What caused this?

[jking2]

Rising warm air from the transformer causes a static charge on the wall which attracts dust/soot from the ambient air.

Jim king

[Brian A. MacNeish]

Originally Posted by Speedy Petey

BINGO!

My mother has had this for years. She burns a lot of candles and just the soot from the candles will do this.
Electric basebaord heaters are one of the worst culprits.

Was being called in for ghosting on walls over electric baseboards in the early 1980's. Was told by one POCO advisor that the worst was certain types of synthetic carpet fibers that burnt at low temperatures around the temps of the elements.