[urbanspaceman]

If you look at the earlier 'genuine' image of the sword hilt (before I photo-shopped it) you will see the areas of copper plating predominate in the lower surfaces and the areas where little to no contact has been experienced.
As far as I am aware, but I hasten to add I am not an expert in this field, copper sulphate or maybe copper oxide, I don't know which, was mixed with mercury and applied to the brass which was then baked so the mercury evaporated leaving a copper plating behind.
This was the same process used to gild those blue and gilt blades that became so popular.
Obviously, this was a hazardous process and life expectancy was poor, which was why it was a luxurious finish.
Electroplating was not established until around 1810.
My suggestion is that it was done for time at sea, apart from the excellent look.
You will also note that the colour I used to 'plate' the image was taken directly from the existing copper finish.