Anyone who has worked with metal, either or purpose or because they were fidgeting with a paper clip, has experienced a process known as work hardening. With most metals, the more you bend, flex or “work” the metal, the harder and more brittle it will become. If you don’t know what I’m talking about, grab a paper clip and start flexing it. You can bend it about 5 times before it breaks in two. Maybe more times if you’re really careful.
There are all kinds of great scientific explanations of why this happens on the internet, but I will sum it up as well as I can (based on my limited understanding). The molecules in the metal are arranged just so. When you bend it, they move around, and that realignment strengthens the bonds between them and causes the metal to become less flexible.
This is something that you manipulate and exploit when you’re working with metal, or in my case making jewelry. You want the metal to be as flexible as possible when you’re forming it – because that’s easier. You want the metal to be as hard as possible when you’re wearing it, because that makes the jewelry more durable.
I recently picked up some anodized aluminum wire at the craft shop to see if I could successfully pound it into bangles. Short answer is – I can’t. The anodized finishes, while permanent, will mare when you hit it with a hammer. For the record, I also mare when hit with a hammer. But that left me with a handful of pretty colored aluminum wire and no jewelry.
Aluminum is really, really malleable. It’s also the third most abundant element on our planet’s crust, so it gets used in all kinds of ways – cans, airplanes, etc. One would there would be a plethora of easy to parse information on the Internet about how to make it harder, but I couldn’t find it. I tried hitting it with a hammer (work hardening) with little success. Based on what information I could find, it appears that aluminum is most frequently stiffened by a process called precipitation or age hardening. For those of us without a degree in thermodynamics, that involves heating it up and cooling it down really fast. Anything to get those molecules moving around, right?
I decided the best course of action for my wire was to use it up on Stitch Witches (stitch markers for knitting) so I cut it into jump rings with a 1 centimeter diameter. It’s so, so soft that even getting the jump rings to stay round was a difficulty. At that point, I decided to try precipitation hardening them in the oven. The melting point for T2 aluminum is somewhere between 700-950 degrees Fahrenheit (as far as I could tell – damn the Internet has a lot of conflicting info on it), and I was aiming for a temperature around 60 percent of that.
Now, I have to the preface the rest of this post with a disclaimer. I think that aluminum might be flammable. Typically, precipitation hardening is performed in an autoclave, which is like an oven with a vacuum inside it. The oven heats, and the vacuum removes oxygen so whatever you’re working on doesn’t burst into flames. It’s possible that even trying this was dangerous, so I can’t recommend it. It also stunk to high heaven, and I may or may not have critically poisoned myself. So, proceed in this manner at your own risk. I have also been known to change sockets out without bothering with switching off the breakers.
That said, I think it worked! I put my rings into the oven, first around 200 degrees F, and then working my way up to 400 degrees F. All told, I probably baked them for about three hours, taking them out every hour or so and dropping them into a glass or water (quenching) and then checking their progress. They’re still not super hard, but they are hard enough that I was able to make my Stitch Witches. They kind of ended up looking like Barbie’s first nose ring, which isn’t what I was going for, but no amount of pseudo-science is going to fix my bad design.