It was thought for a long time that sterling silver clay couldn’t be manufactured, because of the firing conditions required for it to completely sinter. However, since the advent and refining of base metal clays which require the use of carbon for firing, in conjunction with the experiments of combining copper clay with fine silver clay at MCSJ, Mitsubishi finally fabricated commercially viable Sterling Silver clay in 2012.
At first, you might be a little bit daunted by what seems to be a complicated firing schedule; especially if you have only been used to firing fine silver clay. I mean, what’s easier than putting a load of fine silver items in the kiln, switching it on and leaving it there until the firing is complete? It seems a lot more faff to fire sterling silver on an open kiln shelf then go back and fire a second time submerged in carbon. If you think it is, reading this blog might tempt you to reconsider trying Sterling for the first time.
I was dubious about Sterling silver clay to begin with. I had not had a lot of practise with base metal clays like copper and bronze beforehand and I always seemed to have breakages when the items I had so carefully crafted, broke after firing. My first love was fine silver clay and didn’t think that would change. It hasn’t really! But what I will say is that I love both equally now – like both my children, they have good points and not so good points.
So what’s so great about Sterling?
Price and value for money
This is possibly one of the main advantages of Sterling silver PMC; because Sterling is a silver alloy (i.e. it is a mix of silver and copper) it is a little cheaper than fine silver gram for gram. And because it’s a silver alloy you can roll it out thinner; a little goes a longer way. This is because the copper content makes the clay more durable and stronger than fine silver, so you can use less clay than you might otherwise do with fine silver.
When it’s in the greenware state (i.e. dry but not fired) it is stronger than fine silver too and more flexible. So there’s not the awed hush and holding of breath if you’re sanding a particularly delicate piece – although please don’t go hell for leather, it is still clay after all ;o)
Perfect for ring bands
If you like the flexibility and the ease of texturing PMC but have always worried about the tensile strength of it when making ring bands, then this is definitely an answer. Just be aware that you need to make allowances for a larger shrinkage than PMC3. Where PMC3 shrinks about 12 – 15%, Sterling has a larger shrink rate, typically between 15 – 20%. This means that you will have to make the clay band bigger to allow for more shrinkage. For ring making, I generally teach my students in American ring sizes because I use ring pellets to control shrinkage and you only find them in US sizes. So I allow 3 US ring sizes for shrinkage. For example if I wanted a size 8 ring, I would make a ring initially a size 11, but a word of warning, don’t use investment ring pellets for Sterling rings – it can mark the inside and this is extremely difficult to remove without using abrasion. If you want more information about this, have a look at Janet Alexander’s blog.
Texturing, carving and engraving
Like all versions of PMC, the Sterling takes texture very well. It also carves and engraves beautifully – everyone including me seems to agree with this point. Because Sterling PMC is a little bit more flexible and strong, it withstands a little more pressure when carving.
This can be a good or a bad thing depending on your point of view. If, like me you enjoyed the shrinkage rate of PMC Original (now discontinued), PMC Sterling shrinks nearly as much as seen in this picture taken by Claire of Where the Wild Roses Grow. This can be an enormous advantage when making detailed work like Claire’s.
People know what Sterling is!
When you sell your fine silver jewellery, do you tell you customer that it is fine silver? And if you do, how many people know what fine silver is? The uninformed jewellery buyer is not sure of the difference between Sterling and fine, they are not truly interested since they only care what the piece looks like and if it is ‘silver’. But Sterling is a universally known type of silver that needs no explanation.
What’s not so great about Sterling silver PMC?
You need a kiln to fire PMC Sterling, there is no getting away from that fact. If you have been used to firing fine silver with a torch, this may be a real problem, because kilns are expensive. But, if you are kiln-less and interested in trying it, go to a local silver clay artist and ask about kiln time or a firing service. Most do offer this for a small charge; we certainly do here at Bluebell Design Studio and if you are lucky, some might even tumble it for you!
The firing schedule may be considered to be a drawback, if you’re not used to firing the base metal clays, it may seem to be a little bit convoluted. The reason for the 2 step process is firstly, to let the organic binder burn away and the second to allow the clay to sinter. It needs to be in the carbon to limit the amount of oxygen getting to the clay.
Suggested Firing Schedule for Sterling PMC
The firing schedule I use was suggested by Celie Fago who was one of the first artists to get her hands on Mitsubishi’s Sterling PMC and I have tweaked after using it for some time.
30 minutes on an open shelf @ 538oC – longer if the piece is thick.
1.5 hours submerged in carbon @ 815oC
When I submerge in carbon, the pieces are put in a small custom made box (No Flake Foil) fastened with ordinary staples. There is a layer of about 10mm at the base on which the items are placed and then covered with another 10mm of carbon.
I use Magic Carbon from Cool Tools in the US primarily because I was told it was the best for base metal clays but coconut activated carbon works too and this can be obtained in the UK. If your item is delicate you might want to protect it with some kiln paper to stop the carbon granules getting stuck in your textures which can distort and mark them.
I find that once the firing is finished, the items that come out the kiln have a slightly yellowish tinge. I still haven’t worked out why this is the case and my metal clay friends have not reported this to me but there’s a simple enough solution. I pop mine in some citric acid (otherwise known as safety pickle) and hot water. Within 5 minutes they are a lovely freshly fired white. Then I polish. I have to admit, that I bung mine straight in the tumbler and polish until shiny. But the polishing rules that stand for fine silver are the same for Sterling, so polish as you would normally.
So there you have it – hopefully this has given you some insight into Sterling Silver PMC. One of my fellow metal clay artists, Claire from Where The Wild Roses Grow was an avid user and fan of PMC Original and was very upset when Mitsubishi announced its discontinuation. When I suggested she try the Sterling as a possible replacement, she told me she was always a ‘fine silver snob’ and was reticent. She has since put Sterling PMC through its paces and found to her delight, it has a lot of the properties that she is looking for. Easily carved and a good shrinkage, with real tensile strength and when I asked how she felt, she told me she was optimistic!
If you have any questions or comments about this blog, I look forward to hearing them. Until next time, have fun being creative and enjoy!