The Medieval Technology Pages Soap Soap seems to have been Gaulish invention previously unknown to the Romans. It was made by boiling animal fats with natural soda. Soap came into widespread use in Europe during the ninth century. Techniques for producing it improved during the next two centuries, but the product remained soft, more like today's liquid soaps. By the 12th century hard soap came into use.
Hard soap is said to be an Arab development later imported into Europe.
Grind them well and place them in a tank. Pour five times water and stir for two hours. The tank is provided with a plug-hole. When the stirring is stopped and the liquid becomes clear, the hole is opened. When the water is emptied plug the hole again and pour water and stir, then empty, and so on until no taste is left in the water.