Wiring, a relatively modern method of training bonsai trunks and branches into the desired forms, has become commonly accepted. It is often used in place of, or in conjunction with the traditional methods of long-term pruning and hemp-rope binding.
Copper wire that has first been annealed in a low-temperature fire is preferred. After it has cooled, it is wrapped around the branches in the direction the branch is to be bent. The branch should be bent once into its final position so as not to harm the cambium layer under the bark.
The wire should be wrapped taut, but not too tight, and should be removed just before it bites into the branch — between 6 and 12 months. The wire is removed with a bonsai wire cutter by snipping the wire at each turn, thereby allowing the cut pieces to fall to the ground. Never unwind the wire or use pliers to cut the wire, since this will damage the branches.
As a general rule, the thicker the branch, the thicker the wire it will require to hold it in place. Copper wire is stronger than aluminum wire of the same diameter. As a result, aluminum can be easier to work with, but you may need a thicker gauge wire to do the equivalent work that a lesser gauge copper wire could accomplish. One advantage of a thicker gauge wire is that it has more surface area to distribute the pressure of the wire onto the bark with so it may bite into the trunk less than a thinner wire would. For bonsai wiring, the thinnest usable aluminum gauge would be 1mm and .5mm would be the lowest recommended copper gauge. Often times it is better to wire a branch using two wires in parallel than use a heavier gauge wire.