Hair relaxers originated in the early 20th century when inventor Garrett Augustus Morgan, while experimenting with a new lubricating liquid for a sewing machine needle, wiped his hands on a wool cloth and returned the next day to find it smoothed out. He then set out to find how the liquid chemical had changed the texture of the cloth, and tested it on his own head before later patenting the first chemical hair straightener ("G.A. Morgan Hair Refining Cream").
Alkali relaxers, such as that one, have active ingredients of sodium hydroxide. In order for a relaxer to straighten hair, the chemical bonds that connect the strands' protein must be broken. To speak scientifically for a moment, most hair has a natural pH level between the 4.5 and 5.5 range – the range, as you may recall from science class, measures how acidic (low pH numbers) up to how basic (high pH numbers) a substance is. Lye relaxers have high pH numbers (12-14) in order to break those bonds and straighten hair, which is very effective but can also be very damaging. For this reason, a base is put on to protect the scalp and neutralizer is applied after the relaxer is rinsed off to get the pH back to its normal level.
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It's important to have someone who's experienced to perform this; one should never apply a relaxer to hair that's already been relaxed, as this will break down the hair significantly and cause breakage. It's necessary to only touch up the new growth that's come in. Lastly, hair must be deep conditioned back to that natural pH level when finished, and it's crucial to keep hair well moisturized if choosing this method for your hair.