Black Smoke

I glanced out the window yesterday and saw a plume of black smoke coming out of the smokestack across the street. One of my coworkers said that they must be flue sanding. According to her, they periodically toss sand into the working smokestack to clean the built-up crud off the insides. I guess the heat from the fire makes the sand rise, but before it melts, it tumbles against the sides, sanding off the creosote and whatnot. I’d never heard of flue sanding, and a quick Google wasn’t very helpful. However, my coworker said that the smoke would only be black for a few moments, and she was right. I snapped three pictures, and by the third one, the black smoke was entirely gone.

ETA: YouTube to the rescue! This is about trains, but the concept is basically the same.