Wire is used to wrap around branches or used as a guy wire attached to the pot in order to allow them to be shaped and held in position while the form of the tree is stabilised.
The most common type of bosai wire used (for cheapness) is anodized aluminium wire which is brown on the outside with a silver "core". Alternatively annealed copper wire, which is heated red hot to soften it before purchase and is more expensive, is used by the more experienced growers.
Copper is naturally stiffer and when worked becomes stiffer again, continual bending of copper wire will quickly introduce that stiffnes which creates kinks in the wire that cannot be removed until the wire is re-annealed. There is therfore a little more skill required to wire with copper rather than aluminium and getting it right first time becomes paramount. Copper however "case hardens" to a much greater extent than aluminium.
Copper wire comes in varying gauges the most common being : 0.8mm, 1.0mm,1.25mm, 1.5mm, 2.0mm, 2.5mm and 3mm. It is generally sold in 1 Kg rolls because it is heavier than aluminium and so there is less length per gramme. However, because of its greater stiffness once applied smaller gauges can be used. It can also be obtained (less commonly) in 500gram rolls.