Over the ages, people have used a variety of materials to wash things. Water does a pretty good job all by itself. Minerals and plant extracts can be added to improve the detergency of water. Ancient Romans ingeniously covered themselves with olive oil (which dissolved the dirt and grime that is not soluble in water) and then scraped the dirty oil off. But the first mention of a substance that is unmistakably soap appears in Pliny the Elder's first century work, Natural History. Pliny describes an exotic pomade called “sapo” produced by Germans from suet and ash, but the details of production are not given.

Not until 1130 AD do we get a detailed description of the soapmaking method. The Mappae Clavicula, a handbook for artists, describes the leaching of wood ashes with water to produce “lye,” which may be made more caustic by adding lime. Lye can be concentrated by boiling off the excess water, and olive oil or tallow may then be added to the boiling lye, and the mixture cooked until it thickens. Upon cooling, the mixture separates into spent lye (below) and soap (above), which may be skimmed off. Soap produced from wood ashes is a liquid, but the addition of salt converts it to a solid. 

The details of the process changed slowly over the centuries. If wood ashes were replaced with the ashes of kelp or saltwort, solid soap could be produced directly without the addition of salt. It takes a lot of plant material to make a little ash, however, and it takes a lot of ash to make a little lye. And because the kelp industry was dominated by Great Britain and the saltwort industry by Spain, the French government challenged scientists to produce lye directly from salt, a material the French could produce domestically, and the first chemical factories were born.

The cold process traces its roots to a 1622 patent, which eliminated boiling from the method, but the idea did not catch on. Writing in 1807, the eminent chemist Chaptal gives recipes for CP soap made from almond oil, but also says that, “no clear advantage has resulted from preparing them without heat.” It was not until 1822 that Chevreul brought a truly scientific approach to the study of the soapmaking process. Prior to that, soap had been seen as simply a mixture of fat with sodium hydroxide. Chevreul discovered that the fat actually decomposes into soap and glycerin, and that a given amount of fat requires a specific quantity of sodium hydroxide for the reaction to be complete. This specific quantity, the saponification value, is well known to CP/HP soapmakers today.

Soap boiling continued to dominate soap production throughout the nineteenth century, but once saponification values were understood, the cold process fostered a niche market. The key difference between the two processes is that when soap is boiled, the glycerin byproduct remains in the spent lye, from which it may be extracted and sold as a separate product. In the cold and hot processes used by handcrafters, all of the glycerin remains in the soap.

The early soap industry used batches, just as handcrafters do today. Ingredients were added, the mixture cooked and the products removed. The next major development was to move to a continuous process, adding fresh oil and lye while simultaneously removing soap and spent lye. In this way, soap could be produced around the clock, like a chemical assembly line. Soap became a commodity produced by large national and multi-national corporations.

Two revolutions changed the commodity soap industry in the twentieth century. First, fat began to be decomposed (“hydrolyzed”) by steam at high temperatures and pressures. The products of this reaction are fatty acids and glycerin. The fatty acids are then reacted with sodium carbonate (soda ash) or sodium hydroxide. The resulting soap is identical to that produced by boiling—the glycerin is removed as part of the process. The second revolution came with the invention of synthetic detergents. While they may be produced from petroleum or traditional fats and oils, these detergents are not derived from fatty acids. Their chief advantages over soap are that they can be formulated at lower pH, and that they perform well even in hard water.

The cold process has remained the ugly stepchild of the commodity soap industry. In the last quarter of the twentieth century, however, it became central to the handcrafted soap movement. While it lacks the economies or scale of soap boiling or steam hydrolysis, it lends itself to small-scale production of specialty soaps. Handcrafters use a much larger pallet of oils, colors and scents than are used in the commodity industry. And enthusiasts of cold process soap value the retained glycerin as an emollient. Like the cold process, the hot process retains glycerin in the product. The re-introduction of heat to the process speeds up the saponification reaction and allows delicate colors and scents to be added after the sodium hydroxide has been used up.

Today, consumers have more choices in cleaning products than they have ever had before. And while the market for commodity soaps and detergents dwarfs that for handcrafted soaps, small soap companies are able to thrive in an environment where startup costs are low and interest in locally-produced artisanal products is high. The Internet has opened national and even global markets to small companies that would have been unheard of a generation ago. We are truly living in a golden age of soap.