A Summer with Stanley Kubrick | Unframed The LACMA Blog:
Another mystery quickly developed when the studio received a call from the manager of the Loews Capitol Theatre, MGM’s 5,500-seat showcase theater on Broadway (second largest in New York after Radio City Music Hall’s 5,700 seats). The projectionist was threatening to go on strike and close the theater, which meant no more showings of 2001: A Space Odyssey. Someone saying they were from MGM had gone into the projection booth and was using a chisel to file the aperture frame to remove the built up dust from the carbon arc projectors so that there would be sharp, not fuzzy, edges on the theater screen.
The arclight, or carbon-arc lighting, was the illumination source in movie projectors at the time. As the carbon rods burned down, they smoked and threw off dust that would adhere to the edges of the aperture frame with the result of projecting fuzzy edges on the screen. Kubrick did not like the distraction of fuzzy edges, so he brought his chisel into the projection booth to clean the edges so 2001 would be seen with crisp, clean edges on the screen. The mystery of where Kubrick had been was solved, and all future projectionists of 2001: A Space Odyssey would receive written instructions from the director stating how he expected his movie to be projected.