Hot-processed soaps are created by encouraging the saponification reaction by adding heat to the reaction. Saponification is a process that produces soap, such as mixing an acid with an alkali, using fats and lye.
Before making hot processed soap, the soap maker needs to calculate the saponification value of the base fats that are going to be used. 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. You can do this by running your recipe through a lye calculator to measure how much lye will be needed in your recipe.
Then the process begins by adding lye to water. This simple step can be very dangerous, since accidentally adding water to lye will cause a minor explosion. Lye must be added to the water, not the other way around. Next, the lye mixture is added to heated oils or butters. This can be done in a crock pot, large pan or the microwave. The lye/water and oil mixture gets blended together with a stick blender for a while before adding any desired extras, such as any exfoliating products. Then the soap is ‘cooked’.
There are several different ways of doing this. You can bake it in the oven, cook it on top of the stove, cook it in a double boiler, or use a crock pot. Typically a batch of hot processed soap takes an hour or two to complete, and the soap is ready to use as soon as it is cooled. However, using a microwave is considerably faster. A five pound batch is done in about 15 minutes and a smaller batch can cook in as little as 5 minutes.
The mixture is then stirred a little more to make sure it is properly blended. Once it starts to look like gel or applesauce, it then transforms into a kind of mashed potato look. Then the soap is almost done. The mixture is then stirred a little more to make sure that everything is evenly distributed. Once it has cooled slight;y you can add your fragrances and colours before you pour it into molds.
The cooking forces the gel stage to occur faster, evaporates more water and produces a harder bar of soap quickly.
There is little or no curing time for this type of soap, as any excess liquid has evaporated during the heating. This is great if you are impatient and can’t wait to get your finished product, and is also good if you don’t have much space for curing products. Some soap makers still like to let their soap cure for a week or two, to create a harder, firmer bar.
Because you add the fragrances at the end of the cooking they are not effected by the lye, so you can use smaller amounts and still get the same effect.
There is only one disadvantage of hot process soap, which is that the soap is quite thick when it has finished the cooking stage. This can make it quite difficult, although not impossible, to use very intricate molds. It is also difficult to get the top perfectly smooth, and the bars usually require some trimming with a cheese slicer, or potato peeler.
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