Hi Jeff,
Welcome to bonsaiTALK!
Copper wire is annealed to make it soft enough to apply to branches. As it is bent it work-hardens and becomes more rigid and resistant to bending. This is why copper wire cannot be reused or repositioned once it has been applied to your bonsai.
If you walk into a hardware store electrical center, they will have several spools of bright shiny copper wire, and you might be tempted to purchase and use it for your bonsai.
You would find that it is very difficult to bend, because it hasn't been yet been annealed.
There is some
information here on the annealing process.
Aluminum wire, on the other hand, is not annealed. It is significantly softer than copper wire, and has less holding power, but it can be repositioned somewhat, so many beginners prefer to use it until they have developed some expertise at applying wire. Aluminum is also the preferred wire choice of many professionals for some thin-barked trees
like azalea, where the raw holding power of copper isn't as important as the flexibility of aluminum.
Because aluminum is naturally a silver color, it is often treated by an eletrochemical process called anodization to give it a brownish color similar to copper wire. It's easy to tell the difference because the core will still be silver, and the color, a metallic brown, is unobtrusive, but not really that similar to the appearance of annealed copper wire, which is a dark, dull red when applied and becomes darker with time.
You can buy and anneal your own copper wire, or purchase aluminum or copper wire that has been pre-annealed for bonsai work. www.Treebay.com has a large selection of both aluminum and copper wire in the Supplies folder of the website.
Regards,
Matt