Unlike other mummies which are kept dry to ensure preservation, this mummy was exposed to water, which seeped into the casket and turned the fats in his body to soap. Hence the mummy’s nickname: “Soapman.”
Saponification, a chemical reaction used to create soap for millennia, literally means “soap making” in Latin. When water reacts with the fats and oils, a reaction called hydrolysis, the result is glycerol and soap.
This process is common when bodies have been exposed to water. The end result is called adipocere, or grave wax.