No lye, no soap. Almost every soapmaker has stated this moniker at one point or another. And it’s true, you cannot make soap without lye. Both the handmade and commercial processes use lye to make soap. But, what is lye and what does it do in soapmaking? This article will look at both sodium hydroxide and potassium hydroxide.
Merriam-Webster Dictionary defines lye as a strong alkaline liquor rich in potassium carbonate leached from wood ashes and used primarily for making soap and washing. More broadly, it’s a strong alkaline solution (sodium hydroxide or potassium hydroxide), also known as a solid caustic (NaOH).
If that made zero sense to you, don’t fret! I’ll explain the two types of lye, how it is made, what it does in soap, how it’s listed on labels, and how to tell whether a bar is still lye-active (however unusual my process may be).
What Is Lye?
The FDA 21 CFR 184.1763 defines lye (sodium hydroxide, NaOH) as a white, odorless solid or a 50% solution, commercially produced via electrolysis of sodium chloride or by reacting calcium hydroxide with sodium carbonate.
It is labeled “Generally Recognized as Safe (GRAS) for use as a direct food ingredient (pH control agent/processing aid) following current good manufacturing practices,” which is a long-winded way to say that lye is considered safe to use in processing food (pretzels, ramen, olives, etc.).
Lye (sodium hydroxide) is essential for soap-making because it triggers a chemical reaction called saponification, transforming fats and oils into soap molecules, glycerin, and water.
During this process, the caustic lye is fully neutralized and consumed, leaving none in the final, safe, solid bar of soap. It acts as the catalyst, enabling oils to mix with water and creating the actual soap structure, without which you can’t make traditional soap.
Lye is available in two forms: sodium hydroxide NaOH and potassium hydroxide KOH.
Sodium Hydroxide (NaOH)
Sodium hydroxide (NaOH, CAS Reg. No. 1310-73-2) is a highly caustic, inorganic, and strong base, commonly known as lye, soda lye, white caustic, or caustic soda. It is a white, hygroscopic, water-soluble compound, typically available in pellet, flake, or granular form.
Before it gets to a soapmaker, sodium hydroxide is prepared commercially by electrolysis of a sodium chloride solution and by reacting calcium hydroxide with sodium carbonate.
From there, it is widely used in the production of handmade and industrial soap bars, as a cleaning agent, in the manufacture of paper and textiles, and for water treatment (source).
Potassium Hydroxide (KOH)
Potassium hydroxide (KOH, CAS Reg. No. 1310-58-3) is a highly caustic, inorganic, and strong base, commonly known as lye, caustic potash, potash lye, or potassa. It is a white, hygroscopic, and water-soluble compound available in pellets, flakes, sticks, lumps, or powder.
Potassium hydroxide is obtained commercially from the electrolysis of potassium chloride solution in the presence of a porous diaphragm.
KOH is primarily used in industrial liquid or soft soap production, as an electrolyte in alkaline batteries, as a cleaning agent, and in agriculture as a fertilizer (source).
How Was Lye Made Traditionally?
Historically, KOH (potassium hydroxide, also known as caustic potash) was produced by filtering wood ashes to obtain potassium carbonate, which was then reacted with calcium hydroxide.
This causticization method was dominant until the late 19th century. Wood ash was gathered and mixed with water. Then it was leached through a filter made of straw and boiled to concentrate the potash.
If it wasn’t going to be used right away, it could be boiled down until the water evaporated and caustic flakes appeared. This method was labor-intensive and dangerous, but it’s the process you may have read about in pioneer stories, like Little House on the Prairie.
Historical production of NaOH (sodium hydroxide) has undergone several revisions throughout history. It started with a similar wood ash leaching process, then was treated with slaked lime to produce NaOH (sodium hydroxide).
This is known as the Lablanc Process (developed by Nicolas Leblanc in 1791) and was primarily utilized in the Late 18th-19th centuries.
The Solvay Process (late 19th Century) replaced the Leblanc process for mass-producing sodium carbonate, which was subsequently converted to caustic soda. This method produced a more effective, efficient, and pure product and was also more cost-efficient.
Toward the end of the 19th century, the electrolysis of sodium chloride solutions was introduced and became industrially viable, with early plants, such as those by Solvay, adopting this method by 1898.
Today’s sodium hydroxide is much cleaner, more uniform, more stable, and easier to use thanks to the newer technology used to create it.
Are the Two Types of Lye Produced Differently Today?
Both types of lye are produced in factories today through similar industrial electrolysis (chlor-alkali) processes that electrolyze brine from salt water. They differ in raw materials and cost.
KOH utilizes potassium chloride (KCl), while NaOH uses the more abundant sodium chloride. Lye produced today is cleaner and much more standardized, resulting in a far more predictable product than in the past.
Is There Lye In Soap?
Lye is not present in the final soap product. It should be completely used up during the saponification process. This fact leads to interesting discussions about whether to name the ingredient on the label or ignore it.
Over the years, I have heard many claims about “lyeless” soap. Some soapmakers claim, “I don’t have lye in my soap.” They are technically correct, but the statement is a little misleading.
Lye was used to make the soap, but it does not exist in the finished product. The rationale seems to be: since there will be no lye in the final product, why bother mentioning it?
I have also heard people say, “I don’t use lye to make my soap.” After a bit of conversation (and by looking at their soap), it is usually very clear that they are using a ready-made soap base (known as melt-and-pour) to make their soap. They did not make their soap base; however, it was made in a factory where lye was utilized to saponify the fats.
How Do Companies List Lye On Their Labels?
I have also read soap labels with “Lye-Free” prominently printed on the front, listing ingredients like sodium tallowate/potassium tallowate (saponified tallow) or sodium oliveate/potassium oliveate (saponified olive oil).
These are simply the technical terms for tallow/olive oil saponified by sodium hydroxide or potassium hydroxide.
If sodium hydroxide is used, you will sometimes see ingredients listed as “sodium olivate, sodium palmate, sodium cocoate, sodium tallowate, sodium soyate, and so on. It’ll be listed in the format: the word sodium-oil name, combined with -ate.
If potassium hydroxide is used, you will sometimes see ingredients listed as “potassium olivate, potassium palmate, potassium cocoate, potassium tallowate, potassium soyate, and so on. Again, following the same naming format of the word potassium-oil name, combined with -ate.
None of these labeling methods is wrong. But consumers need to be able to decipher the labels so they know exactly what is in that soap.
By the way, here is a handy link to decoding ingredients and products: https://incidecoder.com/
Did you know that not everything you consider soap is actually soap? Learn more in Handmade Soap vs. Commercial Soap: What’s the Difference?
Can Lye Remain In Soap and Hurt My Skin?
Notice I use the term “when correctly made” a lot? Soap must be formulated correctly so that the lye molecules do not outnumber the required number of oil molecules. Then, when those ratios are adequately met, you get a pleasant bar of usable soap.
If there is too little lye, you end up with soap that breaks down and goes rancid quickly because of the abundance of unsaponified fats and oils. Yuck!
If you have too much lye, you get a caustic bar of soap that is likely to cause skin irritation and damage because there aren’t enough oils or fats to neutralize it. Ouch!
This is easy to mess up because every oil has a different saponification value that must be met for proper saponification to occur. You cannot use the same amount of lye in a pure olive oil soap as you do in a soap containing a different oil or multiple oils.
Each value needs to be calculated and added for each oil. It can get complicated, and miscalculations do occur.
How Can I Know If a Bar of Soap Is Safe?
You’ll know a bar of soap is lye-active when your skin tingles and continues tingling after use. If a soapmaker’s calculations were really off, the bar would irritate your skin within a few seconds of picking it up.
If you feel a tingling or burning sensation after holding or using a bar of soap, pour or spray white vinegar on the area. White vinegar (lemon juice works too) neutralizes active lye.
How Does Cadron Creek Soapworks Ensure the Lye Has Fully Saponified?
Lots of experience and multiple test stages. I’ve made thousands of bars over the past few years, and I have learned through trial and error when a soap is not fully saponified. We test our soap batter right after it has been cooked, just before molding.
The most obvious way is to just look at it. If it is not thoroughly saponified, it will look a bit like applesauce: opaque, loose, a little runny. When it is fully saponified, my soap looks like Vaseline: semi-translucent, slick, and stiff.
Also, I can lick the soap. Bet that got your attention. I take a small sample of the batter and touch it to my tongue. If it zaps me like a 9V battery, it is still lye active, and the batter goes back into the oven for another 20 minutes or until it tests negative (the test sample goes into the trash).
Honestly, I can also feel the zap on the inside of my wrist (I test the wrist more than the tongue), but I like to see the look on people’s faces when I describe how to zap test soap batter by licking it.
Please do not go around licking random soap bars at craft fairs and farmers’ markets. That method works best when the soap is still malleable, before it hardens. It is not a surefire way to test solid bar soaps that are already hardened and cured. But it would be a great conversation starter and a memorable way to get kicked out of an event.
We handle our soaps multiple times during cutting, drying, labeling, and packaging… we are always on the lookout for a bar or batch that does not feel right. It is easy to feel the discomfort of the lye if it is still present.
Those soaps are set aside for a longer curing time and tested at a later date. I have only had this happen ONCE in 15 years of soapmaking.
In a Nutshell
- Lye is caustic and should be handled with care.
- Lye is necessary for making soap.
- Sodium hydroxide is best for making bar soap.
- Potassium hydroxide is better suited for making liquid soap.
- Accidental exposure to lye can be neutralized by vinegar or lemon juice.
- Good soapmakers will always ensure their product is consumer-ready.
- Never lick a stranger’s soap.
Sources:
https://www.merriam-webster.com/dictionary/sodium%20hydroxide
https://www.merriam-webster.com/dictionary/sodium%20hydroxide
https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-184/subpart-B/section-184.1763
https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-184/subpart-B/section-184.1631