it comes to soap.  The only thing that really happens over an extended period of time is that the soap's scent tends to fade, and the bars themselves become extra hard.  That's about it.

My conclusion is that reducing the amount of water in the recipe shortens curing time and results in soap that is ready to use sooner rather than later.  To learn more about water discounting here's a link to a detailed description of the process.