Most of my customers call roundels "bottle bottoms". Indeed they look like the bottle of a bottle. Actually the process is similar. At least it use to be when bottles were mouth blown. Today roundels are made one at a time in the same manner they were made in the middle ages. A gather of glass is taken by the glass blower and a slight breath of air is blown into the pipe. The resultant bubble is then transferred to a punty rod. Once on the rod the glass blower returns it to the furnace(glory hole) to keep it hot. Then the glass blower opens the bubble with a pair of wide prong pliers. Then the glass blower spins the bubble, and the centifugal force does the rest. The bubble is changed into a disk shape. Finally the roundel is broken off the punty rod, and cooled in a lair.
There are many designs that incorporate rondels, and roundels come in many sizes, and colors. Historically roundels were clear and used in homes as decorative windows to allow light in. They were also used in secular buildings for design. Some roundels have texture(ripples), some are solid colors, and some have a variet of colors in them. The designs can be roundels placed side by side vertically and horizontally, or they can have separation between them with a variety of designs. There is always a background glass. Again this glass can be clear, obscure,or colored.
We just finished this wonderful design for our clients laundry room. She wanted a decorative window that would hide the washer and dryer, and didn't want to use opalescent glass. In addition to the roundels we used 1" square faceted glass jewels.
The leading holding the roundels and glass together can vary in width and shape as well. In this case we used 1/4" lead. depending on the size roundel and effect we have used 3/16" and 3/8" wide lead came. After the window is assembled, soldered, and puttied reinforcing bars are soldered to the inside to support the weight and pressures.