I have a woodstove which has a glass door on the front. As I live in a cold and snowy location it sees a lot of use in the winter. Which involves cleaning the glass and then letting the build up of soot, or whatever it is, to build up again.
I have noticed that regardless of what I’m burning or how I’m using the stove (damper open wide or closed down for overnight) the build up that occurs always produces a sort of rainbow on the glass. It’s pretty clearly visible and recurs in every circumstance.
It’s got me to wondering what’s going on? Why does it do this? What’s making the rainbow?
I suspect you are getting a very thin film of creosote on the inside of the glass, which is acting to create interference effects in the reflected light. The creosote builds up because the glass is colder than the smoke, and condenses the creosote out of the smoke. Some stove pass the intake air over the glass to keep the smoke away.
I’m not sure what the stuff building up on your window is, but I’d bet on it being creosote, the stuff that gets distilled out of heated wood and condenses on chimneys. It can eventually catch fire if it’s not cleaned out regularly, which is why we have chimney sweeps.
If so, this stuff is plating out on the (relatively cooler) window. It’s “colorless to yellow” (says Wikipedia: Creosote - Wikipedia ), and is likely plating out in a thin, uniform layer. If that’s the case, you’ll get interference betwen the front and back surfaces, with different colors corresponding to different thicknesses of the stuff (this is thing that Duck Duck Goose is referring to – and on prevuiew **beowulff ** has beaten me to.