Sodium hydroxide is a chemical compound that holds or maintains the pH of skincare products, also known as lye. In soap, it’s combined with animal fat or vegetable oil in a process called saponification. Essentially, soap is a mixture of base oils, a diluted lye solution, and herbal additives.
During the soap making process the lye is heavily diluted with water, and added to the oils to create a thick mixture. The mixture is then added to a mold for 24 hours to harden, and set to cure for 4-6 weeks. During the curing process, the soap bars lose water weight, in which the sodium hydroxide is dissolved. Therefore lye is needed for the chemical process that soap goes through, but it's not present in the final product.