Insulator The main part of the insulator is typically made from sintered alumina (Al2O3),The Bosch Automotive Handbook, 8th Edition, Bentley Publishers, copyright May 2011, , pp 581–585. a very hard ceramic material with high dielectric strength, printed with the manufacturer's name and identifying marks, then glazed to improve resistance to surface spark tracking. Its major functions are to provide mechanical support and electrical insulation for the central electrode, while also providing an extended spark path for flashover protection. This extended portion, particularly in engines with deeply recessed plugs, helps extend the terminal above the cylinder head so as to make it more readily accessible. thumb|right|Dissected modern spark plug showing the one-piece sintered alumina insulator. The lower portion is unglazed. A further feature of sintered alumina is its good heat conduction – reducing the tendency for the insulator to glow with heat and so light the mixture prematurely.
Insulator The main part of the insulator is typically made from sintered alumina (Al2O3),The Bosch Automotive Handbook 8th Edition, Bentley Publishers, copyright May 2011, , pp 581–585. a very hard ceramic material with high dielectric strength, printed with the manufacturer's name and identifying marks, then glazed to improve resistance to surface spark tracking. Its major functions are to provide mechanical support and electrical insulation for the central electrode, while also providing an extended spark path for flashover protection. This extended portion particularly in engines with deeply recessed plugs, helps extend the terminal above the cylinder head so as to make it more readily accessible. thumb|right|Dissected modern spark plug showing the one-piece sintered alumina insulator. The lower portion is unglazed. A further feature of sintered alumina is its good heat conduction – reducing the tendency for the insulator to glow with heat and so light the mixture prematurely.