Over the rainbow

I was in the lab yesterday drying and distilling some triethylamine for one of my projects. On a whim, I used a crystalizing dish full of copper shot instead of a heating mantle, oil bath, or sand bath. The resulting rainbow of copper brought a little joy into my day. Perhaps there is a lesson about grad school to be learned here... nah.

The rainbow resulted from the different rates of oxidation across the temperature gradient in the dish (~300 °C to 80 °C). There is a more thorough explanation for the phenomina in "Optical characterization of thin thermal oxide films on copper by ellipsometry" Derin, H.; Kantarli, K. Appl. Phys. A 2002, 75, 391–395, DOI: 10.1007/s003390100989

A copper oxide film grown thermally on a copper surface can be distinguished by the color of reflected light from the
copper oxide–copper film system. If the film grown on copper is cuprous oxide (Cu₂O), the observed color changes from purple to blue, green, yellow and red depending on oxide thickness. However, cupric oxide (CuO) grown on a copper surface is black... The reflectance spectra in the visible region for the thick copper film and copper oxide–copper structures, for different oxide thicknesses, are shown in Fig. 8. The reflectance minimum that begins to show itself after the first oxidation time shifts towards long wavelengths as the thickness of oxide film increases. The observed colors of these samples changed from green to yellow for the oxide thicknesses given in Fig. 8. These colors arise from the reflectance drop passing a minimum at wavelengths between 4000 and 6000 Å in the reflectance spectrum of the Cu₂O–Cu system. If the oxide films were CuO, the reflectance value would be zero for all wavelengths in the visible and therefore all samples of different oxide thickness would be black... The reflectance minima observed in Fig. 8 arise from the destructive interference of rays reflected from the air–copper oxide and copper oxide–copper interfaces.