The cold process is very similar to the hot process, except that the mixture is not heated throughout the entire stirring process. The fat is heated until it is melted, and then mixed with the lye and water, but it is not heated further.
Some soap makers think that the cold process produces a soap that is softer on skin as it has more conditioning properties. Some people prefer the smooth texture of cold processed soap to the more homemade rustic look of hot processed soap. It is really just a matter of preference.
Both the cold and the hot process rely on lye, which is a very dangerous chemical that can burn the skin on contact and is fatal if ingested. Because of this, people who use lye to make soap need to be very careful, wear protective clothing and follow the detailed directions. Many people have tried to find a way to make soap without using lye, but this is impossible. Even the soap in the melt and pour soap bases has been processed with lye, even though all of it has gone by the time the glycerin is left.
To make cold process soap, you first need to look up the saponification value of the fats being used on a saponification chart. The saponification value represents the number of milligrams of potassium hydroxide required to saponify ( turn into soap) 1g of fat under the conditions specified. This will depend on the kind of fatty acid contained in the fat. This value is used to calculate the appropriate amount of lye you need for your recipe. Excess unreacted lye in the soap will result in a very high pH and can burn or irritate skin. Not enough lye, and the soap will be greasy. Most soap makers formulate their recipes with a 4–10% deficit of lye so that all of the lye is converted and any excess fat is left to add skin conditioning properties to the soap.
The process is relatively straightforward. The lye is dissolved in water. Then oils are heated, or melted if they are solid at room temperature. Once the oils are liquified and the lye is fully dissolved in water, they are combined. Lye must be added to the water, not the other way around, as this is very dangerous and can cause an explosion.
This lye-fat mixture is mixed until the oils and water are fully emulsified (blended together). Emulsification is most easily identified visually when the soap shows some level of “trace”, which is a thickening of the mixture. Nowadays, soap makers often use a stick blender to speed up this process. There are varying levels of trace. Different additives can affect trace, so they may be added at different stages of “trace”. After much stirring, the mixture turns to the consistency of a thin pudding. Essential oils and fragrance oils can be added with the base oils at the start of the process, but solid additives such as botanicals, herbs, oatmeal, or other additives are most commonly added at light trace, just as the mixture starts to thicken.
The batch is then poured into moulds, kept warm with towels or blankets, and left to continue to saponify for 12 to 48 hours. (Milk soaps or other soaps with sugars added are the exception. They do not usually need to be kept warm, as the sugar in the mixture speeds up the reaction and the production of heat.) During this time, it is normal for the soap to go through a “gel phase,” when the opaque soap will turn somewhat transparent for several hours, before once again turning opaque.
After the insulation period, the soap is firm enough to be removed from the mould and cut into bars. At this time, it is safe to use the soap, since the saponification is complete and there is little lye left. Excess lye remaining in the soap can burn the skin. However, cold-process soaps are typically cured and hardened on a drying rack for 2–6 weeks before use. During this cure period, trace amounts of residual lye is consumed by saponification and excess water evaporates.
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