Lye

Lye is the common name for the caustic alkali used to make soap from scratch. Bar soap uses sodium hydroxide (NaOH), while liquid soap uses potassium hydroxide (KOH). On its own, lye is a strong alkali that reacts readily with fats, water, and skin, which is why it is handled with care from the moment the container is opened until the batter reaches trace.

In a recipe, lye is dissolved in water or another liquid to form a lye solution, which is then mixed with oils to start saponification. The amount of lye is never a guess: it is calculated from the oils in the recipe and a chosen superfat percentage, most often with a lye calculator.

Standard precautions include working in a ventilated area, wearing gloves and eye protection, adding lye to liquid rather than liquid to lye, and keeping the container labeled and out of reach of children and pets. When buying lye for soap, look for soap-making or food grade sodium hydroxide with no additives, since some drain-cleaner formulations contain other chemicals mixed in. A common mistake is storing lye in an unlabeled container, which risks someone mistaking it for something else.