What is all this talk about "discount" and percentages?
I guess you could figure it this way. It will take a given amount of lye to completely saponify an ounce of a certain fat...with no excess lye or fat left over. This is what the numbers are based on. If you want a cushion of extra fat so that it's milder, you build it in by having 5% more fat in the recipe than you have lye to saponify it...5% excess fat. You can either add on more fat, or subtract some lye...but in the end you want about 5% difference (some people want even more), with the fat being higher. SO, if you figure out how much lye it takes to get it to be a perfect saponification (with no extra of either)...then you take that number and multiply it by .95 (95%), you would then have the amount of lye you want for that 5% cushion. Am I just confusing you more? So, if my recipe said 12 ounces would perfectly saponify the fats, I would multiply 12 oz. X .95 = 11.4 oz. of lye. If I used 11.4 ounces for that recipe, I would end up with it being a 5% discount and would have extra fat in the recipe by that amount. This is more to protect us from errors than anything (measuring types). You can still do a little bit more superfatting with a 5% discount and be okay, but if you get too carried away, your soap could be kind of soft or a bit spongy. The MMS lye calculator is nice because it gives you the measurements in the three areas (although the one that says "proceed with caution" would be fine if you are totally accurate in your measurements).
What complicates this a bit more, is that there is more than one SAP table out there. Elaine White came up with the first one and it is a bit on the higher side for the amount of lye it recommends (and laboratory conditions and the quality of fats can change the numbers a bit too). It doesn't leave much margin for error. The Majestic Mountain Sage people have a slightly altered SAP chart they use for their calculator (don't know exactly how much...but on some of the oils used most often, their numbers are a bit different) and they also show you the quantities you need for certain amounts of excess fat in your recipe. I put up the chart above so that I could use numbers with the 5% built in and figure the recipes without MMS online calculator if I needed to. When I've figured the recipe with milder numbers and the 5% built in, I don't worry about superfatting at the end...unless I put in a tiny amount of castor or vitamin E. or something.
Also something to note: In Susan Miller Cavitch's books she gives a SAP table, but many people fail to notice the the posted numbers are for Potassium hydroxide and not Sodium hydroxide (lye). You have to multiply all of them by a fraction (40/56.1) to get the SAP numbers for lye. I've seen people quote those numbers as the SAP value for lye and wow! Would that ever by a lye heavy recipe if someone used them! I just saw something like this on a site that sells cocoa butter. She had in her specs on cocoa butter a SAP number of 193.8.
Since I use lye in 12 oz. cans, I try to make it come out (on the MMS site) with around "12 oz." shown in the 4-5% range on their chart. If it's not quite right, I back up and change the quantity of oils slightly and refigure until it comes closer.
You can superfat in any number of ways and I don't think it's magical to just add it at the end, assuming it won't get saponified. If the saponification was completed that soon, we could use the soap as soon as it got hard without aging it (like when you cook it). If your superfatting agents have special qualities you want to retain (like vit. E.) then it might be worth adding them at the end when there's not as much free lye present, but for the other oils, I just add them up front and do the recipe with a 5% discount built in. Superfatting in that case is optional.
-Kathy Miller-
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