Precious Metal Clay was developed by scientists working at the Mitsubishi Materials Special Products Division in Sanda, Japan. After years of experimentation, the first patents were awarded in the early 1990s, with many additional materials later joining the family of products. PMC consists of microscopic particles of silver or gold suspended in an organic binder to create a pliable material with a consistency similar to modeling clay.
After firing, the water and binder have completely burned away, so what remains can be hallmarked as .999 silver or gold. Dried out or unwanted PMC objects can be refined just like conventional precious metal.
Under the proper conditions, crystals of metal fuse together in the same way that droplets of water run together to make larger puddles on a window pane. Oxides (tarnish) that form naturally on most metals prevent this from happening. The solution is to use precious or noble metals in their pure state. These do not readily oxidize, so even at the high temperatures needed to induce fusion they remain free of coatings. This explains why there is not a brass or sterling version of PMC - short of firing in a vacuum, it won't work.
Using very simple tools and your own fingers, PMC is rolled, pressed, squeezed, layered and molded into a desired shape. Parts can be added, removed and refined as you go, making this a spontaneous and liberating process. Most work is done with moist clay, but it is also possible to work with PMC after is has dried. You typically get the general idea of a form and then refine it with carving tools or files when it is leather hard. To join parts at this stage you use slip (semi-liquid clay). If the PMC becomes dry as it's worked, you can spray or brush on a little water and cover it with plastic wrap for a few minutes to allow it to rehydrate. If you accidentally add too much water, you set the PMC aside loosely wrapped and allow it to dry out.
After it has dried, the PMC object is fired to a specific temperature. This drives off remaining moisture, then burns the binder. This evaporates as a harmless smoke. At this point the PMC is a fragile, porous metallic husk. At higher temperatures the particles melt into one another to form a solid, dense metal. Depending on the type of PMC, this can take from 10 minutes to 2 hours.
After firing, the object can be handled like any other gold or silver item. It can be soldered, burnished, buffed, tumbled, plated, etc. to achieve the desired finish. After the piece has been tumbled or polished, you can then add a patina to enhance the piece.