Roman glass jar
3rd Century A.D
This jar has an attractive metallic sheen or iridescence, but it was not always like that. When it was first made it was clear. Over the years, water and atmospheric pollution have created many thin layers of glass on the outside. As light penetrates the surface and is reflected from each layer it is split into its component colours to give a rainbow effect. Because the colours come from light, and not added pigment, the effect changes as the viewing angle is altered.