Hard Soap Boiled from Tallow and Soda Lye, Framed and Wire-Cut, Ure 1839

Hard soap is tallow boiled with caustic soda lye; potash would give soft soap. Andrew Ure (1839): run the lye through soda and quicklime, boil the tallow with weak lye, draw the spent lye off and boil again with stronger lye day by day, then ladle the soap into a frame and cut it with a brass wire into bars. Caustic lye burns skin and eyes: goggles and gloves. The picture: a bar of hard soap.