The Chemistry of Soap

July 08, 2011

A reoccurring query that we encounter in relation to our soap is about the use of lye. Generally folks don’t understand what it is and are afraid of it. The word lye, in and of itself, sounds ominous; most people know it as drain cleaner or from the movie Fight Club. After trying to answer questions for people and also to dispel fears, we have come up with a hopefully simple way of explaining lye and it’s role in soap.

First, lye is sodium hydroxide (NaOH) or potassium hydroxide (KOH). It is a alkaline substance that is commercially manufactured, though previously it was leached from hardwood ashes. In soap making, we use a food grade lye because it is more uniform and meets guidelines set by the FDA. Make no mistake, lye is corrosive and it will react with organic material. Which is why it works for soap making but also why you should wear gloves if you handle it.

Does your soap have lye in it?

This is THE most common lye question that we get. And the simple answer, no. Usually when people ask this question, what they really mean is, do you use lye to make the soap? And the answer to that is, yes. Saponification is the process of producing soap by combining fats with lye. The fats that we use are organic plant oils and butters. The lye breaks apart the triglyceride (the scientific term for fats) and what is left is the fatty acid salt and glycerol. The fatty acid salt is the cleaning component of soap and the glycerol is responsible for moisturizing. So, even though lye is used to make soap, it is no longer present in the the finished product because something new is produced, soap! See the following drawing:

(this is saponification)

In our ingredients, we don’t list lye as an ingredient, instead we list the plant oils and butters that we use (completely vegan) as being saponified. This is the most accurate way of describing this process, because technically the fats are a new substance (soap). There is not a single soap that exists that was not made with sodium hydroxide or potassium hydroxide. It’s impossible since by definition soap is this process.

(oils and butters melting)

(Jeff dissolving lye into water)

(combining lye solution and fats, saponification begins)

(pouring into the mold)

(cutting the dried soap)

Okay, so one more little note. If you’re still feeling confused about what lye is, and you’re concerned because it is a chemical compound, think about the following. Baking soda (sodium bicarbonate or NaHCO3) is a chemical compound that most of us are not worried about or confused by. It is a commercially produced ingredient that we use in baking, to brush our teeth, to deodorize our refrigerators, and clean our homes. If you are feeling distant from your high school chemistry class, check out this video to see another familiar and innocuous chemical reaction.

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