As usual, your wanting the right technical info! and quite right too! well ill tell you what ive found, with samples of work done 30 yrs ago I have here.
It lasts indefinately, as the primary colours come from 400C . The secondary come from 600c to around 700, beyond that the oxide is no longer transparent and all you will have is a dark grey colour…
The oxide film , when generated with a neutral soft propane flame, is harder than the metal.
However, the real problem is with the user.
One customer loved her ring, but a few weeks later came to my expo and showed it to me.
I asked how come its so badly scratched?
She said she does a lot of gardening and had been building a rockery, without gloves!
Naturally I had it back, polished out all the scratches and the oxide film, re coloured it and gave it back. She thought the blue colour went right through. I said IF I could do that id be a millionaire!.
At last some sun here in Dorset.
PS, if your thinking of trying it do let me know.
theres more to it than justjust putting it into a flame!.