The first type of metal that was used in this way was pure (24k) gold, developed and manufactured by Mitsubishi Materials. This was followed with pure (.999) silver and marketed to the Japanese craft community as Precious Metal Clay (PMC) - a brand name. Soon another company, Aida Chemical Industries, developed a similar product called Art Clay Silver (ACS).
In 2006 Metal Adventures debuted a bronze version called BronzClay and soon after followed with CopprClay. Since then many more manufacturers have entered the metal clay landscape to offer a variety of base metal clays including rose bronze, steel, and brass. Base metal clays are different from fine silver clay in that they need to be fired in an oxygen free atmosphere to sinter correctly.
All metal clays can be worked wet or dry using a combination of techniques which include pressing or rolling a texture into the moist clay, carving dry clay, and forming 3 dimensional shapes in either stage. During the firing process, work shrinks anywhere between 8 and 30% depending on the type of clay (clay body) and specific firing process used. Gold, silver, and some copper metal clays may be fired with a butane torch or in a kiln, but most base metal clays and alloys of silver must be fired in a kiln, submerged in granulated carbon. PMC carries a sterling silver clay (.925) alloy which must be fired in a manner similar to base metal clays. 'Enriched' sterlings like .960 can be kiln fired without carbon (referred to as an 'open firing').
Some metal clay companies offer powdered metal pre-mixed with binder that the end user combines with water to create a malleable material. Others sell ready-made lump clay. There are also some alloys that one can blend in the studio. One of these is .960 sterling, which is an equal combination of .925 Sterling clay and PMC3, PMC+, PMC Flex or ACS. This alloy is stronger than fine silver alone, and while not quite as strong as .925 Sterling, can be fired on an open shelf with no carbon.
The possibilities of creating jewelry and small scale sculpture with metal clay is endless, and as such a young art form – new designs, techniques, and ideas are being developed every day.
From Bill Struve, inventor of Metal Clay Adventures brand clays
Regarding metal clay shrinkage:
Metal Clay (any brand) is basically metal powder, water, and binder. Drying removes the water and firing removes the binder. Firing also joins the metal powder particles together in a process known as sintering. Since sintering removes the space between the particles, the volume must decrease (shrink). A 15% shrinkage in length, width, and height of a cube results in a 61% shrinkage in the volume of the cube. But who makes cubes? During sintering, pieces are really hot and about to melt (visualize a slurpy). The force joining the particles is weak (think capillary attraction). Any other force, like friction, gravity, ring sizer, etc., will influence shrinkage in one or more directions. Since the volume must shrink 61%, there will be more (or less) shrinkage in the other directions. Have fun with that! Fortunately, any piece fired exctly the same way (including orientation), will shrink the same amount.
About Sintering From Lora
Think of 'sintering' as you think of ice in a freezer during a power outage. First the ice cubes are separate and perfectly formed. As they melt the edges become liquid and if the power comes on again, they re-freeze. The edges are touching and connected to their neighbor, but they are still individual cubes. Depending on how long the power is out the individual cube can be more or less bonded depending on how much melted before they re-froze. If the power is out long enough, the ice would melt completely and when re-frozen would coalesce into a solid block of ice.