A bar is not finished when it is cut
Fresh soap is wetter and softer than it will be a month later. After cutting, the bar keeps changing: water evaporates from the surface, the structure firms up, and the bar gets lighter and a little smaller. It is normal for the shape to shift slightly as this happens. Understanding the process helps you tell the normal from the unusual.
This is one of the reasons soap curing matters. As water leaves the bar, it becomes harder and generally lasts longer in the shower. How long that takes varies, and many makers allow several weeks.
Shrinkage
Shrinkage is the loss of size that comes with the loss of water. A bar can lose a noticeable share of its original weight during cure, and its dimensions shrink along with it. The exact amount depends on the recipe, the water in the batch, the humidity of your space and how long the bars cure.
Bars made with more water, as in recipes that use a lower water discount, generally have more to lose, so they tend to shrink more. Soaps made with a lot of added liquids such as milk or purees may also lose more. If you sell or label soap by weight, remember that the weight at cutting is not the weight at the end of cure, which is relevant when thinking about soap labels.
Some shrinkage is simply a fact of handmade soap. It is not a defect.
Warping
Warping is when a bar bends, curls or twists as it dries. Common contributors include:
- Uneven drying. If one side of a bar is exposed to air while the other sits flat against a surface, the exposed side dries and contracts faster, and the bar can curve.
- Thin or wide bars. A thin slab or a wide, flat bar is more prone to curving than a thick, compact one.
- Soft bars. A soap that is still soft at cutting is more likely to deform under its own weight or as it dries.
- Uneven thickness. Differences in thickness across a bar mean different drying rates in different parts.
- Wet surfaces. If bars sit on a damp shelf or in a humid space, they dry unevenly or very slowly.
The usual remedies are practical. Allow air to circulate around every side of a bar. Turn bars from time to time during the first weeks. Use a rack or a surface that lets air reach the underside, and avoid stacking bars tightly. If your bars warp often, consider whether a different shape, thickness or soap molds would suit you better, or whether the recipe is on the soft side.
A slightly warped bar is generally just as usable as a flat one. Some makers press or trim a warped bar while it is still pliable, but be careful: soap that is not yet cured can irritate skin, so wear gloves when handling.
Cracking
Cracks come in two main types.
Cracks in the top or sides of the loaf. These usually appear soon after the soap is poured, and they are often related to heat. A batch that gets very hot, as can happen with strong gel, expands and then contracts, and the surface may split. Making the batch cooler, or using less insulation, can reduce this. The article on the gel phase describes how heat shapes the look of a batch.
Cracks that appear later in cured bars. As a bar dries and shrinks, tension can build, especially if the surface dries faster than the interior or if the bar is very hard and brittle. Cracks may appear along edges or through the face. A recipe with a high share of hard fats or a very low water amount can make a bar more brittle, and drying in a dry, drafty place can speed up surface cracking. See the article on soap texture for the link between recipe and brittleness.
Some cracks have nothing to do with drying. A crack that runs through a bar along a line where two layers or additions met can come from a weak bond between them, for example in soap layers or embedded pieces, where one layer was too firm when the next went on.
A cracked bar is usually still safe and usable, assuming the recipe was sound. If a crack comes with crumbling, oily patches or a harsh feel, think about other causes before you assume it is just drying, and review the article on lye heavy soap.
Cutting and trimming
Timing also matters. Cutting too early, while the soap is soft, can produce rough, deformed bars. Cutting too late, when the soap is very hard, can lead to chipping and cracks. The article on cutting soap covers the practical details. Many makers cut within a day or two, depending on the recipe and how firm the loaf is, but this varies, so trust your own notes more than any fixed rule.
Normal versus worth investigating
| Observation | Usually |
|---|---|
| Bar is lighter and slightly smaller after weeks | Normal |
| Gentle curve in a thin bar | Normal, mostly cosmetic |
| Fine surface cracks on a very dry bar | Often cosmetic |
| Deep cracks with crumbling | Worth investigating recipe and water |
| Cracks with oily or sticky patches | Worth investigating, and use caution |
Reduce the surprises
Keep bars in a cool, dry, airy place away from direct sun. Give them space. Record the weight at cutting and again after cure if you want to know how much your recipe loses, and make a note of any bars that warped or cracked. Over time, your notes will show which changes to the recipe or to storage helped. The habit of comparing batches is the subject of learning from failed batches.