The first precious metal clays were introduced in the 1990's - made from pure silver powder and organic binders (Mitsubishi Materials Corp. makes PMC = Precious Metal Clay and Aida Chemical Industries makes Art Clay). The silver clay behaved kind of like a ceramic clay, could be molded, sculpted, carved, extruded etc. The metal clay was then 'fired' in a kiln or by torch, to produce a pure metal item.
Fine silver (.999 pure silver) is a noble metal, meaning that it resists oxidation and corrosion. Many noble metals are also precious metal, gold, silver, platinum. Because it resists oxidation, fine silver can be fused by heat in an oxygen (normal atmosphere) environment such as in a kiln or open torch flame. When particles are heated, close to melting temperature, but not melting, they move close together and become sticky, adhereing to each other. This compaction and partial fusing is called 'sintering'. Sintering produces metals that are more porous and lighter than cast metal, also not as strong. Gold clay later became available, but required higher firing temperatures and was very costly.
Precious metals are costly, and in recent years have grown even more expensive. Many people worked on trying to make 'base metal' clays. Base metals, such as copper, bronze (copper and tin) and brass (copper and zinc) do oxidize and corrode. When heated in an oxygen atmosphere, base metal particles oxidize and do not stick together (sinter). Industrially, metal powders are sintered in special kilns filled with inert gases. This is virtually impossible to do in a home or small studio setting. The inventor of BRONZClay (tm) and COPPRClay (tm), Bill Struve, found a wonderful way to fire base metals in a home kiln. He used activated charcoal in a closed, stainless steel container to produce a low oxygen environment for sintering base metal clay. When heated to high temperatures, the charcoal tries to burn and uses any available oxygen in the area, thus preventing (and even reversing) oxidation of metal particles.