You unmold at three days and the bar dents under your thumb. Or you cut at a week and the edges smear. Or it's a month later, the calendar says cured, and the thing still feels like cheese.
Those are not the same problem, and a list of causes is no help until you know which one you have. There is a way to tell, it takes a kitchen scale, and it costs nothing.
First: is it soft, or is it young?
Almost all hardening in the first six weeks is water leaving. So the question "will this bar ever firm up?" has a direct measurement behind it: put a bar on a scale and weigh it again in a week.
- Still losing weight? Your problem is time, and time is already fixing it. Nothing else needs doing.
- Weight has plateaued, and it's still soft? Most of the water that will leave has left, and further curing will change it little. The cause is one of the three below, unless it's a castile or other high-olive bar, which keeps firming for months (see below).
That single fork removes most of the guesswork, and it is the reason to weigh bars at all. The cure guide uses the same measurement to answer the opposite question (when a normal bar is finished) and gives the weight to expect.
Is your recipe even capable of a hard bar?
The standard advice is to use at least 40% hard oils: the fats that are solid at room temperature, meaning coconut and palm, lard and tallow, and butters such as shea and cocoa. It is a reasonable proxy, because those fats carry most of the saturated fatty acids a hard bar is made of. The hardness number is calculated directly from those fatty acids, against the customary range of 29–54, so the rule can be checked:
| Blend | “Hard oils” | Hardness | INS | Verdict |
|---|---|---|---|---|
| Castile100% olive | 0% | 17 | 105 | Soft, and meant to be. Months, not weeks: this is the recipe, not a fault. |
| Bastileolive 80 / coconut 15 / castor 5 | 15% | 25 | 127 | Soft by design. Firms up usefully by 8–12 weeks; will never be a gym-bag bar. |
| Lard barlard 50 / olive 30 / coconut 15 / castor 5 | 65% | 38 | 144 | A hard, long-lasting bar. Lard does most of the work. |
| Palm-freeolive 40 / coconut 25 / shea 15 / almond 10 / cocoa 5 / castor 5 | 45% | 37 | 146 | Perfectly firm, with coconut and the butters doing the work. |
| Classic barolive 35 / coconut 30 / palm 30 / castor 5 | 60% | 45 | 162 | Firm at four weeks. |
"Hard oils" counts every fat this site's oil table files as solid at room temperature: coconut and palm, lard, and the butters. That is the usual reading of the rule; RusticWise's list of hard oils for soapmaking, for one, includes lard, tallow and the butters. Hardness and INS are computed from the fatty-acid profile of the whole blend; the customary range is 29–54, and green means inside it. Per-oil figures are in the oil properties chart. The rule and the measurement agree on all 5 blends.
Counted the usual way, the lard bar has 65% hard oils and a hardness of 38, and the palm-free starter has 45% and lands at 37. Where the hardness number earns its place is near the line, because hard fats aren't equally hard: 40% coconut with the rest olive computes to a hardness of 42, while 40% lard with the rest olive computes to 27, under the 29 floor. The rule counts both as a pass. Use the rule to get close and the hardness number to check. And if your recipe is below 29, expect a bar that starts soft and needs a long cure to firm up; no additive changes what the fats you chose make it do.
Too much water
The commonest cause of a bar that is soft early, and it fixes itself. Water is the largest thing that has to leave: the classic bar at a 25% lye solution carries 431 g of water per kilo of oils against 216 g at 40%, so the same recipe can be firm in ten days or still yielding at five weeks depending on a number you set at the mixing jug. Extra liquid on top of the lye water compounds it: milk, purée or beer added as well as the water rather than instead of it.
If a bar is soft, still losing weight, and you used full water, the answer is patience and airflow. If you want firmer bars sooner next time, a steeper concentration is the lever, and the water discounting guide covers what else it changes.
Too much superfat
Superfat is oil that never becomes soap, and oil is soft. The arithmetic is unforgiving:
| Superfat | Free oil per kg of oils | Share of the batter |
|---|---|---|
| 5% (default) | 50 g | 3.5% |
| 8% | 80 g | 5.6% |
| 12% | 120 g | 8.5% |
| 15% | 150 g | 10.8% |
| 20% | 200 g | 14.6% |
Free oil is the superfat's share of 1,000 g of oils; the last column divides it by the classic bar's whole batch weight at a 33% lye solution. The superfat adjuster recomputes the lye for a different superfat without rebuilding the recipe.
At 5% there is 50 g of free oil in a kilo of oils; at 20% there is 200 g, which is 14.6% of the batter: a bar that is soft and slightly greasy by construction rather than by accident. High superfats have their uses, mostly in very high-coconut recipes, but they are not free, and a long cure at a high superfat trades softness for rancidity risk instead. The superfat guide makes the case for 5% as a floor rather than a preference.
The other suspects, ranked
- Sugars (honey, milk, beer, fruit). They do soften a bar, and the effect is real at the amounts people use. They also heat the batch, so they push it toward gel, which is a separate decision.
- Ungelled soap unmolds later than gelled soap, and can be noticeably softer in the first few days. It catches up.
- Silicone molds hold moisture against every face except the top, so a loaf in silicone does firm up more slowly than one in a lined wooden box. Real, and minor.
- Sodium lactate. The obvious fix, and the wrong first answer. It makes bars release from the mold cleanly a day earlier. It does not turn a soft formula into a hard bar, and reaching for it when the hardness number is below range is treating a symptom you diagnosed wrongly. (If you use it, the additive rate calculator converts the teaspoon rate, which is for the 60% liquid, into grams.)
- Salt and stearic acid do harden a bar, and both change it in other ways: salt cuts lather, and stearic accelerates trace hard enough to need hot process above a few percent. Reformulating is usually cleaner.
What to do about the batch in front of you
- Weigh it and wait. If the weight is still falling, that settles it. Give it two more weeks on a rack with air on every face.
- Check the recipe's hardness. Put the blend into the recipe builder. Below 29 and you have your answer: no additive will change it, so this batch needs a long cure and the next one needs more hard fat.
- Check the superfat you actually made, not the one you wrote. A mis-weighed lye can leave you far above the number in the recipe. The superfat adjuster reconstructs it from the weight you used.
- Rebatch, if it's worth it. Grating and cooking a soft batch drives off water fast and produces a firmer, rustic bar. It cannot make a soft recipe hard, only a wet one dry.
- Use it anyway. A soft bar is still soap. It will dissolve faster in the dish; keep it out of standing water and it will wash exactly as well as anything else.
Check the obvious thing first. A bar that is soft and zaps is not a hardness problem; it's a lye problem in the other direction, and worth ruling out in ten seconds. Then check whether "too soft" means "softer than the bar I bought", because commercial bars are made from different fats by a different process and are the wrong benchmark. A handmade bar at hardness 45 is a good bar; it is not a supermarket bar and it never will be.
Soaps that are supposed to be soft
Some of the best recipes fail every rule on this page on purpose. Castile at hardness 17 is far below the customary range and is not a mistake. It is soft out of the mold and needs cutting early and curing for months, and after six it becomes what soapmakers describe as one of the mildest, hardest-wearing bars there is. The hardness number predicts how firm a bar is out of the mold and through a normal cure; castile is the clearest case of a recipe that keeps changing long after that. High-olive recipes generally behave the same way, proportionally. Salt bars want unmolding within hours and then months on the rack.
So before you add anything to fix a soft bar, check that you did not choose it. The fix for a castile that is soft at four weeks is week twelve.