Castile Soap vs Detergent: What You Need to Know

Difference between castile soap and other liquid soap

I was chatting to a customer this week about castile soap, and she asked a very good question. What is the difference between castile soaps and other liquid soaps?

What is castile soap?

Castile soaps are made from vegetable oil, rather than animal fats or synthetics. They originally came from Spain, from the Castile region. Castile soaps always used to be made with olive oil, but nowdays they can be made with any plant based fat. You'll find coconut, jojoba, hemp oil, palm oil and more.

Castile soaps come in both solid and liquid forms. Taking Dr Bronner's as an example, you'll find that their range includes bar soaps and liquid soaps. Both of these are castile soaps. The difference between a bar and a liquid castile soap is the chemical that's used to change the fat into soap. This process is called saponification.

How are soaps made?

Soap needs three basic ingredients: animal or plant oil, water and lye. When these are mixed together in the right proportions, a chemical reaction called saponification takes place, and you have soap.

Liquid soaps use potassium hydroxide to saponify, and bar soaps use sodium hydroxide (lye). Although the potassium hydroxide and sodium hydroxide are both highly caustic, the chemical process of saponification completely removes these chemicals. Through the magic of chemistry, you're left with a mixture of water, soap and glycerin.

Castile soaps are pure, safe and can be formulated with extra oils to make them moisturising, rather than drying, on your skin. Adding the extra oils is called superfatting.

What are other soaps made from?

Whereas a castile soap is made from plant based fats and oils, other natural soaps are made from animal fats like lard or tallow.

If a liquid soap isn't made with either plant or animal based fats, they're not soaps at all, they're detergents. Detergents are different to soaps in that they’re often synthetic and they work slightly differently.

What is a detergent? A detergent is based on surfactants, which are generally synthetic, but can also be derived from natural sources (cocamidopropyl betaine is derived from coconuts, for example). This doesn’t make them natural, as they’re still heavily processed, but they do come from natural sources.

How to tell the difference between soaps and detergents?

Liquid soaps will list potassium hydroxide, bar soaps will list sodium hydroxide in their ingredients lists. Detergents will use some kind of surfactant. These might be naturally derived, like cocoamidopropyl betaine or polyglucoside. Or they could be synthetic, like sodium lauryl sulfate (SLS).

So if you’re seeing ‘soaps’ with surfactants in them, they’re actually detergents. Generally, shower gels, liquid hand soaps are detergents rather than soaps.

Should you use a soap or a detergent?

Castile soaps are made from natural ingredients, that are pretty simple, and break down easily in the environment. There are lots of ways you can use castile soaps, so you can cut down on the amount of products that you're using.

If you're in an area that has hard water, you may find that soap doesn't work very well. The high amount of calcium and other minerals in hard water reacts with the soap to form a scum. This inhibits the surfactant properties, so it doesn't foam up as easily. You use more soap to make up for this and end up with a film of scum on your tiles and sink. You'll probably find that using a detergent based cleanser designed for hard water areas will give you a better result.

Castile Soap vs Dish Soap

Can you use castile soap to wash dishes? The short answer is yes. However, if you're in a hard water area, you may find that using castile soap on dishes leaves a soap scum.

If you're a fan of Dr Bronner's like I am, you may want to use their Sal Suds instead. This is a detergent, not a castile soap.

You may also find that soap irritates sensitive skin, especially facial skin. Finding a sensitive skin cleanser that is detergent based might be a better choice. This is why you'll sometimes see products that are advertised as being 'soap free'.

It also depends on what you're using the product for. You may choose to use a castile soap for personal cleansing, but stick to detergents for household cleaning. (Although Dr Bronner does say there are at least 18 uses for castile soap!).

The key, as always, is to read the ingredients on the back of the label. Ignore the marketing claims on the front and find out what's really in the products that you're using.

FAQs about Castile Soap

Is castile soap sulfate free?

Pure castile soap is made from animal or vegetable oil, an alkali (either sodium hydroxide or potassium hydroxide) and water. That means that it's sulfate free.

Is there a difference between glycerin soap vs castile soap?

Not really. All real soaps are glycerin soaps, as glycerin is produced as part of the soap making process. Transparent soaps are often confused with glycerin soaps, but it's just real soap where sugar and alcohol are added to stop the soap crystals from turning opaque. When it doesn't crystallise, the soap stays clear.