Does anyone know what the frit seals are made out of in HPS bulbs? I suspect some kind of oxide, but those are usually white in color and the seals I see in use are darker.

Bump!

Tag, you're it! this probably isn't much help,but my GE Lucalox sales brochure has a picture of a lamp with arrows pointing to various parts of the lamp and the arrows at the ends of the arc tube just say "ceramic seals"

Since the lampmakers also produce their own raw ceramic materials, most of them have ended up with their own specific compositions for the frit seals, optimised to match their different sealing processes and arc tube seal geometries.  However one of the more common and classic frit seals for HPS lamps is GE’s G-54 material, which consists of 54% Al2O3, 38.5% CaO, 7.5% MgO.  The lampmaking process causes this to melt, and it is then cooled at a particular rate which keeps it mainly glassy, although it does contain some crystalline phases (the % crystallites and their form and distribution along the seal is highly critical to ensure stress-free seals and long lamp life).  Because it is mainly glassy it has a translucent appearance.  By contrast the Osram and Sylvania frit seals have a more greyish appearance, and Philips seals are usually slightly greenish due to their own different compositions.  The actual compositions of all manufacturers as well as the developments over time can be found in their patents if you are interested to know more.