Fatty acids -- myristic, lauric, stearic, palmitic, oleic, ricinoleic, linoleic, linolenic -- are the building blocks used to make soap. To get an idea about the qualities a particular soap may have, you can look directly at the percentages of fatty acids in the recipe.
Another way to evaluate the fatty acid profile of a soap is to look at the cleansing, bubbly, creamy, etc. numbers calculated by various soap recipe calculators, including SoapCalc and Soapee. The owner of SoapCalc originally created these properties and defined how to calculate the values for each. These decisions were based on input from soap makers from the Soapmaking Forum, according to what I've been told. Based on this history, I will refer to these numbers as the "Soapcalc numbers."
Whichever group of numbers you prefer -- the fatty acid percentages or the Soapcalc numbers -- it is important to remember these numbers are only a general guide. They cannot tell you all there is to know about a particular soap recipe. For example, they do not account for the effect of superfat nor for the influence of additives (sugar, milk, honey, sodium lactate, etc.). The numbers also do not tell you anything about the changes that can result from how the soap is made and cured.
To make things more confusing, Soapcalc's names -- bubbly, cleansing, conditioning, etc. -- are misleading, especially to newer soapers who tend to take these names literally. For example, a person might think a soap with a "cleansing" number of zero will not clean the skin at all, even though the soap will clean just fine.
I want to explain the Soapcalc numbers by looking at the fatty acids for a single fat and explain how you can calculate the Soapcalc numbers from these fatty acids. I have chosen cocoa butter as my example. Cocoa butter has a fatty acid profile that looks something like this:
Palmitic 25-35% (average is about 30%)
Stearic 28-38% (average is about 33%)
Oleic 29-41% (average is about 36%)
Linoleic 2-7% (average is about 4%)
SoapCalc groups these numbers into fewer bits of useful information:
How is the fatty acid profile converted into the Soapcalc numbers? And what do those numbers mean? And what are some of the other factors that affect soap qualities besides the fatty acids?
The fatty acids that contribute to hardness come from the fats that are solid (or solid-ish) at room temperature -- coconut, babassu, palm kernel, palm, tallow, lard, and the butters (cocoa, shea, mango, etc.)
The Hardness number is a measure of the physical hardness-like-a-rock. It also is an estimate of how easy it will be to unmold a particular soap after saponification -- hard soap tends to be easier to unmold.
The Hardness number does not necessarily tell you how long-lived the soap will be, however. I explain more about longevity below.
Cocoa butter hardness --
Hardness from the fatty acid profile = 0% + 0% + 30% + 33% = 63%.
Soapcalc Hardness = 61%.
Is the difference between 63% and 61% important? Nope! A fatty acid profile for a given fat is an average for that fat. The SoapCalc folks calculated their Hardness number from slightly different data than I am using. A few percentage points difference in any of these numbers is not important.
What's a reasonable range for the Hardness number? Frankly, I do not pay much attention to it. I focus on the Cleansing number (myristic and lauric acids) and the Longevity number (palmitic and stearic acids). The Hardness number comes directly from those two answers. Why worry about three numbers if only two are important?
So let me talk more about the Cleansing and Longevity numbers...
Lauric and myristic acids are found in coconut oil, babassu oil, and palm kernel oil. Soap with a high cleansing number has the ability to dissolve easily in hard water, cold water, or salt water.
If you are going to make a salt-bar (soap with fine salt crystals), a brine bar (soap made with a mixture of salt dissolved in water), or a mariner's soap for bathing with seawater, you will want to make a soap high in lauric and myristic acid. A soap that is high in these fatty acids will not last long due to its high solubility in water, especially if you use soap as most people do -- for bathing in warm, not-salty water.
There is another issue to consider. Soap high in lauric, myristic, and similar short-chain fatty acids also has the ability to bind with and remove the natural proteins and fats on the surface of the skin that protect the underlying living tissues. If too much of this normal protective barrier is stripped away, a person's skin will feel tight, dry, and even irritated and reddened.
Not everyone's skin reacts strongly to the stripping effects of soap high in lauric and myristic acid. The response depends on genetics, allergies, skin type, age, gender, occupation, climate, season of the year, air humidity, sun exposure, amount of bathing, etc.
People with sensitive or troubled skin may have to use soap with no lauric and myristic acid, regardless of the amount of superfat or other additives included in the recipe. Never fear -- a soap with a zero "cleansing" score will still clean the skin just fine.
People with normal skin often do well with soap that has 10% to 20% combined lauric and myristic acids, maybe a tad higher. Some people, men in particular, may prefer a stronger cleansing soap with combined lauric and myristic acids ranging from 20% to 35%.
If even more lauric and myristic acid is used in a soap recipe, the superfat is often raised higher than usual -- as high as 15% to 20% superfat -- to help tame the harshness of the soap.
Cocoa butter cleansing number --
Cleansing from the fatty acid profile = 0% + 0% = 0%
SoapCalc Cleansing = 0%
Palmitic and stearic acids -- the fatty acids in lard, tallow, palm, and butters -- create a soap that is hard and fairly insoluble in water.
A moderate percentage of these fatty acids will create a soap will last a decent time in the bath, be mild to the skin, and have a dense, long-lasting lather. Too low and the soap may not last long as you might prefer. Too high and the soap may not lather very well. So what is the happy medium?
A common concern voiced by soapers in the Soapmaking Forum is their soap does not last long enough. Having informally looked at many of these recipes over the years, I see most have a combined stearic and palmitic content in the low 20% range.
A survey done by Kenna of Modern Soapmaking showed a similar trend. The combined stearic and palmitic content in the recipes she looked at averaged about 20% with a range from about 13% to 28%. (1) Just because many recipes have a combined stearic and palmitic content in the low 20% range, does not mean this is an ideal amount, as I have learned from experience.
When I first started to make soap, my recipes also fell in this range. When my soap making mentor critiqued soap from these early batches, she said the soap didn't last long enough in the bath and challenged me to do better. I gradually increased the combined palmitic and stearic content and now most of my general purpose bath soap recipes contain to 30% to 35% combined stearic and palmitic and I am happy with the longevity. Other soap makers shoot even higher -- 35% to 40%.
SoapCalc does not provide a value for longevity. When using Soapcalc, a quick way to estimate the Longevity number is this:
Longevity = Hardness - Cleansing
Cocoa butter Longevity number:
Longevity from fatty acid profile = 30% + 33% = 63%
Longevity from Soapcalc numbers = 61% - 0% = 61%
For a coconut oil soap, the Longevity number is quite different:
Hardness from Soapcalc = 79%
Cleansing from Soapcalc = 67%
Longevity = 79% - 67% = 12%
A 100% coconut oil soap will be hard-like-a-brick, just like a 100% cocoa butter soap, but the coconut soap will not last nearly as long, all other things being equal.
These are the fatty acids from monounsaturated and polyunsaturated oils (olive, avocado, canola, sunflower, safflower, grape seed, castor, etc.) Soap high in these fatty acids is more water soluble and the soap forms a low, dense "whipped cream" lather. Many people think soap high in these fatty acids is also mild and gentle, although that is not universally true. Soap high in unsaturated fatty acids can be irritating or drying to some people's skin.
Cocoa butter conditioning number:
Conditioning from the fatty acid profile = 0% + 36% + 4% + 0% = 40%
SoapCalc Conditioning = 38%
The Conditioning number is not helpful, in my opinion, for three reasons.
Firstly, the name "Conditioning" is misleading because some soap makers interpret this to mean soap can truly condition the skin. I wish the original designers had chosen a more accurate word! Perhaps "mildness" or "unsaturated" -- something other than "conditioning."
Conditioning is the blanket property of adding humectancy (water) to the surface of the skin, providing occlusion (barrier to abrasion or water evaporation), adding emollience (replacing lost fats), as well as smoothing and soothing the skin.
Some additives and the superfat in a soap may provide a small amount of conditioning, but soap itself is a cleanser; it removes dirt and fats from the skin. Soap can be a mild and gentle cleanser or it can be a harsh and drying cleanser, but it cannot also function as a conditioner.
Secondly, the Conditioning number, as defined by the soap recipe calculators I have looked at, does not even do a good job of measuring "mildness." A mild, gentle soap is usually high in palmitic and stearic acid and moderate to low in the other fatty acids -- a 100% lard soap being the classic example.The palmitic and stearic acids are not included in the Conditioning number, so their important contribution to mildness is entirely ignored.
Lastly, when I set the Cleansing number (lauric + myristic acids), set the Longlasting number (palmitic + stearic acids), and decide whether or not to use castor oil (ricinoleic acid) -- then those three decisions automatically define the Conditioning number. So why worry about it?
My goal as a soap maker is to create soap that cleans sufficiently well while also being mild, refreshing, long-lived, and nicely lathery. I focus on getting the cleansing (lauric + myristic acids) and longevity (palmitic + stearic acids) numbers in the ranges that work best for me. Once those values are set, I then look at the individual percentages of the oleic, linoleic, and linolenic acids.
Oleic (oh-lay-ick) acid increases the solubility of the soap and creates creamy lather. Too much oleic acid can give the lather a slimy quality. Too little oleic acid and the soap might not lather well.
Kenna of Modern Soapmaking reported the recipes she studied averaged 36% oleic acid with most recipes ranging from 32% to 41%. (1) My general purpose bath soap recipes typically have 40% to 50% oleic acid.
Linoleic (lin-oh-lay-ick) and linolenic (lin-oh-leen-ick) acids make soap more prone to rancidity (DOS, dreaded orange spots) and can reduce the initial hardness of the soap so it can be difficult to unmold. The rule-of-thumb I use is to limit these two fatty acids to 15% or less in my soap recipes. Most of my recipes run below that limit -- about 5% combined linoleic and linolenic. Other soapers look at the INS value or Iodine number to evaluate the likelihood of softness or rancidity.
More on ricinoleic acid in Bubbly.
Soap that is high in lauric and myristic acids is highly soluble in water, and this soap will make lots of big fluffy bubbles. Most people like lots of fluffy lather, so it is tempting to design a recipe high in these fatty acids. The downside of a recipe high in lauric and myristic acids is the soap can also be harsh and drying for many people's skin.
I do not pay any attention to the Bubbly number. The only difference between the Bubbly number and the Cleansing number is the ricinoleic percentage, and that depends simply on whether I include castor oil or not. So I really only need to worry about the Cleansing number and decide whether to use castor oil.
Ricinoleic (rye-sin-oh-lay-ick) acid, the third part of the Bubbly number, comes solely from castor bean oil. In soap, ricinoleic acid increases the water solubility of the soap and it stabilizes and strengthens the bubbles made by the other fatty acids. This means castor helps the soap to start lathering quicker with less work and the lather lasts longer.
If your soap lathers poorly without castor, it will not lather better with castor. You will see the most benefit by adding castor to a soap recipe that already produces good lather -- the castor may help the soap lather a notch better. A rule of thumb is to use castor at 2% to 5% of the total fats in a typical bath soap recipe, although that is not a hard-and-fast rule.
Cocoa butter bubbly number:
Bubbly from the fatty acid profile = 0% + 0% + 0% = 0%
SoapCalc Bubbly = 0%
Palmitic and stearic acids produce a long-lived lather like whipped cream. While fluffy bubbles are fun, a handful of dense, creamy lather is satisfying too. What is interesting is that oleic acid is not included in the Creamy number, even though a high-oleic soap also produces dense, long-lasting whipped-cream suds.
As mentioned earlier (see Bubbly), ricinoleic acid in soap enhances lather from the other fatty acids, but it does not create lather on its own, so castor oil, the source of ricinoleic acid, is best used in small amounts.
I do not pay attention to the Creamy number any more than I do the Bubbly number. If I get the Longevity number in the range I like and decide whether to use castor oil or not, those decisions define the Creamy number without any extra effort.
Cocoa butter creamy number:
Creamy from the fatty acid profile = 30% + 33% = 63%
SoapCalc Creamy = 61%
Other factors that affect soap quality
The Soapcalc numbers and the underlying fatty acid profiles are helpful guides when creating a soap recipe, especially if you do not pay much attention to the misleading names. I want to stress that you cannot focus on these numbers alone, however. There are other factors to keep in mind when designing a good soap recipe.
Superfat. The fatty acid profile and the Soapcalc numbers will not change if you raise or lower the amount of superfat. That does not mean superfat has no effect on the soap, however. Raising the superfat by as little as 2-3% may noticeably increase the softness of the soap and reduce the lather.
Additives. The Soapcalc numbers also do not account for additives, many of which can change the lather quality, add mildness, increase the superfat, affect bar hardness, and so on. Examples of such additives include sugar, sodium lactate, beer, vinegar, citric acid, dairy, salt, aloe juice, pigmented colors, fragrances, etc.
Method. Soap qualities are also affected by the way in which the soap is made. Some factors include the amount of water-based liquid in the recipe, whether the soap is allowed to gel or not, the type of mold used, and whether the soap is fairly new or well cured.
Water quality. Minerals in hard water react with soap to form insoluble soap scum that sticks to your skin, sink, and bathtub and makes your skin feel sticky, itchy, and dry feeling. Soap that lathers beautifully in reasonably soft water will have average to poor lather when used with hard water. A chelator such as citrate or tetrasodium EDTA reacts with hard water minerals to reduce or prevent soap scum from forming.
(1) Kenna. The Most Popular Fatty Acid Profiles, determined by survey! Modern Soapmaking. Version as viewed on 26 September 2020. https://www.modernsoapmaking.com/the-most-popular-fatty-acid-profiles-in-soapmaking/
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