Liquid soap dilution
How much water to add to a potassium soap paste to reach the concentration you want. The paste already holds water from the cook, so the amount is not obvious, and dilution is a one-way step.
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Water already in the paste
— g. That is why the water to add is less than you might expect.
Explain every number
The formula, and where it comes from
solids = oils + pure KOH # mass is conserved in saponification paste % = solids ÷ paste × 100 diluted total (g) = solids ÷ (target ÷ 100) water to add (g) = diluted total − paste
- Conservation of mass in saponification: the soap and glycerol formed weigh exactly what the fat and hydroxide weighed. Solids in a paste are therefore taken as oils + pure lye; the remainder is water.
- Failor, Catherine. Making Natural Liquid Soaps. Storey Publishing, 2000. Paste method, dilution ratios and sequestering.
How to dilute without overshooting
Weigh the paste, work out the target, and add most of the water hot — boiling is fine — then leave it. A paste takes hours to dissolve and looks stubbornly thick for most of them. Come back the next day before deciding it needs more.
Hold back the last tenth of the water. If the soap ends up thinner than you wanted there is nothing to do but cook another batch, whereas a slightly thick soap takes thirty seconds to fix.
Use distilled water for the dilution as well as the cook. Liquid soap is a much better solvent for the minerals in tap water than a bar is, and hard water shows up as cloud.
If it stays cloudy
Cloud is usually unsaponified oil, which is why liquid soap is made at 0–3 % superfat rather than the 5 % a bar takes. It can also be a genuinely lye-heavy or lye-light paste; a zap test and a pH reading will tell you which. Sequestering with a little borax or a chelator clears mineral haze but will not fix excess superfat.
Thickening
Diluted soap is thin by nature. Salt solution thickens some formulas and thins others depending on the oils, so add it a gram at a time from a 20 % solution and stop the moment it moves. A high-oleic formula often will not thicken with salt at all.
Common questions
Why can I not just add water until it looks right?
You can, and plenty of people do. The reason to work it out first is that dilution takes hours, not minutes — a paste that looks too thick at midnight is often perfectly diluted by morning, and water added in the meantime cannot be taken out. Knowing the target weight tells you when to stop.
What counts as the soap solids in my paste?
The oils plus the pure potassium hydroxide. Mass is conserved in saponification: the soap and glycerol that come out weigh exactly what the fat and hydroxide weighed going in, and both stay in the pot. So the solids are the oils plus the lye, and whatever else the paste weighs is water. If you want to account for glycerol separately you can, but for working out how much water to add it makes no difference.
What concentration should I aim for?
Most finished liquid soaps land between 15 and 25 % soap. Lower than about 15 % and it is watery and needs a preservative more urgently; higher than about 30 % and it will not pour. Start at 20 %, live with it for a week, and adjust the next batch.
Why does my diluted soap go cloudy?
Usually superfat. Liquid soap is normally made at 0–3 % superfat because unsaponified oil does not dissolve in the finished soap and shows up as cloud or a separated layer. It can also be hard water, which is why distilled water is worth using for the dilution as well as the cook.
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