Several methods may be used to make soap, but they all obey to a common chemistry process called saponification.
It is a chemical reaction which transform fat into soap. Fat can be vetegal oil or aninal fat. As we see in the following formula which describe saponification, the chemical reaction not only gives soap but also glycerin as a by-product.
Fat + Caustic soda = Soap + Glycerin
The manufacturing process of Marseille soaps
The particularity of a Marseille soap is found in its manufacturing process that involves cooking in a cauldron at 100 or 110 degrees (about 220 Fahrenheit) and several washing of the dough with salt water. It is an ancient method of saponification which requires patience but it guarantees a final product of the highest quality.
It takes a minimum of 20 days to make a raw cube of extra pure Marseille soap.
Making a soap paste
It is a slow chemical reaction to begin. In a large cauldron, caustic soda and fats are mixed and heated. Fats are olive oil and/or palm-coco oils. Some soap from a previous cooking is also added, as well as sea salt.
A salted lye removes leftovers of caustic soda. By stopping the boiling, the caustic soda goes down and the soap rises to the surface. This operation is called epinage.
Cooking the soap paste
For about 10 days, the Marseille soap paste is slowly and carefully cooked. Modern technology can make this step goes a lot faster but a soap factories who stand with the traditional process of Marseille will never skip this important step. In just a few hours, the soap would not have time to form well enough and soapmakers would end up with a low quality product. Ten days of cooking is what makes a Marseille soap perfectly usable for several decades after it is being manufactured!
Cooking is complete when the caustic soda does not deteriorate further more. The master soapmaker then tastes the soap (he really tastes it!), check the appearance of the dough and verify its elasticity. An art and a knowledge that can not be learned in books.
Washing the soap
One last time, caustic soda is introduced and the mixture is boiled. A final epinage is made and the paste is washed with clear water, causing the impurities to be drawn down to the bottom. The soap, then clean and pure, swim to the surface and it is settle down for two days. The soapmaker finally proceed a final check of the viscosity.
Filling the pool
The pools are 40 cm depth (almost 16 inches) and receive the hot soap from the cauldrons. They are quickly filled up to prevent pipes from clogging. The soap is spread with a long spatula to get a smooth surface. Being as hot as 110 degrees Celcius, the hot soap must cool down for 2 days. When it is colder and dryer, lines are drawn and the soap is divided into blocks of 35 kg (about 70 pounds). These blocks will then takes the direction of the cutting machine. Marseille cube soaps are born!
Stamping and drying
The soaps need to be stamped on six sides. Usually, a soapmaker engrave the following informations: weight of the soap at the time of manufacturing, the famous savon de Marseille extra pur 72 % d'huile, the type of oil that has been used as a base and sometime the name of the soap factory.
At least, the cube soaps are put to rest on woodden shelves so they can dry for about 10 days in a room naturally ventilated with the outside air. From time to time they will be turned over to make sure that the six sides are drying well.
Scented and colored soaps
A Marseille soap being a cooked soap, it is difficult to add fragrances during the manufacturing process. The trick is to use raw soap and grinds it. We obtain soap flakes that are passed through a blender where fragrances can be incorporated. The soap, now perfumed and colored is passed through a roller mill in order to have a homogeneous soap paste to work with. Finally, soap sticks are made thanks to an extruder. By cutting and molding those sticks, any desired shaped can be obtained. Naturally, scented Marseille soaps do contains glycerin.
Watch how Marseille soaps are made