The biggest difference lies in the temperature of the saponification reaction:
Handmade soap is at 60 ° C or less, industrial soap is often in the 90-105 ° C or so, the difference in temperature directly affects the reaction rate. For most chemical reactions, the higher the temperature, the faster the reaction, and roughly speaking, every 10°C increase in temperature roughly doubles the reaction rate.
In other words, in the case of handmade soap, the entire reaction is slow, and saponification is not completed within 20 or 30 minutes, whereas the industrial process ensures that most of the raw materials are converted into products.
Is handmade soap less alkaline than industrial soap?
During the process of making handmade soap, the handmade soap that solidifies after being poured into the mold is not fully reacted and still contains a lot of raw materials that have not had time to react, and in order to be fully reacted, all handmade, cold-produced soaps are referred to as being "left to ripen for a few weeks". Unfortunately, even after a few weeks, a small amount of sodium hydroxide may still be left in the soap that has not reacted. How can washing your face with such a strong alkaline soap be gentler than industrial soap?
However, there is a process of washing industrial soap with salt water, which aims to wash away the unreacted sodium hydroxide, at least most of it; and blowing out the soap base with hot air also helps to reduce the strong alkali residue in industrial soap.
Is handmade soap more natural?
Handmade soap retains glycerin, the "all-natural glycerin" that is essential for moisturizing the skin. But if you think about it, most people wash their face with soap, and then use water to wash the soap foam, and this process of washing water will also bring the glycerin in the foam and wash away, its moisturizing effect may not be effective.
Industrial soap, generally will also add a lot of extra fragrances, plant extracts, glycerin and other substances. On the contrary, handmade soap has a lot of strong alkali throughout the manufacturing and maturing process, and these fragrances and "natural nutrients" are likely to react and decompose under the action of sodium hydroxide, and will be destroyed to a much higher degree.
Cold, handmade soaps tend to have higher levels of strong alkalis that cannot be accurately detected, so using them to wash your face may not be a good idea. If you do want to use it for washing your face, it's best to leave it out for a longer period of time. In addition, if you want to use pH paper to test the pH of handmade soap, you may want to drill a few small holes in different parts of the soap, and take the sample out and dissolve it together before you measure it.