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Discounted water cold process method (DWCP) (2)
Back to the Overview
How to discount water
To calculate the amount of water needed for a certain recipe, I decide the concentration of the lye solution based on the type of oils or fats used in the recipe. Recipes with higher percentages of soft oils will accept stronger lye solutions; recipes with high percentages (>50%) of hard oils or fats (such as palm, coconut and tallow) will need weaker lye solutions  i.e., a higher amount of water.
There is no ideal concentration level, and there's no quick and easy way for suggesting the "perfect" water discount. It all depends on the oils and fats used, and on which other fillers or extra ingredients, if any, are added to the soap mixture (for instance, as already mentioned, milk and honey soaps typically call for more water than others). With discounted water recipes more than others, knowing the properties of your oils, and the possible reactions with other ingredients, is really the only way to go! Also, I should mention I use little or none fragrance oils, and never in CP  so if you are a FO user, you need to make sure your fragrance oils will not create seizing problems.
Now the important point to remember is, the amount of water is determined based on how much sodium hydroxide is needed  *not* on the weight of the oils. So the first step, when formulating your recipe, is calculating the lye (sodium hydroxide) and then using this value, in grams or in ounces, to calculate the water.
"No discount", when it comes to concentration of the lye solution, corresponds approximately to a 27% lye solution. This means that the mix of water and caustic soda contains 27% sodium hydroxide and 73% water. Therefore, any lye solutions that contain more than 27% caustic soda and less than 73% of water are "discounted water" solutions  but of course the difference will only be noticeable as the water or NaOH percentages move closer to 50%.
A popular formula for calculating a discounted water lye solution is the following:
( NaOH weight * 2 ) + 2
This formula, which has become popular on several soaping mailing lists thanks to SuziQ, of Moon Mountain Soap, was initially suggested by the late (and sorely missed) JD Anderson, and is widely used by many "discounted water" soapers. It is certainly a handy shortcut, and, translated into percentages, it corresponds approximately to a 33% lye solution. I tend to work with more extreme water discounts (if I can), and I prefer to work within a range, with hard fats at one extreme and soft fats at the other. My range goes from a 33% lye solution (for hard oils) to a 48% lye solution (soft oils). These are the *extremes*, which means that different recipes, with different percentages of soft and hard oils, and different extra ingredients, will use different lye solution strengths.
To calculate the amount of water for a 33% lye solution, use the following formula:
NaOH weight / 33 * 67
For a 48% lye solution, the formula is:
NaOH weight / 48 * 52
The intermediate range of lye solution strengths is calculated adjusting the formula as needed  i.e., dividing the weight of the caustic by the desidered percentage, and multiplying it by (100  desired percentage).
A good rule of thumb for those who approach water discounting for the first time, is to start with low discounts  for instance, a 36% lye solution (where water = NaOH weight / 36 * 64 ) would give you a pretty good idea of what to expect, and will work with most recipes  as well as with Double Boiler Hot Process soap.
For those who use recipes with a relatively high amount of unsaturated oils (e.g., 45% or more of olive oil), my suggested "safe discount" to start with is a 40% lye solution strenght. In this case, the amount of water can be calculated in one of two ways:
water = NaOH weight / 40 * 60 , or
water = NaOH weight * 1.5 (which gives the same result as the above formula).
The above discount ratio (40% lye solution strenght, or, as it's more commonly referred to, water = lye * 1.5) is what I normally use for most of my soaps, and recommend to experienced soapers who want to explore the potential of water discounting. This is because it has the obvious advantage of being particularly easy to calculate and remember, and also works with most recipes  but as already mentioned, I would suggest that it's always first tested on a "safe" combination of oils, before applying it as a general rule!
And now.... before rushing to the kitchen to make your first batch of DWCP.... please invest another few minutes on the theory, and read some IMPORTANT WARNINGS!

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