Handcrafted soaps made from scratch require three ingredients to become soap: oil (animal or vegetable oil, not petroleum-based oil), water and lye. These three ingredients, mixed together in correct proportions, combine and chemically change into soap - a process called "saponification". They may include other ingredients to provide additional benefits or to color or scent the soap, but these are not part of the chemical process that results in basic soap.
An accurate recipe is necessary, in order to ensure that the right amount of lye is used for the specific oils selected. A lye calculator can be used to calculate the right amount of lye, although it can also be calculated based on the type and quantity of oils being used.
The solid lye is mixed with water to dissolve it. The amount of water depends on the particular recipe, but is usually 2 to 3 times the amount of lye, based on weight. When the lye is added to the water the solution gets hot. It is important to remember that lye is highly caustic and can cause severe burns if it comes in contact with skin for more than a few seconds. Proper protective gear, including at least gloves and goggles, should be used when handling lye or incomplete soap.