Aquadag

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Aquadag is a trade name for a graphite based coating commonly found in cathode ray tubes manufactured by Acheson Industries a subsidiary of ICI.

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.

Aquadag is a water based colloidal graphite suspension, and may be painted on to items to avoid build up of static, or provide electrical screening.

Aside from its use in the production of CRTs, Aquadag is used in many types of high voltage lab apparatus where the creation of a highly predictable electrostatic field is required. The surfaces of some metals (most notably aluminum) can develop nonconductive oxide layers which tend to disrupt the electrostatic field produced around the surface of the metal when used as an electrode. Aquadag is not subject to such effects, and provides a completely uniform equipotential surface for electrostatics.