Friday, September 23, 2016

The Truth about Work Hardening and it's Pal, Annealing

This is my metalsmith persona talking here. Please excuse the length of this but I wanted to explain something to folks who have no background in moving metal.

Ahem. there are only four ways to work harden non-ferrous metals: rolling in a rolling mill, drawing wire through a draw plate, hammering (usually between steel hammer heads and steel bench plates--sometimes a hard plastic, like a delrin mallet will compress metal over steel) and lastly, by bending/twisting. That’s it. Period. (There are some kiln heat treatment procedures that can harden sterling and Argentium--available to find online)

Tumblers have a burnishing action and not an action like a hammer. Tumblers rub. A hammer head has weight and mass and as it falls through the air to its target, it picks up velocity which gives the strike more power. That power compresses the internal microscopic structure of, say, a piece of sterling sheet.  A tumbler, even if you tumble for 3 days straight, will only compress the surface molecular layers and not all the way through the piece of metal--that's what gives the shine. If you tumble fine or sterling silver or base metal clays, you will merely "case harden" the object. Kinda like an M&M's thin candy shell!

Fine silver has no copper added like sterling alloy does. The addition of a copper (7.5%) to fine silver (92.5%) makes an alloy that is still very malleable but able to withstand abrasion better and able to be work hardened--the quality of springiness that a forged neckpiece has is work hardened by the use of hammers and steel surfaces to create the form.

A fairly thin (18 ga., say) piece of silver metal clay (fine or sterling) that is tumbled will look shiny but will be able to be bent by hand. A thin piece of sterling clay that is subjected to one or more of the four methods of work hardening mentioned above will be quite difficult to bend. So for sure, a thin wire-like form in fine or sterling will be easily bent by hand after lots of hours of tumbling.

Fine silver, some say, cannot be work hardened as it doesn't contain that bit of copper. I think, technically speaking, that is correct. Fine silver will change its "temper" when continually compressed with a rolling mill, for instance, but nothing at all like a similar piece of sterling.

Here's the point of all this jabber: If you are making something out of fine silver clay and it needs to be springy or unable to be distorted in any way, you need to choose a different material (metal) to make it. OR make it thicker, OR use sterling or 960 clay OR use a base metal clay (they tend to be less maleable), OK rant done, sorry