[julianburke]

Here's the "official" answer: Has anyone ever had a window break in their car? No matter how much you vacuum or clean you almost always find a glass shard that mystically appears! RCA was always big on this prospect and most plumbicons would come shipped with a gimbal to keep the tube UPRIGHT and NOT FACE DOWN for fear that GLASS OR METAL particles (not dust) that may have lodged in the unit could fall down and hit the TARGET. (NO PUN INTENDED) These tubes were all 100% hand made and no matter how hard you clean the assemblies, debris can be present. Iconoscopes usually were never a victim of this unless they were broken or dropped in some way and also were too rugged to be bothered by this phenomenon. Most all image orthicons were labeled to not store or ship face down.