Cathedral glass is the name given commercially to monochromatic sheet glass. It is thin by comparison with slab glass, may be coloured, and is textured on one side.
The name draws from the fact that windows of stained glass were a feature of medieval European cathedrals from the 10th century onwards.
The term cathedral glass is sometimes applied erroneously to the windows of cathedrals as an alternative to the term stained glass. Stained glass is the material and the art form of making coloured windows of elaborate or pictorial design.
Traditional methods of making coloured glass
Very early architectural glass, like that sometimes found in excavations of Roman baths, was cast. The molten glass was poured into a mold of wood or stone to make a sheet of glass. The texture of the mold material would be picked up by the glass.
Casting came back as a common technique when rolled glass began to be manufactured in the mid-1830s and as glass jewels (also used for architectural glass) became popular. Rolled glass is not as rich and translucent as hand-blown glass, but it is much cheaper and is made in a variety of colors and textures, making it a useful decorative material.
Modern methods of making cathedral glass
This type of rolled glass is produced by pouring molten glass onto a metal or graphite table and immediately rolling it into a sheet using a large metal cylinder, similar to rolling out a pie crust. The rolling can be done by hand or machine. Glass can be "double rolled", which means it is passed through two cylinders at once to yield glass of a certain thickness (approximately 3/16".) Glass made this way is never fully transparent, but it does not necessarily have much texture. It can be pushed and tugged while molten to achieve certain effects. For more distinct textures the metal cylinder is imprinted with a pattern that is pressed into the molten glass as it passes through the rollers. The glass is then annealed.
Rolled glass was first commercially produced around the 1830s and is widely used today. It is often called cathedral glass, but this has nothing to do with medieval cathedrals, where the glass used was hand-blown.
Cathedral glass comes in a very wide variety of colors and surface textures including hammered, rippled, seedy, and marine textures. It is made in the USA, England, Germany, and China.
Cathedral glass has been used extensively in churches (often for non-pictorial windows) and for decorative glass in domestic and commercial buildings, both leaded and not, often in conjunction with drawn sheet glass and sometimes with decorative sections of beveled glass. It lets in light while reducing visibility and is an inexpensive, but useful and decorative material. While it doesn't have the richness and versatility of hand-blown glass, it has been used successfully for the creation of modern stained-glass windows in which the texture of the glass is treated, with the color, as a significant design element.