Sorry to get back so late. Porcelain is much denser than concrete and maintains a greater degree of contact with the drill tip resulting in a hotter piece of carbide in a shorter amount of time. Using a hammer drill on the porcelain will not be as effective as using one on concrete bcause of the difficulty in chiseling porcelain (or glass).
Concrete has more voids and air allowing the hammering action to crush the spaces and cut the material away uch easier.
Ceramic brick (the dense kind, not the cheap flakey stuff) cause the same problem. Clay and glass are all vitrified and denser (most of the time) than concrete ans make drill bits hotter. If you can drill straight down and keep water on the bit at all times, you wont run into any problems, but if you drill on a wall and cool the bit intermitantly it can crack it from the sudden change in temperature.
Thats a bigger hole than I thought you were talking about, and I would guess you could get that big a glass cutting bit. Good luck
