Superfat is literally extra fat that you have in your soap recipe that does not become soap. It is NOT saponified because there is not enough lye in the recipe to make ALL the oils turn into soap. The oil is kind of floating in between the soap molecules, so when you use the soap, not only does it clean, but it also deposits a thin layer of fresh oil on your skin.
This is a great cartoon that helps explain superfat & what is going on in the bowl/pot while you are making soap.
Bunnies & Wolves
In a Cold Process (CP) recipe the lye is not finished working when you mold the soap. Unfortunately, that means that the lye gets to pick which oils it will saponify. Maybe you want to add a rather expensive luxury oil as a superfat. There is no guarantee in CP that the oil YOU want to make the superfat will wind up as the superfat.
In Hot Process (HP), the lye is finished working, as a result of using heat to accelerate the reaction, when the soap is put in the mold. You can test the pH of the soap or do a "zap test" to make sure the soap is "done", completely cooked, & no lye remains. The zap test is where you touch the tip of your tongue to a small bit of cooled soap. If it "zaps" you like a battery would, then there is still active lye. If it just tastes like soap, then it's done. I f you DO get a zap, make sure you rinse out your mouth with some water & spit. When the HP soap is done, there is no further reaction taking place between the alkaline lye and the acids in the oils. The soap has finished the saponification process. Now, if you add your superfats, they will not be touched by the lye because there is no lye, anymore. The oils & lye have all been turned into soap. You can add your superfat & be assured that it will remain intact as your superfat. Whatever oil(s) you chose to add as your superfat you can believe that it will remain your superfat. In other words, if you make a soap out of lard, but want apricot oil as a superfat, if you add the apricot oil after the cook it will remain the superfat. There will be no sneaky action from the lye to turn your apricot oil into soap & give you back some lard as superfat.
Most soap calculators have a space for you to put in the superfat % you want. This is based on the weight of the oils in your recipe. In CP soap if you are using 16 oz oils and set your superfat % at 5%, the calculator will deduct 5% (0.8 oz) of oils from your recipe which will not be saponified. You can see the difference in the lye amount if you run the recipe through the calc at 0% superfat, then change it to 5% superfat. Soapmakers used to believe that if you added the superfat at trace in CP then there was less chance that the lye would saponify it. Unfortunately, this is not true. The lye takes the easiest oils it can grab onto for the chemical reaction. Whatever the most difficult oils to saponify in your recipe are, will most likely be your superfat. If you are making CP soap just let the calculator figure your superfat. Put the percentage you want in the clculator & follow the recipe it gives you.
If, however, you are making an HP batch, you will probably want to calculate the superfat on your own. Using the same example of a 16 oz batch. There IS a way to calculate this so you wind up with 16 oz oils total, but it is not very easy & may make your brain hurt. Here is an easy & accurate way to do it. Remember I said that superfat was a percentage of the oil weight.
16 oz X 0.05 (also known as 5% or 5 hundredths) = 0.8 oz
Ok, so is it really a big deal? You wind up with 16.8 oz, not 16 oz exactly. So if your mold only holds 16 oz, put the 0.8 oz in a yogurt cup :D
I am not trying to imply in this article that 5% superfat is ideal. It depends on YOUR skin or hair. I happen to like 3% superfat in my shampoo bars, (recipe in link, below)
Liz's Shampoo Bars
I have older dry skin & prefer 8-12% superfat in my body bars. Coconut oil makes a nice soap but is VERY cleansing. That means it strips all the oil off your skin. I don't like it, but a lot of people find it a fabulous soap. It is usually superfatted at 20-25% for body soap because of the high cleansing of the bar. Coconut oil makes a GREAT laundry soap. You don't want extra oil (superfat) on the clothes you are washing so it should be made at 0 or 1% superfat. 1% is just to be safe & make sure there is a pinch of extra oil for the lye to saponify. A safety net, so you don't have any lye left over.