Soap boiler

Soap is made by heating fats or oils with an alkali. In the old method, tallow or oil was boiled or heated with lye or alkali from plant ashes and stirred, sometimes for days, until it thickened into soap.

In England soap was heavily taxed, and its making was closely supervised by revenue officials. A law even set a minimum batch size for soap boilers, which kept small makers out of the trade, and soap stayed a luxury there until the mid-1800s.