Making
PAR lamps is 75% science, and 25% magic-- that's according to one of my coworkers who used to work as an engineer in the PAR64
line in Winchester, KY back when Sylvania was making their PAR56 and PAR64 lamps there. The Glass
reflector and
lens are VERY difficult to flame seal together without creating stress in the quartz, which is what happened here . When they get hot or subject to temperature shifts, the thick quartz can shatter along those stress points. It doesn't mean anything was done wrong by anyone, it's just going to happen a lot more often that it did in the past. Back in the day, GE and Sylvania used Corning glass ($) that was very low expansion, and better able to survive. [Most] modern PAR64 lamps are made in China, often using
Borosilicate glass (
Pyrex anyone?) or Soda Lime glass which has a slightly higher expansion coefficient. This glass is cheaper and doesn'
t handle expansion due to high heat as well. That's why some modern "
Pyrex" ovenware explodes, and the stuff your Grandmother had lasted forever.
The reason the lamp is sealed is that it has a nitrogen (inert) gas fill, which help prevent oxygen from getting at & attacking the Moly Foil/weld in the press seal area of the lamp capsule, which in turn increases lamp life. It's the same reason some
metal Halide lamps (
HMI, HSD, HSR) all have outer envelopes and inner capsules. It will still work without that environment, but it will result in shorter life.