Saponification: The Art & Science of Soap

To understand soap making, one must first understand soap itself.

Soap is a salt.  

The soap molecule has two sides and is shaped like a tadpole. The head sticks to water (hydrophilic), and the tail repels water and attracts oil (hydrophobic). It is a type of molecule known as amphiphilic, whose Greek roots mean “Both” (Amphi) and “Attract” (Philic).

Water alone can’t wash away oil on your skin because water doesn’t mix with oil. Soap acts like an adhesive tape which binds water to the oil allowing it to be washed away. Soap molecules surround the oil droplet by attaching their tails and forming a Micelle. Once the oil droplet is fully encased with the water loving heads facing outward, the water can bind to it and wash it away.

The soap molecules encapsulate the oil droplet

The tails stick to oil because of their similar composition which allows them to attract. This is why soap is made from fat and is essentially a hybrid molecule consisting of fat on one side (hydrophobic) and salt on the other (hydrophilic).

The two basic ingredients of soap are Triglycerides (fats) and Lye, also known as Sodium Hydroxide (NaOH). Simply put, when Lye is added to fat like olive oil, coconut oil, or tallow (beef fat), the Triglyceride molecule breaks into 3 fatty acids and picks up a Sodium (Na) atom to form a new molecule called Soap, a Fatty Salt. This process is known as Saponification. A natural byproduct is Glycerin, a clear slippery substance which can be added back for slickness, and also is a humectant which attracts and retains water and is thought to soften and moisturize the skin.

Two processes have developed over time for making soap: The Cold and Hot Processes.

Cold Process

The oils (Olive, Coconut, Palm) or fats (Tallow) are mixed with Lye at room temperature. For Saponification to take place the mixture must usually cure for 4-6 weeks until hard. One of the key characteristics is that the glycerin stays inside the soap adding softening and moisturizing properties.

Hot Process

The mixture is cooked which speeds up saponification. The soap can be used as soon as it’s cooled, although 1-2 weeks of curing is recommended to allow for hardening and improved lather. Another advantage is the glycerin can be removed by salting the mixture which causes the soap to float to the surface where it can be removed.  

Hot Versus Cold Process Soap Manufacturing

The Marseille Hot Process

The most famous of the hot soap making processes is the Marseille Hot Process, which was developed in Southern France in the Middle Ages. By the 17th century, Marseille’s proximity to olive oil from Provence and sea salt from the Mediterranean made it the soap capital of France where it developed a process whereby pure olive oil was cooked for days in large cauldrons then purified of glycerin and other contaminants with sea water.

In 1688, Louis XIV’s minister, Jean-Baptiste Colbert issued an edict officially protecting the name Marseille Soap (Savon de Marseille) and requiring it be made 100% from oil olive. In 1811, under Napolean Bonaparte, a decree was issued relaxing the 100% olive oil requirement and expanding it to other vegetable oils. In 1906, a chemist named Francois Merklen at the soap maker Savonnerie Charles Roux Fils, wrote a paper establishing the tradition that Savon de Marseille consist of no less than 72% vegetable oil.

The Marseille Hot Process

Today, most artisan shave soaps use the Hot Process, but not the Marseille Hot Process which removes glycerin needed to retain moisture. Martin de Candre’s shave soaps take inspiration from the Marseille Hot Process, but mainly in the way they are heated, and without being purified with sea water. Saponificio Varesino also claims inspiration from the Marseille Hot Process, but again only in the heating. They also triple mill their soap, shredding it into flakes and pressing it in steel rollers three times to remove excess moisture and impurities. Milling a soap creates a harder, denser, puck, which tends to last longer.