How To Set Temperature-Sensitive Stones into Original PMC

Straight from the PMC Guild archives! Follow this clever how-to by Tim McCreight to set temperature-sensitive stones into original silver PMC designs.

Last edited: 10/31/2018

One of the really cool things about PMC is the way certain stones can be fired into place. Of course this only works with gems that can tolerate the relatively high temperatures of the firing process. What about natural gems like jade, turquoise, and agates? Besides soldering on bezels after firing, is there any way to set these in original silver PMC? Yes! The trick is to use a plug of high-temperature material to hold a place for the stone. Let’s see how this will work ... 

For our purposes, imagine an oval turquoise cabochon that measures 10 x 14mm. This stone would be absolutely destroyed by the firing process, so some alternative is needed.

You’ll need to make a plug of some material that will stand up to the sintering temperature of 900°C (1650°F). This could be a piece of fire brick, investment (jeweler’s plaster), or potter’s clay. Whichever you choose, create a plug that has the same "footprint" as the stone, in this case a 10 x 14mm oval. I use a plastic template or simply trace around the form, then I use files to carve the plug.

 

Notice that the sides of the plug are straight, and that it is about twice as tall as the stone. After some experience with original silver PMC, most people get a sense of the shrinkage and develop a knack for understanding how much the size will change. To help with this, try using a copier with reduction settings. Make a sketch of the piece the size you intend to make it and set the copier to about 60%. It’s also possible, if you are gentle, to set the actual piece onto a copier and do the same thing. The alternate version works well, too: Make a drawing of the piece the size you want it when finished, then set the copier to 140%. This will tell you what size you should make the PMC original.

If you don’t want to take the time to get to a copier (I’m usually in too much of a hurry to get on with my work), you can make a quick sketch and do some simple math. In the case of this example, for instance, I multiply the dimensions of the stone by .4 (40%) and come up with 14 by 20 as the pre-fired size that will be needed to contain the turquoise. I make a quick sketch on a piece of scrap paper to help me visualize the starting bezel. I build a rim of clay roughly this size from fresh PMC and attach it to my work.

My educated guess is that this rim will shrink down to fit the stone. To ensure this, however, I set my plug in the center of the oval; no attachment is necessary. When the PMC shrinks in firing, it will close in on the plug and stop there. Even if my guess was off by a little, the plug will stop the PMC at exactly the correct size.
 

After firing, I lift the plug out to reveal a straight-walled compartment that is a perfect fit for the stone! I finish the piece in the usual way and complete all soldering and polishing, then lay the stone into place. I use the cut-off handle of a toothbrush to press the bezel down onto the stone, working alternately around the stone until the wall lays down smoothly. I achieve final shaping with a small file and polish with a burnisher.

Courtesy of the PMC Guild archives.


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