Lye is the common name of variousalkaline solutions, includingsoda lye (a solution ofsodium hydroxide) andpotash lye (a solution ofpotassium hydroxide).[1][2] Lyes are used as cleaning products, as ingredients in soapmaking, and in various other contexts.
The wordlye derives from the Germanic root *lau, meaning "to wash" (comparelave,lather) and hascognates in all theGermanic languages.[3] The word could appear in older English texts asley.
Traditional lye-makers leachedwood ashes in water, producing an alkaline liquor rich inpotassium carbonate or potash. Addingslaked lime could increased the alkalinity, causing the solute to become potassium hydroxide or caustic potash.[4][5]
Lyes are used tocure many types of food, including olives andcocoa (making them less bitter), cannedmandarin oranges,lye rolls,century eggs,pretzels, candied pumpkins, andbagels.
Lye is the curing agent and namesake of the traditional Nordiclutefisk, a type of preserved fish. 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. In the Philippines, lye is used in the making ofkutsinta, a type of rice cake, andpitsi-pitsî.[6] InAssam, extensive use is made of a type of lye calledkhar in Assamese andkarwi inBoro, which is obtained from the ashes of banana stems, roots, and skins. It is used in cooking, 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.[7] Chocolate was firstdutched using lye as a way to alkalize the cacao bean.[8]
In theUnited States, food-grade lye must meet the requirements outlined in theFood Chemicals Codex,[9] as prescribed by theFood and Drug Administration.[10] Lower grades of lye that are unsuitable for use in food preparation are commonly used asdrain cleaners and oven cleaners.[10][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.
Lye, like otherhydroxide-based products, is also used as a hairrelaxer and straightener. Lye relaxers break a third of the chemical bonds in the hair strand and cause the hair to swell. Rinsing and then heating the hair straightens the strands.[11] Due to their highpH, they often cause scalp irritation.[12][13] Though no-lye solutions, which useguanidine orlithium hydroxide, produce less scalp irritation, they may cause brittleness due to calcium deposits.[14]
People of African descent have used lye to relax or straighten hair since the nineteenth century.[15] Black men used lye-based formulations toconk their hair, and black entrepreneurs sold such formulations under names like Konkalene;The Autobiography of Malcolm X includes an account of Malcolm X receiving his first lye conk.[16] TheJohnson Products Company introduced industrially produced lye-based relaxers forAfrican American salon hair care in the 1950s; Ultra Wave was marketed to men and Ultra Sheen was marketed to women, and the company represented 80% of the chemical relaxer market through the 1960s. In 1971, Johnson Products became the first black-owned company on theAmerican Stock Exchange.[17]
A number of alkaline substance including sodium hydroxide have been used to adjust thepH during factory processes. It is regularly used to make fruit juices alkaline so that the pulp and sediment will clump, or formflocs, and can be removed.[18] This results in a clear fruit juice that has been clarified. Due to low cost, it has been the main way thatchicken scalding tanks are kept sanitary, with an added benefit that it helps the feathers come off.[19]
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,[20][21][22] 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.[23][24] Sodium hydroxide is frequently used in the process of decomposingroadkill dumped in landfills by animal disposal contractors.[21] 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.[25] In Mexico, a man who worked for drug cartels admitted to having disposed of more than 300 bodies with it.[26]
A 3–10% solution ofpotassium hydroxide (KOH) gives a color change in some species of mushrooms:
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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.[27]
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.
