Silver clay is a wonderful material, but it has its constraints. It is difficult to avoid cracks. In my opinion main crack causes are shrinkage, porosity, and contamination. Shrinkage manifests itself mainly in small cracks around the stone (or other hard material) which has been set in wet clay and co-fired. Inherent porosity of the PMC3 should not present a problem since the pores are very small. However, when kneading and shaping the piece larger cavities can emerge, leading to internal and external pits and holes. Contamination is really big evil. None of us works in a sterile environment. The sawdust collected for recycling inevitably contains contaminating particles such as grit from sanding paper, hairs from brush and simply dust. When mixed with the main piece all these particles burn out, leaving pits on the surface. I call them ‘pox blemishes’, and they are real spoilers.
|A surface crack|
Luckily, all these cracks can be repaired with a wonderful material – art clay silver oil paste. Everyone who orders his or her first pack of PMC or Art Clay Silver, should order the paste for repairs too. I have been using it for years already. First I was strictly following the instruction: dry 24 hours, kiln fire at 800°for at least 30 minutes. This process works very well with bezel cracks and other cracks discovered right after the first firing. But what if a crack has been discovered during polishing when the piece has been soldered? NEVER put a soldered piece in a hot kiln, especially at 800 degrees. Since the melting temperature of the hardest solder is about 780°, the solder will flow and burn a hole in your piece, much larger than the crack you want to repair. Besides, if the piece contains sterling silver elements, they will be also damaged by the heat (the maximum temperature sterling can stand without becoming brittle is about 760°).
The fine silver cat on the picture had been already soldered to a bezel with middle solder (melting temp 730°), when some annoying cracks were revealed by polishing. I did not feel like de-soldering, so I decided to deviate from the manual. After the silver oil paste had been applied and the piece was dried, the kitty was packed in fire-resistant fiber, which is also a great insulation material. So when I heated the cat with the torch, the rest of the piece was barely heated. Afterwards there was almost no scale on the sterling silver parts. The cat has been heated with a torch for about 5 minutes (according to the timer). It is very difficult to measure the temperature when firing with the torch, so you have to trust your eye and feeling. Just don’t let the piece become red, and you will be fine.
The result turned out to be quite nice. The cracks were gone, and did not reappear even after a rigorous polishing with all kinds of disks.
This method can be applied to any element which can be isolated from the rest of the piece. However I wouldn’t apply it to repairing bezel cracks since the stone can be easily scorched during firing with the torch.
And, last but not least, use this metal melting temperatures table, since it can be very helpful.