Lye is a term that can refer to the liquid obtained by leaching ashes (containing largely potassium carbonate or "potash"), or to a strong alkali which is highly soluble in water producing caustic basic solutions. It is commonly the alternative name of sodium hydroxide (NaOH) or historically potassium hydroxide (KOH).
Today, lye is commercially manufactured using a membrane cell chlor-alkali process. It is one of the highest volume industrial chemicals with world wide annual production of approximately 40 million tons. It is supplied in various forms such as flakes, pellets, microbeads, coarse powder or a solution.
Lye is used to cure many types of food, including lutefisk, olives (making them less bitter), canned mandarin oranges, hominy, lye rolls, century eggs, and pretzels. It is also used as a tenderizer in the crust of baked Cantonese moon cakes, in "zongzi" (glutinous rice dumplings wrapped in bamboo leaves), in chewy southern Chinese noodles popular in Hong Kong and southern China, and in Japanese ramen noodles.
In the United States, food-grade lye must meet the requirements outlined in the Food Chemicals Codex (FCC), as prescribed by the U.S. Food and Drug Administration (FDA). Lower grades of lye which are unsuitable for use in food preparation are commonly used as drain de-cloggers and oven cleaners.
Both sodium hydroxide and potassium hydroxide are used in soap making. Sodium hydroxide is often used to make solid soap while potassium hydroxide is used to make liquid soap. Soaps made of potassium hydroxide are softer and can more easily be dissolved in water than sodium hydroxide soaps.
When used in making soap, sodium hydroxide cannot be substituted for potassium hydroxide and vice versa because soap making recipes will have different quantity requirements for 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 hydrated potash.
The "Cold Process" method is often used to make soap from lye. The lye is mixed with water and then base oils, butters, and fragrances are added to give the soap depth. The lye forms with the oils during the process and causes the oils to harden over time.
Lye is also valued for its cleaning effects. It is commonly the major constituent in commercial and industrial oven cleaners and clogged drain openers, due to its grease-dissolving abilities. Lye decomposes greases via alkaline ester hydrolysis, yielding water soluble, easily removed (e.g., rinsed away) residual substances.
As is common with other corrosives, the major safety concern with lye is its potentially destructive effects on living tissues (e.g., skin, flesh, and the cornea). Solutions containing it can cause chemical burns, permanent injuries, scarring and blindness, immediately upon contact. Lye may be harmful or even fatal if swallowed; ingestion can cause esophageal stricture. Moreover, solvation of dry solid lye is highly exothermic; the resulting heat may cause additional burns, or ignite flammables.
The reaction between sodium hydroxide and a few metals is also hazardous. Aluminium reacts with lye to produce hydrogen gases. Since hydrogen is flammable, mixing a large quantity of lye (e.g., sodium hydroxide) and aluminum in a closed container is dangerous—especially when the system is at a high temperature, which speeds up the reaction. In addition to aluminum, lye may also react with magnesium, galvanized zinc, tin, chromium, brass or bronze—producing hydrogen gas.
Manufacture of sodium hydroxide consumes salt (sodium chloride) and electrical power, both with significant environmental footprints. Release of sodium hydroxide into the environment causes damage as a result of its reactivity.
Personal protective equipment including safety glasses, chemical-resistant gloves, and adequate ventilation are required for the safe handling of lye. When in proximity to lye that is dissolving in an open container of water, the use of a vapor-resistant face mask is recommended. Be aware that adding too much lye to water too quickly can cause the solution to boil and gush.
Lye is a deliquescent and has a strong affinity for air moisture. Lye will deliquesce or dissolve when exposed to open air, absorbing a relatively large amount of water vapour. Accordingly, it should be stored in air-tight plastic containers. Glass is not a good material to be used for storage as lye is mildly corrosive to it. Similar to the case of other corrosives, the containers should be labeled to indicate the potential danger of the contents and stored away from children, pets, heat, and moisture.
- McDaniel, Robert (1997). The Elegant Art of Handmade Soap: Making, Scenting, Coloring, and Shaping. Iola, WI: Krause Publications. ISBN 0-87341-832-8.