Glass

The materials definition of a glass is a uniform amorphous solid material, usually produced when a suitably viscous molten material cools very rapidly to below its glass transition temperature, thereby not giving enough time for a regular crystal lattice to form. A simple example is when table sugar is melted and cooled rapidly by dumping the liquid sugar onto a cold surface. The resulting solid is amorphous, not crystalline like the sugar was originally, which can be seen in its conchoidal fracture.

The word glass comes from Latin glacies (ice) and corresponds to German Glas, M.E. glas, A.S. glaes. Germanic tribes used the word glaes to describe amber, recorded by Roman historians as glaesum. Anglo-Saxons used the word glaer for amber.The remainder of this article will be concerned with a specific type of glass-the silica-based glasses in common use as a building, container or decorative material.

Glass Art

Even with the availability of common glassware, hand blown or lampworked glassware remains popular for its artistry. Some artists in glass include Lino Tagliapietra, Sidney Waugh, Rene Lalique, Dale Chihuly, and Louis Comfort Tiffany, who were responsible for extraordinary glass objects. The term "crystal glass", derived from rock crystal, has come to denote high-grade colorless glass, often containing lead, and is sometimes applied to any fine hand-blown glass.

There are many techniques for creating fine glass art; each is suitable for certain kinds of object and unsuitable for others. Someone who works with hot glass is called a glassblower or lampworker, and these techniques are how most fine glassware is created. Glass that is manipulated in a kiln is called warm glass, and traditional stained glass work is commonly called cold glass work. Glass can also be cut with a diamond saw, and polished to give gleaming facets.

Objects made out of glass include vessels (bowls, vases, and other containers), paperweights, marbles, beads, smoking pipes, bongs, and sculptures. Colored glass is often used, and sometimes the glass is painted, although many glassblowers consider this crude. A significant exception is the collection of pieces by the Blaschkas.

The Harvard Museum of Natural History has a collection of extremely detailed models of flowers made of painted glass. These were lampworked by Leopold Blaschka and his son Rudolph, who never revealed the method he used to make them. The Blaschka Glass Flowers stand as an inspiration to glassblowers today. See the Harvard Museum of Natural History's page on the exhibit for further information.
Stained glass is an art form with a long history; many churches have beautiful stained-glass windows.

Architectural Glass

Float (Annealed) Glass

90% of the world's flat glass is produced by the float glass process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is annealed by cooling in a temperatured controlled oven called a "lehr". The finished product has near-perfect parallel surfaces.

A very small amount of the tin is imbedded in the glass on the side it touched. The tin side is easier to make into a mirror. This "feature" quickened the switch from plate to float glass.

Glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly thicker glass is pushed back and not permitted to expand as it cools on the tin.

Annealed glass is considered a hazard in architectural applications as it breaks in large, jagged shards that can cause serious injury. Building codes across the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, in door panels, fire exits and at low heights in schools.

Sheet Glass

Before Pilkington's invention, flat glass panels were generally made as plate glass or sheet glass. Sheet glass (sometimes called window glass or drawn glass) was made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardened just out of the vat. This film or ribbon was pulled up continiously held by tractors on both edges while it cooled. After 12 meters or so it was cut off the vertical ribbon and tipped down to be further cut. This glass is clear but has thickness variations due to small temperature changes just out of the vat as it was hardening. These variations cause lines of slight distortions. You may still see this glass in older houses. Float glass replaced this process.