This is a round glass dome that measures 12 x 18″ high.
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.
Most new round glass domes have a bead on the bottom. The bead gives greater strength to the glass. This bead is created by holding the glass dome at the top and rotating it while applying a gas flame near the point of its height. The glass melts at that point with the bottom dropping off and leaving a bead to seal the work. Oval glass domes do not have this bead generally since the flame can not be held in place as the dome rotates.
Many of our customers have the feeling that older glass domes will have more bubbles. This is generally not the case. It is the desire of all glass blowers to not have bubbles in the glass.