Acid: Compound that reacts with a base to form a salt and has a pH of less than 7. In soapmaking, this refers to oils and the fatty acids that comprise them.
Base: Compound that reacts with an acid to form a salt and has a pH of greater than 7. In soapmaking, you use a base, or alkali, solution to react the acids, or oils, into soap.
Caustic: A substance that burns or destroys organic tissue by chemical action. Two caustics used in soapmaking are lye and sodium hydrochloride.
Cold-process: This is the basic form of handmade soapmaking. A blend of oils is mixed with a solution of lye and water, stirred until thickened, and poured into a mold. This technique is called “cold” process because there is no cooking involved, beyond heating the oils enough to liquefy them.
Hand-milled: In this soapmaking process, you take finished cold-or hot-process soap, grate it, heat it, mix in additives, scoop the mixture into molds, and let it set. Some people call it “rebatching,” but that term usually describes the rescue of an otherwise failed batch. Soap projects that start out with the intent to use this technique are most often called hand milled.
Hot-process: This type of soapmaking is basically an extension of the cold-process technique. After you follow the cold-process steps, you cook the still-caustic soap batter for several hours, forcing it to neutralize. This eliminates the need to wait several weeks for the soap to cure. You can make solid, liquid, clear, and cream soaps using this procedure.
Lye: A strong alkali used to make soap. To make solid soap, you use Sodium Hydroxide, abbreviated as NaOH. To make liquid or cream soap, you use Potassium Hydroxide, abbreviated as KOH.
Mise en place: This term, borrowed from French cooking, refers to the practice of measuring out all the ingredients ahead of time and putting away everything except what you'll be using in your recipe.
pH: The measure of acidity and alkalinity of a solution that is a number on a scale on which a value of 7 represents neutrality and lower numbers indicate increasing acidity and higher numbers increasing alkalinity.
Rotting: When making cream soap, the soap will change in texture during a period of “rotting.” It is an old-fashioned soapmaking term, brought into use for home soapmakers by Catherine Faillor.
Saponification: Literally, this means “to turn into soap.” It is a reaction between a lye solution and oils. A soap is said to be “fully saponified” when there is exactly enough oil and lye to fully react. Since you usually want a little extra oil in your soap, for gentleness and moisturizing benefit, most soaps are formulated with slightly more oil than will completely saponify.
Soap casting: Also known as “melt and pour” or “melt and mold,” soap casting is a wonderfully accessible method of making soap at home. In this method, a simple meltable soap base, commonly called glycerin soap base, is melted down and poured into a mold.
Superfatting: This is a process to make a gentler soap by using less lye than needed to completely saponify the oils. Another way to create a gentler soap is to superfat, by adding extra oils at the end of the stir, before you pour. Overly lye-discounted or superfatted soap is softer and prone to rapid spoilage. Lye-heavy soap is a worse problem, as it makes harsh, caustic, and unusable soap.
Trace: A key stage in the soapmaking process. When your soap is “tracing” you know that the chemical reaction is working properly. The term trace refers to the presence of traces of the soap mixture on the surface of the mass when some is taken up on your stirrer and dribbled back in. If the dribble makes no mark, your soap has not traced. When it leaves a little lump on the surface that sinks in quickly, it's beginning to trace. Opacity, a slight graininess, and a subtle soap smell are other clues that your soap is tracing. A trace state is described from “light” to “heavy.” You add color, scent, and other materials at varying levels of trace. A light trace may be like a thin pancake batter; a medium trace is like a medium-thick gravy. If your soap gets gloppy, you've got a heavily traced batch, which you need to get into its mold as soon as possible.