What a lye masterbatch is
Instead of dissolving lye for every batch (the hot, fuming, careful part), you dissolve a lot of it once, at a known strength, and keep the solution in a labeled jug. The usual strength is 50%: equal weights of sodium hydroxide and water. Then on soaping day you weigh out solution instead of dry lye, add whatever extra water the recipe needs to reach its own concentration. You avoid dissolving dry lye for each recipe, but the stored solution is still caustic and dilution can release heat. Use the same protective equipment and handling precautions as for freshly mixed lye.
The recipe calls for 143.8 g of NaOH and 292.0 g of water (33% lye concentration). Every gram of 50% solution is half lye, so you need 143.8 ÷ 0.5 = 287.6 g of solution. That solution already carries 143.8 g of water, so add 148.2 g more (292.0 − 143.8) to land exactly where the recipe was. Same lye, same water, same soap, and the lye was already dissolved.
Is 50% concentration the same as 50% lye purity?
No. Stock concentration describes the mix of lye and water in the jug. Dry-lye purity describes the material you weighed before dissolving it. This tool uses the recipe's dry-lye weight and a stock made from that same material. Match its purity in the recipe builder first. For a commercial solution specified as active NaOH, reconcile that assay with the recipe's pure-NaOH requirement; do not apply a dry-flake purity correction twice.
Solution and extra water for common lye weights
| Dry lye in recipe | 50% solution to weigh | Extra water at 28% | at 30% | at 33% | at 40% |
|---|---|---|---|---|---|
| 50 g | 100 g | 79 g | 67 g | 52 g | 25 g |
| 75 g | 150 g | 118 g | 100 g | 77 g | 37 g |
| 100 g | 200 g | 157 g | 133 g | 103 g | 50 g |
| 125 g | 250 g | 196 g | 167 g | 129 g | 62 g |
| 150 g | 300 g | 236 g | 200 g | 155 g | 75 g |
| 200 g | 400 g | 314 g | 267 g | 206 g | 100 g |
| 250 g | 500 g | 393 g | 333 g | 258 g | 125 g |
Extra water = the recipe's water at that concentration, lye × (1 − c) ÷ c, minus the water already in the solution. The solution column is simply lye × 2; the tool handles any other strength.
The one case a masterbatch can't do
A masterbatch fixes the minimum water: every gram of lye brings its own water with it. A 50% solution can make any recipe at 50% or weaker, which is every normal recipe. But a 33% masterbatch, which some people keep because it stays liquid in a cold garage, cannot make a 40% recipe: it already has more water per gram of lye than the recipe wants, and there is no way to take water back out. The tool says so in red rather than printing a negative number. Mix the masterbatch at least as strong as the strongest recipe you make; 50% covers everything.
A 50% NaOH solution starts to crystallize around 12 °C (54 °F); if the jug sits in an unheated space over winter it may go cloudy or grow crystals. Do not pour off the liquid above crystals: it no longer represents the whole jug's concentration. Follow a suitable warming procedure until the crystals dissolve and the solution is uniform. Do not assume an unknown white deposit is only frozen lye. What does change it is an open lid: sodium hydroxide pulls carbon dioxide out of the air and becomes sodium carbonate, which doesn't saponify. That's the white crust, the same compound as the soda ash on a loaf. Keep it sealed, labeled LYE 50% — CAUSTIC, out of reach, and in HDPE or polypropylene. Never use glass, which strong caustic etches and can crack.
Mixing the jug
A kilo of 50% solution is 500 g of lye dissolved in 500 g of water, enough for 3 of the classic 1,000 g batches with 69 g to spare. Weigh the water first, add the lye in stages with stirring, and let it cool with the lid loose before sealing. Distilled water keeps it clear. Write the strength and date on the jug: if you ever change strengths, an unlabeled jug is a batch of lye-heavy soap waiting to happen.
Additional milk or other liquids must fit within the remaining water allowance; use the milk split calculator to account for their water and solids. Keep acid additions separate from a plain-water stock calculation; the acid tool handles their lye demand. The masterbatching method covers cooling, storage and weighing a dose.
Masterbatching the oils too
The same idea works for oils: melt and blend a large batch of the recipe's oils in their proportions, store it, and weigh out what a batch needs. Then a soaping session is two weighings and a stick blender. The lye math doesn't change (the blend's SAP is the same at any quantity), so the recipe builder figures are still right; just make sure the oils are thoroughly remixed before you weigh, because butters settle.