lye /oil sap formula

[Wyndham Dennison]

Hey folks, another newbie getting into the soap vat. I am trying to get a feeling of how the whole cp comes together with the lye ratios etc. I printed out a oil SAP value chart and used Brambleberry's lye calc to confirm a recipe I had found on the net, it was spot on.
My question is about a formula for doing old school math of each oil , say canola oil with a SAP of .132. What does this number relate to. Or put it another way how to use this number to get the lye amount.
I don't mind using the calc program I'd just like to understand the math behind it.
My other thought since the lye calculus does not take in the SAP value of the superfat put in after trace, can the superfat with it's SAP number, adversely affect the curing time of a batch.
BTW, I'm a potter here in NC and have made soap dishes for a local soapmaker for some time and looks like the soap may have rubbed off on me. Thanks for any help Wyndham

 

[Anstarx]

Canola oil has a SAP of 0.132 means it takes 0.132 grams of lye to react with 1 gram of canola oil.
Say if you want to make a 100% canola oil soap, and you want to use 1000g canola oil. In order to calculate the lye amount you need, you times the amount of oil and its SAP value. Some thing like this:
1000g x 0.132=132g
So that means you will need 132g NAOH for 1000g canola oil to make a 0% SF soap.

If you have multiple oils in your recipe, you need to calculate each oil's lye amount individually, add them together, then minus the lye discount.
For example, my recipe is 50% olive, 25% coconut, 25% palm, 5% SF, and the total oil amount is 1000g.
You will need 500g olive, 250g coconut, 250g palm. Still with me?

500g olive needs 500gx0.135=67.5g lye
250g coconut needs 250gx0.183=45.75g lye
250g plam needs 250gx 0.142=35.5g lye
Add them together you get 67.5+45.75+35.5=148.75g lye
For SF, a 5% SF means deduct 5% from your total lye mount.
So the final lye amount you need for this recipe is 148.75g x(100-5)%=148.75g x 0.95=141.31g

I just ran this recipe through soapcalc and the given amount of lye is 141.57g
Close enough I say. For some reason their calculation result is always different from mine by 0.5g or so.
Boy do I love math lol.

Also, each oil has different SAP value for NAOH and KOH so watch out for that.
Auntie Clara has written a very good tutorial poston this: https://auntieclaras.com/2018/09/lye-calculation-tutorial/

 

[CatahoulaBubble]

So you're calculating how much lye to use for the oils you are using. So if canola oil is .132 the you take .132 and multiply that against how many ounces of oil. That gives you the lye amount that should be used for the batch. So if the batch of oils is 12 oz you will do 12 X .132 =1.58 oz of lye. When using multiple oils you would calculate each saponification value times the ounces of each oil and then add the totals to get the total amount of lye that should be used for the whole batch. This is lye with no liquid added so the raw lye vs oils but you still need to calculate the liquid that you dissolve the lye in.

The rule of thumb is
weight of oils × Saponification Value of the oils = amount of pure Lye
weight of lye ÷ 0.3 = Total Weight of Lye Water Solution
Weight of Lye Water Solution− Amount of Lye)= Amount of Water

 

[Wyndham Dennison]

Thanks for the info.

 

[Steve85569]

Much simpler for me to use one of the many lye calculators available rather than hand calculating every batch.
http://soapcalc.net/calc/SoapCalcWP.asp
SoapMakingFriend.com

etc...

https://www.soapmakingforum.com/threads/soap-making-friend-fully-operational-thank-you.76481/

is a better link for SoapmakingFriend,,,