I bow down to the wisdom of the people at Cool Tools, who have published an incredibly detailed and enormously helpful article guiding us through the infinite complexities of firing silver clay: Cool Tools Ultimate Firing Guide
For those new to it, Silver Clay is a molecular powder of silver mixed with a binder. When fired, the binder burns off, and the silver molecules "sinter" together. Sintering is a magical process that involves metal atoms moving along their crystal boundaries (this is the magic of physics though, not crystal waving!) If sintering is not complete throughout the entire piece of jewellery, it may crack or break. The problem is that different types and thicknesses of metal clay (and the substrates it's built on) all need different firing schedules, and if the clay is combined with other items such as sterling, porcelain, glass or gemstones, the firing schedules change again, with some pieces needing to be fired several times at different heats for varying periods.
Be ignorant of this at your peril! I can sadly attest to sterling silver not just melting but disappearing completely when fired at a schedule that was fine for the silver clay. I've also had pieces made from silver sheet on a large burnable core being destroyed by the heat of the core burning.
So, I'm very grateful for the laboratory work done by Cool Tools and their generosity in sharing their findings with us all. Now my more adventurous pieces are much more likely to come out of the kiln alive!