patina

In insulating single glazing, insulating precious metal nanoparticles are melted into the glass. When it is darker inside than it is outside, these particles can sometimes render a hue in the reflection of the glass that resembles a patina. Colours become more visible as it becomes dark(er) inside and light(er) outside.

In the photo here, with a dark interior, you can see the exterior of the heavily distorted Van Ruysdael traditional glass type VR22.29 and the colours of a patina emerge. The colours are not visible when looking outside from inside.

patina