Watch Dome Group
There are a couple of ways to make oval or rectangular glass domes. The best way is to make a round dome and press in the sides to make it oval. The advantage is that the glass thickness is more uniform this way. Rectangular domes are then pressed on all four sides.
One way to make oval glass domes is to make a mold the size of the dome that you need and blow the molten glass into the mold. The problem with this method is that the glass is generally thicker on the rounded part than the flat sides.
One way to tell if a glass dome is old is by checking the color of the glass. Today we have very pure raw materials. In the older days this was not the case. Many of the older glass domes will have a slight blue or gray tint to the glass. It generally does not detract from the item displayed.
Most of the antique glass domes will have a paper tape on the bottom (or a visible trace of it once being there). The paper tape had two purposes. First if gave a slight cushion when setting the dome down. Second it covered the chipped bottom of the dome from it being cut to a particular height.