The disc technique
The disc technique entailed blowing a sphere which was then attached to a prop at the opposite end of the blowing tube. Once the tube was detached, the prop was progressively opened from the hole left by the blowing tube. When heated intensely and spun rapidly, the hemisphere opened into a disc by the centrifugal force; this technique is still used today in the production of artisan dishes, open bowls and shield polychrome discs connected to lead. Once detached from the prop, the disc, if small in dimension (up to about 20 cm in diameter) was used as it was. The bigger discs, which could be larger that a metre in diameter, were cut into squares. The thickness is irregular (thicker in the centre, less at the edges), but the surfaces are clearer, because they were formed in the air without contact with other surfaces.
The disc technique, like the cylinder one, started in the first centuries AD and their production continued up to the 19th century. There is still some limited production today (cathedral glass) of artistic stained-glass windows.