What Is Lye?

Lye, also known as caustic soda, is a basic ingredient in soapmaking. But what really is lye? Aside from soapmaking lye is also used for food curing, known as food-grade lye, and also for tanning hides and biodiesel production.

Lye in Acid-Base Chemistry

Let us first try to understand what lye is through acid-base chemistry. Lye is the common name for sodium hydroxide, one of the strongest bases in nature, a chemical compound that can dissolve fats. Lye is the opposite of hydrochloric acid, one of the strongest acids, a chemical compound used in dissolving metals and some salts. Sodium hydroxide is useful for cleaning jobs inside the house such as drain cleaning, dissolving hair that clogs in the shower drain and dissolving fats deposits in the kitchen. Hydrochloric acid, on the other hand, is used mostly for cleaning concrete and removing rust from iron.

However, lye and hydrochloric acid are both dangerous to handle and can be destructive to the skin. It is always important, that when handling both chemicals, to use gloves and/or goggles to protect the eyes. In case you accidentally get sodium hydroxide or hydrochloric acid on your skin, reduce the effect by using vinegar and baking soda, respectively. The fastest first aid is to put the affected part under warm running water for about 5 minutes until all the chemical is washed off.

Lye in Soapmaking

In soapmaking, there are two types of lye being used: sodium hydroxide and potassium hydroxide. Sodium hydroxide is used to make solid soap while potassium hydroxide is used to make liquid soap.

Sodium hydroxide, commonly known as caustic soda, is mainly used as drain cleaners or de-cloggers. It is also used as an ingredient in the production of methamphetamine, an illegal and dangerous drug. Thus keeping large quantities of lye is controlled or regulated in some areas. However, for soapmaking, only pure sodium hydroxide is used. Lye comes in different solid dry forms such as powder, micro beads, granules, flakes, pellets and in liquid form or as a solution mixed with water.

Potassium hydroxide, commonly known as caustic potash, is mainly used in commercial chemicals such as fertilizers, alkaline batteries, potassium carbonate, agrochemicals and dyes. It is harder to find than sodium hydroxide and are usually used in manufacturing chemicals. Soaps made of potassium hydroxide are softer and can easily dissolve in water than sodium hydroxide soaps.

In soap recipes, sodium hydroxide cannot be substituted with potassium hydroxide and vice versa because soapmaking have different quantity requirements of these two chemicals depending on the kind of soap being manufactured. In addition, the quantities required for soap saponification differ when using caustic soda and caustic potash.

Lye Handling and Storage

Both types of lye are dangerous to both humans and surfaces like metal, paint, cloth, plastic and aluminum. It should be stored safely in a cool, dry place and away from the reach of children. It should be properly stored and labeled. Even the fumes of lye can be dangerous and must not be used when there is not enough air circulating around the area. This means that you should not close the door and windows in the toilet when using lye to de-clog the shower drain. Lye is not combustible when dry but when it comes in contact with water it can ignite and can start a fire.