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How I Make Soap: An overview of the hot process soapmaking method

tea tree oil soap: no more lye

Making soap is like magic. Seriously, think about it: You take stuff that makes things feel greasy, mix it with stuff that would eat your face off and, if you do it just right, the end result is a wonderful bar of non-drying, skin-loving, fluffy-lathering soap.

Soapmaking is fun, but safety is a priority when dealing with sodium hydroxide, aka lye, aka the eat-your-face-off stuff. This is just a brief overview of the soapmaking process I use. Please do not use this as a complete how to guide. If that’s something you’re interested in, please, leave a comment and I will direct you to some more thorough resources on soapmaking.

Melting the Oils

Creating soap takes two basic things: oils or fats and an alkaline solution. In order to combine the oils and alkaline solution, the oils have to all be in liquid form and uniformly mixed. So, step one is measuring and melting the oils. For precision, all measuring is done by weight. Since I use a Crockpot for my hot process soapmaking, I add all my weighed oils to the Crockpot, then melt and mix them in the pot. For soapmaking, I generally use the low setting, but if I need to jump start the melting process I will start it on high then turn it down after a few minutes. This Crockpot is similar to the one I use.

Melting the Oils
Melting the Oils

Measuring the Lye

While the oils are warming in the Crockpot, I weigh the lye. It is important to be very precise when measuring the lye. Too little and the soap will be too soft. Too much and the soap could burn your skin. This is also why you need to be very careful about the soap recipes you use. If they are not correctly formulated, the end result could be dangerous.

Before I measure the lye, I measure the water or other liquid for the lye solution, also by weight. I do that first to limit the time I have the lye out.

Weighing the Lye
Weighing the Lye. Gloves and goggles on.

Lye Safety

I make sure to measure the lye into glassware and all containers and utensils that come into contact with lye or the lye solution are reserved solely for that purpose. Once the lye is out, I never leave it unattended. Lye crystals resemble table salt, which would be a potentially deadly mix-up. That’s also why my soap production has slowed since Thaddeus was born. Until they are old enough to understand the importance of staying out of the room and can be trusted out of sight but in earshot for the time it takes to get the soap cooking, I only make it when they are either out of the house or asleep with Chris there to tend to them if they wake up during the process.

Mixing the Lye Solution

Once my lye is measured, I add the lye to the water and stir with a wooden spoon until dissolved. Lye fizzes up when mixed, so it is important to have the liquid in a container with plenty of room. The solution with also get really hot and put off fumes, so be prepared. I like to have a window open or fans and the vent a hood running. Some soap makers mix the solution outside, but I like to stay close to my work space to limit the chance of spills.

Blending the Oils and Lye Solution

For cold process soapmaking, it is important to have the oils and lye solution at about the same temperature. With the Crockpot hot process method I use, I find I can mix the lye solution into the warmed, melted oils without having to measure the temperatures first.

When you first add the lye solution to the melted oils, the color will change from clear to opaque. In order to properly combine the lye and oils, I use a handheld stick blender. You can stir by hand, but it is much harder to get everything properly blended and it takes f-o-r-e-v-e-r. I have had a stick blender die in the middle of making a batch of soap and it is a crazy long arm workout. I do not recommend it.

Adding the Lye Solution
Adding the Lye Solution

Goal of Blending

The goal for all the hard mixing is called “trace”. Basically, I’m wanting to be able to lift my blender out of the mixture and trace a design on top. With cold process, there are various stages of trace that guide when to add any fragrance, essential oils or other add-ins. Since I use the hot process method, I add almost everything after the cook. Because of that, I can mix my batches to a full, hard trace where my traced design doesn’t melt back into the mix. This limits the possibility of a false trace, or the mix seeming to quickly come to trace when it isn’t thoroughly mixed. When that happens, it might separate or have lye heavy spots.

Hard Trace
Hard Trace

Once it is at a hard trace, I stop mixing and put the lid on the Crockpot. If the mix seems too hot, for example, I had a false trace that I had to stir through, I will either turn the Crockpot to warm or off for a few minutes. Otherwise, I make sure it is on low and busy myself with cleaning up, lining my soap molds, and preparing essential oils and other additives.


Any lingering lye solution or soap mixture on the blender is still a risk for lye burns, so I’m cautious with the cleanup. For the whole soapmaking process, I keep a good amount of water with vinegar and a bit of dish soap ready in the sink. The vinegar helps neutralize the lye, so anything that comes into contact with lye goes directly into the vinegar solution, and I use a similar vinegar solution to wipe down my soapmaking area, just in case.

Stages of Saponification

During the cooking, the soap mix will change from an opaque, milky color to a shiny, translucent gel like texture. A pool of liquid also forms on top. that pool of liquid is glycerin, a byproduct of the saponification process. This transformation starts at the edges of the pot and happens in a wave moving towards the center. I find it really fun to watch.

Soap Waves
Soap Waves

Finishing the Batch

When the entire soap mixture is translucent and the consistency of mashed potatoes without lumps, I turn off the heat and stir the soap by hand for just a couple of minutes to let it cool. At this point, if done correctly, the mixture completely transformed to soap. I don’t want it to cool too much, or it would be hard to add the essential oils, but too hot and it will burn off the scent.

Glycerin Pool Forming in the Middle
Glycerin Pool Forming in the Middle

I then add any essential oils blended in my chosen carrier oil and any other add-ins I’m using in that batch, stir thoroughly by hand, and plop it into my molds. Since it is thick, I generally have to tap my molds on my counter firmly to eliminate air pockets.

Adding the Blended Essential Oils and Carrier Oil
Adding the Blended Essential Oils and Carrier Oil

Curing the Final Product

Because the saponification process is completed during the cooking, the soap is totally safe to use as soon as it is cool enough to touch. It needs to cool in the molds for 12-24 hours or so to hold it’s shape, though. Once it is firm enough to cut, I cut and wrap the bars. While not necessary, hot process bars still benefit from curing for at least a week or two to allow excess moisture to evaporate. As I discussed in my Proper Care and Feeding of Your Bar Soap post, the firmer and drier a bar of soap is, the longer it will last. That’s one of the reasons I wrap my soap in cloth rather than plastic. Cloth allows the soap to continue to harden for a longer lasting bar.

Soap Log in the Mold
Soap Log in the Mold

The batch I photographed for this post is my Tea Tree Oil Soap. Find it and my other handmade soaps for sale here:

1 thought on “How I Make Soap: An overview of the hot process soapmaking method

  1. […] Hot process is very similar to cold process, except that the lye and oil mixture is cooked over a low temperature until the reaction is complete.  Fragrances, essential oils and herbs can be added after the cook. The cooked mixture is then poured into molds and left to harden. The soap is safe to use immediately after it hardens, although it is best to allow the soap to cure for at least a week or two. Since some of the water evaporates during the cooking process, hp soap doesn’t have to cure for as long.For an overview of how I make my hot process soap, read How I Make Soap: An overview of the hot process soapmaking method […]

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