A soap dish has exactly one job, and most soap dishes fail at it.
The job is not to hold soap. Holding is easy; a saucer holds soap, a folded cloth holds soap, the flat ledge at the back of a sink holds soap until the moment someone reaches past it and knocks it into the basin. The job is to get water away from the bar. Everything else a soap dish appears to do is incidental to this one demand, and the dishes that ignore it, the smooth ceramic dipped slightly at the centre, the shallow enamel tray, the cupped shell bought on holiday, are not soap dishes at all. They are puddles with ambitions.
The problem is standing water
Soap dissolves. That is the entire point of it; a bar that refused to give itself up to water would be useless. But a bar is meant to dissolve in motion, between wet hands, for the few seconds it is in use, and then to dry. Left sitting in a film of its own runoff, it dissolves in the wrong way, slowly, from the underside, until the contact surface goes soft and pale and faintly slick, and the bar that should have lasted a month is gone in a fortnight, much of it down the drain having done nothing at all.
So the design problem, stated plainly, is drainage. Lift the bar off the wet surface and let air reach all of it. This is the whole of it, and it is solved in a handful of ways that have been arrived at independently across centuries and cultures, because the physics does not vary.
Ridges, which raise the bar on a few thin lines of contact and let the rest drain beneath it. Slots, which let water fall straight through to a tray below. A draining stone, diatomaceous earth, most often, which pulls the water out of the bar’s footprint and into itself, then gives it back to the air. Or simply a tilt: any surface angled enough that water runs off it before the bar can sit in a pool. Each is a different answer to the same sentence. Move the water.
What follows from drainage
Once you accept that drainage is the requirement, the rest of the soap dish stops being a matter of taste and becomes a matter of consequence.
Material follows. Unglazed stoneware and draining stone hold no surface water; glazed ceramic and glass keep it. Wood drains and dries quickly but needs its own care. The handsome dishes tend to be the worst performers, because handsome, in this category, usually means smooth and sealed and flat.
Position follows too. A soap dish belongs slightly away from the direct line of the tap, where it will not be refilled with splash every time the water runs, and somewhere air moves. Pressed into a tiled corner with no airflow, even a well-drained bar will stay damp. The dish is only half the system; the shelf it sits on is the other half.
There is a quiet satisfaction in this. The soap dish is among the most overlooked objects in a house, bought last, chosen by colour, replaced without thought, and it turns out to be the thing standing between a considered bar and a slow, slumped death by its own moisture. A good bar made on a coast in the west, cold-pressed and cured for weeks, deserves at least a surface that does not undo the work in a week.
It is a small object. It is not a trivial one.