Sure I could easily follow someone else's recipes or use a lye calculator to do the math for me, but I prefer to design my own soap. If you're as excited as I am about knowing the mathematical formulas to create your own soap recipes, then this is the article for you.
I'm using two recipes to demonstrate the concepts and calculations throughout this article:
~Recipe A will be a single oil soap made from olive oil with a superfat of 2%.
~Recipe B will have a superfat of 10% and includes a blend of multiple oils: coconut oil, castor oil, and sunflower oil.
Note: Since we're only focusing on solid soap here, all references to "lye" refer solely to sodium hydroxide. When I get into liquid soaps in later articles I'll address recipes with potassium hydroxide, which is different.
Okay, let's talk "superfat"! Superfatting (also known as "lye discounting") is an important consideration in the composition of your soap. During the chemical reaction between lye (sodium hydroxide) and fat (plant or animal), it's important for all of the lye to be converted into the soap molecule or it will irritate, and possibly even burn, your skin. In order for this to happen there needs to be enough fat present. Superfatting refers to the amount of fat leftover in the soap after all the lye has been transformed. A superfat of 0% means that there is just enough fat to convert all of the lye without any fat molecules leftover. A superfat of 5% means that 5% of the original amount of fat still remains after the soap is formed. Having some of the extra fat leftover provides beneficial moisturizing and nourishing benefits to the skin. Everyone has their own superfat preferences. Some only like 2-3% while others go as high as 15%.
Keep in mind: A superfat that's too high comes with some issues. If too much extra fat is present, the soap bar will become softer, the amount of lather is reduced, and the likelihood of the fat going rancid and ruining your lovely soap increases. Inversely, I don't recommend a superfat of 0% for soap used on the body because it can be overly drying on skin, or worse yet, have some remnants of lye that haven't been converted to soap. With the exception of my ultra moisturizing soaps (which go up as high as 15%) and my house cleaning soap (which stays at a strict 0%), I prefer to keep my superfat between 2-10%.
Ready for the fun stuff? Don't freak out. Designing your own soap recipes is actually a pretty easy process once you understand some basic calculations.
Let's get scientific! Every type fat has a saponification (SAP) value, representing how much lye it takes to transform 1 gram of fat into 1 gram of soap. Here's the formula:
(amount of fat) x (SAP value) = (amount of lye needed)
For example recipe A, I'll use 16 ounces of olive oil. Olive oil has a SAP value of 0.135. Plugging this into the equation...
(16 oz olive oil) x (0.135 olive oil's SAP value) = (oz of lye needed)
16 x 0.135 = 2.16 oz lye
Applying this equation to a multi-fat soap, lets look at recipe B. The first thing to always do is decide what the total volume of oil you want to start with is. Say I want 16 ounces of oil for this recipe also. I want 50% to be coconut oil, 25% to be castor oil, and the remaining 25% sunflower oil. How many ounces of each oil will I need to make a total of 16? 8 oz coconut, 4 oz castor, and 4 oz sunflower. To figure out the amount of lye needed I'll have to use the SAP equation on each oil because they all have different SAP values.
Coconut oil: (8 oz oil) x (0.184 coconut oil's SAP value) = 1.472 oz lye
Castor oil: (4 oz oil) x (0.129 castor oil's SAP value) = 0.516 oz lye
Sunflower oil: (4 oz oil) x (0.136 sunflower oil's SAP value) = 0.544 oz lye
Now, add all the products together: 1.472 + 0.516 + 0.544 = 2.532 total ounces of lye
The amount of lye from the above equations will produce a 0% superfat, meaning that pretty much all the oil and lye will be turned into soap. But didn't I say I'm making 2% and 10% superfatted soaps? Yes! The next step is another simple formula:
(amount of lye from initial SAP equation) x (1 – amount of superfat desired converted into a decimal) = (actual amount of lye needed)
I know it looks like a lot of words in that equation, but let me break it down starting with recipe A, the 2% olive oil soap. Notice that 2% becomes 0.02 when converted into a decimal...
(2.16 oz of lye from the initial SAP equation) x (1 – 0.02) = (new amount of lye needed)
After subtracting the 0.02 from the 1, the final calculation looks like this:
2.16 x 0.98 = 2.117 oz lye actually needed
Note: I'm going to use the rules of rounding for easier measurements (2.1 oz lye).
This same equation applies to multi-fat formulas, such as recipe B. Because I already figured out the overall lye amount needed, I only need to do one calculation using the final product of the lye amounts added together. In this recipe I want 10% superfat, converting 10% to 0.10.
(2.532 oz lye from the initial SAP equation) x (1 – 0.10) = (new amount of lye needed)
After subtracting the 0.10 from the 1, here's what I get:
2.532 x 0.90 = 2.279 oz lye actually needed
After rounding, I end up with 2.3 oz lye.
So, I know the amount of oil I'm using and now the amount the lye needed. But what about the amount of liquid to use? Liquid, usually distilled water, facilitates the chemical reaction between lye and fat. During the curing stage, much of the water will evaporate, leaving a firm bar of soap. It's important to note that the amount of water used affects the outcome of the soap. Too much water = soft spots, plus takes it longer to trace, set, and cure. Too little water = hotter temperatures, quicker thickening, and can be difficult to work with. Water affects the temperature, as well as the gelling phase.
There is no standard percentage of water used in soap recipes. Every soap-maker and lye calculator has their own preferred percentage – some a little higher, some a little lower. However, all water calculations are based on the amount of lye (never fat), after all, you're creating a lye concentrated solution when making lye water. The easiest and most common method is to use a ratio. Typical ratio's used are 1:2, 1:3, and 1:4. For example, using a 1:4 ratio, four times as much water is mixed with the lye (a 20% lye solution). So if your recipe calls for 1 oz lye, you would mix that with 4 oz of water, and the lye would make up 20% of the overall solution. A ratio of 1:3 is a 25% lye solution (1 part lye to 3 parts water). And a 1:2 ratio is 33% lye solution. However, no matter what, you will end up with functional soap, even if it isn't pretty.
Heads up: Some recipes use "water discounting" where a calculated reduction of water is used to produce certain effects or off-set the liquid found in additive ingredients (such as pureed fruits). Be careful never to go below a 1:1 ratio because lye needs at least twice as much water to dissolve and to facilitate the chemical reaction. Personally, I don't recommend going below a 1:1.5 ratio. Demo recipes A and B here don't have any other additives or special needs so no water reduction is necessary.
For recipe A, I'll use a water ratio of 1:4, which has the most liquid in it, because at a 2% superfat the soap will tend to be harder without much excess oil to soften it up. Using a higher water content will add a little more moisture to the process. Since water calculations are based on lye, I'll use the final product from the previous lye discounting equation.
(2.1 oz lye) x (4 water ratio modifier) = (amount of water needed)
2.1 x 4 = 8.4 oz water
In recipe B, the higher 7% superfat will create a softer soap so a lower water ratio (I'll go with 1:2) will help balance that out. Also, with a lower water content, this soap will probably reach trace quicker than recipe A. Again, I'm taking the final product from the previous lye discounting equation.
(2.3 oz lye) x (2 water ratio modifier) = (amount of water needed)
2.3 x 2 = 4.6 oz water
To sum it up...
1) Choose the amount of fat to use
2) Determine the lye needed to create a 0% superfat using the SAP equation on each type of fat, then add all products: (amount of fat) x (SAP value) = (amount of lye)
3) Choose your superfat percentage and calculate the actual amount of lye to use with the lye discounting equation: (lye from SAP equation) x (1 – superfat% converted into a decimal) = (actual amount of lye needed)
4) Determine the amount of water needed: (amount of lye using) x (water ratio modifier, your choice: 2, 3, or 4) = (amount of water)
Always consider finding balance:
Higher superfat = softer soap; less lather; longer to trace
Lower superfat = harder soap; quicker to trace
Higher water = softer soap; lower temperatures; longer to trace, set and cure
Lower water = quicker to trace; higher temperatures; more gelling
There we have it! Now, let's put both recipes A and B together in easy to read formats.
Recipe A: 3% superfat olive oil bar soap
16 oz olive oil
2.1 oz lye (sodium hydroxide)
8.4 oz distilled water
Recipe B: 7% superfat multi-oil bar soap
8 oz coconut oil
4 oz castor oil
4 oz sunflower oil
2.3 oz lye (sodium hydroxide)
4.6 oz distilled water
Take a look at this easy-to-use SAP value chart of many common oils.
Now, let's get into the nitty gritty process of making Single oil soap and Multiple oil soap!
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