How Is Soap Manufactured?
Soap is made largely from fats or oil, with a variety of other ingredients. Before the introduction of soap in the 1st Century A.D. people “washed” themselves and their clothes with fuller’s earth, a fine clay-like substance that loosens oil and dirt. People first made their own soap by saving scraps of fat and boiling them in an iron pot.
They added an alkaline solution, made from wood ash, called lye. This formed a yellow “soft soap”, the yellow coming from the potash in the lye. Hard soap was made by boiling for longer, and by adding salt, usually from sea-water.
Soap is still made in much the same way, but on a far larger scale in modern factories. The chief things that go into its manufacture are still fat or oil (but oil from coconuts or cotton seeds), lye containing potash, or sodium, and salt. Colored dyes, perfumes and super-fats, such as almond oil and glycerine, are added to make the expensive toilet and shaving soaps.
The industrial production of soap involves continuous processes, such as continuous addition of fat and removal of product. Smaller-scale production involves the traditional batch processes. The three variations are the cold process, wherein the reaction takes place substantially at room temperature; the semi-boiled or “hot process,” wherein the reaction takes place near the boiling point; and the fully boiled process, wherein the reactants are boiled at least once and the glycerol is recovered.
There are several types of semi-boiled hot process methods, the most common being DBHP (Double Boiler Hot Process) and CPHP (Crock Pot Hot Process). Most soapmakers, however, continue to prefer the cold process method. The cold process and hot process (semi-boiled) are the simplest, and are typically used by small artisans and hobbyists producing handmade decorative soaps.
The glycerol remains in the soap and the reaction continues for many days after the soap is poured into molds. The glycerol is left during the hot-process method, but at the high temperature employed, the reaction is practically completed in the kettle, before the soap is poured into molds. This simple and quick process is employed in small factories all over the world.
Handmade soap from the cold process also differs from industrially made soap in that an excess of fat is used, beyond that needed to consume the alkali (in a cold-pour process, this excess fat is called “superfatting”), and the glycerol left in acts as a moisturizing agent. However, the glycerine also makes the soap softer and less resistant to becoming “mushy” if left wet. Since it is better to add too much oil and have left-over fat, than to add too much lye and have left-over lye, soap produced from the hot process also contains left-over glycerol and its concomitant pros and cons.
Further addition of glycerol and processing of this soap produces glycerin soap. Superfatted soap is more skin-friendly than one without extra fat. However, if too much fat is added, it can leave a “greasy” feel to the skin. Sometimes, an emollient additive, such as jojoba oil or shea butter, is added “at trace” (i.e., the point at which the saponification process is sufficiently advanced that the soap has begun to thicken in the cold process method) in the belief that nearly all the lye will be spent and it will escape saponification and remain intact.
In the case of hot-process soap, an emollient may be added after the initial oils have saponified so they remain unreacted in the finished soap. Superfatting can also be accomplished through a process known as “lye discount” in which the soap maker uses less alkali than required instead of adding extra fats.