Identifying soap as being glycerin or lye soap can be misleading--in the production of lye soap, the lye--a caustic sodium product--is cooked out of the finished bar of soap. Lye is combined with lard and water to produce a soap when cooked. This is an effective method of producing soap, and has been used for years--homesteaders in the United States were accustomed to using the fat from animals to make their own soap. There is a little bit of risk when making lye soap because of the lye's caustic and corrosive properties, but the list of ingredients itself is very simple.
Lard
Lard is animal fat and/or plant oils. It can be purchased as a block at grocery stores. It is a common cooking ingredient, but in soap it is key to the chemical reaction that results in the unique properties that comprise soap. Most recipes call for four times as much lard as lye. Once the cooking process is completed, there actually is no lard remaining in the soap, although some of the properties of fat remain in the soap--lard is what gives soap its slick texture.
Lye
Lye is a common name for sodium hydroxide--the chemical used to make bar soap--or potassium hydroxide, used in liquid soap. Lye reacts with lard in a simple chemical process called saponification, in which these ingredients are transformed into glycerin and salt. This chemical reaction enables soap to feature both hydrophobic and hydrophilic chemicals. Hydrophobic molecules are not easily combined with water, while hydrophilic compounds easily bond with water, helping to draw hydrophobic compounds like oils off your body by bonding to them.
Essential Oils
Scent is a common feature of many soaps. The essential oils of numerous plants can be used to give soap an aroma, whether it's lavender, avocado, mint or any other kind. These usually comprise a small amount of the recipe since essential oils are very potent.
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