Ancient Glass

© Copyright 2002 by Lois Fruen

This article accompanies the textbook The Real World of Chemistry 6th ed by Lois Fruen Kendall/Hunt Publishing ISBN 0-7872-9677-5

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Glassmaking originated in the Syro-Palestine area around the third millennium BC and was developed in Egypt in 1500 BC. The Phoenicians became the greatest glassmakers and exporters of the ancient world. This was because of the rich deposits of silica-based sand, which contained a substantial amount of lime, found along the coast of Lebanon (James 464 and Fleming 138).

Glass was rare. It was used in artistic pieces and given the same status as semi-precious stones by artisans. Beautiful bright red and yellow opaque glass and cobalt-blue glass ingots were used as beads for jewelry and in figurines and decorative vessels like the ones shown to the right.

Ancient people probably discovered the technique for making glass when firing faience. Faience is a type of ceramic pottery glazed with a sodium alkaline flux. If the glaze was mixed with the crushed silicon clay before firing, a glassy substance would have been produced in the body of the clay (Bowman 33).

Ancient glass vessels were produced in molds. The earliest datable example of molded glass was found in the tomb of Thutmose III’s three foreign wives. The tomb yielded a molded glass vessel and a large number of glass beads and inlays, as well as two more unusual vitreous vessels. This has led some archaeologists to speculate that glassmaking came to Egypt from the Syro-Palestine area during the reign of Thutmose III (Lilyquist 194).

Ancient glass was made from sand quartz (SiO2). Soda ash (Na2CO3) was added as a flux to lower the melting point of the quartz from 1723°C to 850°C (Bahn 322). If only soda and sand were used, the resulting glass would have been very low quality. So, limestone (CaCO3) was added as a stabilizer. This resulted in much higher quality soda glass with the chemical formula Na2SiO3.CaSiO3.3SiO2 (Bahn 288).

5SiO2 (s) + CaCO3 (s) + Na2CO3 (s) Na2SiO3.CaSiO3.3SiO2 + 2CO2 (g)

Ancient glass often contained iron and manganese impurities that gave it color. The iron and manganese impurities came from the soda ash or silica. The iron ions gave the glass a light-green tinge, and the manganese ions produced pink to violet colors. The beach sands used to make glass also contained sulfurs in the form of sodium sulfate (Na2SO4) that gave colors from yellow to dark green depending on the percent sulfur in the sand. The picture to the right shows a small light-green glass vessel colored with ferrous oxide (FeO) (Fleming 138).

Ancient glassmakers also purposefully added minerals to color class. Blue glass was made by adding copper compounds such as azurite (Cu3(OH)2(CO3)2), chrysocolla (CuSiO3.2H2O), and chalcopyrite (CuFeS2). Powder-blue shades resulted from using Egyptian blue (CaCuSi4O10). Darker blues were produced by added cobalt-rich minerals, such as asbolite (CoO). Green glass came from iron (II) compounds. Hues from pink to violet were the result of different manganese oxides (MnO). Yellows and umber were produced with iron oxides and carbon. A brilliant-yellow was achieved by mixing antimony and lead (in the form of bindheimite Pb2(Sb, Bi)2O6), which resulted in a yellow precipitate of lead pyroantimonate distributed throughout the glass mixture (Lambert 113, Bucat 288, Flemming 140, Hill 321).

Complicated redox chemistry was used by ancient glassmakers to produce red-opaque glass. To make the red-opaque glass, the glassmakers mixed copper and lead compounds and then had to keep the firing furnace oxygen free (reducing atmosphere). Even a whiff of air would have oxidized the Cu1+ ions back to Cu2+ ions, and the glass would have become blue. In the reducing atmosphere, cuprous oxide (Cu2O) crystallized throughout the glass mixture, giving the glass a rust-red opaque color. Even though making red-opaque glass involved complicated chemistry, near-eastern glassmakers were routinely making it by the ninth century BC (Fleming 143-4).

Not until Roman times was a source of silica found that was almost iron-free, so until then ancient glassmakers added antimony in the form of sibnite (Sb2S3) to decolor glass. This enabled them to produce almost colorless glass (Fleming 142).

In 63 BC, the Romans conquered the Syro-Palestine area. They brought back glassmakers to Rome. Soon after, the first transparent glass sheets were produced in Rome. The word vitrum, meaning glass, entered the Latin language (Lambert 113).

The final and greatest ancient technical achievement of glassmaking was developed around 50 BC. That was glassblowing. The technique was probably developed in Palestine, since the earliest blown glass was found at Jerusalem. Before glassblowing, only the very wealthy could afford glass items. Suddenly, glassblowing made the production of large numbers of cheaply made vessels possible (James 464).

With glassblowing, glassmaking techniques quickly spread to other parts of the Roman Empire and flourished until 400 AD. In fact, Roman output of glass was not matched again until the Industrial Revolution (Bahn 323).

 

Sources

Bahn, Paul. Archaeology: Theories and Methods 2nd ed. London: Thames and Hudson, 1996.

Bowman, Sheridan ed. Science and the Past. Toronto: University of Toronto Press, 1991.

Bucat, R.B. ed. Elements of Chemistry vol.1. Australian Academy of Science, 1983.

Fleming, Stuart J. Roman Glass: Reflections on Cultural Change. Philadelphia: University of Pennsylvania, 1999.

Hill, John. Chemistry for Changing Times 7th ed. New Jersey: Prentice Hall, 1995.

Lambert, Joseph B. Traces of the Past: Unraveling the Secrets of Archaeology through Chemistry. Reading: Addison-Wesley, 1997.

Lilyquist, C. and R.H. Brill. Studies in Early Egyptian Glass. New York: The Metropolitan Museum of Art, 1992.

James, Peter and Nick Thorpe. Ancient Inventions. New York: Ballantine Books, 1994.

"Making Glass." www.sis.gov.eg/pharo/html/glass.htm