Homemade soap needs to cure for two reasons. 1.) The saponification process needs enough time to convert the fats, oils, and lye into soap; and 2.) Curing hardens the bars of soap so they last much longer.
What is Soap Curing?
Curing can be broken out into two processes as follow:
Saponification
The first is called “saponification.” This means “to make soap.” It is the process that occurs when the lye solution has an organic bio-chemical reaction with the fats and oils in the mix. This neutralizes the base (lye) and makes the fats and oils gel and then harden into usable soap.
Although many soapers consider the removal of moisture and bar hardening as actual “curing,” saponification is widely accepted as the first stage of curing. Its real purpose is to create the soap to the degree that it is safe to use as a mild cleaning agent for people. At this point the pH should be between 7 and 9. A pH of 10 or higher can be too high and is likely to cause skin irritation and even a burning sensation.
Once a batch of soap has fully saponified, it is safe to use. Completion of the saponification process and curing times in general differ by the soap making process used as well as the ingredients used in the batch.
Hardening
The second aspect of curing soap is the removal of excess moisture. Eliminating the extra moisture results in a firmer bar. Besides feeling good when used, it also lasts far longer than soft bars in the bath.
When a typical batch of cold process soap is poured in the mold, 10% to 15% of the weight and volume is excess fluid. This might be water, beer or whatever liquid was used in eth soap making process. The removal of that excess moisture is what most people think of as true curing.
When the curing process occurs naturally, it can take weeks for the bar to be firm enough for long-lasting use.
Starting the Soap Curing Process
Technically, as soon as the fats, oils and lye solution have been thoroughly combined and the saponification process is underway – the soap will begin to cure. There are, however, ways to speed and simplify the process. In a moment we will explore curing times, but for now here is a general rule for hot process and cold process method steps for curing:
Cold Process
Cold process soap making is a simple method that involves combining fats, oils, lye solution and any exfoliates, colors, and fragrances in exact proportions and letting the concoction turn to soap through a natural occurring thermal reaction that occurs between the lye solution and fats/oils.
It is called “cold process” because there is no need to add an external heat source. The entire batch can be made in a bowl. Pot or bucket as long as the container can withstand the 200F+ degree temperature the soap reaches while saponification occurs.
Hot Process
The hot process of soap making uses an additional heat source to speed the saponification process and reduce cure time by an average of 70% or more.
Often done on a stove top or in a slow cooker (like a Crockpot) – hot process tends to produce more rustic looking bars of sop that are ready to use in days versus the weeks needed for clod process.
Let the Curing Begin
Whether the cold or hot process is used, the next steps are the same…
- Combine the ingredients in order according to the recipe
- Pour the finished soap batter from its container (pot, bowl, Crockpot, etc.) into the soap mold. It is easiest to work with a silicone lined rectangular mold when you start out. They are very forgiving. Simply pour in layers a few seconds apart and tap the sides or bottom of the mold to help the batter settle with air bubbles.
- Here you might cover the mold, insulate it, or even just leave it open to the air. It depends on the process and the recipe. The important thing is you do not want to cut the soap loaf into bars until the saponification process completes. For hot process this generally takes 12 to 24 hours. For cold process it typically takes 24 to 36 hours. Again, follow the recipe instructions.
- Once the mix has fully saponified, cut the loaf into bars. The thickness will depend on preference, however 1” thick is a standard many people use.
- Next, place the bars on a surface that provides the best possible airflow. Wire baking cooling racks work great, but you can also get specialty stackable curing racks made specifically for the purpose. The trick is to space the bars far enough apart to allow for good airflow. About 0.5” open area all around each bar works well for my batches – especially when I use a small low-power oscillating fan to move the air.
- Now place the racks in an area with minimal humidity and good airflow. Be sure to flip the soap on its different edges and even turn the bars every so often to even out the curing of surfaces.
Tip: Gentle fans to move the air along with dehumidifiers or silica water traps to remove moisture can help speed the curing process significantly.
Cure Time by Soap Making Process Type
The amount of extra moisture in the soap is the first deciding factor in how long it will take to fully cure the bars.
In general, cold process soap has several times more leftover moisture than hot-process soap. This is because the added heat in hot process evaporates most of the remaining liquid from the lye solution. Of course, if you add liquid to hot process, to make it thinner and easier to work with, you will have longer cure times.
The second key factor in cure time is the type of oils used. Hard oils, such as coconut oil, tends to cure faster than soft ones such as olive oil. Following is a very general chart that details curing time by process and fat or oil type.
| Process | Time to Saponify/Ready to Cut | Fully Cured |
| Cold Process | 24 to 36 hours | 3 to 10 days |
| Hot Process | 12 to 24 hours | 30 to 120 days |
| Melt & Pour | NA | 1 to 3 hours |
Pre-Cured Soap
As you probably noticed, a new soap type was added in the above table. Melt and pour, or glycerin soap, is specially made soap that comes to you already fully saponified and cured. It has some additional glycerin and alcohol solutions in place to make it easy to re-melt and add any colors or fragrances you want.
Just note that with melt and pour soap, the added glycerin tends to suck up as much moisture from the air as possible. This means it forms a “glycerin dew” that looks like sweat. That said, you will want to wrap this in an airtight package as soon as it has hardened sufficiently – especially if you live in a humid area.
Speeding the Curing Process
Cold and hot process soap will continue to cure for months or longer when exposed to air. This is good. It helps create a rock-hard bar of soap that lasts a very long time in the shower. Some people, however, want to get the soap cured as quickly as possible. This is especially true of those who make their living making and selling soap.
The best way to do this is to let the bars cure in their racks with one or more fans providing a gentle breeze of dry airflow. This can be taken a step further by placing a dehumidifier or silica water traps in the curing area.
I converted an old wardrobe cabinet, that would have been headed for Goodwill, into a curing closet. Doing it on the cheap, I cut a three-inch hole in each side (one near the top; one near the bottom) and used a hot glue gun to mount two old computer fans from my husband’s junk pile of old computer parts he swears he cannot live without. The one on top draws air into the cabinet and the one at bottom sucks it out.
I bought a few silica water-traps at the Dollar Store and placed it in the bottom of the cabinet. It lasts a lot longer than I thought it would. You can buy them just about anywhere in the laundry section because people use them for reducing mustiness in laundry closets.
Reduced Curing Times
Using this old wardrobe, I can cure up to 360 bars at once, spaced ½” apart on wire racks (rubber coated wire type closet shelving from Home Depot). I choose however to do a few less bars and add more of the small silica water-traps so I don’t need to change them as often with so many bars curing at one time.
Here are the reduced times:
| Process | Time to Saponify/Ready to Cut | Fully Cured |
| Cold Process | 24 to 36 hours – NO CHANGE | 2 to 5 days |
| Hot Process | 12 to 24 hours – NO CHANGE | 30 to 75 days |
| Melt & Pour | NA – NO CHANGE | 1 to 3 hours |
Please note the saponification times do not change and there is no change to cure time for melt & pour. It is already super-fast at 1 to 3 hours.
Water Weight
This is important – especially for labeling your soap. You need to use the bar weight of cured soap on your labels. Curing typically removes 5% to 20% of the bar weight by removing the excess water. It is usually safe to assume 10% for hot process and 15% for most cold process. Having 20% extra water weight usually only occurs in batches that had even more liquid added to make the batter more malleable for doing swirls or filling the nooks and crannies of intricate molds. This is called thinning.
Here is a chart that provides good averages for estimating post-curing bar weight:
| Process | % of Extra Water Weight |
| Cold Process | 10% to 15%; 12% average |
| Hot Process | 5% to 10%; 8% average |
| Thinned | 12% to 20% – no average |
I mention the weight posted on the label to ensure you remember to label with the intent of the consumer having an accurate understanding of what they are getting.
Items such as bar soap are often calculated, and compared, based on cost per gram or ounce. If you sell a 4-ounce bar that feels rock-solid but still has 5% added water weight – the customer is only getting the value of 3.8 ounces.
In Closing
Curing is a key component to soap making and should be treated that way. Do not fall into the trap of simply cutting the bars and putting them aside for months at a time. Although curing bars do not need a lot of attention – they do need some. It would be a shame to lose a bar or even a batch to neglect from dreaded orange spots (DOS) or another issue that could have otherwise been caught early on.
Here is a link to plenty more articles on how to make the soap you just learned to properly cure.
Happy Soaping!