I think it is tedious to measure and mix the lye solution for each and every batch of soap, so I normally make a masterbatch lye solution that is large enough to make several batches of soap.
Masterbatching is a method of measuring and mixing enough ingredients to make multiple batches of a product. You can make a lye masterbatch (this article) or a fat masterbatch as you like.
If stored in a tightly closed, lye-safe, airtight container, a lye masterbatch can be safely stored for months without losing its strength.
There are two schools of thought about the lye concentration to use for a lye masterbatch. Both methods are valid, but one may make more sense to you based on the way you like to make soap:
50% masterbatch -- Some soap makers make a 50% lye solution (1:1 water:lye ratio) as their masterbatch. Extra liquid is added later to dilute the lye solution to the concentration desired. Advantages -- Liquids other than water (beer, aloe, milk, etc.) can be used for dilution. A wider range of lye concentrations can be made from a 50% masterbatch.
Ready-to-use masterbatch -- Other soap makers make a masterbatch at the concentration they normally use for making soap. For example, if you make soap using a 33% lye concentration (2:1 water:lye ratio), then you might choose to make a 33% lye solution as your "ready to use" masterbatch. Advantages -- Less math and measuring. Ideal for making many batches using the same base recipe.
Most small-scale soap makers make solid bar soap so most lye masterbatch tutorials are based on using only NaOH (sodium hydroxide, caustic soda) to make the masterbatch.
A lye masterbatch can be made with other alkalis, however, For example, a liquid soap maker could make a lye masterbatch using only KOH (potassium hydroxide, caustic potash). Or a lye masterbatch could be a blend of NaOH and KOH for making dual-lye soap, also called hybrid soap.
The general method described below will work to make any type of lye masterbatch.
If you want to make a dual-lye masterbatch that contains both NaOH and KOH, read "Extra credit 2" at the end of this article to learn how to correctly calculate the weights of NaOH and KOH for a dual-lye masterbatch.
1. Choose a sturdy, lye-safe storage container that is easy to grasp and not overly large.
Lye solution weighs quite a bit more than the same volume of water. If the container is too heavy, too big, awkwardly shaped, or slippery, it will be difficult to hold and pour safely.
Make sure the container is clean and dry and has a secure, liquid-tight cap. The cap should be a screw-on cap for safety, not be a snap-on or flip-top cap.
A laundry detergent jug (below) is an ideal choice because it is made of sturdy lye-resistant plastic and has an easy-grip handle, secure screw-on cap, and drip-resistant spout. A jug that holds about 100 fluid ounces (about 3 liters) is a good size.
2. Calculate the volume of the storage container
Read the markings on the container if present. Look for the volume in "fl oz" (fluid ounces) or "liters".
If the volume is not marked on the container, fill it with plain water and use a measuring cup to find the volume. Measure the volume in fluid ounces or milliliters, not cups.
To make the lye solution easier to pour and reduce the chance of drips, I recommend filling the storage container only about 80% full. Calculate this reduced volume --
Lye solution volume = 80 / 100 X Full volume
If the volume is in liters, then convert liters to milliliters --
Lye solution volume in mL = Volume in Liters X 1000
If the volume is in ounces or milliliters, ignore this calculation.
3a. Decide what lye concentration you want to make
You can make a masterbatch lye soluion at a 50% lye concentration and dilute this solution later to the concentration you need for making soap.
Or you can make a "ready to use" masterbatch at the same lye concentration that you normally use to make soap.
You get to decide!
3b. Estimate the total weight of lye solution to fit your container
For a lye concentration from 37% to 50%, the approximate total weight of lye solution will be --
Lye solution weight = 1.5 X Lye solution volume
Lye concentration from 27% to 36% --
Lye solution weight = 1.4 X Lye solution volume
Lye concentration of 26% or less --
Lye solution weight = 1.3 X Lye solution volume
Important -- If your volume is in fluid ounces, the lye solution weight will be in weight ounces. If your volume is in milliliters, the lye solution weight will be in grams.
4. Calculate the water and alkali weights for the masterbatch
For a masterbatch that is all NaOH or all KOH:
Alkali weight = Lye concentration / 100 X Lye solution weight
Water weight = Lye solution weight - Alkali weight
For a dual-lye masterbatch (a blend of NaOH and KOH), read "Extra credit 2" to calculate the weights of NaOH and KOH.
Caution! Use water only to make a lye masterbatch-- do not use vinegar. For more information, see "Extra Credit 1" below.
5. Get prepared
Check that you have enough alkali and distilled or demineralized water to actually make the masterbatch.
If your storage container has a small opening, you may need to make the solution in another container with a large opening so it is easier to stir and make sure all the lye is dissolved.
Plan to control the lye mist and water evaporation by lightly covering the container with a paper towel or something similar (below).
I also run my stove ventilation hood when I mix lye solution, because the hood will vent any mist directly outdoors. If the hood for your stove recirculates air back into the house, it will not be helpful.
You can let the lye solution cool down naturally or you can make a cold-water or ice-water bath to help the solution cool faster.
Last but not least, plan to wear eye protection and lye-proof gloves.
6. Make the lye solution
Measure room-temperature distilled or demineralized water into the container.
Caution! Do not use warm or hot water -- room temperature water only!
Measure the solid alkali in a separate, dry container.
Slowly pour the alkali into the water while slowly stirring. A silicone or plastic spoon or spatula is ideal. Stainless steel is next best.
Keep the container lightly covered as much as possible while stirring and cooling to reduce the amount of lye mist and water vapor that escapes into the room air.
Caution! Do not seal the container tightly when the lye solution is hot, because pressure will build up.
Let the lye solution cool while lightly covered until the lye solution is cool enough to handle safely. Pour the lye solution into the storage container. Cap the container securely, label it well, and store it out of reach of pets, children, and unsuspecting adults.
If your masterbatch is at 33% to 50% lye concentration, store the solution at 65F / 18C or warmer.
If concentrated NaOH solution is allowed to get colder than about 60F / 16C, some of the alkali will crystallize out of solution.
The result will be a glass-like sheet of solid NaOH on the bottom of the storage container. If this happens, the solid alkali must be fully dissolved before it can be used to make soap. Speaking from experience, this is a real mess to clean up.
Prevent this problem by storing the solution in a reasonably warm place that is at least 65F / 18C.
7. Use the lye masterbatch to make soap
Ready-to-use masterbatch
Check your soap recipe and add the alkali weight (sometimes called the "lye" weight) and the water weight together. This answer is the total weight of lye solution you need for your soap batch.
Gently swirl or stir the masterbatch solution to mix any settled impurities into the liquid (see "Extra Credit" below).
Measure enough of the masterbatch lye solution to equal the total lye solution weight.
50% masterbatch
First, multiply the alkali ("lye") weight in your soap recipe by 2. This is the total lye solution weight.
Gently swirl or stir the masterbatch solution to mix any settled impurities into the liquid.
Measure enough of the 50% masterbatch lye solution to equal the total lye solution weight.
Subtract the alkali weight from the total water weight. The answer is the additional water needed to dilute the 50% lye solution to your desired lye concentration.
Measure enough water or water-based liquid to equal this amount.
If the math of using a lye masterbatch is daunting, try the SoapmakingFriend soap recipe calculator. If you tell it the lye concentration of your masterbatch, it will do the math for you.
You can also make a masterbatched fat blend...
More about lye masterbatching
https://www.soapmakingforum.com/threads/lye-solution-master-batch.18098/
https://www.soapmakingforum.com/threads/masterbatching-help-please.62496/
https://www.soapmakingforum.com/threads/pre-mixed-lye-water.57711/
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Extra credit 1: Frequently asked questions about lye masterbatching
Can I use vinegar instead of water to make my masterbatched lye solution?
I do not recommend using vinegar when making lye solution that you plan to store for awhile.
A vinegar-lye mixture will thicken into an unpourable gel a day or so after it is made. The gel can be used to make soap, but it is more difficult to use than a pourable liquid.
It is best to use a vinegar-lye solution the same day it is made.
Why mix impurities into the lye solution before using the masterbatched lye?
NaOH and KOH always contain impurities, particularly sodium carbonate or potassium carbonate. Mixing these impurities into the masterbatch will ensure the lye solution stays consistent from the first pour to the last drop.
If you did not mix these impurities into the solution, the impurities will accumulate in the masterbatch container. Your last batches of soap will contain more impurities and somewhat less NaOH or KOH than the first batches.
Should I warm the lye solution before I use it to make soap?
I never warm my lye solution for safety's sake. If I think my ingredients need to be warmer, I heat the fats rather than the lye solution.
Doesn't the lye temperature need to be close to the fat temperature?
It is a myth that the lye solution and fat temperatures must be within a few degrees before a person starts to make the soap. This is totally not necessary from a chemistry point of view.
I agree it is certainly easier to see if the lye solution and the fats are both at almost the same temperature, then the soap batter will also be about that same temperature. But the only reason to match temperatures like this is to reassure the soap maker.
If the goal is to start with a soap batter at 100F, the lye solution could be 70F and the fats could be 110F.
Extra credit 2: Calculate alkali weights for a dual-lye masterbatch
Some soap makers use a mix of NaOH and KOH to make their soap. Learn more about dual-lye soap...
When I make a dual-lye recipe, I normally use my 50% NaOH masterbatch and add dry KOH to the masterbatch at the time I make the soap. But a dual-lye masterbatch can also be made. Here is one way to get the correct weights of NaOH and KOH to use in the masterbatch:
Create a "recipe" using a soap recipe calculator that allows the user to create dual lye (hybrid) soap recipes. Soapmaking Friend and LyeCalc are two calculators will allow this.
For my example, I want to make a masterbatch at 50% lye concentration. I want to use a target weight of 1000 grams NaOH to make this masterbatch. I want to make a dual-lye blend of 95% NaOH and 5% KOH. I will use LyeCalc (https://www.lyecalc.com/) to do these calculations.
To create ths recipe, first set up the proportions of NaOH and KOH.
For my example, I'm using 95% NaOH and 5% KOH.
Decide about how much NaOH you want to use to make the masterbatch. This is your "target" weight.
I'm going to use 1000 g NaOH to make my example masterbatch.
Set up the lye concentration (or water:lye ratio) to be the concentration you want for the masterbatch.
I will enter 50% lye concentration. This is the same as a 1:1 water:lye ratio.
Create a recipe using one fat. Any fat will do.
I chose to use coconut oil for my example.
Set the superfat to zero percent (0%). All the other settings don't matter.
Set the starting weight of the fat at some number. It can be any number you like -- just pick something.
I will start with 2000 g coconut oil.
Calculate the recipe. Look at the calculated NaOH weight. Is the calculated NaOH weight at your target weight?
My target is a "recipe" that uses 1000 g NaOH.
I see the calculated NaOH weight for 2000 g of coconut oil is a lot lower than my target NaOH weight of 1000 g.
If the calculated NaOH weight is not the same as the target weight, increase or decrease the weight of fat and recalculate the recipe. Repeat as needed.
I had to increase the coconut oil weight to 5850 grams to until the calculated NaOH weight equaled my target of 1000 grams NaOH.
When the calculated NaOH weight is equal to the target weight, you're done. Use that weight of NaOH, the calculated weight of KOH, and the calculated weight of water to make your masterbatch.
The screenshot below shows the LyeCalc results for my example masterbatch. This dual-lye masterbatch solution will contain 1000 grams NaOH, 75 grams KOH, and 1075 grams water.
Note the weight of KOH in this example is not 5% of the total alkali weight. This is not an error.
The amounts of KOH and NaOH in a dual-lye solution is based on the correct proportions of alkali molecules, not on the alkali weights. More about this: Calculating a dual-lye soap recipe...
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