Thank you Frank for your interest and pertinent question . I'd be happy too share what little i've learned about this material . I don't often shape or work the sheets much . The heat patinas become muted in colour on anything other than a smooth flat sheet , for the same reasons that a puddle's surface creates rainbows with oil .
This isn't necessarily always bad from the perspective of the artist , in fact , some of my new experiments will concern annealing a whole sheet ' crumpling ' it up , and then flame painting it .

Hi Colin,
I'm a tad confused by your response. The color variations on the oil film are due to a variation in the film thickness as you would also observe on a bubble of soap film.The bubble surface is essentially a sphere(not flat), with gravity causing a film thickness variation along with currents.Wave interference from light reflected from the surface with light reflected after it passes through the film and back causes the colors, which vary according to the film thickness, as the different colors represent different wave lengths.I'm not sure why the copper color variation occurs,but will speculate that it is a crystal size variation on the surface, not the orientation of the crystals.Regardless, your work is beautiful, and I am looking forward to seeing more.
Peace,eh?
Frank

No problem , I will try to clarify . I see you are fully aware about refraction /wave interference principles . Now imagine a still puddle , drop some oil in it and watch it spread out it's colours evenly and uniformly. Drop afew rocks in to roil up the water a bit and you get chaotic mixing eddies of colour. This is analagous to what happens when you fracture the smooth surface of the copper.

The oxidation which refracts the light is easy to 'bloom' and control when you have a large flat expanse , but terminates quickly when hemmed in .Any abrasion or bends tend to scatter the light and mute the colours in my experiments .

As to why this is , I must confess I am feeling my way intuitively in this field , I lack the trade language to express the process adequately , and am not always aware of the exact science behind everything I'm doing . Hope I haven't muddied up things more ( a video demo would clear all this up) , I appreciate all of your patience ,
Colin

Hi Colin,
In my experience shaping metal, I learned that a rubber hammer moves the metal(sheet steel) more efficiently.I find this true for both shrinking and stretching the sheet.I am not speaking of achieving fine detail.The rubber hammer surface does not disrupt the surface like the pebbles(metal hammer) in the oil film.
Perhaps shaping the copper sheet in a similar fashion would still (somewhat) preserve the effects you achieve in the flat undisturbed surface.
I think what you are doing on the flat copper sheet is beautiful. I am not suggesting that you need to change;just exercising the gray stuff.
Frank