What do we mean when we talk about letting soap “cure”? It’s a term of art, to be sure, and can be a bit less than fully obvious in meaning, even when the immediate sense is made clear.
Freshly made cold process soap is about 20% water, which makes it very soft. Within a day or three of first being made, it is safe to use, in the sense that the lye has essentially all reacted with with the oils and the new soap is no longer caustic. However, it’s not really very good soap yet. It’s soft enough to be difficult to use properly, and not only that, due to it already being partially dissolved in its own water, it gets used up much more quickly than it would if it were let alone for a while before using. High-quality soap is soap with most of the water gone from it.
So why do we call it “curing”? Wouldn’t “drying” work as well? Sort of; it’s not drying in the sense of drying dishes, after all. You can’t wipe it off or point a hair-dryer at it and get it done faster, as it’s already dry on the surface and the hair-dryer will mostly just melt the soap and make a mess. It’s more like drying in the sense of drying meat or fruit; exposing to dry air (and sometimes gentle heat) until enough of the moisture from inside the meat, fruit, or soap has diffused out toward the surface and evaporated away, leaving beef jerky, raisins, or properly-cured soap!
Having established the value of curing soap before using it, how does one go about the process? Well, soap will mostly cure by itself in a few weeks if left in a place where air can circulate around it well enough. In my soap room I have several screen-lined shelves as well as a selection of baker’s racks, which I arrange just-sliced soap on and leave them for a while. I try to accelerate the process by dehumidifying the room and keeping it a bit on the warm side, and sometimes the soap is fully cured and hardened in as little as two weeks, but when possible I try to keep it on the racks for four. In some cases a soap will turn out to need extra curing for some reason, so it stays on the shelves even longer. Soap never goes on sale until I’m satisfied it’s hard enough to be an easy to use, long-lasting bar.