Uncoated Copper dust after heating

Hello, I have heated up (300ºC or more) one 3D printed part of uncoated copper in vacuum and after test there was an orange dust attached to the surface of the part. It does not look like oxidation (which should be red/black for cooper) and I do not have any idea of what it is. Does anyone have an idea?

Hi Adria, sorry for the late answer.
It might have been coated by mistake and that you see the PU coating. Do you have still an image of the dust?
tx,
WiM