Glycerin is a by-product of soap making. It can come from other sources, but soap making is by far the most common source. When you combine lye and water with oils, it saponifies and becomes soap. (Check out our tutorial on making cold process soap.) Actually, what you get is ⅔ soap and ⅓ glycerin. The glycerin is then usually taken out and sold to other companies such as fertilizer and explosives manufacturers. If you refine the solid soap, add sugar and alcohol, and skim off the impurities, you get glycerin soap. Here’s the actual process: