A spark plug is provided having a resistor. The resistor is made from resistor glass material containing an alkali free barium alumino-silicate glass mixed with mullite. In one embodiment, the resistor is a 15 to 30 wt % alkali free barium alumino-silicate glass and 10 to 25 wt % mullite. The resistor material provides for a greater processing kiln temperature range with reduced resistor variability and improved durability performance.