The most common soap making process today is the cold process method, where fats such as rendered lard react with lye. Some soapers also practice other processes, such as the historical hot process, and make special soaps such as clear or transparent soap, which must be made with ethanol or isopropyl alcohol.

Soap makers sometimes use the melt and pour process, where a premade soap base is melted and poured in individual molds. While some people think that this is not really soap making, the Hand Crafted Soap Makers Guild does recognize this as a legitimate form of soap crafting.

Handmade soap differs from industrial soap in that whole oils containing intact triglycerides are used and glycerin is a desireable byproduct. Industrial detergent manufacturers commonly use fatty acids, which are detached from the gylcerol heads found in triglycerides. Without the glycerol heads, the detached fatty acids do not yield glycerin as a byproduct.

Lye

Reacting fat with lye (sodium hydroxide) will produce a hard bar soap. Reacting fat with potassium hydroxide will produce a soap that is either soft or liquid. Historically, the alkalis used were sodium hydroxide, potassium hydroxide, and sodium carbonate leeched from hardwood ashes.

Fat

Soap is made from either vegetable or animal fats. Sodium tallowate, a fatty acid sometimes used to make soaps, is derived from tallow, which is rendered from cattle or sheep tissue. Soap can also be made of vegetable oils, such as palm oil, olive oil, or coconut oil. If soap is made from pure olive oil it may be called Castile soap or Marseille soap. Castile is also sometimes applied to soaps with a mix of oils, but a high percentage of olive oil.

An array of oils and butters are used in the process such as olive oil, coconut oil, palm oil, cocoa butter, hemp oil and shea butter to provide different qualities. For example, olive oil provides mildness in soap; coconut oil provides lots of lather; while coconut and palm oils provide hardness. Most common, is a combination of coconut, palm, and olive oils.

Process

Cold process soap making is done without heating the soap batter, while hot process soap making requires that the soap batter be heated. Both processes are further described after the general soap making process description.

General soap making process

Soap making requires the use of saponification charts to determine the correct lye/fat ratio. If excess unreacted lye remains in the soap, the resultant high pH can burn or irritate skin.

Conversely, a high proportion of excess fat will result in greasy sludge that will not form solid bars of soap, although some soap makers deliberately “superfat” their soap so that some oils will remain in the finished bars of soap. This can be done by either adding a small (5-10%) excess porportion of fats, or by discounting the formulated amount of lye to 90-95%.

The lye is dissolved in water; as this is an exothermic process, the solution will spontaneously generate heat and may even boil. The oils are heated separately (to the point of liquefaction if they are solid at room temperature). Once fats and lye water have both cooled to 80-100° F, they are combined. This mixture of lye water and fats is stirred until “trace” occurs and the mixture becomes a soap batter. There are varying levels of trace: a light trace implies a thinner soap batter and a heavy trace implies a thicker soap batter. Additives, such as essential oils, fragrance oils, botanicals, clays, colorants or other fragrance materials, are combined with the soap batter at different degrees of trace, depending upon the additive. With elapsed time and continued agitation the soap batter will continue to thicken. The cold process soap batter is then poured into molds, while hot process soap batter is poured into a double boiler or crockpot to sustain a high temperature.

Cold-process

Although cold-process soapmaking takes place at room temperature, the fats are first heated to ensure the liquification of the fats used. Then, when the lye water solution is added to the fats, it should be the same temperature of the melted oils and both are typically between 80-90° F. An external heat source is not necessary but the molded soap should be incubated by being wrapped in blankets or towels for 24 hours after being poured into the mold. Milk soaps are the exception and do not require insulation, which may cause the milk to sour. The soap will continue to exothermically give off heat for many hours after being molded. During this time, it is normal for the soap to go through a “gel phase” where the opaque soap will turn semi-transparent for several hours before turning opaque again. The soap may be removed from the mold after 24 hours but the saponification process takes several weeks to complete.

Hot-process

Unlike cold processed soap, all hot processed soap experiences a “gel phase” as a result of being heated, such as in a double boiler or crockpot. Hot process soap may used soon after being removed from the mold because the higher temperatures accelerate the saponifcation process and also drive off excess water.

Purification and finishing

The common process of purifying soap involves removal of sodium chloride, sodium hydroxide, and glycerol. These components are removed by boiling the crude soap curds in water and re-precipitating the soap with salt.

Most of the water is then removed from the soap. This was traditionally done on a chill roll which produced the soap flakes commonly used in the 1940s and 1950s. This process was superseded by spray dryers and then by vacuum dryers.

The dry soap (approximately 6-12% moisture) is then compacted into small pellets. These pellets are now ready for soap finishing, the process of converting raw soap pellets into a salable product, usually bars.

Soap pellets are combined with fragrances and other materials and blended to homogeneity in an amalgamator (mixer). The mass is then discharged from the mixer into a refiner which, by means of an auger, forces the soap through a fine wire screen. From the refiner the soap passes over a roller mill (French milling or hard milling) in a manner similar to calendering paper or plastic or to making chocolate liquor. The soap is then passed through one or more additional refiners to further plasticize the soap mass. Immediately before extrusion it passes through a vacuum chamber to remove any entrapped air. It is then extruded into a long log or blank, cut to convenient lengths, passed through a metal detector and then stamped into shape in refrigerated tools. The pressed bars are packaged in many ways.

Sand or pumice may be added to produce a scouring soap. This process is most common in creating soaps used for human hygiene. The scouring agents serve to remove dead skin cells from the surface being cleaned. This process is called exfoliation. Many newer materials are used for exfoliating soaps which are effective but do not have the sharp edges and poor size distribution of pumice.