silicon glaze

A form of contamination on aircraft spark plugs that can lead to engine failure. Silica is found in sand and dust, and if a carburetor air filter leaks and allows sand or dust to get into the engine, the silica can form a hard, glasslike deposit on the nose core insulators of the spark plugs. Silicon glaze is an insulator at low temperatures, and will not cause a spark plug to show up as bad on a tester. But when the spark plug gets hot, the silicon glaze becomes conductive, and short circuits the high voltage to ground before it builds up high enough to jump the gap between the electrodes.