"Hard soap or curd soap is a kind of soap; examples are Aleppo soap, Castile soap, and Marseille soap or savon de Marseille. During the preparation ...
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of the soap, common salt (sodium chloride) is added to the liquid soap mass. This leads to the soap mass separating from glycerin, resulting in a harder soap. It can be made using sodium hydroxide." - (en.wikipedia.org 29.02.2020)