Setting stones in pmc standard: before and after firing

According to this gemstone firing quide many natural and synthethic gemstones can be set into metal clay and fired in place. Certainly white cubic zirconia should do fine, especially those cut in the cabochon shape. In my experience some faceted CZ have been ‘smoothened’ after firing at 900 degrees Celsius for 2 hours (“the pmc golden standard”). They lost their sparkle and hue. One should be very careful with the choice of stones for firing in place in pmc.

But has anyone looked into the issue of setting stones in pmc not from the point of view of the stone, but from the point of view of the metal? It is known that pmc shrinks: pmc standard and pmc+- the ‘classic’ pmc types – shrink up to 25-30%. How does it affect a piece with a (larger) stone? Will it deform? Will the bezel survive the deformation caused by pressure and show no cracks?

Here are different jewelry pieces made from pmc standard. In each of these pieces a different stone setting technique has been applied. In the hoop earrings white CZ cabochons of various size (from 2 to 6 mm) have been set in wet clay, furnished with bezels (made from pmc ‘snakes’) and fired in place. As you can see, there has been slight deformation: the small ‘craters’ on the reverse side just underneath the stones. There has been some horizontal deformation too caused by the stones. But the fact that the stones protrude on the inside of the ring is due to the design (I wanted the stones to be set exactly in the middle of the circles). They did it in the unfired clay already. If a piece is reasonably thick (mine was 4 cards only, while 5 cards should be the minimum for texture), the texture and the proportions should not be distorted by the stones as seen in the circles pendant below. And the bezels show no cracks at all.

In this circles pendant I was afraid to co-fire the beautiful blue spinel so I just soldered the bezel after firing. The same thing I did with the ‘dew drop leaf pendant’ shown on the right. Soldering pmc standard is quite challenging, especially when a bezel is serrated. It is so easy to scorch the tiny teeth. You have to warm up the larger piece, not the bezel, use as hard solder as possible and burnish the pmc piece very well to prevent it from ‘soaking’ solder. And if the piece has stones which have been already set before firing you have to be careful not to burn them – even if they have survived 2 hrs at 900 degrees, they can get burned by the topical flame produced by the torch. I discovered that fine silver findings survive the soldering much better than the ones made from sterling silver.

Some suppliers offer a nice compromise solution: prefab fine silver bezels (tubes, serrated bezels and prong findings) which can be embedded into wet clay and fired in place. I inserted two serrated fine silver bezels to accommodate the 4 mm  smaller lapis lazuli stones in this bracelet. When setting the bezels in wet clay I left a narrow space of appr. 1/2 mm around the perimeter to allow for shrinkage. It was just a guess, but it was right. After firing the bezels were gripped firmly by the surrounding metal. I wonder what would have happened if they had been set tight in the clay? Any deformation because the bezel would resist the shrinking clay?

The larger bezel holding a 10 mm turquoise stone has been made from fine silver bezel wire and soldered afterwards. I have a problem with setting bezel wire into wet clay. Namely, the bezel has to be ‘closed’ to stay in place. Wet clay has bad cohesion with metal silver. However ‘making both ends meet’ by soldering the bezel before firing won’t work at all. There is no solder which can survive the temperature of 900 degrees. Even the hardest one has melting temperature of 760 degrees Celsius.

Note that this blog describes an ‘extreme’ scenario of pmc standard. Precious metal clays with less shrinkage such as PMC 3 may show different behaviour.

Conclusion: there are different solutions for setting stones in pmc. The deformation depends on the type of pmc and its shrinkage.  Whether the stone can be fired in place depends not on the stone only but on the design of the piece too. And fine (not sterling) silver bezels and findings are the first choice for soldering on pmc fine silver.