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Hot Process Soap Making

Hot Process Soap Making

pouring fresh soap into a moldThe process for making soap via the Hot Process method is quite similar to the Cold Process method. In fact many of the same recipes can be use interchangeably between the two methods. In the extended entry below, I have listed the materials needed to start the soap making process. The basic premis behind soap making is to take an acid (fat) and mix it with a base (lye) to form a salt like product that is able to change the surface tension when mixed with water making it ideal for whisking away particles of dirt and oil.

Safety Equipment
Safety First! You will need a pair of safety goggles and a pair of safety gloves. Lye is extremely caustic. You’ll notice just how caustic if you leave it open on a humid day. You might want to also consider an apron specifically for making soap. Unless you’re like me and you think holes in your jeans are cool.

Measuring Equipment
A Scale is a must. All quality soap making formulas depend on the weight of the ingredients and not the volume. I use a $9 postal scale. A measuring cup for the liquid you are going to use. You could measure the water by weight as water has a 1:1 ratio of weight to volume (English and metric). A good set of measuring spoons is always good to have handy for fragrances and/or colors. You will also need a thermometer to check the temperature of the oils and lye solution before mixing them together.

You’ll need 2 containers that are heat safe and lye safe. I use two heat safe Pyrex glass jars. Glass is great for lye if and only if it is heat safe. When you mix the lye with water it will heat the water up very quickly to 212°+F/100°+C and will cause glass to crack and you will have a strong lye solution all over. Other containers you could consider would include cast iron, steel, ceramic, etc. Never use aluminum! Lye will eat right through it. You’ll also want to have 2 cups (plastic is fine here) to use to hold your oils and lye as you measure them on the scale. Make sure you label these for soap making only. You don’t want to cook with any of these no matter how much you clean them.

Mixing Equipment
Most places suggest a wooden spoon. Really any spoon that is lye safe and heat safe will work. The problem with wooden spoons is after a while they will start to dissolve because of the lye. I would also suggest you invest in a stick blender. This will make the process of mixing your soap 100 times easier.

There are many things you can add to your soap (I’ve even seen formulas that call for ketsup). The basic things you will need are water, lye, and oil.

Oil (lipid): The type of oil you choose will make major differences in the type of soap you get. Each oil has its own properties that it adds to the soap. You can check out the sap table on this site for a list of oils and the properties they have. The table shows the iodine value (the lower the number the harder the bar), type of lather, conditioning, and a general hard/soft bar designation.

Lye: There are two types of “lye” that can be used for soap making; sodium hydroxide (NaOH) or potassium hydroxide (KOH). Sodium hydroxide is often used in making ‘bar’ soaps and potassium hydroxide is often used to make liquid soaps. Both are not readily available anymore. The first place to check is a local hardware store or backing supply store (Lye was/is often used in making soft pretzels a golden brown outside but leaving the inside soft). If you cannot find it locally, your only other option is to call scientific supply stores or search the Internet. You will notice, though, that if you buy more than 2lbs of lye you will be slapped with a $50 shipping charge because of the caustic nature of lye.

Water: A clean bar of soap starts with clean water. I recommend starting with distilled water. You can steep teas in the water to add a variety of properties to your finished soap. You can also replace the water with a variety of different milks. If you are using anything besides distilled water, I would suggest making a small batch of soap to see how it turns out.

Soap Mold
After you have made your soap you need somewhere to put it as it cures. A plastic lined desk drawer works just as well as any fancy mould you can buy. This is another place in soap making where creativity is the limit.

Quick Reference

  • Safety Goggles
  • Safety Gloves
  • Scale
  • Measuring Cup
  • Thermometer
  • 2 dishwasher safe jars/buckets/pots (not aluminum)
  • A large mixing spoon (wooden)
  • Lye
  • A variety of oils
  • distilled water
  • Soap Mould (mold)

Making the Soap

Now that you have all of your equipment you are ready to make some soap! Setup your work area so that you won’t be disturbed. Open a few windows if you are inside. Line your moulds with wax-paper or plastic and set them aside. I like to set the oils and oil jar I will be using on my left, scale in the middle, lye and lye jar on my right. I also put a few sheets of wax paper under the scale in case of small spills. Measure your oils and keep a record of the weight of each oil. After you have combined all of your oils in your oil container you will need to figure out the amount of lye and liquid you need.

The liquid is easy. Just multiply the total weight of the oil by 28%-40% (.28 – .40). The more liquid you use the longer it will take for the bar to harden. I usually use 30%-33% (.30 – .33). Next you need to figure out the amount of lye. You can do this by hand using the sap table. Take the amount of each oil and multiply it by the SAP # then add the products together to get the total amount of lye. I also have a lye and liquid calculator you can use if you don’t want to do the calculations by hand. The calculator will also give you the amount of lye to use when superfatting your soap (superfatting or leaving some oil in the soap is recommended @ 4-5% to allow for a moisturizing soap that is not harsh on the skin). Put on your safety goggles and gloves (if they are not already on) and measure out the liquid you need. Put it into the empty lye container. Open the lye and measure the amount on the scale that you calculated. Close the lye. Start mixing the liquid and slowly pour the lye into the liquid. Keep stirring until the lye is completely dissolved. The mixture will heat up (and may start to boil). It will also start to release fumes. Lid the container and set it aside to cool.

As the lye solution is cooling you will want to heat up your oils. You want to oil and the lye to be very near the same temperature. The temperature you are shooting for is 100%°-120°F/38°-49°C. A double boiler works great for heating the oils or if the container will fit (and is safe) the microwave at 15-30 second bursts.

Once your lye solution and oils are at or near the same temperature begin stirring the oils then pour the lye solution into the oils while you are stirring. What will happen now is the lye will begin to break the chemical bonds in the oils and create essentially a salt. This is called saponification. While the oils is saponifying you are looking for it to trace (thicken). If you are stirring by hand it can take anywhere from 15-45min to trace depending on the oils you used. You can speed this process up by using a stick blender. If you use a stick blender be cautious of bubbles in your trace. You’ll want to mix until you reach a custard type consistency.

The Hot Process and Cooking the Soap

Once the soap reaches a full trace, it’s time to cook the mixture and force the gel. You will want to put the soap into a crock pot (you could even start the oils out in the pot to heat them up) Put the lid on the crock pot and leave it to cook on low for awhile. The soap will heat up and start to bubble around the edges of the crock pot. Keep an eye on the soap and stir it down gently only if it starts to bubble over. The mix will begin to take on a clear vaseline like look. Once the whole mix has this look, you can test it to see if it is done. Take a small sample of the soap and rub it between your fingers. It should have a waxy feel. Test the soap by touching it to your tongue. Keep cooking….if it ‘zaps’ like a nine volt battery, it’s not done.


At this point you can add fragrances, extra oils, colors, exfoliants, etc. Once your additives, if any, have been incorporated pour your mixture into the mould(s) you have. Hot process soap is a lot like rebatching when it comes to molding. It doesn’t pour. It is a thick goopy mass (like mashed potatoes) that must be scooped into the mold quickly. Make sure to tap the mold on the counter to get out any air pockets.

Once your soap has reached room temperature remove the mixture from the mould. If you are using a large mould that requires cutting the bars, now is the time to do it. After cutting the bars allow the soap to further cure and harden in a well ventilated area. You may even need to turn them a few times to be sure all sides are exposed to the air. It can take up to six weeks for a bar to be completely finished saponifying. The longer the cure time, the milder the soap will be.

See, that was easy and fun!

Melt and Pour Soaps

Melt and Pour Soaps

melt and pour soap arrayFor some soap makers, the melt and pour method is a simple alternative to the cold and hot process of actual soap making. The different in the melt and pour method is that no saponification actually occurs in the process. This limits the ability of the soaper to control the quality of the soap because the oils that go into the soap are not chosen. Some soap crafters choose to use this method because it removes the need to use caustic chemicals like lye during the process of making soap. This process has already been handled before the soap base has been purchased. The process of melt and pour soap crafting is actually fairly simple. Once you have obtained a melt and pour soap base, you simple place the base into a double boiler and heat up the product. At this point additives such as fragrance, essential oils, colorants, exfoliants or moisturizing agents can be put into the mixture. While the mixture is still hot, you pour it into individual molds or trays just as you would in an actual soap making process. Once the mixture is cooled, it can be sliced and used.

Spring Eclipse

Spring Eclipse

Lipid Amount
in oz
Olive 12 19.67
Palm 12 19.67
Coconut 76° 12 19.67
Soybean 12 19.67
Castor 5  8.20
Flaxseed 4  6.56
Cherry Kernel 2  3.28
Rosehip 2  3.28
Total Weight 61

Fluid (mint tea) needed:
17.08 – 22.57 fluid oz

Lye (NaOH)
in oz
0 8.88
1 8.79
2 8.71
3 8.62
4 8.53
5 8.44
6 8.35
7 8.26
8 8.17
9 8.08
10 7.99
Rainy Morning Soap

Rainy Morning Soap

Lipid Amount
in oz
Palm 12 22.22
Coconut 76° 12 22.22
Olive 12 22.22
Soybean 8 14.81
Castor 4  7.41
Blueberry Butter 4  7.41
Lanolin 2  3.70
Total Weight 54

Fluid (water) needed:
15.12 – 19.98 fluid oz

blue coloring

Lye (NaOH)
in oz
0 7.91
1 7.83
2 7.75
3 7.67
4 7.59
5 7.51
6 7.43
7 7.35
8 7.28
9 7.20
10 7.12
Fresh Verdant Soap

Fresh Verdant Soap

Lipid Amount
in oz
Coconut 76° 13 20.97
Palm 12 19.35
Soybean 10 16.13
Olive 8 12.90
Hemp 5  8.06
Castor 5  8.06
Green Tea Butter 3  4.84
Shea Butter 2  3.23
Meadowfoam 2  3.23
Rosehip 2  3.23
Total Weight 62

Fluid (mint tea) needed:
17.36 – 22.94 fluid oz

green coloring, rosemary for preservative and lilac and sage scents

Lye (NaOH)
in oz
0 9.01
1 8.92
2 8.83
3 8.74
4 8.65
5 8.56
6 8.47
7 8.38
8 8.29
9 8.20
10 8.11
Simple Castile Soap

Simple Castile Soap

Lipid Amount
in oz
Olive 43 66.15
Palm 11 16.92
Coconut 76° 11 16.92
Total Weight 65

Fluid (water) needed:
18.2 – 24.05 fluid oz

Lye (NaOH)
in oz
0 9.40
1 9.31
2 9.21
3 9.12
4 9.03
5 8.93
6 8.84
7 8.74
8 8.65
9 8.56
10 8.46
Saponification of Oils into Soap

Saponification of Oils into Soap

The processes of making soap are all centered around the chemical process of saponification. Soap makers have for years had huge lists of SAP (saponification) tables that describes the basic process of converting fats into soap. For new soap makers, the process may seem almost like magic because of all the mathematical formulas and conversions. Really, what every soap maker is doing when he or she makes a batch of soap is stepping into an organic chemistry lab and forcing a chemical reaction to take place.

The Chemicals Involved

The basic ingredients in any batch of soap are a strong base (lye, caustic potash, etc), water and an oil of some kind. When you put the lye into water, the water molecules ionize the bond between the sodium and hydroxide forming: Na+(aq) + OH(aq). The final ingredient is where the complexity and the artistry of soap making enters the equation. Most oils are comprised of triglycerides. This basically means that oils are made up of three fatty acids with a glycerol backbone. The glycerol molecule has three hydroxyl (HO-) groups. Each fatty acid has a carboxyl group (COOH), a central long unbranched aliphatic chain (saturated (CH2)n or unsaturated (CH2)nCH=CH(CH2)n) and an omega carbon at the end that has 3 hydrogens (CH3). It is in the middle chain that the properties of the oil come from. Because of the limitless number of repeats and configurations of this middle chain, there are literally an unknown number of fatty acids in the world. Fortunately for us, when mother-nature finds a formula she likes, she reproduces it quite a bit in different places. The most common fatty acids that make up vegetable and animal lipids (at least the ones soap makers are interested in) are Lauric, Linoleic, Linolenic, Myristic, Oleic, Palmitic, Ricinoleic and Stearic acids. So, it are these three ingredients, when broken down to their base molecules, that form the palette for a bar of soap.

The Saponification Reaction

As any kid playing with vinegar and baking soda knows, when you mix an acid and a base together, you get a reaction. When soap makers mix their lye water into a vat of oil, what they are doing is really just this simple process of mixing a base with an acid. The first thing that takes place, is that the hydroxide (OH) attacks the carboxyl group of the fatty acid which is attached to the glycerol. This causes the carboxyl to break away and form carboxylic acid. The lye base is attracted to the carboxylic acid and forms a salt with the aliphatic chain hanging off the side. At this point a single molecule of “soap” has been formed. This chemical reaction continues until all the lye or fatty acids are spent. Soap makers can play with this knowledge and discount/superfat the soap. That is to say, they can reduce the amount of lye they add to the oils so that the reaction stops before all of the oil has been converted to soap. While this saponification process is going on, a lot of heat is released during each of the reactions. This is why soap goes through a “gel” phase before it hardens into its final product.

Adding Fragrance to Homemade Soap

Adding Fragrance to Homemade Soap

In a lot of my soap making videos, I simply show myself adding a particular fragrance to the soap without explaining much about the different options there are to making your soap smell like it just came out of a designer spa but at a far cheaper price. In this article I hope to explain some of the basics to adding fragrance to your soap so that you can feel more confident to experiment with your own scents as you join the wonderful community of soapmakers.There are basically two chances you will have to add fragrance to your soap. The most common way is to add either a fragrance oil or a pure essential oil to your soap. The second way in which to add scent to your soap is to use various aromatic liquids like tea or coffee. There is a third way that is sometimes suggested by adding herbs or flowers to your soap but I have never had much luck with this method and if the cuttings are not heat safe, you will find most of the time they end up being burned by the saponification process.

Fragrance Oil vs Essential Oil

There is a long debate in the soap making community between those that use fragrance oils and those that use essential oils to scent their soap. The basic difference is that one is considered synthetic and the other is considered natural. In the United States fragrance oils are formulas or special combinations of chemicals or essential oils that produce a unique scent and can be protected by the FDA. On the other hand essential oils are tend to be a single ingredient that is extracted from a plant or other piece of verdure. Because the ingredients are known the essential oils tend to be considered more natural. The decision to use either material is really up to you but here are some of the facts you will need to know for soap making.

Fragrance Oils

Fragrance oils are by far cheaper to use and have a far wider variety of scents you can add to your soap. The drawback to using fragrance oils is that some of the oils can lead to a failed soap batch. Because the ingredients are often not disclosed, it is hard to tell the probability of failure of your soap when adding a new fragrance oil. That being said, I have used fragrance oils for some of my soaps and have never had a batch seize on me.

Essential Oils

Essential oils are considered more natural but the process by which they are extracted may be anything but natural. So if one of the reasons you are using essential oils is to be greener, be sure to know how the oil was extracted and be sure the plant that is providing the oil is being harvested in environmentally sustainable ways. Essential oils provide a stronger scent and often times last longer in finished soaps. If you plan on having your soap giving a room aroma for months and only on display, you’ll more than likely want to uses some combination of essential oils. Another benefit of essential oils is that they are a known ingredient and because of that the probability of a failed soap batch is known ahead of time. Unfortunately, because some plants don’t offer much in the way of oil getting your hands on some essential oils can cost an arm and a leg.Essential Oils that WorkHere is a list of essential oils that tend to withstand the saponification process quite well and come out smelling like what they did before they were turned into soap:

  • Almond
  • Cinnamon (be careful with this one)
  • Citronella
  • Cloves
  • Eucalyptus
  • French Lavender
  • Jasmine
  • Orange
  • Patchoili
  • Peppermint (this can irritate the skin)
  • Rose
  • Sage
  • Vanilla

How much should I use?Probably the most important question to ask is the amount to use to get the scent you want after your soap has set. This is actually a personal question for you to answer. It will depend on how the soap is being used (or not used). If the soap is merely for decoration, then you can go ahead and load it up with fragrance. On the other hand if you actually want to use the soap, you need to be aware of how the fragrance/essential oil you used will react to your skin. Having a cinnamon scented soap for general shower use might not be a good idea but having it for a foot wash would be ideal. I tend to follow the general rule that for every 30 oz of soap oil I used 1.5 to 2 oz of fragrance. I’ve found this gives me enough scent to come through the soap making process while still being easy on my skin.The next most important question is that of timing. This is actually a simple answer. Unless otherwise specified in a recipe, the fragrance (and all other additives like color, herbs or old soap pieces) right before the soap has reached full trace. Once you add the additives only a few more mixes of the soap is needed. If you mix more, there is more of a chance the things you just added will seize your soap or cause it to streak.

Liquid Scenting

Another way which you can add scent to your soap is by using a special tea or coffee as the liquid base for your soap. When you are making the liquid, you will want to make sure you make it very strong. This liquid is not for drinking and would probably taste horrible because it is so strong but what you are after are the qualities of the tea/coffee/juice as well as the scent. I’ve found that sage tea makes a good scent as well as coffee. The only thing to be aware of is that theses liquids can change the pH of the water. This will effect how your lye processes the oil. More often than not the liquid will tend to become more acidic. This in turn will cause a stronger reaction when you add the lye to your water.As you make more and more soap batches, you will become more comfortable adding fragrance to your soap and coming up with fabulously creative ideas! Good Luck!

Crazy Woody Soap

Crazy Woody Soap

Lipid Amount
in oz
Coconut 76° 15.2 24.72
Palm 15 24.39
Soybean 12 19.51
Olive 8 13.01
Hemp 6  9.76
Castor 3.3  5.37
Shea Butter 2  3.25
Total Weight 61.5

Fluid (water) needed:
17.22 – 22.755 fluid oz

Lye (NaOH)
in oz
0 9.14
1 9.05
2 8.95
3 8.86
4 8.77
5 8.68
6 8.59
7 8.50
8 8.41
9 8.32
10 8.22