Lye (sodium hydroxide) is the alkali that makes soap possible. When combined with fats or oils, it triggers saponification — the chemical reaction that transforms raw ingredients into real soap. The lye itself is fully consumed in this process; a properly made, fully cured bar contains no active lye whatsoever. Understanding saponification is fundamental to understanding what real soap actually is, and why it behaves so differently from the synthetic detergent bars that dominate commercial shelves. Wikipedia’s overview of soap covers the chemistry clearly if you want to dig deeper.
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