If your soap is supposed to be natural then why is caustic soda listed as an ingredient?
Caustic soda has a few common names. It is also known as Lye or sodium hydroxide.
While it is commonly listed as an ingredient in soap, this can also be a little confusing. In the soap-making process, lye is an ingredient that is used for the process of saponification to create soap by reacting with fat. Once the saponification process is completed, you technically no longer have caustic soda in your soap. However, labeling requirements combined with ethics results in many soap makers listing caustic soda as an ingredient.
How is Soap Made?
Soap is made by combining fats (animal or plant) with a strong alkaline (Caustic soda). The ingredients react with each in what is known as the saponification process and combine to form a soap. This process takes anything from a couple of hours to days and even months depending on the fats used and the process of soap making that has been used.
In the image above the process of soap production is illustrated. Acid is the fat or oil used in the soap. The most common fats (fatty acids) used in soap are olive oil, palm oil, or coconut oil. Of course, any oil or fat can be used. This fatty acid is combined with a strong base (Caustic soda) using precise calculations to prevent any excess base from being left in the product.
Once combined a process called saponification occurs. This is when the acids and the base become chemically altered and form a salt (soap). Any water formed or used in the process is usually evaporated out of the soap during the curing process of the soap.
Some soap makers will state in their ingredients that they used Palm oil and caustic soda (Sodium Hydroxide, Lye) while others will take a shortcut and express this by the technical name of Sodium Palmate. As such you may come across ingredients with names such as sodium cocoate, sodium stearate, sodium laurate, sodium oliveate, etc.
Traditional Soap-Making without Caustic Soda
You may be asking if it is possible to make soap without using caustic soda.
The short answer is that it is possible but at the same time, it is not practical or advised.
When making soap, the amount of caustic soda used is carefully calculated using the specific fats saponification value. Each fat requires different amounts of caustic soda to be converted into soap. If the incorrect amount of base (lye) is used in the process, the end product may still contain unreacted lye which could irritate or burn your skin. If too little lye is used you will have a soap that leaves a fatty layer on your skin and which does not clean properly.
It is thought that soap was originally invented from the fire cooking days. Fat that was spilled onto the rocks around the fire mixed and reacted with the ash from the fire and eventually formed soap. Someone realized that this foamed and helped to remove dirt and grease etc. As such soap was born.
When using alternative sources of base such as soda ash or ash from your fire, the exact quantities to be used are difficult to determine and therefore you need to overuse the base. Once the saponification is complete the excess base then needs to be carefully and painstakingly washed out of the soap. This is a difficult and time-consuming operation that some die-hard hobby soap makers employ for fun and the challenge but it is not a commercially viable process. It is also not a process that I would place my full trust in.
Do Liquid Soaps Contain Caustic Soda?
As a quick response to this question, all soaps have been through the process whereby fat is combined with a base to form salt, even liquid soaps.
Each fat used and each base used in the above process results in a soap with differing qualities. Soap may be gentle or harsh, hard or soft, give great lather or foam, clean well or clean gently, etc.
Typically natural liquid soap is made using potassium hydroxide as the base instead of sodium hydroxide. However many shower gels, shampoos, and liquid soaps are also made using ingredients such as SLES (sodium lauryl ether sulfate) or SLS (sodium laureth sulfate) or similar. While it is so much easier and more practical to use SLES or SLS in the making of liquid soap, these products are not natural and there are many concerns about the health aspects involved with using these ingredients.
Wrap up about Caustic Soda in Soap.
Water has a chemical composition of H2O. Two parts of hydrogen and one part of oxygen. However, you can not breathe water – you will drown. So too, soap is made from caustic but once soap is made there is no longer any available caustic in the soap. You can not extract the caustic back out of the soap either.
In conclusion, it is safe to say that while caustic is used to make soap, it is no longer an active ingredient in soap. However, labeling laws require you to list it as an ingredient.