Precious Metal Clay
Precious Metal Clay is made of very small particles of precious metals, which can be worked in the same way as modelling clay. The particles of silver or gold are less than 20 microns in size, and it would take around 25 of them to equal the size of a grain of salt.
These metal particles are suspended in a binding agent which is pliable and easily worked with simple hand tools and the fingers. When the design is complete, the metal clay is heated (with a torch or in a kiln) which causes the binding agent to burn off and the metal particles fuse together.
The design is then comprised entirely of the precious metal within the material. It can then be sanded, polished, soldered or worked in the same way as any silver or gold jewellery made by conventional methods.
Pictures courtesy of Maggie Bergman from Silver Clay Art who conducts Gold Coast Jewellery Classes
Objects made with silver PMC can be hallmarked 999 pure silver after the water and binding agents have been burned away during the firing process. Gold PMC can be hallmarked as 22 carat after it has been fired. Silver is also available in a 90/10 alloy of silver and copper, and variations of pure silver where the amount of binding agent is increased or decreased.
Shrinkage after firing ranges from about 10% to 30% depending on the ratio of the filler to the silver particles present in the different types of clay. The more filler, the greater the shrinkage experienced. Firing times can take between 10 minutes and two hours depending on the type of PMC and the temperatures used.
The tools to get started using PMC are pretty basic. Toothpicks, plastic tubes (for a roller), needles, paint brushes, straws, a knife and a ruler are all useful to have at hand. A hands free magnifier, files, sandpaper, tweezers, cutting templates and playing cards (as a thickness guide) may also be useful.
DO NOT use tools made of aluminium or allow the clay to touch any object made of aluminium as it can cause discolouration, warping, brittleness and flaking in the clay.
The PMC is supplied in foil pouches to keep the clay moist and you should only take out what you will use in a few minutes. Left over clay should have a drop or two of water added before putting it back in the pouch. It also helps to moisturise your hands with a few drops of olive oil before you begin work. A sheet of glass makes a fine working surface.
Making flat sheets of PMC is easily done by creating two stacks of playing cards side by side with a gap between them. Make them of even height and place the clay between the two stacks, then roll over the cards with a plastic tube. To make thicker or thinner sheets, use more or less cards.
The finished sheet can then have shapes cut out with a knife or needle tool. Using a shape template can help to create uniform pieces. The cut pieces can be lifted from the glass with the edge of a playing card (plastic cards are best).
To join pieces, place them close together and apply a drop of water using a fine bristled paint brush. Wait a few seconds for the water to be absorbed and press the pieces together. Textures can be added by pressing shapes into the clay; leaves, bark, twigs, grainy timber, bottle caps, fabric, fly wire, there are hundreds of possibilities.
Normally the clay is used when it is moist, but it can also be worked after it has dried. (Joining dry pieces is done with a product called ‘slip’, which is comes in a tube). When the clay feels like leather, it can be filed, wire brushed or carved to finish the moulding you did when the clay was more supple. If you feel that the clay has dried too much, spray a fine mist or lightly brush over a small amount of water. Cover with plastic wrap for a few minutes to allow the water to penetrate. Don’t get it too wet, but if you do, loosely wrap it in plastic so air can enter and give it some time to dry.
You can let the clay air dry, or you can use a hairdryer or a slow oven to speed the process. Be sure to set your work on a bunched up paper towel or a screen to allow air to reach all sides of your work.
Slip can be used for decorating, setting stones, creating a mesh as well as an adhesive and as reinforcement. Care must be taken when setting gems to ensure the temperatures required to fire the PMC will not cause them harm.
Synthetic rubies, synthetic sapphires and cubic zirconias are created at high temperatures and generally won’t be harmed whilst firing the clay (some may discolour badly when exposed to high temperatures however). Most organic materials will not be suitable (such as wood, opal or shells) and glass* and any ‘doublets’ are also not suitable because of the temperatures required for firing. Non-fireable stones can be bezel set after firing.
*Glass can be used if the clay is fired at lower temperatures, which can only be done in a kiln.
After the piece has been fired it will have a fine powdery skin that needs to be removed by burnishing to show the bright metal below. This can be done by hand with a stainless steel burnisher by dipping it in a little water to lubricate it and rubbing the surface of the metal in every direction. Finishing papers and very fine brass brushes can also be used, but a small rotary tumbler filled with stainless steel shot is highly effective (and hands free).