The waste cooking grease being saved over a period of time without the benefits of refrigeration usually become rancid, so this cleaning step is very important to make the grease sweeter. It will result in a better smelling soap. The soap made from rancid fats or grease will work just as well as soap made from sweet and clean fats but not be as pleasant to have around and use. To render, fats and waste cooking grease are placed in a large kettle and an equal amount of water is added. Then the kettle is placed over the open fire outdoors.
Soap making is an outside activity.
The smell from rendering the fats is too strong to wish in anyone's house. The mixture of fats and water are boiled until all the fats have melted. After a longer period of boiling to insure completion of melting the fats, the fire is stopped and into the kettle is placed another amount of water about equal to the first amount of water. The solution is allowed to cool down and left over night. By the next day the fats have solidified and floated to the top forming a layer of clean fat.