Aquadag

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Aquadag is a trade name for a graphite based coating commonly found in Cathode ray tubes.

The coating can be on the inside of the tube, where it helps to maintain a constant electrical field near the screen. Tubes intended for television use often will have an aluminum flashing which replaces the internal Aquadag, and also acts as a reflector on the back of the screen.

Aquadag is also painted in a band on the outside of many tubes. The glass of the tube between the internal coating and the outer Aquadag can then be used as a capacitor to filter the high voltage supply. Normally there is a spring contact that touches the Aquadag. When this contact is missing or loose, arcing will at times be observed.

When removing a CRT from a TV set, it is wise to discharge this built in capacitor to avoid a shock hazard when handling the tube.