Clear As Glass

The very first flat glass was made by the Romans, who rolled partly melted glass on a flat surface, or cast it into molds. The resulting small panes were cloudy and uneven, but they were transparent.

Through the late 1800s, flat glass was made with the Crown method in which blown spheres were reheated, punctured, and spun into a nearly flat disc. These circular panes had a defect at the center called the bullion. In the early 1900s, rolled glass panes were produced by factory machines. But in 1959, a new method made all the rest obsolete.

Today, optically clear panes are made by the "Pilkington float" method. Molten glass is poured onto the surface of a bath of molten tin, under a chemically controlled atmosphere. The glass spreads out into a perfectly flat layer and solidifies into a clear pane.

Source: Thought & Humor Blog.