Denise was out over the weekend helping to make a batch or two of soap. Look at that girl! She color coordinates her outfit with the soap! She never takes a bad picture and she's rocking my new mold! Don't you love how the sides are clear too?!
I think she was maybe a bit surprised to find out how much math and science is involved with making soap. For the next few posts, I’m going to talk about some of the math, science, problem solving and critical thinking that goes into soap making.
I’m telling you – never get between a soap maker and her soap calculator! This is our lifeblood when creating recipes.
A soap calculator figures out how much lye is needed to turn oils and fats into soap.
Too much lye makes a very harsh soap and elicits stories about “grandma’s laundry soap” and how it made their hands red and raw. Not enough lye and you have an oily slimy mess.
Calculating Lye Amounts
Today, we know the SAP value of the oils and fats we use in soap making. For example, olive oil has a SAP of 135 and coconut oil has a SAP of 183. The SAP value is the amount of lye in milligrams needed to turn 1000mg of oil into soap. By knowing the SAP value and the amount of oil we will be using, we can now calculate the amount of lye needed. If I make a batch of soap that has 2000 grams of oil (1000 of Olive and 1000 of Coconut), I would need 318g of lye to make soap.
But soap makers formulate their recipes to be "superfatted." This means that a certain percentage of fats and oils won't be turned into soap. This allows us to make a more gentle bar. To superfat a soap we reduce the amount of lye by a certain percentage. A 5% superfat would reduce the amount of lye in our example above to 302 grams.
The fringe benefit is that what's good for the soap maker is also good for the customer as it helps us make a safer product.
(Right now, Denise is so happy that there is such a thing as a soap calculator!)
Formulating a recipe
Many of today's soap calculators will help a soap maker formulate a recipe with by evaluating the properties of each oil selected. A soap calculator will “score” your recipe on --
Iodine - indicator of hardness/softness in soap. Higher number = softer bar.
INS (an overall predictor of a good bar of soap from a technical perspective)
Look how different the properties on Olive and Coconut Oil are! Coconut wins hands down in Hardness, Cleansing and bubbly lather, but olive oil rocks in conditioning.
How do those numbers impact recipe formulation? For a “mechanic’s bar” a soap maker would want a high cleansing value, but for a person with cancer they would probably make the cleansing number as low as possible if their skin is fragile. Oily skin or babies might be another reason to shake up the numbers to come up with an special formulation.
A well balanced soap
But what if you are looking to make a well balanced soap? In that case, we may try to get our numbers to fall within some suggested ranges. If you look back our example oils, coconut and olive, you see that they fall out of the range. In coconut oil the hardness and cleansing values are higher than desired and the conditioning is much lower than desired. In olive oil, the hardness and bubbles are both low.
By adding oils with specific properties in varying percentages we are able to tweak a formula to get a well balanced bar. (But just so you know, we are not adverse to breaking the rules.)