Saponification is the chemical reaction between a base (such as the caustic soda) with an acid (such as oil) in the presence of water. The product of this reaction is a salt, in this case a soap. Depending on the oil, the reaction either requires heating or can happen at a cold temperature. Types of oils used in the soap industry range from vegetable to animal to mineral oils. Each with its virtues and disadvantages. Among the vegetable oils, those most commonly used are either palm or olive oils for their availability and affordability. Palm oil which is solid at ambient temperatures has the advantage of not requiring heating beyond 40°C to jump-start the reaction. Olive oil which is liquid at ambient temperatures will depend on its quality to calculate its demand in heating. For example, extracted oil from olive pomace is corrosive and requires a lot of heating while a good quality oil doesn’t require heating at all. With palm and olive both being vegetable oils, their effect on the reaction is not the same, and their benefit to the skin is totally different. My advice to you is to stay away from soaps that market themselves as “100% vegetable oil” because most probably they’re hiding another flaw in their recipe. As a rule of thumb, to produce a good quality soap choose an oil that is both fluid at ambient temperatures and of moderate acidity.