Annealed solid copper wire is the choice of bonsai masters for bonsai training and refinement. Annealed copper wire has the unique property of changing its stiffness as it is applied to a branch.
Annealed copper wire is very soft for easy application when wrapping a branch. As it is applied to the branch, annealed copper wire work hardens (greatly increases its stiffness) giving superior holding power. No other wire has this peculiar and useful characteristic. It is easier to apply than aluminum for a specific holding power, requires less wire, and is therefore less expensive per tree.
Copper has the additional benefit of a dull reddish brown color that blends with the natural color of the bark for a more attractive look. Forethought and correct wiring are encouraged since the softness is lost on the first bending. To avoid bark injury, wire should always be removed with wire cutters.