Lye is a hydroxide, eithersodium hydroxide orpotassium hydroxide. The wordlye most accurately refers to sodium hydroxide (NaOH),^{[citation needed]} but historically has been conflated to include other alkali materials, most notably potassium hydroxide (KOH). In order to distinguish between the two, sodium hydroxide may be referred to as soda lye while potassium hydroxide may be referred to as potash lye.

Traditionally, it was obtained by usingrainwater toleach wood ashes (which are highlysoluble in water and stronglyalkaline) of their potassium hydroxide (KOH). Acausticbasic solution is produced, calledlye water. Then, the lye water would either be used as such, as forcuring olives before brining them, or be evaporated of water to producecrystalline lye.^[1][2]

Today, lye is commercially manufactured using amembrane cellchloralkali process. It is supplied in various forms such as flakes, pellets, microbeads, coarse powder or asolution. Lye has traditionally been used as a major ingredient insoapmaking.

The English wordlye/ˈlaɪ/ hascognates in allGermanic languages, and originally designated a bath or hot spring.^[3]

Lyes are used tocure many types of food, including the traditional Nordiclutefisk,olives (making them less bitter), cannedmandarin oranges,hominy,lye rolls,century eggs,pretzels, candied pumpkins, andbagels. They are also used as a tenderizer in the crust of baked Cantonesemoon cakes, in "zongzi" (glutinous rice dumplings wrapped in bamboo leaves), in chewy southernChinese noodles popular in Hong Kong and southern China, and in Japaneseramen noodles. Lye provides the crisp glaze on hard pretzels. It's used inkutsinta, a type of rice cake from the Philippines together withpitsi-pitsî.^[4] In Assam, north east India, extensive use is made of a type of lye called khar in Assamese and karwi in Boro which is obtained by filtering the ashes of various banana stems, roots and skin in their cooking and also for curing, as medicine and as a substitute for soap. Lye made out of wood ashes is also used in thenixtamalization process ofhominy corn by the tribes of the Eastern Woodlands inNorth America.

In theUnited States, food-grade lye must meet the requirements outlined in theFood Chemicals Codex (FCC),^[5] as prescribed by theU.S. Food and Drug Administration (FDA).^[6] Lower grades of lye that are unsuitable for use in food preparation are commonly used asdrain cleaners and oven cleaners.^{[6][page needed]}

Both sodium hydroxide and potassium hydroxide are used in makingsoap. 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.^{[citation needed]}

"Hot process" soap making also uses lye as the main ingredient. Lye is added to water, cooled for a few minutes and then added to oils and butters. The mixture is then cooked over a period of time (1–2 hours), typically in aslow cooker, and then placed into a mold.

Lyes are also valued for their cleaning effects. Sodium hydroxide is commonly the major constituent in commercial and industrialoven cleaners and cloggeddrain openers, due to itsgrease-dissolving abilities. Lyes decompose greases viaalkaline ester hydrolysis, yieldingwater-soluble residues that are easily removed by rinsing.

Sodium or potassium hydroxide can be used to digest tissues of animal carcasses. Often referred to asalkaline hydrolysis, the process involves placing the animal carcass into a sealed chamber, adding a mixture of lye and water and the application of heat to accelerate the process. After several hours the chamber will contain a liquid with coffee-like appearance,^[7][8][9] and the only solids that remain are very fragile bone hulls of mostlycalcium phosphate, which can be mechanically crushed to a fine powder with very little force.^[10][11] Sodium hydroxide is frequently used in the process of decomposingroadkill dumped in landfills by animal disposal contractors.^[8] Due to its low cost and easy availability, it has also been used to dispose of corpses by criminals.Italianserial killerLeonarda Cianciulli used this chemical to turn dead bodies into soap.^[12] In Mexico, a man who worked for drug cartels admitted to having disposed of more than 300 bodies with it.^[13]

A 3–10% solution ofpotassium hydroxide (KOH) gives a color change in some species of mushrooms:

• InAgaricus, some species such asA. xanthodermus turn yellow with KOH, many have no reaction, andA. subrutilescens turns green.
• Distinctive change occurs for some species ofCortinarius andboletes

When a person has been exposed to lye, 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 as well as contacting emergency services.^[14]

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. Adding lye too quickly can cause a runaway thermal reaction which can result in the mixture boiling or erupting.

Lye in its solid state isdeliquescent and has a strong affinity for moisture in the air. As a result, lye will dissolve when exposed to open air, absorbing large amounts of atmospheric moisture. Accordingly, lye is stored in air-tight (and correspondingly moisture tight) containers. Glass is not a good material to be used for storage as severe alkalis 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.

The majority of safety concerns with lye are also common with most corrosives, such as their potentially destructive effects onliving tissues; examples are theskin,flesh, and thecornea. Solutions containing lyes can causechemical burns, permanent injuries, scarring andblindness, immediately upon contact. Lyes may be harmful or even fatal if swallowed; ingestion can causeesophageal stricture. Moreover, thesolvation of drysolid lye is highlyexothermic and the resultingheat may cause additional burns or ignite flammables.

The reaction between sodium hydroxide and some metals is also hazardous.Aluminium,magnesium,zinc,tin,chromium,brass andbronze all react with lye to producehydrogen gas. Since hydrogen isflammable, mixing a large quantity of lye with aluminium could result in an explosion. Both the potassium and sodium forms are able to dissolve copper.

• Slaked lime (calcium hydroxide)

• McDaniel, Robert (1997).The Elegant Art of Handmade Soap: Making, Scenting, Coloring, and Shaping. Iola, WI: Krause Publications.ISBN 0-87341-832-8.

• "Federal Food, Drug, and Cosmetic Act". US Food and Drug Administration.
• "Food Chemicals Codex". United States Pharmacopeia. Archived fromthe original on 1 February 2012. Retrieved30 January 2008.