Let's look at the difference between a
water discount and a
lye discount:
- a water discount means that you use less water than the recipe calls for. Most recipes call for an amount of water that is around 32-38% of the total weight of the recipe. Leaving out some of the water can result in a bar that cures harder, faster - BUT the trade off is a water discount can affect the outcome of the recipe. I wouldn't recommend playing with water discounts until you get a bit of experience and can consistently produce your recipe with good results. (Edit to add: you would
purposely discount some water if you were going to use another liquid such as goat's milk or coconut milk. Example: if you wanted to do a 50% goat's milk, you would purposely leave out 50% of the water the recipe called for because you are replacing it with the GM. But milk recipes are a totally different animal so let's ignore those for now!)
- a lye discount refers to the amount of oil that is not converted to soap by the lye. For example, a 5% lye discount means that 5% of the total oils will be left unsaponified. We do this on purpose - for two reasons: first, if you mismeasure a bit, it gives you a margin for error. If you were to use a 0% lye discount (also called 'Full SAP') and had even just a teensy bit of unreacted lye, the finished soap would be 'lye heavy' and could do anything from irritating your skin, to causing a rash, to causing outright chemical burns. Second, having the little bit of oil left over makes a bar that is less likely to strip your skin totally bare of its natural oils, and it is less likely to be too drying. Most soaps should fall into the 4-8% lye discount range with a few exceptions. Any higher and you risk the DOS - dreaded orange spots that form when the leftover oil goes rancid. Yucky!
My personal 'Walmart' recipe calls for a 5%
lye discount. Like I said before, I wouldn't take a water discount on any recipe until you know how it behaves and you can reproduce it consistently.
Have you played with a
soap calculator?
www.soapcalc.net is my favorite. Check out their tutorial.
Note: "Lye Discount" and "Superfat" do not mean exactly the same thing, but they are used fairly interchangeably. You'll see the term "Superfat" a lot more often.
HTH!
