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Soap making. Historical artwork of men attending to kettles in a soap making factory. Soap is made by melting a fat, in this case tallow, and an alkali together. After the mixture is bought to a boil the fat reacts with the alkali, producing soap and glycerine. To separate the soap from the glycerine salt is added. This causes the soap to rise to the top and the glycerine to sink to the bottom. After the removal of the glycerine a caustic solution is added and boiled to remove any fats that have not reacted. After this the kettle is boiled once more until it separates into a layer of neat soap and impurities. The soap is then taken off the top and cooled in moulds. Engraving from "Grands Hommes et Grands Faits de l'Industrie" (1880).
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