Updated: Jul 22, 2019
"Do I have to cure my hot process castile soap for a whole year? Can I just put them in a box in the garage and pretend they aren't there? What about a plastic box in the closet?"
I get asked these questions a lot so I thought I would open the topic up for discussion. In regards to curing hot process castile soap, there seems to be a significant debate out there- to cure or not to cure... or to cure for a year. Some say it has to be cured for the average 4-6 weeks, some say 6 months and others say a full year... or even two! I know hundreds of soap makers who will argue to the death that castile soaps absolutely must cure for a year, but when I ask them why, the only response that I get is that "it makes them better" or "because it makes them gentler". But what exactly does this mean, is this statement true and does it have any scientific merit?
I have had different experiences with castile soap and have done a few experiments with water concentrations and temperatures, but my results have not concluded any profound answers. Even after my tests and years of research, I have yet to find a quality and science-backed answer that provides the exact amount of time needed to cure castile soap.
Aw man... Come on! I thought you were going to share an actual answer! Although I can't provide the exact answer, I can tell you what I know for sure. These are the things that I consider when determining how long I cure my castile soaps.
1. Castile soap becomes more gentle over time. Castile soap has an initially higher alkaline pH, with an average of 11.2. Yep! You read that right! The more alkaline pH is because of the high oleic acid content (80%) in olive oil. When oleic acid saponifies, it creates sodium oleate soap that has a pH of 11.2. Over time, and as the soap cures, the pH is lowered and becomes less alkaline. This is because a process called acidification occurs with exposure to carbon dioxide. This process reduces the pH of your soap, making it more gentle by converting some of the alkali to free fatty acids. Without the exposure to carbon dioxide, say, keeping your soap in an enclosed or sealed container, you aren't reaping the benefits of carbon dioxide's acidic nature. We want our castile soaps (actually, all soap), to be exposed to the air from our environment during the cure. This need for air circulation is one of the reasons why Aleppo soap companies stack over 1,000 soaps in such a way that allows air flow.
2. Castile soaps experience water evaporation over time. Soap is a combination of solid soap crystals and liquid. New soaps have more liquid and older soaps have less liquid. Older soaps have less liquid because the water from the soap evaporates until it reaches a point of equilibrium with the moisture concentration of the environment around it. We want our soaps to lose water content through evaporation, especially our castile soaps (talk more about this later), because it not only makes our soaps harder, but also begins a process called precipitation crystallization which allows more of the soap crystals to solidify because there is less water for them to dissolve in. This provides a harder and longer lasting soap, requires less soap for a quality lather and also provides a better and more stable/voluminous lather. Soaps that are kept in closed containers experience what is called a state of dynamic equilibrium, or the state at which water evaporation becomes equal to the rate of condensation in the container. It may or may not be visible, but without the ability for water to escape as a gas (water evaporation), the moisture content remains the same. (Reason number 2 why we want to keep them in an open space, even if its just a closet that is opened every now and then or on a curing rack)
3. Sodium oleate in the liquid phase. Castile soaps have a very high oleic acid content which we know creates sodium oleate. Sodium oleate is one of the largest and most "bulky" soap molecules and it has a funky bending shape because of the double carbon bond. This "odd" shape can prevent it from initially forming a solid crystalline structure with the other saturated sodium soaps, which are more straight in nature due to the lack of double carbon bonds. Instead a large portion of sodium oleate molecules remain in the liquid phase. Also in the liquid phase are some other soap molecules like our stearic and palmitic soaps and other water soluble chemicals like glycerin. As the water in your soap evaporates, these other soap molecules are able to solidify into solid soap crystals. When this happens, we are left with even more oleic soaps in the liquid phase and it creates a better lather and a longer lasting bar.
4. Sodium oleate also changes phases over time. But if castile soaps are mostly oleic acid, won't there be a lot of sodium oleate in the liquid phase? This is one of the reasons a longer curing time may be beneficial. Because there is such a high concentration of sodium oleate soaps, water evaporation is essential. There have been studies that show that sodium oleate doesn't change phases until a very low water concentration is reached. As the water evaporates from your castile soap, the soaps are able to solidify and crystallize. As the water concentration reaches an even lower level of moisture, even more of the sodium oleate soaps are able to crystallize. From my tests, and because we know these things, I can conclude that a water discount and the use of hot process can significantly affect the cure time and reduce the total time necessary for this process to happen.
5. There are subtle differences between hot process castile and cold process castile. Castile soap that is made by hot process is slightly different than castile soap made by cold process, not only in the proces that makes them, but also including chemical composition and soap phases. HP castile soap will already have a low pH when molded and cut because it has saponified. This is compared to castile soap made with cold process which will have an initially higher pH for much longer (often weeks) both internally and on the surface. HP castile soap can be cut and unmolded an hour later, whereas CP castile soap may take a week. By unmolding and cutting the soap faster, it also begins the curing process faster. Castile soap made by hot process undergoes different phase changes than cold process due to the high heat. HP castile soap will also soak up less water and will have a lower wear rate than cold process castile. These are just a few of the differences in process, but the list can continue on and on.
I personally don't cure my castile soaps for a full year and I don't feel the need to. I use a water discount, very high processing temperatures and ensure that my soaps are stored in a dry and well-ventilated area. I make sure that I use the appropriate amount of sodium hydroxide and use a 8% superfat for a mild bar. When my castile soaps no longer have any additional water loss, and after two additional weeks waiting time, then I feel comfortable gifting/selling them. My typical castile soap made at 28% water cures for around 6-8 weeks total.
These are just some thoughts to get the conversation started. The answer to the second half of the question, "Can I store my soaps in a sealed container?" should be very apparent. After the initial curing period, or if your soaps have been out for longer than 6 months, then it might be acceptable to box them up, but I find it best to still allow ample air flow.
I haven't found an exact answer for the amount of time that castile soap should cure from a more scientific standpoint, but I am still looking! If you have any information or resources you would like to share, please do so!
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