Some people are surprised that soap contains sodium hydroxide.
All soap does. It cannot be made without it.
Companies generally disguise this unpleasant bit of news and don’t list sodium hydroxide alone as an ingredient.
So tallow mixed with sodium hydroxide becomes sodium tallowate; sodium hydroxide mixed with olive oil is sodium oleate, etc.
When soap is made with the correct proportions of ingredients there is no sodium hydroxide left in the bar so is safe to use.
If too much sodium hydroxide is used or the soap is not mixed enough the soap will be brittle, it may have streaks of sodium hydroxide in it and it must not be used.
Soap will be too soft if not enough sodium hydroxide is used.
Ingredient proportions are critical and each oil needs a different percentage of sodium hydroxide to break it down into soap.