Tom I love to work raw copper. As you form copper, say with a hammer, to simplify, the butter soft copper "hammer hardens" as you form and heat (from say a torch or oven) has to be applied to the workpiece "annealed" to prevent the metal from cracking. Depending on the size and intricacies of the workpiece, the heating process to anneal is repeated, air cooled a bit and planish continued, until the desired configuration is formed. This beat and repeat cycle may cover many repetitions.
The completed formed piece can be reheated to relax the metal and slowly cooled in a preheated sand bed to cool over several hours, but this super slow cooling is more typical on harder metals such as steel. One example of which is in firearm actions and the application of case hardening these & the sand may be replaced with other types of insulating materials to produce non-cyanide "color" to the firearm parts being hardened. SA Colt handguns get the cyanide gas treatment as well as sand during slow cooling. This creates the mottled blue finish which typifies the Colt Army actions. The Parker shotguns were however buried in wood ash, bone and leather. Specifics of the exact types of materials was a closely guarded secret, but the Fall colors on the finished piece were awesome.
On a factory formed piece, such as a piece of copper large diameter tube purchased at a box store, the copper is already hard and requires annealing to facilitate much of a radius bend. Water lines/gas tube is sufficiently soft to allow bending straight off the roll.
Different metals require varying techniques when it comes to annealing and are used for varying reasons unique to the chosen metal and purpose for which it's intended.
Hope this helps somewhat as it's quite an involved subject of metallurgy which is arcane to most, but those hand forming metal employ routinely.
Rick C.
