Mechanism of PMC turning into solid metal (silver)
The state of clay
Very fine silver particles, binder, and water are evenly scattered.
The water evaporates leaving the silver particles and binder. Because the shape and intensity is supported only by the binder, the structure may be damaged by any impact.
Beginning of firing
The binder is burned, and the silver particles begin to bind with each other. Yet, each particle is still independent and there are gaps. The piece has not yet shrunk and is less strong.
As time passes by, broader areas come in contact with other particles and the boundary becomes less defined. The gaps become smaller (meaning shrinkage during firing).
To make the slightly uneven surface mirror-smooth, polish thoroughly with a spatula or other tool.