Glass is one of the most difficult things to photograph because, well, it's invisible. When we look at glass – clear glass that is – we don't see the glass itself, but only what it reflects. Therefore when photographing glass, the lighting setup is critical because it determines what will be reflected and what the reflections will look like. Creating and adjusting the setup to produce the look you are after can take a long time, but it's worth it.
Another important element of the photo studio setup for glass is the background. If the background is dark, the relfections must be light to provide contrast. Conversely, if the background is light, the reflections must be dark. Observe the two product photographs below. Both are of the same object (a hand-blown glass ice bucket that my wife and I received as a wedding present eons ago), but the backgrounds and reflections are opposite of each other. Where there are white relections on the dark background, there are black relections on the white background. The second photo adds a little twist because it contains water which is also invisible, but the same rules apply.
For an excellent lesson on how to photograph glass, read Light Science & Magic by Hunter, Biver, and Fuqua.