With the development of leaded petrol in the 1930s, lead deposits on the mica became a problem and reduced the interval between needing to clean the spark plug. Sintered alumina was developed by Siemens in Germany to counteract this. Sintered alumina is a superior material to mica or porcelain because it is a relatively good thermal conductor for a ceramic, it maintains good mechanical strength and (thermal) shock resistance at higher temperatures, and this ability to run hot allows it to be run at "self cleaning" temperatures without rapid degradation. It also allows a simple single piece construction at low cost but high mechanical reliability. The dimensions of the insulator and the metal conductor core determine the heat range of the plug. Short insulators are usually "cooler" plugs, while "hotter" plugs are made with a lengthened path to the metal body, though this also depends on the thermally conductive metal core.
With the development of leaded petrol in the 1930s, lead deposits on the mica became a problem and reduced the interval between needing to clean the spark plug. Sintered alumina was developed by Siemens in Germany to counteract this. Sintered alumina is a superior material to mica or porcelain because it is a relatively good thermal conductor for a ceramic, it maintains good mechanical strength and (thermal) shock resistance at higher temperatures, and this ability to run hot allows it to be run at "self cleaning" temperatures without rapid degradation. It also allows a simple single piece construction at low cost but high mechanical reliability. The dimensions of the insulator and the metal conductor core determine the heat range of the plug. Short insulators are usually "cooler" plugs, while "hotter" plugs are made with a lengthened path to the metal body though this also depends on the thermally conductive metal core.