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Fashioning Glass, Part I

Dateline: 05/11/98

By Alan Bruzel

Glass is a state of matter that is neither solid nor liquid. Solids are rigidly bound molecular structures; the crystalline solids are ordered in molecular lockstep. Liquids have no particular order, and are not rigidly bound. Glass is somewhere between the two, having the rigidity of solids and the disorder of liquids. Robert Herrick's words of 1648 speak to this dreamland between solid and liquid:

Whenas in silks my Julia goes
Then, then (methinks) how sweetly flows
That liquefaction of her clothes.
From "Upon Julia's Clothes"

In recognition of its intermediary position, glass has been variously described as a highly viscous liquid, an amorphous solid, or, more simply, a unique state of matter. The term supercooled liquid, however, is out of favor. Here we may abolish a popular misconception. The notion of old windows being thicker at the bottom due to the slow flow of glass over time is incorrect. Manufacturing processes of bygone years involved spinning the molten glass. This created a disc thicker around its circumference than its center. Panes cut from this disc were installed with their thicker regions at the bottom.

For millions of years, the heat of volcanic activity created volcanic glass, also called obsidian. Its sharp cutting edges were used as tools by humans 75,000 years ago. It required about 70,000 years more for man to learn how to produce his own glass. The raw material for glass is plentiful. Good deposits of quartz sand, more than 99% silica (SiO₂, silicon dioxide), exist today in southern Jordan. However, silica's extremely high melting point (1700 ^oC; 3100 ^oF) made it unworkable by early civilizations. Pure silica is still difficult to work with today, but its melting point may be lowered by adding metal oxides. This was done in 4,000 B.C. by the Egyptians, who included plant ashes and alkali to their glass formulation. Technical refinements, including glassblowing, accelerated the worldwide spread of glass consumer items by the Roman Empire beginning around 50 B.C.

Today's commercial processes require the inclusion of fluxes and stabilizers that allow the preparation of specialty glasses. Fluxes are compounds such as soda (Na₂O, sodium oxide) and potash (K₂O, potassium oxide) that lower the melting point of silica. Stabilizers are compounds such as alumina (Al₂O₃, aluminum oxide), lime (CaO, calcium oxide), and magnesia (MgO, magnesium oxide) that impart useful characteristics to the finished product. Addition of barium or strontium gives protection against ionizing radiation. Glass so treated is advantageous in video display screens. Color screens and large screens use strontium because of its better X-ray resistance and its ability to strengthen glass.

As may be expected, glass melting furnaces require a non-silicate refractory substance to withstand the 1500 ^oC temperature needed to melt most commercially produced glass. A material composed of 94% zirconium dioxide (ZrO₂), 4.5 % silicon dioxide, 0.8% aluminum oxide, and 0.4% sodium oxide is suitable for this purpose.

Silicate glass is grouped with clay and cement in the traditional ceramics; advanced ceramics include non-silicate glasses, carbides, and nitrides. After 6,000 years of use, everyone should be familiar with the commercial silicate glasses, but the terminology is at times unclear. Here is a short vocabulary used in glass manufacturing and retailing.

Annealed glass: Left to its own devices, the surface of molten glass will cool faster than its center, resulting in internal stresses. Placed in the annealing oven (lehr), molten glass is allowed to cool gradually, relaxing these stresses.

Float glass: This is the latest glass manufacturing technology. Molten glass (1500 ^oC) is layered onto a bath of molten tin (melting point, 232 ^oC; boiling point, 2300 ^oC), producing a smooth, clear, glass sheet that is then fed into the annealing oven.

Frosted glass: Light bulb glass is frosted (etched) with hydrofluoric acid (HF) or ammonium bifluoride ((NH₄)HF₂).

Glass: The Celtic word glas means blue-green, and describes the color obtained from woad (Isatis tinctoria), used by early Britons to dye their bodies. The Latin word for glass is vitrum.

Heat strengthened glass: Annealed glass is cut to size, and brought to near its softening point. It is then cooled more slowly than in the tempered glass process. Heat strengthened glass is twice as strong as annealed glass.

Insulating glass: Two or three panes of glass are separated by an air space, followed by application of a sealant to their edges.

Laminated glass: Also known as compound glass, this article is produced by placing a plastic sheet between two glass plates and heating. If broken, the glass safely adheres to the plastic layer.

Plate glass: Unlike sheet glass, plate glass is formed by rolling, not pulling, the molten glass. Optical distortions are thus reduced, but subsequent grinding and polishing steps are necessary to render the glass fully transparent. Plate glass is no longer produced in North America.

Safety glass: This is a term for laminated or tempered glass.

Sheet glass: This material is produced by drawing molten glass upward. Sheet glass is transparent when formed, but has some optical distortions. Sheet glass is no longer produced in North America.

Softening point: The temperature at which glass deforms under its own weight.

Tempered glass: Annealed glass is cut to size, then brought to near its softening point. Rapid surface cooling produces surface compression that balances internal tensions. Tempered glass is three to five times stronger than annealed glass; breakage yields small, granular fragments.

A final word on silica must include two important physical characteristics. Melted silica – like melted boric oxide (B₂O₃), a compound used in glazing – has the attractive property of not crystallizing upon cooling. This phenomenon is responsible for silica's transparency. Silica's second important characteristic is a very low thermal coefficient of expansion, which is shown in the following table.

Thermal coefficients of expansion of glasses and other materials

With a low coefficient of expansion, objects made from pure silica can withstand extremes of heat and cold without cracking. As one would imagine, the exposed portions of the US Space Shuttle make good use of silica and silica-containing compounds. The reusable surface insulation tiles are 99.8% silica. The outer and middle panes of the forward windshields are 100% fused silica; the inner panes are tempered aluminosilicate.

Next week's article will discuss the chemical composition and properties of aluminosilicate, borosilicate, fused silica, high silica, lead, and soda-lime glasses.

Recommended Web resource for additional information:

Fashioning Glass, Part II
An article from this Web site detailing the chemical composition of silicate glasses.

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