Convergent Series

learning, using & teaching metal clay, and other aspects of life

Posts Tagged ‘strength’

Some Thoughts on Finishing: Strength!

Posted by C Scheftic on 2012/10/08

One more comment, now that I’ve started down this route. And then I really have to get back to other things….

I have heard people say that they tumble their pieces to work-harden them. Personally, I don’t believe it works quite like that. And it’s not just a theory; it is based on at least one example (well, one series of examples) from my own personal experience.

When I got my first rotary tumbler (the one with the smelly barrel that I hated, which I mentioned two posts back), part of the reason I did so was because I’d heard people talking about work-hardening via tumbling. And I’d seen how mixed stainless steel shot did appear to bang on pieces as they tumbled, so it seemed to make sense. Since I’d started playing around with making rings, I wanted them to be as strong as possible.

Especially because I wanted to make a few adjustable / by-pass / wrap-around rings: the kind that do not have what I think of as the “extra strength of internal connectedness.” (If there is some official technical term for that, I’d appreciate learning what it is! What I’m referring to is how much harder it is to bend the band of a continuous ring, compared to that of an adjustable one.)

And, yes, I do understand that fine silver will never “work harden” to the extent that sterling silver will. (But it does get harder with working than it is after being annealed, and heat anneals it, and firing uses heat, and hammering will un-anneal it even if it does not get it quite as “hard” as work-hardened sterling.)

So there I was, working with fine silver metal clay, and I was willing to make thicker bands to help compensate for its softness; I simply wanted my rings to be as strong as I could make them. And I’m here to tell you that, in teaching myself to make them, I found out that tumbling does NOT yield full strength. If you think it does, you are wrong! I’m not saying it has no effect in that regard: tumbling is better than doing nothing. Yes, in the past few posts I’ve said that the shot does bang on pieces as they’re tumbled, but perhaps I should have used the word peck instead. Thousands, even millions, of little pecks does not equal a handful of good hammered whacks! Those pecks do appear to add a little bit of strength [later clarification: but just to the surface, not the whole way through the piece] which may be enough for earring elements that are not going to take much of a beating while worn. I have not found it to yield the strength needed for things that will get knocked around, such as rings, bracelets, cuff links, etc.

I know that some people will tell you that tumbling is adequate. But you can verify my statements by doing something very much like what I did: Just make yourself a pair of wrap-around rings. (They are the easiest project to use for this test. If you don’t know how to make them, there are sample projects online. Here is a very simple one that is actually pretty good, except for the line that then suggests tumbling to work harden the final results, sigh.) Your two rings need not be identical in design. (I’d suggest making them at least slightly different so you can clearly tell them apart!) But, for this test to be meaningful, they should be very close in size and thickness. Fire them to the maximum (2 hours at 1650 degrees Fahrenheit). You can curve them in the moist clay state and dry them in that shape, but it’s probably better if you fire them flat and carefully curve them after firing: bending a fired-flat piece into shape can be another very useful step in the work-hardening process! (I’d guess that’s the reason Nettie’s project, linked above, has you do that.) But if you are more comfortable shaping them in the moist-clay state, that’s OK; just make both of them the same way so that doesn’t add an extra “effect” to your test.

After your rings have been fired, you should shape or adjust them, as needed, to get a good fit. Tumble one for as long as you think is necessary. Then, take a hammer and a bench block and really work-harden the other one. Or, if you prefer, you can tumble both and then just hammer-harden one of them. (If you don’t know how to do that, either find a local metalsmith who can teach you, or let me know and I can make some suggestions. You really have to know what work hardening means for this test to be valid; but as you keep working on the piece, you should figure it out!)

Finally, with your bare hands, try to open or twist each of them. If you work hardened one correctly, you will feel the difference immediately!

(Full disclosure here: it did take me several tries to really grasp how to do this. At first, I was way too gentle with my attempts to hammer-harden my by-pass rings. But I kept thinking that they were just too soft: no way I’d sell them like that, I didn’t even want to risk wearing them myself. But I also wanted to keep the lovely metal-clay-style texture on my rings. So it took some experimenting with different hammers and mallets, steel blocks with and without some protection (e.g., a bit of leather), and more, even including some consulting with a local metalsmith (and author, Jan Loney) before I really grasped all this. A well-hardened ring will still be “adjustable” but it will become noticeably, ummm, harder to do so.)

For that matter, if you want to take this experiment to the next level, make one pair out of fine silver metal clay and a second pair out of the sterling form, fire each pair as recommended for its type, and do the same test with each pair. The kind of knowledge you can get from such an experiment will go a long way to building your understanding of the metals involved.

And you’ll get a nice collection of rings in the process: to wear yourself, to give as special gifts, or even to sell to help recover the cost of the clay used in this learning episode! (Be sure that any test-rings you pass on to others have really been work-hardened as much as possible…)

Happy claying to all!

p.s. I learned that lesson early on in my explorations with metal clay, when only the fine silver clays were available, though I did play around with annealing and hardening bits of both copper and sterling silver to get a feel for what was going on. All those results had such a physical feel to them, ones I had no idea how to capture in a photo so I did not even try. But I will close with a photo of a more recent ring; it’s about a year and a half old now. This one does have a connected, continuous band but, because all the flower-petal layers spin around the post holding the ruby, it still posed some particularly interesting challenges in my quest to find effective ways to work harden each and every the element of all my creations….

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