Hi everyone, thanks for your comments. Just to clarify a few points about this process:
The gold isn't reacting with anything or oxidising - it can't. (Thats why we love gold so much, it doesn't oxidise)
So the two components of the model that we are interested in are the stainless-steel and bronze.
Stainless-steel doesn't easily react with much - which is why we call it 'stainless'.
But it can , if heated enough, slowly create a thin surface oxide layer that acts as a diffraction grating , making it look yellow/blue/purple. This is a well-known effect - you can read about it on the web:
http://www.bssa.org.uk/topics.php?article=140
Because stainless is so unreactive, this oxide layer doesn't grow over time and the colour is stable in air.
Bronze on the other hand reacts well with other things, so you can use traditional chemical patination techniques on it. These however need coating/waxing to prevent further oxidation/reaction over time.
Because our model is made of a mix of the two things, you see both effects. When you heat it, the stainless component can colour, while the bronze component will darken.
The trick seems to be to stop heating after the stainless has coloured but before the bronze darkens . . .
Note:
The different steel materials do seem to have a different stainless/bronze ratio. If you file into a Bronze Steel Model, its that bronzey colour inside. If you file into a Gold Steel or Grey Steel model, its bright silver inside.
The coloured surfaces - Matt Black, Grey, appear to be chemical patination surface layers.
So for heat-treatment the best starting point is Stainless Steel or Gold Steel , whereas for patination experiments you should start with Bronze Steel.
Hope this inspires you all, do post pics here of your results!
Ben