[Would a glow plug with a plain steel red hot filament fire? I'm guessing not, if it's really just a chemical reaction of the platinum and methenol? - Joe
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In discussions with the late Great Johnny Clemence (SP?) of Texas. He showed me the first spark plug that functioned as a glow plug in the 40's or 50's He had filed it down to get a hotter spark, and the tip began to glow red hot. So when he removed the coil/ignition the engine kept running.

It's just the heat from the coil not a chemical reaction. Platinum is used because it will stay hot.