Thorium is IN the glass, not ON the glass... Got it!
The reason I understood things to be the way I said is because xenon lamps and carbon rods are often doped with thorium and/or other rare earth elements to improve their spectral emission characteristics.
The electrodes inside a high pressure xenon lamp are made from sintered tungsten that has a small percentage of thorium mixed in or coated on. I forget which.
Regardless, I was led to believe that as long as you don't eat them or carry them around in your pocket, moderate amounts of thorium-treated (thoriated?) materials are mostly harmless and that you stand more chance of contracting cancer from a sunburn than you do from (conscientiously handled) thorium.
Anyhow, it's good to know more about thorium and, now, I have something new to go read about.