Figuring Out a Castille Soap Recipe

A castille soap is simply an olive oil soap. It’s an easy soap to get started with since you don’t need to meld any oil and it reacts slow enough to see what’s going on. It takes a bit more time to harden than some other soaps to harden, but it will harden up nicely while being a wonderfully mild soap.


The only things you really need for this recipe is olive oil and lye. I usually use cheap olive pomace oil for soap. Pomace oil is obtained from the last pressing of the olives and is poor quality for food, but is great for soap. Since there are more impurities, you won’t need to stir as much. You could always try with the best quality extra-virgin oil, but you will have to stir alot before your soap is ready. Also, make sure that what you buy is 100% olive oil by checking the label carefully : some cheap brands sells olive oil mixed with canola oil and the results are unpredictable in that case.

The maths

Now, we need to figure out how much lye you need to use. Soap recipes are always in weight of oil. We’re going to do a small 1 lb (16 oz) batch, so we already know that we’ll need 16oz of olive oil.

Then we’re going to use a ratio called SAP that express the amount of lye needed to make the oil react completely. This ratio is different for each oil, but since we’re going to use only oil this is going to be easier. The ratio for the olive oil is 0,134 and is expressed in oz, so :

16 oz of olive oil X 0,134 = 2,144 oz of lye

With this amount of lye, 100% of the olive oil will react, and we don’t really want to do that. You need to add a little safety margin to make sure all the lye has reacted with oil, with some extra in the soap so it will be milder. You don’t want too much though, or the soap will be soft and mushy. A 5% superfat, or 5% extra oil, is calculated this way :

16 oz of oil X 0,134 X (1 – 0,05) = 2,0368 oz of lye

It’s not a huge change since the batch is small, so you’ll have to measure as precisely as you can. The only thing left is to calculate how much water will be used. This is not as important since the water will evaporate, but too little will make it hard to dissolve the lye and too much will increase the time the soap takes to harden. A 40% solution is a nice compromise for most soaps. So, to calculate it :

(2,0368 oz of lye / 0,4) X (1 – 0,4) = 3,0552 oz of water


The final recipe

To sum it up, the recipe will be :

1 lb (543,6 g) of olive oil
2,0368 oz (57,74 g) of lye
3,0552 oz (86,61 g) of water

You can double-check those calculations with the MMS Lye Calculator. The default values are fine, just write the amount of olive oil (in oz by default) and press “Calculate Lye”. The amount will not be identical, since SAP is an approximation and I don’t know which numbers they use. Next time, we’re actually going to make this soap.

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4 Responses

  1. Great post, I’ll have to try it.

  2. Ali P in the Qc says:

    Now I want to make soap. Cin, you are SUCH an enabler!!!!!! And an inspiration to a scaredy cat like me. Plus, I love your gummies when you come to knit night…LOL

  3. Cindy says:

    Great to see that you like it, I’m sure you can make great soap. The final post should be up soon, I’m a bit busy since I’m moving in May.

  1. February 12, 2011

    […] the last post in the series, we figured out a simple castille soap recipe […]

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