There are two major types of lye: sodium hydroxide (NaOH, caustic soda, or
potash), and potassium hydroxide (KOH, or caustic potash). Sodium hydroxide is the
one typically used when making solid soap bars; potassium hydroxide makes much
softer soap, and is sometimes utilized for making liquid soaps, or shaving cream.
The lye reacts with the triglycerides in the fat or oil molecules, and the fatty acids
are released from the glycerin, and bind to the lye molecules. Both the alkali and the
acids are neutralized. The byproducts of this reaction are a salt (soap) and glycerin.
When the process is complete, there will be a ratio of approximately three soap
molecules to every molecule of glycerin. Put another way, glycerin makes up
approximately 25% of the finished product that results from the saponification
process.
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