A lye is a liquid obtained by leaching ashes (containing largely potassium carbonate or "potash"), or a strong alkali which is highly soluble in water producing caustic basic solutions. "Lye" is commonly the alternative name of sodium hydroxide (NaOH) or historically potassium hydroxide (KOH).
Today, sodium hydroxide 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.
Lyes are used to cure many types of food, including lutefisk, olives (making them less bitter), canned mandarin oranges, hominy, lye rolls, century eggs, and pretzels. They are 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 making soap. Sodium hydroxide is often used to make solid soap, while potassium hydroxide is used to make liquid soap. Potassium hydroxide soaps are softer and more easily dissolved in water than sodium hydroxide soaps. Sodium hydroxide and potassium hydroxide are not interchangeable in either the proportions required or the properties produced in making soaps.
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. The gradual chemical reaction between the lye and the fats eventually produces a solid soap.
Lyes are also valued for their cleaning effects. Sodium hydroxide is commonly the major constituent in commercial and industrial oven cleaners and clogged drain openers, due to its grease-dissolving abilities. Lyes decompose greases via alkaline ester hydrolysis, yielding water soluble residues that are easily removed by rinsing.
Sources recommend immediate removal of contaminated clothing/materials, gently brushing/wiping excess off of skin, and then flushing the area of exposure with running water for 15-60 minutes while contacting emergency services.
As is common with other corrosives, the major safety concern with lyes is their potentially destructive effects on living tissues such as skin, flesh, and the cornea. Solutions containing lyes can cause chemical burns, permanent injuries, scarring and blindness, immediately upon contact. Lyes may be harmful or even fatal if swallowed; ingestion can cause esophageal stricture. Moreover, the solvation of dry solid lyes 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 lyes to produce hydrogen gases. Since hydrogen is flammable, mixing a large quantity of a lye such as sodium hydroxide with 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, lyes 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 lyes. When in proximity to a lye that is dissolving in an open container of water, the use of a vapor-resistant face mask is recommended. Adding lye to water too quickly can cause the solution to boil.
Solid lyes are deliquescents and have a strong affinity for air moisture. Solid lyes will deliquesce or dissolve when exposed to open air, absorbing a relatively large amount of water vapour. Accordingly, they should be stored in air-tight plastic containers. Glass is not a good material to be used for storage as lyes are 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.