Updated: Jan 15, 2020
Currently there is a giant, and sometimes very heated, debate in the soap making world: Do hot process soaps need to cure? If you scroll through any of the Facebook soap making groups or try to find an answer online, you will find a variety of different answers, all with unique explanations as to why or why not hot process soap should be cured and for how long. Many hot process soap makers will state that because hot process soaps are pH safe and hard, they do not need to cure and are ready to be sold the next day. Many soap makers, even very experienced and lifelong soap makers, may not have experience with hot process soap or may not understand the process of curing on a more in-depth level. Because there are so many suggestions and opinions out there, to better understand and help answer this question, it is important to take a more comprehensive look at what precisely curing is and why it is done in the first place.
What exactly does “cure” mean? According to Webster Dictionary, to cure is to “prepare or alter, especially by chemical or physical processing, for keeping or use”. In soap making, we cure our soaps over a period of time to alter both chemical and physical properties for later use. There is no special process or requirements to do this; we simply store finished soap in a cool, dry location.
In general, soap is cured so that these things happen:
2. Water evaporation
3. Increased hardness (crystallization)
To review, saponification occurs when oils are combined with NaOH to break down the chemical bond between the fatty acids and glycerol to form a sodium fatty acid salt or soap. The chemical reaction is:
Oil + 3 NaOH = Glycerol + 3 Soap
In cold process soap making, oils and lye (a solution of water and NaOH) are combined and mixed with an immersion blender until an emulsion is formed. The formation of an emulsion is visible by the appearance of “trace”. The emulsion is then mixed with any desired fragrances or additives, poured and designed. Contrary to popular beliefs, the saponification process in cold process soaps is often complete within the first 24 hours, many within the first hour. In hot process soap making, heat is used to accelerate the chemical reaction rate of saponification. This significantly reduces the total reaction time, sometimes down to only a few minutes, depending on the on the initial and sustained temperature. Therefore, a cure time is not necessary for the saponification process to complete in hot process soap (and most cold process soaps!).
Saponification & Water Concentration
If you take another look at the chemical reaction of saponification or think back to our chapter on Water in Soap Making, you will remember that the chemical equation for saponification does not include water (or H2O). There is no set relationship between the number of molecules of water needed per molecule of oil. Water is simply used to disassociate the NaOH molecules so that it can react with the oils.
When we make handcrafted soaps, the final product will contain excess water and water that is not chemically bound. This water will move to the surface of the soap and evaporate. This water evaporates from the soap because the excess water concentration is not in equilibrium (balanced) with the water in the air around it.
When water exists at a temperature below its boiling point, it can exist in both a liquid and gas phase, and it has a temperature-dependent vapour pressure. Vapour pressure is the pressure at which a liquid and its vapour are in equilibrium at a given temperature. At this pressure, the vapour is said to be "pushing" against the atmosphere. The higher the vapour pressure, the faster a liquid evaporates. The temperature-dependent vapor pressure of water represents a point of equilibrium between liquid water wanting to evaporate and water vapour wanting to condense.
When the excess liquid water meets dry air, it is not in equilibrium, or it is not balanced. Because of this, water molecules evaporate off the soap’s surface until the amount of water in the air creates enough vapour pressure to achieve equilibrium. When this equilibrium is reached, the soap will no longer continue to lose significant water weight, although it may fluctuate slightly depending on the water concentration in the air around it. Soaps that are stored/cured in a location that has high humidity will contain more water than soaps stored in a dry, arid climate. This is because the amount of water in the air is different and will cause different points of equilibrium.
The amount of water used in your recipe can be increased or decreased to better help control the soap’s qualities both during and post-production because the amount of water used affects things like the rate and viscosity of trace, the overall reaction rate, the possibility of gelling, the ease of unmolding, time spent in the mold after production, and so much more, all of which were covered in great detail in previous chapters.
If we know that all soaps will experience water evaporation up until the point at which it reaches equilibrium with the environment, we can predict how low, medium, and high-water concentrations will react during the curing process. We can expect low water soaps to harden at a much faster rate and contain a lower water concentration, compared to soaps that are made with a higher water concentration. The more water that is used in your soap, the more amount of time required for it to evaporate.
Let’s explore this concept a little deeper. If you start with a lower water concentration, your soaps will initially be harder and have a lower moisture content, compared to those with a higher starting water concentration. This remains true even after a significant amount of time, often more than eight weeks. (This is true for both hot process and cold process soaps). Kevin Dunn did a very informative experiment regarding starting water content and its effect on moisture content and hardness. He tested four different soap recipes, each with a unique blend of oils, and made these four recipes with three different water concentrations. In almost every soap recipe tested, it took approximately two weeks for the higher water concentration soaps to catch up ad reach the initial hardness and moisture content of the low-water concentration soaps. In almost all the soaps, even after 8 weeks of time, the low-water concentration soaps still remained harder and contained less moisture than those made with higher water concentrations.
(See images and read Kevin Dunn’s “Water Discounting” at http://cavemanchemistry.com/WaterDiscount-Dunn.pdf)
Why is this information important? If a recipe is made with a low-water concentration, compared to one with a higher water concentration, even after the most commonly suggested “four-week week cure time,” the higher water concentration soap may not reach the level of hardness or low weight of the lower water concentration recipe. For example, in the Palm oil recipe in Kevin Dunn’s experiment, it took the high-water concentration recipe four weeks to reach the initial hardness level of the low-water concentration. That is four weeks of cure time required to reach the same amount of hardness that the low-water concentration had on the first day. It took two weeks for it to reach the same moisture content. That is two additional weeks of cure time required for the use of more water.
Heat & Water Evaporation
What about water loss from heat in hot process soap making? The heating of water causes the water molecules to move faster and faster until they break free from the bonds that hold them in place as a liquid. This results in water evaporation and boiling, which occurs at a higher rate as temperature and length of time increases. At sea level, water begins to boil and evaporate at 212F (100C), right below the processing temperature of HTHP/HTFHP.
I tested three different soap making methods with the same recipe and the same 33% water concentration. The three soap making methods were cold process, LTHP (160F) and HTHP (215F). This test was completed with three different recipes, with different combinations of saturated and unsaturated fatty acids. The fatty acid combinations were made from coconut, palm, and olive oil. Immediately after and one week later, the cold process soap had the highest moisture concentration, LTHP was in the middle and HTHP had the least. The increased temperature increased the amount of water evaporation during the process so the bars had less water in them immediately after unmolding. After 4 weeks and 8 weeks, the cold process still had the highest moisture concentration, LTHP was second and HTHP had the lowest. CP lost an average of 15% total weight, LTHP lost an average of 9% and HTHP lost an average of 6%. This means that during the cure time, the HTHP soaps only lost 6% of their total moisture concentration which was almost three times less than the moisture loss by cold process.
What does that mean? They were not only harder, but also contained less moisture after the cook and less water evaporated. Kevin Dunn’s students did a very similar experiment where their HTHP soaps lost 8% and their cold process soaps lost 15%, still, very similar and with the same concluded results.
There are some graphs and charts circulating the internet right now that are very well put together and detailed; however, they can be slightly misleading. Some of them state that because their hot process soaps continue to lose water weight after 4 weeks compared to their cold process soaps which stopped losing water weight, it must mean that hot process soaps must be cured for more time than their cold process soaps. This is not an accurate statement, especially if the water concentration used in the recipe is different, which is most often increased in hot process soaps, especially in fluid hot process soaps.
The total amount of water evaporation and weight loss is primarily dependent on the recipe and recipe water concentration, not the process used. In both soap making styles, water concentrations can be decreased or a “water discount” can be used to affect the initial hardness and moisture content, thus decreasing the amount of time necessary to reach the desired hardness and moisture content goals and increase the rate of precipitation crystallization. It is important to take this into consideration, in addition to the decrease in initial water concentration and total water evaporation in hot process soaps compared to cold process soaps (6% in my HTHP compared to 15% in my tests over 8 weeks). Curing hot process soaps should not be thought of as longer or shorter than cold process, but rather determined by the amount of time necessary for that particular recipe.
All soaps will continue to decrease in moisture concentration until the total concentration reaches a point of equilibrium with the moisture content in the air, regardless of their creation process. Water evaporation and soap moisture content are directly dependent on recipe water content; however, it has been shown that hot process soaps made at the same water concentration will lose water weight more quickly and decrease in moisture content as the temperature increases. If a hot process soap is made at a higher temperature and/or with a lower water concentration, it will increase in hardness and decrease in moisture concentration more quickly than the same soap made with cold process.
As stated at the very beginning of this chapter, cure time is often a very heated debate topic. Many lifelong soap makers haven’t had the opportunity to do an in-depth analysis of the chemistry and physical science of soap making, including the chemistry of curing soap and soap crystallization. This means that there are many different personalized philosophies out there regarding the process of curing, especially crystallization.
What does crystallization mean and how does it apply to soap? A crystal is a substance, typically a solid substance, in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating three-dimensional pattern. Crystallization is the process of forming crystals. Soaps are comprised of solid crystals and a liquid-soap mixture. When soap is first created, it exists in a somewhat “jumbled mess” that undergoes chemical changes to form solid soap crystals, or regularly ordered solidified soap molecules, in addition to a liquid phase and any other added chemical compounds, like those from included additives.
As you recall from the Fatty Acids chapter, each fatty acid has its own unique shape based on the number of carbon atoms in the tail and the types of bonds. When trying to imagine the crystallization process, I find it helpful to imagine a game of Tetris, where the soap molecules come together and coalesce into a more organized and solid repeating structure. Saturated fatty acids are straight in shape and smaller in size and these readily coalesce and crystallize. They are like the straight small pieces in a Tetris game that easily fit into the open spaces to form a uniform or solid base.
Unsaturated fatty acids, on the other hand, have kinks and bends and have longer carbon tails that don’t fit compactly into the pattern right away. These are slower to crystallize and are more likely to remain in the liquid portion of the soap after production, along with some of the other less soluble soaps like sodium stearate and very soluble soaps like sodium laurate (think back to our eutectic mixture).
When we pour our soap recipe into the mold, it is in a liquid neat phase that is easily swirled and designed, yet the finished product is a hard bar of soap that is used in the shower. This is because the finished product of our soap making process creates a mixture of primarily solid soap crystals and liquid. A finished bar of soap is a complex mixture of solid soap crystals that come in unique sizes and phases that are bonded together in different 3D arrangements and are all surrounded by a mixture of liquids.
You may hear some people state that a soap bar “must cure for four weeks so that it can turn into crystals." Although soaps do crystallize over time, this statement is not always accurate and can be misleading. It suggests that the crystallization process has finished after four weeks and that no more will continue.
The notion that after four weeks the crystallization process is over and that there are no more changes to the solid soap composition is misinformed. It suggests that there is no crystallization that existed before that four-week point and that the crystallization process has completed after four weeks. Before hot process soap is even put into the mold, it is already in a type of crystal phase, a liquid crystal phase, called a lamellar lyotropic crystalline phase of soap and water. When our soaps are hot, they exist in neat phase, and as they cool, they transition into new soap phases and develop new solid crystalline structures and patterns.
The crystallization process may not be done in six weeks, eight weeks, or even twelve weeks. In fact, the full crystallization process can take months, especially for soaps that have a much higher water concentration, additives that may slow the process, or a higher concentration of unsaturated fatty acids that don’t readily fit into the pattern. The rate of crystallization varies from each soap because each is made with different crystallization-affecting factors that include things like temperature, water content, chemical composition, and additives. Curing your soap for exactly four weeks specifically so it “turns into crystals” is not a valid concept.
Water evaporation not only creates a harder, longer-lasting bar but as the moisture content in your soap gets closer to the equilibrium of water concentration in the air, several things begin to happen. When the water concentration in your soap diminishes, the glycerol and other water-soluble chemicals in the liquid portion of your soap become more concentrated because there is less water. When the water evaporates, it triggers a process of precipitation crystallization or what some may call this “salting out”. Because there is less water left in the soap as water evaporates, the less soluble fatty acid salts like sodium stearate (stearic acid soap) and sodium palmate (palmitic acid soap) that are in the liquid portion will amalgamate into solid soap crystals, while the more water-soluble soaps will remain in the liquid matrix. For this process to happen, the water concentration of your soap must decrease, therefore, water concentration and precipitation crystallization are correlated. A decreased water concentration may increase the rate of precipitation crystallization both during the standard four to six-week cure period and for much longer after, often months.
What does precipitation crystallization do for our soap? When the less soluble soaps solidify rather than remain dissolved in the liquid portion, this increases the lifespan, hardness, and longevity of the bars. We already know that sodium stearate and sodium palmitate aren’t the best lathering soaps, so the soaps that do remain in the liquid portion of our soap, like our sodium oleate and sodium laurate, provide a much better lather. Wait a minute, do you notice something about those two soaps? Remember way back to The Ultimate Lather section of our Fatty Acids chapter at the beginning of the book. When combined, these two sodium soaps create our eutectic mixture, which produces an awesome lather. Soaps that are allowed to fully crystallize are not only harder and longer-lasting, but they also experience a faster forming and more voluminous lather.
Another claim that you may hear regarding soap curing is that “only a long cure time with soap crystals will make the soap gentler”, but what exactly does this mean and is there any scientific evidence to back this claim? Soap crystallization benefits the lathering and hardness properties of the soap, but something else during the curing process also helps increase the soap’s “gentle” qualities, a process called acidification.
NaOH is very alkaline with a pH of 13.0. When lye is combined with oils and fats that are acidic (fatty ACIDs), the recipe is neutralized and the pH of finished soap averages between 8.0-10.0. If the acid mantle of the skin has a pH of 5.5, why does soap have such a “harsh” pH? For soaps to “work”, they must have a high enough pH and enough ions to form micelles. These micelles are our little cleansing buddies which work to remove the dirt and oil from our skin. In other words, we WANT our soap to be alkaline because that is what allows them to function as a cleansing agent and what makes soap… well, soap.
For commercial soaps, detergents are added to quickly lower the pH of their products. For homemade soap makers, we create informed recipes and control the pH by acidifying soap. But how exactly do we acidify soap? By doing nothing! After the saponification process and during the cure time, the atmospheric carbon dioxide naturally present in the air reacts with any excess alkalinity in the soap. Carbon dioxide is naturally acidic and after it neutralizes the sodium hydroxide, it begins to neutralize the soap. As soap is cured, it becomes less alkaline in the process, which is called soap acidification. Soaps that are less alkaline are much gentler on the skin, so yes, the notion that soaps become gentler over time is most certainly true.
An additional factor that makes soap gentler after time is the increase in glycerol concentration, which we touched upon earlier. As water evaporates from your soap, the glycerin concentration also increases. Glycerin acts as both a humectant and an emollient and can aid in skin hydration, improved cutaneous elasticity, and epidermal barrier repair. Glycerin acts as a soap solvent and will increase the rate of lather formation and the longevity of the bubbles.
Note: It is important to note that just because a soap has a low pH does not mean that it will be gentler than a soap with a higher pH. There have been multiple tests and significant research that have been done that have shown that although a lower pH often does create a gentler bar, some soaps with a higher pH have been reported as being gentler. Some of the most popular sensitive-skin and baby soaps have higher pH levels of almost 11.2. If you remember reading about fatty acid soaps in the previous chapter, you will know that sodium oleate has a pH of 11.2 and it is considered one of the most conditioning soaps. This means that castile soap, which has a very high concentration of sodium oleate soaps, will also have a higher pH. One of the reasons castile soap becomes gentler over time is the acidification that occurs which lowers that pH, in addition to an increased glycerol concentration and a change in lather.
Now that we have taken a closer look at what curing is and how it affects the final soap, we can better answer the question “Does hot process soap need to cure?”. The answer is very clearly: YES. If you want a harder, longer-lasting, better lathering, and gentler bar, it is important to cure your soaps, both cold and hot process. The next logical question then becomes, “How long does hot process soap need to be cured?” and “Is it the same amount of time for cold process soaps?”.
Almost every soap making book and the majority of soap making blogs that provide directions for hot process soap claim that it does not need to be cured and can be used, sold and gifted the same day, so these resources don't really help us with recommended cure times. Most soap making books and resources will suggest that cold process soaps should be cured for four weeks before they can be used, sold or gifted. This is the standard amount of time recommended by the soap making community, yet it does not take into consideration the soap’s fatty acid composition, the additives used, the water concentration, or any other aspect of its production process. Using all of the information and knowledge that you have gained from reading this book, in addition to the previous paragraphs about Kevin Dunn’s “Water Discounting” experiment, it would seem silly that all soaps would cure in the same amount of time. We already know that soaps made with less water will harden and decrease in water concentration at a significantly faster rate than those made with more water and the same recipe.
If four weeks isn’t a “one-bar-fits-all” time requirement, what should be the amount of time required to cure a hot process bar for its optimum composition and performance? If you are selling or gifting your soaps, a sample bar of each recipe should be weighed at weekly intervals until there is no longer a significant change in weight. This will not only allow you to sell your soap at the correctly labeled weight, but it will also ensure that the bar will be harder, have an increased lifespan, will have a lower and more gentle pH, a better quality lather, and so much more. Although a soap may be safe the same day of production, so long as it is soap-neutral, a longer cure produces a better bar.
Not everyone can cure their soaps for 8-12 weeks, especially for those who run a soap making business, so although not all soaps will experience the same cure properties at four weeks, it is certainly a good starting minimum. Are there things that can be done to accelerate the cure? A dehumidifier? A food dehydrator? Curing takes time. Crystal growth takes time. A dehydrator is not a bad idea if you live in a climate that has high humidity, as humidity can cause increased water retention in the soap and can also increase the risk of oxidation, including DOS and other signs of rancidity. Although a food dehydrator may help remove excess moisture, it can also warping, increased alkalinity, and more.
It has been proven that hot process soaps increase in hardness and decrease in moisture content more quickly than cold process soaps made from the same recipe. This means that hot process soaps will need less time to cure for hardness and water evaporation than cold process soaps of the same recipe and although this difference is not exceptionally significant, there is still a decrease in the required time for the water concentration to decrease (approximately 9%), which should be noted. Recipes made with less water will require less time to decrease in moisture concentration and will be harder, initially and often many weeks after than those made with a higher water concentration. If you are selling your soaps, a sample should be weighed at weekly intervals until there is no longer a significant weekly change so that you can sell your soaps at the correctly labeled weight.
My average minimum cure time for my HTHP soaps made at 30% water concentration is typically 3-4 weeks. My average minimum cure time for CP recipes made at 30% water concentration with the same recipe is typically 6-8 weeks. The hot process soaps are initially harder and remain harder until the full 8 weeks, often up to 12 weeks, and they contain less water from the cut and up until the full 8 weeks as well. My average minimum cure time for my fluid hot process soaps made at 38% water is usually around 6 weeks. Every recipe is different. I make sure that I take this into consideration when determining the amount of time necessary for curing my soaps and never use a generic timeline, but rather treat each recipe individually.
When I state average minimum cure time, this is regarding the least amount of time cured before selling/gifting. I know that after that amount of time, for my recipes, that I will have a quality product that I feel comfortable and confident selling or gifting to others. Even though I can sell/gift them, with the knowledge that we now have regarding the curing process, we know that additional time creates even better soaps. I prefer to cure my soaps for the longest amount of time possible. If I can keep my soap on the cure rack for six months before use, I try to do this as often as possible. When I make Christmas soaps, I do so in August. If you make soap for yourself and do not sell/gift, use a sample bar after the process and keep the rest curing for extra amazing soap. Not everyone can cure for extended periods of time, especially if you are running a new or fast-paced business, and thus, minimum cure times should be accounted for. Weighing your soaps is an accurate way to monitor the water reduction and possibly investing in or creating a hardness measuring tool.
There is a new debate in the soap making community about whether a cure is even necessary at all, for all types of soap, both hot and cold. If curing a bar of soap is as simple as leaving a bar in a cool, dry and dark location, can’t a customer do this same thing in their own homes? Can’t we just include a note on the label or tell a customer to “wait for six weeks before using”?
Purely from a legal standpoint, curing soap is mandatory because the weight of the soap changes as it cures. By allowing the water to evaporate, sellers can apply the correct weight to the label, which is in strict adherence to federal regulations. However, there is a way around this. Instead of curing a soap, you could include a label that would have the lowest soap weight possible, as if the excess water had already evaporated. This would accommodate for any additional water loss and prevent misrepresentation and misbranding.
Although we certainly could do this, I don’t recommend it. Many soap makers are now suggesting to customers that they, as stated on one website, “cure our fresh-made handcrafted soap on your bathroom counter and wait until our recommended use by date.” When I was at a craft show and purchased a bar of soap from another soap maker, I noticed this on the label and politely asked him what his thoughts were on curing soap and he responded “You don’t need to cure soap, it has no benefits. It would be the same thing as me (a professional soap maker) curing the soap in my soap studio.”
This “cure on the counter” option forces the customer to make a decision- to use the soap now or to wait six weeks as directed. This internal debate is directly associated with a subject that is highly studied in Psychology known as delayed gratification. Delayed gratification is when someone resists the temptation of an immediate reward in preference for a better reward at a later point. One of the most popular studies done on this was the Stanford Marshmallow Experiment that took place in the late 1960s. In this experiment, students were left alone in a room with a single marshmallow and told that if they waited for 15 minutes, they could have an additional marshmallow, or two marshmallows total. As you can probably guess, many of the students ate the first marshmallow instead of waiting for a second later treat. This same test has been repeated for more than 60 years with donuts, cookies, candy and more, and most test subjects decline a better-delayed gratification and instead choose instant gratification.
Your customers probably won’t do an in-depth analysis of the benefits of soap curing, and it would be silly to expect them to. If the purchased bar feels hard enough, smells good, bubbles, and cleans, they might not see any value in waiting an additional six weeks to use their purchase. I personally have very little impulse control and I absolutely love to use things as soon as I purchase them. That is the whole point in buying something that has already been made- I don’t have to wait to use it.
At an absolute minimum, curing creates a harder and longer-lasting bar of soap that is less soluble. These are facts and can easily be tested and measured by any soap maker. This is of course in addition to all the other things that we just learned. If a customer purchases a bar of soap, and it soaks up a lot of water, develops a thick layer of mush, softens when gripped, quickly dissipates, has a reduced lather, and feels more drying than other previously used bars, especially when compared to another soap maker’s product or a commercial syndet bar, you can bet that they won’t be back. Unfortunately, the bars of soap that I purchased from the “cure on the counter” soap maker experienced all of these things and while I absolutely love to support other soap makers, you can bet I won’t be back.
When I purchase a bar of soap, I expect that it lasts for a minimum of three to four weeks with daily use. By curing my soap, in addition using a quality and informed recipe, I can ensure that all of my soaps will do just this. If you wouldn’t wait, would you really expect your customers to? It is your responsibility as a seller to ensure that the absolute best product is provided to customers. By not providing a hard and long-lasting product, and instead focusing on profits and turnover, you devalue your product and devalue the handcrafted soap making industry.
It is silly to assume that everyone who makes soap sells soap. We have learned that cure time is important for many different reasons, but what if you don’t sell your soap? Is it ok and safe to use your hot process soap without curing it? Yes, and I absolutely think you should. Even if you sell your soaps, using a bar the day after is perfectly acceptable, and I absolutely encourage it; however, it should absolutely not be sold or gifted as such and should only be used for fun while the rest cures. Part of the hot process experience is watching your oils and lye turn to soap right in front of your eyes and then being able to cut it later that same day. In just a few hours, you can make solid bars of soap that are pH safe, bubbly, hard and just begging to be used. If your bars are hard enough to be used, go for it! Next-day bars will still leave your skin feeling soft, clean and beautifully fragranced while creating a fun, bubbly and fluffy lather. While you use your new HP creation for the next several weeks, the rest of your bars will have time to cure, and you will most certainly notice that as your soaps begin to age, they will become even harder, bubblier and much longer-lasting.
Hopefully, this information will provide you with a little more insight as to why we cure soaps and how it affects your soap’s qualities and recipe results. We explored the importance of water concentration and how that affects the length of cure time and soap hardness. We learned that hot process soaps often decrease in moisture content more quickly and are initially harder than cold process soaps of the same recipe, which may or may not decrease the overall length of time you decide to cure your lovely HP creations. We also learned that curing hot process soaps has lots of benefits, including creating a harder, longer-lasting, more durable, gentler and better lathering bar. You can use this information to find out what works best for you and your soapy creations by taking good notes and keeping records of every batch made. Your own personal records, in addition to the fundamental knowledge of soap curing, will provide you with the most accurate and detailed information regarding the cure times necessary for your individual soaps and should be considered the standard. Use your hot process sample and take advantage of all of the wonderful things that curing can do for your hot process soaps!
For more information about soap science, recipe formulating, hot process soap and fluid hot process soap, be sure to get your copy of The Ultimate Guide to Hot Process Soap today by visiting our bookstore now!