The simple theory of annealing copper wire
The hows and whys of annealing bonsai wire.

The hows and whys of annealing bonsai wire.


Properly annealed copper wire has a lot of advantages over aluminum. Annealed wire is very ductile until it is mechanically stressed by bending. The simple act of bending affects the crystalline structure of copper and renders it dramatically less ductile (harder). The advantage is that it is soft to apply, but, becomes hard as it bends. That means a small gauge of copper holds a branch in place better than a thicker gauge of aluminum. It’s more difficult to accidently reposition a branch. Unlike aluminum, copper will not fade, instead it darkens as it ages making it blend in better. The one drawback is that because copper is stiffer, it must usually be cut off instead of unwrapping to avoid damaging the tree.


Ok, what is annealing and how do I do it? Copper has a crystalline structure. When the crystals are aligned it is ductile and bends readily. When the crystals are dislocated by bending it is far less ductile and resists further movement. Without going into the physics suffice it to say that annealing (aligning crystals) occurs at temperatures above 760F. At that lower level it takes much longer for the crystals to physically relocate than at higher temperatures.


So how do you do that? First start out with bare copper wire. Strip any insulation. I tend to haunt recycling companies. They almost always have random lengths of stranded high voltage transmission wire. There will be up to 25, or even more, individual wires twisted together. For the recycler’s convenience they cut these lines into lengths of three to ten feet. You need to untwist these strands into single wires. I wind the individual wires around a flower pot so I end up with coils about 6” in diameter. Now you’ll need a heat source capable of more than 760F. It’s best to heat the copper to at least dull red, glowing red is better and faster. A self cleaning oven probably doesn’t get hot enough. A gas stovetop, gas barbeque grill, or the burner of a turkey roaster will work if the wire is close enough to the flame. A campfire will probably work. I’d go for a half hour of heat to start. If that doesn’t soften the wire, try longer. Copper will not become tempered if rapidly cooled so you can drop hot wire into a bucket of water without losing the annealing. That makes the wire instantly usable. Also, quenching causes surface impurities to flake off leaving clean wire. If you have access to a pottery kiln, you load it, ramp the temperature up to 1300F over a period of an hour and a half, turn off the heat, fish out the wire and dunk it in cold water. With some trial and error you’ll find what you have to do to make your heat source work.


Remember, any movement of the annealed wire causes the hardening process to start, so bend it as little as possible until you are actually wiring a branch.