Truth about Lye

One of the key ingredients used in making soaps is lye. Lye is also known as caustic soda or sodium hydroxide. The chemical formula of sodium hydroxide is NaOH.

Lye powder
We handle lye when we craft homemade soaps, and that's why it's very important for soap crafters to understand what lye is all about and what it does in soap making.

It is important for two main reasons:

First of all, if you understand the chemical reaction between lye and the fatty acids that you use in making soap, you will know how to control the amount of lye in your soap recipe for desired results.

Second, lye is very caustic and hazardous if handled incorrectly. A few very simple rules in handling lye will make soap making a safe and pleasurable experience.

Uses of Lye

Lye is cheap and easy to produce making it readily available in a wide variety of applications. In fact, lye is so prevalent, that we come across products and foods that have been manufactured and processed using lye, in our daily lives.

Lye is a strong, water soluble alkali or base, that is perfectly suitable for industrial and household cleaning purposes as it readily binds acidic scales forming harmless salts, and removes them. In its pure form, lye is used as a drain cleaner. Lye is so caustic that nothing lives in it making lye an ultimate disinfectant.

Lye or sodium hydroxide has numerous industrial applications. It is used for oil drilling and in oil refineries, biodiesel production, paper manufacturing, plastics, textiles, extraction of aluminum oxide and silicone from earth, rubber recycling, etching and electroplating, and more.

Lye is also widely used in food industry. It's used for softening pretzels and bagels, making noodles, cleaning and chemically peeling fruit and vegetables, processing of cocoa, chocolates, and soft drinks.

One of the most important use of lye is making soaps and detergents.

A Bit of Chemistry

Lye is one of the most basic substances on Earth having pH 14. Another chemical with pH 14 is potassium hydroxide or caustic potash. Interestingly, potassium hydroxide is also used to make soaps and it was called lye in the past. The difference between the two bases is that sodium hydroxide, or lye, is used for making solid soap (bar soap), and potassium hydroxide is used for making liquid soap.

How is it possible that the same chemical that is used as an oil drilling agent or a harsh drain cleaner, is also used to make skin soothing soaps and delicious food?
Isn't it extremely dangerous for us to eat pretzels that have been in contact with the substance that is also used in oil refineries and paper mills?

No it's not dangerous at all. Sodium hydroxide is a pure chemical. Because it is highly reactive and water soluble, it does not leave noxious byproducts from reactions with acids. The resulting product of chemical reaction with lye depends only on the acid that is used in the mixture.

For instance, when sodium hydroxide is combined with hydrochloric acid, the resulting products are table salt and pure water.

NaOH + HCl => NaCl + H20

In soap making, you combine lye with oils and fats which are made up of fatty acids. When lye reacts with fatty acids, it forms stable salts and glycerin as a byproduct. Glycerin is not removed from the soap. It improves the soap because of its beneficial skin moisturizing properties.

The key in soap making is to have the right amount of lye in your oils and fats to make sure that all lye is used up in the soap making reaction called saponification. If you use too much lye in your soap recipe, you will have residual lye in your soap. Your soap will be very harsh and irritating.You don't want to use too little lye either because too much residual fats in the soap will leave fatty film on your skin causing discomfort. Too much fats in soap may also become rancid.

When you shop for lye, you may find different forms of lye, such as flakes, chips, granules, pellets, or a solution. For making soap, you will need to purchase pure 100% sodium hydroxide or food grade lye that comes in flakes or granules.

Storing Lye

Very little lye is used in soap recipes. You will have leftover lye for your next batches of soap. Lye is highly soluble in water and easily reacts with air and should be stored in airtight containers. If lye is exposed to air it will absorb carbon dioxide and moisture from the air. It won't be pure lye any more and your weight measurements will be off. If stored improperly, lye may also clump and become unusable for soap making. When storing lye, make sure it's properly labeled and out of reach of children and pets.

What Equipment is Used with Lye?

What kind of bowls and spoons should be used when working with lye?
Glass, heat proof ceramic or stoneware, enamel, plastic, wood, and steel bowls are fine to use with lye and lye solution. Wood, metal, plastic spoons can also be used. Do not use aluminum with lye. Remember, lye is used to extract aluminum from soil. Lye reacts with aluminum forming toxic gases. Lye also reacts with tin and zinc.


Many people make lye soap at home and it's a lot of fun. Your soap making experience will just as enjoyable practice care and safely when handling and mixing lye.

Avoid Contact with Lye

Lye is a very caustic chemical, which may cause skin lesions if it comes in contact with your skin. It's damaging to the eyes and may cause permanent blindness. Protective goggles, gloves, apron, long sleeved clothing, and enclosed shoes must be worn when working with lye. Never touch your face when you handle lye.

Have lemon juice or vinegar readily available if your skin becomes exposed to lye. If lye comes in contact with skin, it causes itching at first, then burning. Wash the affected area with acid first to neutralize lye, then rinse it with copious amount of water. After washing, rub the affected area with your finger tip. If it feels oily it means that you still have residual lye on your skin. Repeat acid and water wash until it doesn't feel oily any more.

Do not touch lye in the dry form or solution, do not even try to smell or inhale lye or the mixture. Lye can cause irritation in your nose, throat, and lungs. In case you are curious, lye by itself is odorless, it has no smell. Ingestion of lye may be fatal.

Lye can damage walls and flooring. It lye solution is spilled, wipe it off with used rugs and wash it with vinegar and water solution. Make sure to use gloves when cleaning.

Mixing Lye with Water: ALWAYS ADD LYE TO WATER!

When you make soap, you will be mixing lye with water. It is very important to know how to do it.

When lye is mixed with water, it produces a lot of heat in the exothermic reaction. Mixture of lye and water can cause severe burns and it can ignite inflammable materials. When lye reacts with water, it produces noxious vapors. Make sure your room is well ventilated when you work with lye in your soap making process.

Volcano Effect is what happens if water is added to lye. It is because when water is added to lye, the heat producing reaction causes a lot of heat and pressure build up in the bottom layer of lye. When the pressure rises, it causes explosion of hot lye like a volcano.
I will be very happy if you take home this one very important message about lye. NEVER add water to lye!

When mixing lye and water ALWAYS ADD LYE TO WATER!