The Afshar Group

How is Bar Soap Made?

October 13, 2020

The first time I made soap, I understood immediately that the process of creating handmade soap is one where science and art come together. After lots of hours in the chemistry lab during my premed years, I knew that the chemistry of soap making would be uncompromising: you goof the recipe, you end up with #notsoap. But once you’ve nailed the formula, the options for creativity with colors, textures, and scent are endless—and endlessly fun.

Bar soap is made by combining oils or solid fats (in our case, plant fats) with lye (sodium hydroxide, or NaOH). The oils are heated, allowing any solid oils like shea butter or coconut oil to melt. Meanwhile, the lye gets added to water to make a solution. When both the oils and the lye are at the right temperature (90-110 degrees Fahrenheit), they are mixed together until they start to bond together in the process called saponification.

People often ask if we use lye in our soap because many have the impression that lye is a toxic chemical. In fact, lye is used in all soap making—if it’s not made with lye, it’s not technically soap. While lye is a chemical manufactured in a laboratory, it’s really just a salt made of sodium, hydrogen, and oxygen, and is “toxic” only because of its high pH. The pH of lye is 14, neutral pH (where our bodies function) is 7, and the pH of hydrochloric acid is zero. If pure lye touches your skin, it will burn the same way pure acid will. This is also the reason your Grandma Louise’s “lye soap” didn’t feel good—she probably made lye-heavy soap, where the ratio of fats to lye was incorrect, leaving leftover lye in the finished bar. As a result, the bar would have had a high pH, which can irritate the skin.

The good news is that we have our soap science figured out, and even though lye goes into the soap, it’s not present in the final bar. (Chemistry is actually just poorly marketed magic.) The lye, comprising a sodium molecule, a hydrogen molecule, and an oxygen molecule, gets disassembled and rearranged in cold-process soap making into soap molecules and glycerin. The soap molecules work by sticking their little tails into grease and dirt, and then letting themselves get rinsed down the drain with the water. The glycerin molecules don’t rinse away as much, and work throughout the day to attract water to your skin.

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