I would say it is old oxidised paint. It will come good after a good thorough clean or two.
I remember a sash window nightmare, though not my own thankfully.
I was working on an old art school in Bath. The sash windows were easily 7ft + tall. I had to really stretch to reach the tops (pre wfp).
I was working with "Boner", not the lickiest of people that day. There were 4 of us and we all picked a floor each to work on. Boner decided that the top floor was favourite as the windows were smaller and there were less of them.
Half an hour into the job and boner comes belting, nearly falling down the stairs. He was as white as a sheet and shaking (not too unusual for boner), he was terrified. After calming him down enough to find out what was going on we discovered that he had seen a ghost, and got seriously spooked.
After we had finished rolling about laughing, it was time to get back on the job. Boner flat refused to go back up the stairs, so two of us fetched his gear and set him on a lower floor while we went up for the tops.
Before we got out of the room there was a huge smash of glass, and as we looked round, there was Boner, even whiter than before and trembling in his boots. The first window he came to he flicked the latch and thop came whistling down like a guillotine, and smashed every single pane of glass.
We decided that he could just go for a stroll around the town to soothe his nerves.
A real sash window nightmare............
