Soap: From Ancient Cleansing to Modern Manufacturing

Background

Soap is a classic saponification product, created by reacting animal fat or plant oil with a strong alkali such as sodium hydroxide. When dissolved in water, it emulsifies and lifts dirt, making it an essential cleansing agent throughout history. Beyond its modern use as a simple cleanser and fragrance enhancer, soap has been employed to treat skin conditions, dye hair, and serve as a topical ointment.

While the precise origins of soap remain unclear, Roman accounts date its use back to at least 600 B.C., when Phoenicians mixed goat’s tallow with wood ash. The Celts also produced soap in ancient Britain. During the Roman Empire, soap was primarily used medicinally, and references to it as a general cleanser appear only in the second century A.D. By the eighth century, soap production had spread to France, Italy, and Spain, yet it remained scarce across the rest of Europe until the seventeenth century.

In England, soap manufacturing began in the late twelfth century. High taxes imposed on soap makers caused the government to lock boiling pans overnight to prevent illicit production. Because of these levies, soap was considered a luxury item until the tax was repealed in 1853, after which it became widely affordable and popular across Europe.

Early producers boiled a mixture of wood ash and animal fat. In 1790, French chemist Nicolas Leblanc pioneered the extraction of caustic soda from common table salt, eliminating the need for ash. The term “saponification” was coined by Eugene‑Michel Chevreul in 1823 to describe the chemical reaction that turns neutral fats into fatty acids and, in the presence of alkali, into soap, leaving glycerin as a by‑product. By the end of the nineteenth century, industrial processes had replaced home‑made soap, although rural communities in places such as the American West still produced soap by hand well into the twentieth century.

Raw Materials

Soap production relies on two primary inputs: fat and alkali. Sodium hydroxide is the most common alkali today, but potassium hydroxide is also used. Soap made with potassium hydroxide is more water‑soluble, earning the name “soft soap”; it is frequently blended with sodium‑based soap in shaving preparations.

Modern manufacturers source fats that have been refined into fatty acids, which removes impurities and yields water as a by‑product instead of glycerin. Vegetable oils such as olive, palm kernel, and coconut oil are also widely used.

Additives enhance soap’s appearance, texture, and scent. Fragrances are incorporated to mask residual odors and leave a pleasant fragrance. Abrasives like talc, silica, and volcanic pumice improve texture. Historically uncolored soap appears dull grey or brown, but contemporary manufacturers add dyes to make products more appealing.

The Manufacturing Process

Small soap makers often still employ the traditional kettle method, which can take between four and eleven days and yields variable quality due to the diversity of oils used. Since the 1940s, large manufacturers have adopted a continuous process that produces soap in a streamlined, hour‑long cycle, offering greater consistency and efficiency.

The Kettle Process

Boiling

• Fats and alkali are melted in a steel kettle, a multi‑story tank capable of holding thousands of pounds of material. Steam coils heat the mixture to a boil, where the fat reacts with alkali to form soap and glycerin.

Salting

• Salt is added to separate the soap from glycerin; the soap rises to the surface while glycerin sinks and is subsequently extracted.

Strong Change

• A concentrated caustic solution is introduced to ensure any remaining un‑saponified fat is converted into soap. The mixture is boiled again, and a second salting may follow.

Pitching

• During pitching, the soap is boiled with water, causing it to split into a neat top layer (~70% soap, 30% water) and a lower layer called nigre, which contains impurities and excess water. The neat soap is removed, cooled, and then processed further.

The Continuous Process

Splitting

• Natural fat is introduced into a vertical stainless‑steel column known as a hydrolizer. Hot water at 266 °F (130 °C) and high pressure enters from the opposite end, splitting the fat into fatty acids and glycerin. The two streams exit continuously and the fatty acids are purified by distillation.

Mixing

• The purified fatty acids are blended with a precisely measured amount of alkali, along with abrasives and fragrances. The resulting hot liquid soap may be whipped to incorporate air.

Cooling and Finishing

• The soap is poured into molds or cooled in a specialized freezer. It then hardens into slabs, which are cut into bars, stamped, and wrapped. The entire process can be completed within a few hours.

Milling

• Many commercial soaps undergo milling, where the cooled soap is rolled and kneaded through heavy rollers. This step improves lather quality and texture. Perfumes are best added during milling because the cold mixture preserves volatile oils. The milled soap is then pressed into smooth cylinders, extruded, cut, stamped, and wrapped.

Byproducts

Glycerin, a valuable byproduct of soap manufacture, is used in hand lotions, pharmaceuticals, and as the main ingredient in nitroglycerin, a key component of dynamite.