Simply put, soap is made by mixing a vegetable oil or animal fat with a lye solution.
The lye solution is a very strong alkali, made from sodium hydroxide - NaOH - (caustic soda) dissolved in water.
When the lye is mixed with the oil a chemical reaction takes place and the sodium (Na) joins up with part of the oil molecule to produce a sodium soap.
The OH part of the lye joins up with the remaining part of the oil molecule to form glycerin.
Different oils require different amounts of lye to produce what is called full saponification, which occurs when all of the oil has reacted with the lye.
The amounts are calculated with reference to the 'saponification tables' which lays out a formula so that exact amounts of lye can be determined.
By varying the lye content it is possible to create a superfatted soap (i.e. contains excess oil) that is mild and gentle on your skin.
Once the quantities are determined the selected oils are mixed with the lye, usually both at the same specified temperature.
Mixing continues till the mixture thickens and reaches what is called "tracing" stage.
Times for this to occur will vary, depending on the mix of soapmaking oils used.
Once tracing occurs, additives such as nutrient oils, colouring and vegetable matter are stirred into the mix.
Lastly, fragrances (essential oils are recommended over fragrance oils) are thoroughly mixed in and the batch poured into moulds to set.
Once the soap has hardened enough to handle it can be removed from the moulds, cut into useful sizes and left to cure.
Curing generally takes 4 to 6 weeks to complete.