A machine has overheated in an air filled enclosure, and a major brass component of mass around 1kg turned black. It was probably not in the high temperature regime for more than 2 hours at a guess and perhaps much less.
So, can anyone estimate what the approximate temperature might have been?
The black color is due black copper(II) oxide:
It can be formed by heating copper in air at around 300–800°C: $\ce{2 Cu + O2 -> 2 CuO}$
Possible range of temperature seems quite wide.
Appearing and intensity of brass blackening is combination of temperature and duration. It can be formed faster with growing temperature due faster kinetics of formation, but getting unstable at very high temperature due thermodynamic decomposition.
As organic substances are not reportedly present (above natural trace background), carbon from pyrolyzed organic matter is not considered.
A metallurgist would guess at the temperature with a metallographic examination (grain size, twins, etc.) and hardness tests. (It would anneal to some degree depending on temperature.)
The first step would be to determine the brass/bronze alloy. Common wrought brass products are 70:30 or 60:40, and interestingly the 70:30 is most yellow. The 60:40 picks up a pinkish cast.
The blackening of copper alloys (e.g., basic brass is 33% zinc with 67% copper) is due to the formation of black oxide of copper (the process is known by the trade name Ebonol C: Accordingly, if the copper alloys is containing 65% or more of copper, a black oxide treatment can be applied to alloy surface to blacken by converting the copper to cupric oxide $(\ce{CuO})$. According to Wikipedia, the temperature required to blacken brass alloy is about $\pu{400 ^\circ F} \ (\pu{204 ^\circ C})$.
It is also noteworthy to mentioned here this somewhat cool educational demonstration. According to this Illini website:
[...] penny coins, starting in 1983, were made of zinc with a thin layer of copper plated on the surface. If these coins are heated, the zinc will diffuse into the copper layer, producing a surface alloy of zinc and copper. These alloys are brasses. Not only does the zinc change the properties of copper, but also the color of the brasses changes with zinc content - reaching a golden yellow color at around 20% zinc and golden at 35-40% zinc. Copper also oxidizes when heated in air, producing a black layer of copper oxide $(\ce{CuO})$. Thus when heated, there is a competition between the rate of oxidation (making the surface black) and the rate of diffusion (making the surface a golden-yellow color).
You may use this demonstration to find out the exact temperature when the coin starts blackening (hopefully)!
