This section is from the book "The Steward's Handbook And Guide To Party Catering", by Jessup Whitehead. Also available from Amazon: Larousse Gastronomique.
To one pound of potash, add three gallons of water. Boil the potash until it is all dissolved. Then add three pounds of any kind of soap-grease - the cleaner the better - to the lye, and set it to boiling. It usually becomes soap after boiling from one to five hours. Then add nine gallons of water, and stir well together. When cool it will be beautiful white soap, if the grease was clean.
Take one pound of potash, dissolve in twelve quarts water in a kettle calculated to make the soap, add to the lye five pounds of grease and boil slowly, add water to the kettle as it boils down. Keep about the same quantity in the kettle until it becomes soap. Then add eight ounces common salt, and boil ten minutes, which will separate the water from the soap. If rosin is wanted, melt that in a separate kettle. Add lye with it until it becomes thick, which will take about one quart to a pound of rosin. Then add the rosin to the boiling soap before the salt is added. The salt causes the soap to separate from the water and rise upon the top. When cold it can be taken off or dipped into moulds while hot. The above hard soap may be perfumed just before it is cool. When lye is too strong it will not make soap; therefore, if the kettle, while boiling, is not kept about the same quantity, the lye will boil down too strong.