A Short History of Glass

This is a short history of glass.

Naturally occurring glass, such as obsidian, has been used since the Stone Age. Glass making instructions were first documented in Egypt around 1500 BC when glass was used as a glaze for pottery and other items. In the first century BC the technique of blowing glass was developed.

During the Roman Empire many forms of glass were created, usually for vases and bottles. Glass was made from sand, plant ash and lime. The earliest use of glass was as a colored, opaque, or transparent glaze applied to ceramics before they were fired. Small pieces of colored glass were considered valuable and often rivaled precious gems as jewellery items. As time passed, it was discovered that if glass is heated until it becomes semi-liquid, it can be shaped and left to cool in a new, solid shape.

In the first century BC, at the eastern end of the Mediterranean glass blowing, both free blowing and mould blowing, was discovered.

The color of "natural glass" is green to bluish green. This color is caused by naturally occurring iron impurities in the sand. Common glass today usually has a slight green or blue tint, arising from these same impurities. Glassmakers learned to make colored glass by adding metallic compounds and mineral oxides to produce brilliant hues of red, green, and blue - the colors of gemstones. When gem-cutters learned to cut glass, they found clear glass was an excellent refractor of light. The popularity of cut clear glass soared while that of colored glass diminished.

Glass objects from the 7th and 8th centuries have been found on the island of Torcello near Venice. These form an important link between Roman times and the later importance of that city in the production of the material. About 1000 AD, an important technical breakthrough was made in Northern Europe when soda glass was replaced by glass made from a much more readily available material: potash obtained from wood ashes. From this point on, northern glass differed significantly from that made in the Mediterranean area, where soda remained in common use.

The 11th century saw the emergence, in Germany, of new ways of making sheet glass by blowing spheres, swinging these out to form cylinders, cutting these while still hot, and then flattening the sheets. This technique was perfected in 13th century Venice.

Until the 12th century, stained glass (i.e., glass with some coloring impurities, usually metals) was not widely used.

The center for glass making from the 14th century was Venice, which developed many new techniques and became the center of a lucrative export trade in dinnerware, mirrors, and other luxury items. Eventually some of the Venetian glass workers moved to other areas of northern Europe and glass making spread with them.

The Crown glass process was used up to the mid-1800s. In this process, the glassblower would spin around 9 lb (4 kg) of molten glass at the end of a rod until it flattened into a disk approximately 5 ft (1.5 m) in diameter. The disk would then be cut into panes. Venetian glass was highly prized between the 10th and 14th centuries. Around 1688, a process for casting glass was developed, which led to its becoming a much more commonly used material. The invention of the glass-pressing machine in 1827 allowed the mass production of inexpensive glass articles.

The cylinder method of creating flat glass was first used in the United States of America in the 1820s.

Art is sometimes etched into glass via acid or other caustic substance. Traditionally this was done after the glass was blown or cast. In the 1920s a new mould-etch process was invented, in which art was etched directly into the mould, so that each cast piece emerged from the mould with the image already on the surface of the glass. This reduced manufacturing costs and, combined with a wider use of colored glass, led to cheap glassware in the 1930s, which later became known as Depression glass.