One method for making the lye solution for liquid soap is to heat KOH in glycerin until the KOH dissolves. This method was developed by pharmacists some time ago as an alternative to dissolving the KOH in water to make soap for medical purposes.
The advantage of using glycerin rather than water is the KOH-glycerin mixture saponifies fats very quickly. The obvious downside to the "pharmacist's method" of dissolving the KOH in glycerin is the mixture must be heated carefully to get the KOH to dissolve. The mixture foams furiously while it is heated, so the liquid must be stirred constantly and the heat controlled to keep the mixture from boiling over.
A cautionary comment on a YouTube video about the pharmacist's method was brought to my attention this past weekend. The author advised against using the pharmacist's method and suggested soapers instead use the alternative method of dissolving KOH in water at room temperature. Glycerin can later be added to the KOH-water mixture to gain most of the benefits of the pharmacist's method with fewer of the risks.
The obvious risk of the pharmacist's method is the danger of heating a strong alkali (KOH). If the hot caustic mixture spills onto the skin, severe chemical and thermal burns will result in seconds. Prompted by information given in the YouTube comment, I looked deeper into the chemistry involved with hot KOH-and-glycerin mixtures and found other serious risks to this method that I want to share with you.
The heated mixture bubbles and foams so furiously because the KOH and glycerin are reacting with each other to form bubbles of water vapor and hydrogen gas. The amount of combustible hydrogen gas is large enough that this reaction has been studied as a way to recover useful energy from waste glycerin formed by commercial biodiesel and soap production. Needless to say, it is not safe to mix combustible hydrogen gas with an open flame or hot burner! Hydrogen is a light, buoyant gas, so it will most likely rise safely away from the stove burner, but I'm not sure I want to trust the idea that hydrogen gas will always rise out of harm's way rather than burn or explode.
When glycerin decomposes, it can also form toxic and irritating byproducts such as acrolein, acetic acid, formaldehyde, and formic acid as well as carbon dioxide, water, and combustible gases. Although pure glycerin decomposes only when heated to temperatures that would be unusually hot for a kitchen stove, when mixed with other chemicals, glycerin decomposes at much lower temperatures.
Acrolein is one of the most hazardous chemicals formed by this decomposition. It is a colorless to yellow liquid with an acrid, disagreeable odor and is a severe respiratory irritant. It is flammable and unstable and can form explosive compounds. The US Occupational Safety and Health Administration (OSHA) limits acrolein exposure to a maximum of 0.3 ppm in one short term exposure and an average of 0.1 ppm per 8 hour exposure.
http://www.cdc.gov/niosh/docs/81-123/pdfs/0011-rev.pdf
In summary, the pharmacist's method carries the risk of severe chemical and thermal burns from a spill, the risk of combustible hydrogen gas near open flame or hot burner, and the risk of toxic, irritating compounds being released into the air.
I do not think most of us soapers, even the ones who use the pharmacist's method with good results, are fully aware of these potentially serious risks. It was certainly an eye opener for me. I am sharing this information not to stir controversy, but so people have the knowledge to make an informed choice whether to use the pharmacist's method or to dissolve KOH in water at room temperature.
Thanks -- I appreciate your taking the time to read this!