Cold process soap is made by combining oils and sodium hydroxide (lye). The lye causes an exothermic (hot!) chemical reaction called saponification, where the triglycerides in the fats and oils break down and are converted into soap and glycerol. 

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All oils and fats have a "saponification value" that corresponds to the amount of lye needed to neutralize them into soap with no lye left over. For each recipe a soap-making calculator is used to ensure the ratios of water, oils, and lye are correct and will produce a soap that is safe and cleansing.

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Cold process soap then needs to cure for 4 - 6 weeks to allow excess water to evaporate. Curing creates a firmer bar that lasts longer once the soap is introduced to water in the sink/shower/tub.