“Do you use caustic soda?” is a question I get asked from time to time. Yes, is the answer – it’s the only way to make soap.
“Is there caustic soda in your soap?” is another one. The answer to that one is no.
Although that seems contradictory, both are true.
When you mix an acid and an alkali, a chemical reaction occurs, and a salt is produced. Soap is made my mixing a combination of fats – the acid – with sodium hydroxide, otherwise known as caustic soda, or lye* – the alkali. The process is called saponification, and when it is complete, the alkali is neutralised – there is no caustic chemical left in the product.
Different combinations of acids and alkalis result in different salts:
Olive oil (acid) + sodium hydroxide (alkali) = Sodium Olivate (salt)
Palm oil (acid) + sodium hydroxide (alkali) = Sodium Palmate (salt)
Coconut oil (acid) + sodium hydroxide (alkali) = Sodium Cocoate (salt)
That is why the ingredient labels on my soap list sodium-this and sodium-that, rather than olive oil, coconut oil etc. It’s a legal requirement to list them that way. When olive oil and sodium hydroxide go in, sodium olivate comes out. That’s also the reason why sodium hydroxide, or caustic soda, is not listed on the label – it doesn’t have to be, because it is not present in the finished soap.
“Ah, but you CAN make soap without lye!” is one common response. Whilst it is true that you can buy a pre-made base that you can simply melt down at home, and add whatever colours and fragrances you like, this melt-and-pour base is initially made with lye. It simply has other chemicals added, to enable it to me more easily re-melted.
So, there is really no need to be concerned about lye. You simply can’t make real soap without it.
*To clarify, sodium hydroxide, caustic soda, and lye are all the same thing. Whilst technically, “lye” refers to the liquid made from dissolving the caustic soda in water, the terms are generally used interchangeably.