It has always struck me as incredible that soap is made from oils and sodium hydroxide, when we want soap to clean away oily substances and sodium hydroxide (NaOH) is actually another name for caustic soda, which is a dangerous irritant. So how is it possible? Well, thanks to the extraordinary and rather complex chemical reaction that is saponification. To simplify this reaction: Oils are made up of triglycerides which, when they encounter the hydroxide portion of NaOH are broken down into fatty acids and glycerol. Following this the sodium of NaOH combines with the fatty acids to produce soap. If you have kept up this far, you will have realised that a specific number of triglycerides is required to react with all the NaOH. To make matters more complicated each oil has a different triglyceride composition which means the amount of NaOH will be different depending on which oils or butters you use. So, olive oil, coconut oil, shea butter, tallow etc. all have different saponification numbers which are used to calculate how much sodium hydroxide to use.
