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Facts of the Matter Richard Brill |
Looking back
at glassmaking
through the agesGlass is one of our most common, and often overlooked, technologies.
But that hasn't always been the case. Glassmaking has a history as long, or longer, than history itself.
Glass occurs naturally in the form of obsidian, formed when liquid lava is cooled too quickly for crystals to form. Stone Age man shaped it to be used for cutting tools, but it took thousands of years to learn how to manufacture and fashion it from raw materials.
Almost all of the world's glass is man-made. And glass is made from sand, but not just any sand. The essential component of glass is silica, found in the form of quartz. On continental beaches the grains of sand are mostly quartz, which is pure silica derived from the weathering of granitic rocks. Beach sand in Hawaii is nearly quartz-free because the low silica basaltic rocks do not contain quartz.
STAR-BULLETIN / 1999
Metal oxides vary the properties of glass or give it color. Lead oxide, for example, adds weight and brilliance.
Although there are many different compositions of glass, all are based on silica (silicon dioxide) with some type of alkali added to lower the melting point and lime (calcium oxide) added as a stabilizer. Common alkalis are soda (sodium oxide) and potash (potassium oxide). Ninety percent of common glass is soda-lime glass, which is used for bottles, jars, everyday glassware and window glass. It has low resistance to high temperatures and corrosive chemicals but is cheap to produce.
Other metal oxides in the right percentages vary the properties of the glass or give it color. Manganese oxide clears the glass and removes the slight green tint caused by minute quantities of iron oxide. Lead oxide adds weight and brilliance, and gives glass a soft surface that is well suited for grinding, cutting, polishing and engraving. Boron gives glass thermal, electrical and corrosion resistance, and as such, borosilicate glasses are used in chemistry labs and the pharmaceutical industry, for cooking utensils in the home, and in bulbs for high-powered lamps.
Optical glasses contain barium, which increases the refractive index allowing for thinner lenses. Cerium in the glass absorbs infrared radiation, and various metallic oxides such as cobalt (blue), uranium (red), or copper (green) add color.
Nobody knows for sure exactly when people began to make glass, but glassmaking is among the first technologies of the Bronze Age and one of the oldest arts.
Legend has it that Phoenician merchants carrying a cargo of saltpeter accidentally discovered how to make glass around 5000 B.C. It is said that they could not find rocks to make a fireplace, so they used mounds of saltpeter (sodium nitrate) to rest the cooking pots on. The heat of the fire melted the nitrate, and when it mixed with the sand on the beach, an opaque liquid formed that hardened into glass.
Despite the legend, more than likely the earliest experiences with glass were as glazing material on pottery. Since alkali lowers the melting temperature of silica, a mixture of calciferous sand and quartz sand inside a pottery kiln would easily melt to form a bead of glass.
The earliest examples of man-made glass are opaque beads from ancient Mesopotamia and Egypt dating from nearly 5,500 years ago. The oldest glass vessels date from the 16th century B.C. when glass was crafted by dipping a compacted sand or sculpted clay mold at the end of a stick into molten glass. Rolling the mold caused molten glass to adhere to it. The glazier could then roll the molten ball on a rock to smooth and decorate it as it cooled. Chipping away the mold after cooling left behind a hollow glass bowl.
A major innovation was the discovery of glassblowing sometime between 27 B.C. and 14 A.D. when the Romans began blowing glass inside molds, making possible a wider variety of shapes. Within the city of Rome, the most important buildings had cast glass windows, albeit with less than ideal optical properties.
Early in the first century A.D., across the Mediterranean in Alexandria, artisans learned to add manganese dioxide to clear the glass, which turned out to be a major breakthrough in the manufacture of fine glassware.
During the first millennium, gradual changes took place in the techniques in producing glass of different qualities and styles, although little is known about the first 800 years.
In the 11th century, German craftsmen developed a technique for producing sheets of glass by swinging a molten, hollow glass sphere vertically. Swinging caused the sphere to elongate into a hollow cylinder, which was cut lengthwise and laid flat to form a sheet.
In the Middle Ages, glazing was a luxury, and only royal palaces and churches were likely to have glass windows. As with any technology, glazing saw an increasing number of uses with the passage of time, first in public buildings, then in private homes of the wealthy and slowly in other more pedestrian uses. As recently as a hundred years ago, a frosted glass panel in the front door of a home was a rare and expensive symbol of status.
Venice developed into a center for glassmaking, a distinction it held from the 13th to 17th centuries. The city passed a protective ordinance that banned imports of foreign glass and glassmakers, thereby assuring that the local craftsmen would preserve the unique style and retain their role as the glassmaking center of the Western world. The Venetian crystal known as Cristallo remained the standard of quality glassware all over Europe.
In northern Europe the glass was heavier and sturdier but not as clear as Cristallo. The late 16th century saw many Venetians migrating to northern Europe with hopes of earning a better living. They established factories there and made Venetian-style glass but gradually merged the two styles.
The next major innovation was the creation of lead crystal, which was patented in England in 1674 as the result of a commission to find a substitute for the Venetian-type crystal that was made from pure quartz and potash. By adding higher proportions of lead oxide instead of potash, the glassmaker produced the brilliant glass crystal with high refractive index that we know today.
Around the same time, the French were developing a new technique for producing plate glass for use in mirrors. The molten glass was poured into molds on a flat surface, then ground and polished until smooth.
Once scientific research and development entered the picture, change came fast and furious as automated processes became commonplace. Such innovations as the automatic bottle-blowing machine and the glob feeder assured more consistency in size and quality, and increased the speed of mass production.
Modern flat-glass technology began in 1905 with the development of equipment that could vertically draw a continuous sheet of glass of consistent width from a tank. This method was in full production by 1914. The years following World War I saw the development of an even more efficient process that poured the glass from a pot directly through two rollers, producing glass of uniform thickness and smooth surface.
These processes were refined and plate glass was made this way until 1959 when the "float process" was perfected. In this process, molten glass is poured across a bath of molten tin, then spread and flattened before being drawn horizontally in a continuous ribbon into the annealing oven. This produces a flat, smooth surface of uniform thickness that requires minimal grinding and polishing to produce a finished product.
In the past 50 years, glassmaking has become highly automated with computer controls that produce precision our ancestors could have never even dreamed of. Twenty-first-century technology produces an astounding variety of specialty glasses of different compositions and properties such as transparent glass ceramics, infrared-transmitting glass used in night-vision goggles and the tiny threads in the fiber-optic cables that carry information in the form of laser pulses.
Although glassmaking is one of the oldest technologies, the uses and varieties of glass today are more widespread than ever. We take it so much for granted that it has become virtually and truly transparent.
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu
