Lye, whether it be Caustic Soda (Sodium Hydroxide) or Potash (Potassium Hydroxide) is one of two essential ingredients in soap making. The misunderstanding about lye, lays in the ignorance of the chemical process that creates soap.
In well made soap, is there any lye? No.
Was lye used to make it? Yes.
If anyone tells you no lye was used, they either have detergent (petrochemical) or soap nuts etc, but they don't have soap. That it is not listed in the ingredients of soap is not saying it was not used.
It is impossible to make soap without lye. Soap is the result of a chemical reaction called saponification between fats/oils and lye. The fatty acids combine with the strong alkaline of lye to form glycerine and salts of particular fatty acids and alkaline used. This forms a molecule that has one end fat loving and the other water loving. This allows the water to break through the tension of the oily grime, breaking it up and allowing for it to rinse off.
Charts give a precise amounts of lye needed for each oil so complete saponification occurs without excess lye remaining in the soap. In natural soap making a small excess of oil is used and this will not be changed into soap but remain as free oil in the soap. This is one of the benefits of natural soap as it remains on the skin for conditioning.
Great grandma's soap may have been harsh. This was simply because it was difficult to get an exact concentration of homemade lye and perhaps a lack of understanding in how to fine tune the process. Even in history, soap makers got it right without the benefits of calculators and litmus paper.
Both lye's are highly caustic and caution is needed when handling them. Rubber/plastic gloves are a must, glasses also. Even when it is mixed into the oils, the batter is still highly caustic and will burn. I have some pitting on the concrete floor in the shed from soap batter that spilled. As the lye and oil molecules come into contact with each other they change.
My pseudonym, "No Lye" is a play on words, not an ingredient in my soap. I have had several batches of soap turn out with lye present but they are all tested and those were fixed. No lie.
I have seen experts insist non professionals should not attempt soap making. They are protecting business not people.
Some make a mistake and get scared and now are fear driven to save us all from their mistake. Or one person has an allergy and tries to prevent everyone else from a good thing.
I have seen partial information given with the curious logic that people may try it if all the information is given and hurt themselves. Personally I think this attitude will lead to more mistakes because now we all have to reinvent the wheel. This was the strength of guilds that held secret knowledge while common people were left to struggle on without or with inferior products.
This was the same attitude that kept the world in darkness by withholding the Bible from the people through the dark ages. The establishment argued that the unlearned would do harm with this knowledge (even though priests had been slaughtering their way through Europe for hundreds of years, Cardinals had been poisoning off rivals, and rape and pillage was all done in the name of God). Yes there is hypocrisy such as infamous TV evangelists sucking up the peoples money(now where did they learn that?). None the less, knowledge has brought liberty to those who genuinely seek.
The withholding of knowledge allows a minority to control the majority.
If you burn yourself with the Lye you have to take the responsibility for that but there is no reason why you can't end up with a superior soap than what you can buy at the shop (most of that is not soap as the glycerine has been extracted and chemicals added to make up the loss).
So lye, love or hate it you have to have it to make any soap.