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General usage Hair relaxing, or lanthionization, can be performed by a professional cosmetologist in a salon, or at home with relaxer kits purchased from discount stores and pharmacies. As with hair dye, the treated portion of the hair moves away from the scalp as the new growth of untreated hair sprouts up from the roots, requiring periodic retreatment (about every 6–8 weeks) to maintain a consistent appearance.
The relaxer is applied to the roots of the hair and remains in place for a "cooking" interval, during which it alters the hair's texture by a process of controlled damage to the protein structure. The hair can be significantly weakened by the physical overlap of excessive applications or by a single excessive one, leading to brittleness, breakage, or even widespread alopecia.
When the relaxer has worked to the desired degree, the hair is rinsed clean. Regardless of formula, relaxers are always alkaline to some degree, so it is prudent to neutralize or even slightly acidify the hair with a suitable shampoo immediately afterward. The prompt use of hair conditioner is also important in order to replace some of the natural oils that were stripped away by the process.
 Types of hair relaxers  Alkaline and Lye relaxers Alkaline relaxers were informally discovered in the United States during the 19th century when Garrett Augustus Morgan, an African-American, observed that it is possible to change the basic structure of the hair shaft when certain chemicals penetrate the cortical layer. Hair relaxing products often require washing and combing with soap which had been made with excess lye.
A lye relaxer consists of sodium hydroxide (also known as NaOH or lye) mixed with water, petroleum jelly, mineral oil, and emulsifiers to create a creamy consistency. On application, the caustic "lye cream" permeates the protein structure of the hair and weakens its internal bonds, causing the natural curls to loosen out as the entire fiber swells open. No special deactivation step is required after washing the lye cream out, other than the routine pH adjustment and hair-conditioning.
Manufacturers vary the sodium hydroxide content of the solution from 5% to 10% and the pH factor between 10 and 14.
 "Base" and "no base" formulas Entirely distinct from the chemical concept of base as a wider definition for "alkaline", lye relaxers may be labelled as "base" or "no base". In this instance, the "base" refers to a preliminary coating of petroleum jelly onto the scalp to protect it from being irritated or burned by the lye cream. "No base" creams have a lower concentration of lye and may be applied directly to the hair roots without requiring the protective "base" layer, although these weaker products may still irritate the skin of some people who must therefore coat their scalps beforehand anyway.
 "No lye" relaxers Due to increasing awareness concerning the potential dangers of sodium hydroxide found in traditional relaxer formulas, many women have begun abandoning them. "No-lye" relaxers have become the preferred formula for those unwilling to give up relaxers completely. "No-lye" relaxers are of three main types. One type operates on the same general principle as lye relaxers but uses a slightly weaker alkaline agent, such as potassium hydroxide, lithium hydroxide, or guanidine hydroxide. The last of these is not pre-formulated, but rather is generated at the time of use by combining a cream containing calcium hydroxide (slaked lime) with an "activating solution" of guanidine carbonate.
Another type of "no-lye" relaxer uses ammonium thioglycolate, which is also known as perm salt for its use in permanent waves. Perm salt is a chemical reducing agent which selectively weakens the hair's cystine bonds instead of disrupting the entire protein, but strips out the natural oils even more thoroughly than the alkali hydroxide products. Afterward, the thioglycolate must be oxidized with a special solution of hydrogen peroxide or sodium bromate.
Lastly, in most relaxers sold for home use, the active agents are ammonium sulfite and ammonium bisulfite (the two compounds are interchangeable, depending on the surrounding pH). These also selectively reduce the cystine bonds, but are much weaker and work more slowly. Nevertheless, their mild action minimizes (but does not entirely eliminate) collateral irritation to the skin.
 Commercial sale Early in the 1900s hair relaxing products emerged, such as "G.A. Morgan's Hair Refiner Cream." Sale of "lye relaxers" began in 1971 by companies such as Proline. Proline also produced the first commercial "no lye relaxer" using potassium hydroxide in 1973.
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