A couple of weeks ago, just as I was pulling into a parking bay at the local mall here in Cape Town, my niece #4, who is in her final year of high school in Finland, called me on the phone.
“Clara, I’m doing a project on soapmaking for school”, said she.
“Excellent!”, said I.
“I think I should make some soap for the project”, said she.
“That’s a very good idea”, said I.
“Could you share a recipe, please?”, asked she.
“Nope”, said I.
At this point you might be rolling your eyes thinking “What a mean auntie!”
But wait, there’s more. Before you judge do read on because the story continues.
Instead I told her what I’ve told many others: good soapmaking is not about having a good recipe, good soapmaking is about understanding what makes the recipe good.
I told her that what she needed more than a recipe was to actually understand how lye is calculated for soap. To do that she needed to google ’saponification chart’ and start by finding the sodium hydroxide saponification coefficient for olive oil.
It took her all of 7 minutes to message me back with the correct number for olive oil. Now I suggested that she calculate how much lye she would need for a small batch of e.g. 650g of olive oil – and show me her calculation. Three minutes later I get the correct calculation on Whatsapp.
Then I told her to google ’superfat’ and a few minutes later she was able to tell me what superfat is. I asked her to do a 5% superfat aka lye discount on her 650g olive oil batch and a couple of minutes later she messaged me her calculation. Correct again.
Instead of quickly brushing off my niece with a ready recipe to copy, I had taken a few minutes (while shopping) to point her in the direction of some tools to put together her own recipes – complete with lye and superfat calculations.
A couple of days later she made her first soap from a recipe she had formulated herself (I checked that the formulation and the calculations were sound and correct) and by all accounts the soap turned out beautiful and is now curing nicely.
So let’s look at how it’s done. Let’s see what it is that your lye calculator does when it calculates lye for your recipes.
First Lye Calculation
Let’s pick a mock recipe and work through the steps of calculating lye for it. When I say ’mock recipe’ I really mean that: the recipes in this article are NOT meant to be examples of good soap formulation so don’t try soaping them at home. Instead, they were chosen to illustrate how lye calculations and lye discount calculations work.
So, let’s say our first recipe looks like this:
30% coconut oil
40% castor oil
We have a nice new mould and we know that it fits a batch of 730 grams of oils. I’m using grams, but you could just as well do the calculations in ounces – or any other weight unit. In soapmaking we always go by weight, never by volume.
Our first task is to find out how much of each oil we need for our 730g batch. To keep our numbers in order let’s keep them in a table. We could create a neat excel sheet, but since we don’t need a computer for this I’m doing it in good old paper-and-pencil style:
To get the weight of each individual oil we’ll multiply each percentage by 730. You can do the calculations in your head, on an abacus, on a slide rule; anything that gives you correct answers. I’m doing it on the calculator on my phone:
30% = 30/100 = 0.3 so if you don’t have a handy %-button on your calculator you can do it like this and it will give you the same answer:
There we go. Here we have figured out the weight of each individual oil in our recipe:
Now we need a saponification chart. A saponification chart is a list of oils and fats and their respective SAP (saponification) values.
On technical data sheets saponification values are expressed as milligrams of KOH per gram of oil. They tell you how many milligrams of potassium hydroxide you will need to fully saponify one gram of a particular oil. These values are often given as a range of numbers, typically somewhere between 100 and 300.
On saponification charts for soapmaking the values are given as coefficients (numbers) that can be used with any weight unit you choose. For each oil you will have one coefficient for KOH (potassium hydroxide aka caustic potash) and another one for NaOH (sodium hydroxide aka caustic soda). These numbers will all be fractions of one, i.e. zero point something. E.g. the NaOH saponification coefficient for olive oil is 0.135
In reality saponification values for oils vary depending on factors such as growing conditions, varietals, extraction method etc. The values you will find in any saponification chart are approximations and may vary slightly from one chart to another depending on the source. For convenience I’m using values from SoapCalc.net on this saponification chart of the most common soaping oils and fats:
We are making bar soap so from the chart above let’s pick out the NaOH values for our chosen oils and add them to our table:
Then it’s time to multiply our SAP values by our oil weights to get the amount of NaOH needed to saponify each of our oils:
If we take a look at the NaOH weights we can see how it takes a lot more lye to saponify 219g of coconut oil than to saponify 219g of lanolin. If we were to swap the coconut oil for an oil that requires less lye without adjusting our calculation, our soap would end up with with an excess of lye – which can be dangerous. This makes it obvious why it’s so important never to swap or omit oils without doing the corresponding adjustment to the lye calculation.
So, we need 94.097 grams of sodium hydroxide to fully saponify our 730 grams of oil. This leaves us with a 0% superfat. But if we’re making skincare soap we should make sure to have a little bit of superfat too.
What is Superfat?
The superfat in a soap is unsaponified oil (fatty acid) remaining after saponification is complete. In cleaning and laundry soap we don’t want any unsaponified oil, but in skincare soap we want superfat – for two resons:
- Unsaponified oil in soap helps condition skin by adding oils instead of the natural skin oils that get washed off by the soap. It makes the soap more conditioning while making it less cleansing. It also inhibits lather to some extent.
- A slight surplus of oil gives us a safety margin in our lye calculation. To make sure that we never end up with an excess of lye, e.g. due to slight measurement errors, we allow for a slight surplus of oil instead.
So, in order to be left with some unsaponified oil in our soap we discount the lye. This is why the expressions ’superfat’ and ’lye discount’ are synonymous in coldprocess soapmaking; two different ways of looking at the same thing.
In our recipe we want to allow for a 5% superfat. A 5% superfat means a 5% lye discount, i.e. we will use 100% – 5% = 95% of our lye.
Here I get a number with fractions of a gram, but since my scale will not weigh units smaller than one gram I will simply round down that number:
To make this recipe with a 5% superfat I need 89 grams of sodium hydroxide.
That’s the whole calculation and that’s how simple it is.
- You multiply the weight of each oil in your formula with its respective NaOH saponification coefficient. This gives you the amount of NaOH needed to saponify that particular oil.
- You then add up all the NaOH amounts to get the total amount of NaOH needed to fully saponify all the oils in the formula.
- Finally you discount the total NaOH amount by your superfat percentage of choice – and round down the resulting number to a unit you can weigh on your scale.
Now that we know how this works, let’s try another recipe. Again, I recommend that you don’t try to make this recipe at home; it’s only formulated to illustrate lye calculation – not to make great soap.
For this bar soap recipe we are planning a total oil weight of 3270g with the following oils and proportions:
13% beef tallow
7% shea butter
31% stearic acid
12% cocoa butter
and a superfat of 8%
Instead of going through all the steps separately, let’s just look at the full calculation here:
Here my oil weights came up as numbers with fractions of a gram. Since my scale doesn’t weigh fractions of a gram and I need to be able to weigh each oil exactly, I want to round the numbers to the closest full gram before I calculate the lye. In contrast, the NaOH weights are added up including all the decimals and the total is only rounded down after the lye discount (superfat) has been calculated. (Please note my error here: the saponification coefficient for tallow is 0.143 as per my saponification chart – not 0.140 as written down in this example. A good reminder how important it is to always check and double check recipes and calculations. Thank you Katherine for paying close attention and spotting the error!)
A final example. Here we know our individual oil weights and we want to do a 3% superfat:
100g olive oil
250g avocado oil
250g grape seed oil
200g sunflower oil
200g soy bean oil
With a 3% superfat this recipe requires 129g of NaOH.
If you run these ‘recipes’ through SoapCalc.net you’ll get slightly different numbers because Soap Calc uses more decimals and doesn’t round oil weights, but the differences are very small. Yet, the differences confirm the value of a slight superfat as a safety margin in soapmaking.
What about the water?
Soap is made by saponifying oils with lye. In order for lye and oil to combine lye crystals need to be dissolved in water. The water activates the lye and makes the saponification reaction possible.
The amount of water necessary to facilitate the saponification reaction is equal weight to lye. I.e. our example recipe above which requires 129g NaOH would require a minimum of 129g water. Less water than that and we risk that the lye crystals don’t fully dissolve. This may leave our soap with pockets of unsaponified lye and we want to avoid that.
The upper limit is a little more fluid but in general when making coldprocess soap you don’t want to go higher than three times the weight of the lye. Beyond that the risk of separation increases as does the risk of soap volcanoes and overheating – not to mention that the freshly saponified soap might be very soft and take a long time to dry out.
So the water range is from 1:1 water:lye to 3:1 water:lye. For beginners it’s usually prudent to stay in the middle of the range and use water at 1.5 or 2 times the weight of lye. More water does not make the process safer either for soapmaker or soap so it’s not worth adding extra water for that reason.
‘Water discount’ is a misnomer since we can’t reduce water from the necessary 1:1 water:lye ratio. Any water amount in excess of a 1:1 water:lye ratio is in fact a surplus. The expression refers to a discount from the water default in SoapCalc.net which at 38% of oil weight is rather excessive for coldprocess soap.
Abacus vs Lye Calculator
If it’s this simple to caculate lye with a saponification chart and a calculator (or an abacus, or just your head), can’t we just abandon lye calculators altogether? The answer is no.
While the lye calculation itself along with lye discount and determining water content are simple operations, a good lye calculator offers additional functions that are both handy and important in formulating good soap. A lye calculator should at least give a fatty acid breakdown, along with INS and iodine numbers for each formulation. In addition soap properties like hardness, lather and cleansing numbers are broad but valuable guidelines for formulation. And unless you have remarkable computing skills, making adjustments, tweaks, and changes to formulations will be a lot quicker on a lye calculator than by means of pen and paper.
Yet, being familiar with the steps involved in lye calculation gives a more detailed understanding of the soapmaking process. It can a be a very valuable tool for troubleshooting not to mention how useful it is for those who don’t have access to smartphones or the internet.
And next time your favourite lye calculator does the calculation for you you’ll be able to say “I see what you did there!”