This a quote from DougR "Starting is much more detrimental to the life of the bulb than running. Especially for those of us with probe start bulbs, which will be 97% of those using magnetic ballasts. The biggest issue is that when a probe start bulb fires up, it "sputters" Which means that some of the tungston in the starting probe will vaporize each time. It will then deposit on the inside of the arc tube. That's the black deposits you see as the bulb gets older"