A metal is fully annealed when the atoms diffuse (particles becomes uniformly spread) this is why the metal becomes much more ductile and easier to work with.

 

An annealed metal is in its most relaxed state and therefore has less energy at this resting state. All metals will work towards an diffused crystal structure however if left to do naturally it would take a very, very long time. This is why we use heat to speed up the process.  

 

Limit the heating time to less that 10 mins as any more then that can cause grain growth which can make finishing the piece difficult.

 

Copper, silver, brass can be quenched. Metals with a good amount Iron (ferrous metals) need to be cooled slowly.

 

The infographic is a rough guide to the temperature and therefor colour to look for when wanting to anneal or melt a metal.