While we usually think of "soap" as a cleaning agent that bubbles and lathers, there are actually a number of different substances that are technically soap - it just depends what they are made of. In this article we will mainly deal with the chemistry of the bubbling, cleaning kind of soap.

Soap, Technical Definition

There are several ways that commercial soaps are produced. The method used depends on the facilities of the soap manufacturer and the type of desired soap. Once the basic soap is made, it can then be perfumed, coloured, filled, shaped and packaged.

Full Boiled Method

Probably the most common method of soap manufacture is referred to as the "full boiled" method. This is a several step process:

First, a large amount of alkali (lye) is combined with oils. Any kind of animal and vegetable oils can be used (even kitchen grease), although the most common oils for bar soaps are coconut, olive and palm oils. Heat is applied and the lye and oils are thoroughly mixed until the oil is saponified.

Second, a series of steps are taken to remove any impurities, neutralize or remove any excess lye and separate out and recover the glycerine which is released naturally during saponification. These steps are called "changes" and consist of adding a saltwater solution to the soap mixture. This causes the mixture to separate with the soap on the top and the impurities, excess lye and glycerine on the bottom. Once separated, the bottom liquid is drained off and the glycerine recovered from it. There may be three or four "changes" before the soap is fully saponified and ready.

Semi-Boiled Method

The semi-boiled method is fairly simple: the desired oils of whatever kind are combined with the exact amount of lye necessary for saponification. The lye and oils are mixed with external heat applied until full saponification occurs. The glycerine is not extracted, but is instead left in the soap. This is the method used when the soap is going to be made into transparent soap.

Some handmade soap is made by the semi-boiled method--this is also referred to as "hot process". When Grandma made soap in a big pot over the fire, she was using the semi-boiled method.

Prior Glycerine Removal

In some cases, the oils are processed to separate the glycerin and fatty acids prior to making soap. The glycerine is purified as needed and then sold or used elsewhere. Only the fatty acids are used in the soap making procedure. When lye is then added to the fatty acids the result is soap, without any glycerine in it.

Continuous Process

All of the methods of commercial soap making listed above process the soap in batches, but there is also a method of making soap by a "continuous process". This is an on-going process where oil and lye are continuously added in and as the saponification occurs the finished soap is drawn off in a steady stream. This process was developed by one of the large soap manufacturers and is now licensed for use by others.

Processing the Soap Base

Once basic soap (also known as "soap base") has been produced by any of the above methods, it can then be processed into its final form and packaged for sale. Sometimes this is done at the factory where the soap was produced; sometimes the soap base is sold and the final steps are completed by another party.

Fillers, designed to add bulk and weight to the soap but of no particular cleansing or cosmetic benefit, can be added to the soap base. Builders, which are designed to add some benefit to the soap (such as pumice or water softeners) can be added. Synthetic detergents are also often added to bar soaps. And, of course, colorants and perfumes can go into the soap base.

Once any additional ingredients are incorporated into the soap, it can be shaped into its final form. In olden times, this just consisted of cutting the soap into bars. Nowadays, however, most soaps are milled into fine ribbons, mixed with colorants, perfumes and additives and then pressed into a unique shape, sometimes with a logo imprinted on them. For example, that's how "Dove" gets its well-known design.

Transparent soap, sometimes referred to as "glycerine soap" is made by adding alcohol to the soap, which makes it clear (depending on the type of oils used).

Soap powders, which aren't seen too much in modern stores, were made by spraying liquid soap out of a very fine sprayer nozzle about three stories high. When the tiny particles of soap hardened on the way down, piled up at the bottom as powdered soap.

Many products used in everyday life contain commercially produced soaps. Shampoo, cleaning products, toothpaste, pharmaceuticals, vitamin tablets - even roads were once sprayed with a soap-and-oil mixture to reduce dust (although this practice has been discontinued in recent years due to environmental concerns).

This article is from the Chandlers Soap (US) website.