I get a lot of questions about soap and toiletries. I answer anywhere between 120 – 200 emails, every day. My staff answer that or double that daily. The most frequently asked question I’ve gotten this year has got to be about making soap from biodiesel glycerin. Glycerin is a natural byproduct from the biodiesel refining process. In the photo above, the glycerin is on the right. The biodisel is on the left. I personally would not ever use this glycerin as an ingredient in soap but there are a lot of people that would like to, purity concerns be damned. I am posting my thoughts on this interesting subject for all to see, in the hopes that it educates and possibly stems the endless tide of “I know this is a really strange question, but I make biodiesel and I was wondering ….” emails.
I do not personally recommend making soap with the byproduct glycerin from biodiesel manufacturing as I don’t think it’s entirely clean and pure and has some extra things like methanol in it. However, we do have a local customer in Bellingham, WA that does make a soft, smushy, gray looking bar of solid, slightly lathering soap with their byproduct bio-diesel. They have shared a bar with me and while it is soap, it is very soft and fairly poor in terms of lather. This is probably due to the fact that it is 100% glycerin which is not entirely saponifiable by lye.
If you really want to use the byproduct glycerin, I would recommend a more modified, traditional recipe such as:
Lye 9.4 oz.
Water 16 ounces
Please not that you must do a certain amount of safety precautions to make lye and I strongly recommend researching how to make cold process soap prior to attempting to make it. My favorite books The SoapMaker’s Companion by Susan Miller Cavitch to learn about soapmaking or The Natural Soap Book, also by Susan Miller Cavitch. My favorite DVD on the subject (by moi) would also be a good starting point for learning how to make CP soap.
The recipe above will be soft but will lather decently and last much longer than 100% glycerin soap.
Also, you might think about other things like fire-starters with your by-product liquid glycerin.