Lye, superfat, and why soap recipes are calculated rather than copied
Soap is the product of a chemical reaction between fats and a strong alkali. That single fact explains most of what beginners find confusing about soap recipes: why you cannot swap one oil for another by eye, why every recipe quotes a different amount of lye, and why "a bit extra oil" is a deliberate design choice rather than sloppiness.
Safety first, because this is the part that actually hurts you. Sodium hydroxide and potassium hydroxide are caustic. They burn skin and eyes, and the reaction with water is strongly exothermic — the solution can reach near boiling within seconds and releases fumes worth avoiding. Always add lye to water, never water to lye, which would let the first contact boil violently and spit. Wear sealed goggles and gloves, cover your arms and feet, and work where there is airflow and where children and pets are not. Take the same care weighing dry flakes as mixing the solution — the dust is caustic too.
Containers matter more than people expect. Lye attacks aluminium, zinc and tin, releasing flammable hydrogen gas, so no aluminium pans, no galvanised or tin-plated steel, and no aluminium utensils. Use stainless steel, or heavy plastic that is specifically HDPE (recycling mark 2) or polypropylene (mark 5). Avoid glass: hot caustic solution etches it over time, and a weakened glass vessel full of near-boiling lye is a serious accident waiting to happen.
If it gets on you. Flush the area immediately with plenty of running water and keep flushing — this is one where more water and more time is simply better. For eyes, irrigate immediately and get medical attention urgently. Do not try to neutralise a burn with vinegar or lemon juice: it is a folk remedy that adds an acid–base reaction, and its heat, to already injured skin. Water first, always. Keep the tap accessible before you start, not after.
Saponification: what the lye is for
Fats are triglycerides — a glycerol backbone carrying three fatty acids. The hydroxide splits those bonds, producing the salt of each fatty acid (soap) and free glycerol (glycerine, which stays in the bar and is part of why handmade soap feels different from many commercial bars).
Which alkali you use decides what kind of soap you get. Sodium hydroxide (NaOH) makes hard bar soap. Potassium hydroxide (KOH) makes soft or liquid soap. They are not interchangeable, and a recipe written for one is wrong for the other.
SAP values: why every oil needs a different amount of lye
Each fat has its own molecular makeup, so each requires a different mass of alkali to saponify fully. That requirement is the fat's saponification value, usually given as the grams of hydroxide needed per gram of fat.
Coconut oil needs substantially more lye per gram than olive oil, for example, because its fatty acids are shorter and therefore more numerous per unit weight. This is why substituting oils without recalculating is genuinely dangerous rather than merely inexact: swap coconut oil for olive at the same weight and you are left with excess unreacted lye in the bar, which is caustic. Swap the other way and you get a greasy bar that may never firm up.
A further wrinkle for liquid soap: KOH is not normally sold pure. Technical grade is commonly around 90% pure, with the rest mostly water and carbonate, so recipes and calculators either assume a purity figure or ask you for one. If you use a 100% figure against 90% flakes, your soap will be under-alkalised.
Correcting for it is one division: actual weight = ideal weight ÷ purity. If the calculation calls for 90 g of pure KOH and your flakes are 90% pure, you weigh out 100 g. Purity varies by supplier, roughly 85–95%, so it is worth knowing what you have rather than assuming.
Superfat: deliberately leaving oil unreacted
If you add exactly enough lye to saponify every last gram of fat, any error in measurement, any variation in the oils, or a SAP value that is an average rather than an exact figure, can leave you with leftover lye. Soap makers therefore use less lye than the theoretical maximum, so a small fraction of the oils survives unsaponified. This is superfatting, also called a lye discount.
Five percent is the usual starting point, meaning about 95% of the theoretical lye requirement is used. It provides a safety margin against measurement error and leaves free oil in the bar, which most people find gentler on skin. Higher superfats — 8% or more — feel richer but soften the bar and shorten its shelf life, since free oils are what eventually go rancid. Very low superfats are used for laundry and cleaning bars where mildness matters less.
Note that superfat does not let you choose which oil remains free. In cold process soap, saponification does not neatly reserve your expensive shea butter; the free fraction reflects the mixture. Claims that a specific luxury oil is "the superfat" are marketing rather than chemistry, unless the oil is added after the reaction has substantially finished.
Water: the variable that changes everything except safety
Water does not participate in saponification's stoichiometry — it is the solvent that lets the lye meet the fats. That means water content is a process choice, and recipes express it in several incompatible ways:
- Water as a percentage of oils — the traditional way, often around 33%. Simple, but it makes lye concentration drift as the recipe changes.
- Water:lye ratio — for example 2:1, meaning two parts water per part lye.
- Lye concentration — the percentage of lye in the lye solution, commonly somewhere around 28–33%.
The last two are directly convertible and are what most experienced makers use, because they pin the thing that actually matters: how concentrated the solution is. More water gives a longer working time and a softer initial bar that needs longer to lose its moisture; less water speeds up trace and firming, and is usual when a recipe wants to be unmoulded quickly.
Trace, cure, and why waiting is not optional
Trace is the point where the mixture emulsifies and thickens enough that a drizzle leaves a momentary trail on the surface. It signals that the oils and lye solution will no longer separate, not that the soap is finished.
The bulk of saponification happens over the following day or so, but cold-process soap is normally cured for around four to six weeks before use. Curing is mostly water evaporating: the bar becomes harder, milder and much longer-lasting in the shower. A bar used after a week is not usually dangerous if the recipe was correct — it is simply soft, and dissolves quickly.
Confirming that a batch is safely saponified is where a lot of well-meant advice goes wrong. The intuitive move is to reach for pH strips, but they are the weaker test: soapmaking references describe strips and phenolphthalein as notoriously inaccurate on soap, and the industry's actual method for detecting excess alkali is a total alkalinity titration, not a pH reading.
The reason is that pH is the wrong question. Finished soap is alkaline by nature — properly made, skin-safe soap ranges from roughly 9.5 to 11.5 depending on the fatty acids in the recipe — so a high reading tells you almost nothing. What you actually want to know is whether free, unreacted alkali remains, and a soap slurry's pH is dominated by the soap itself.
That is why the old "zap test" survives: touching a damp fingertip that has been swiped on the bar to your tongue detects free alkali as an unmistakable sensation like a nine-volt battery. Wait two or three days after making, since fresh soap zaps before saponification has finished, and swipe with a finger — do not put the bar against your tongue, which risks a chemical burn if the batch really is lye-heavy.
A bar that zaps goes back to be rebatched or thrown away. A bar that does not, made from a correctly calculated recipe, is fine.
Running the numbers
Every batch needs its own calculation: the oils you actually used, their SAP values, your chosen superfat, and your water figure. This is the one place in soap making where doing the arithmetic by hand is a false economy: use a lye calculator you trust, and specifically one that lets you set superfat, lye purity and your water figure explicitly rather than assuming them for you.
Whatever you calculate with, weigh everything — including the water — on a scale, and recalculate whenever the oils change. Soap making is forgiving about fragrance, colour and technique, and unforgiving about the ratio of alkali to fat.