it is difficult to imagine how anyone could apply pressure to an object while it's being fired (particularly in antiquity up to the 19th C.).

Sintering, as I understand it, pertains to the fusing of grainy bodies, generally quartz granules and glass granules (these being already melted quartz, but in a ground-up state) one to another. Sintering explains what happens when these granules are heated enough so that individual granules form a glass around their circumferences, allowing one to stick to any that touch it. In faience-making, this holds the granules of quartz together. In powderglass beadmaking this holds the granules of glass together.

Because I am weaker in ceramics technology and strategy, I can't say what happens with the fusion of clays—though I understand there is a great deal of science devoted to this. But I would be surprised if it were (essentially) sintering, as happens with quartz and glass. Perhaps it is.

Jamey