Hard soap or curd soap is a kind of

soap Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are use ...

. Examples are Nablus soap, Aleppo soap, Castile soap, and

Marseille soap Marseille soap or ''Savon de Marseille'' () is a traditional hard soap made from vegetable oils that has been produced around Marseille, France, for about 600 years. The first documented soapmaker was recorded there in about 1370. By 1688, Loui ...

or ''savon de Marseille''. During the preparation of the soap, common salt ( sodium chloride) is added to the liquid soap mass. This leads to the soap mass separating from

glycerin Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...

, resulting in a harder soap. It can be made using sodium hydroxide.Hard Soap
Merriam-Webster Dictionary

References

{{Reflist Soaps