Borax In Glass - Ancient Or Modern?
From the Abingdon paper delivered by Robert A. Smith
January 1997

Just how long borax has been used by man is a question unlikely to be resolved. According to legend, Babylonians brought borax from the Far East more than 4,000 years ago to be used by goldsmiths, and writings have frequently cited ancient Egyptians as using it in metallurgy, medicine, and mummification, but none of this can be substantiated. The nitron baurak of the Greeks, the borith of the Hebrews, the baurack of the Arabians, the boreck of the Persians, and the burack of the Turks might all appear to express the same substance, borate of soda (i.e. borax). However, there is little evidence to show when or whether these names described the substance we now know as borax (Na2B4O7.10H2O). In fact, they are all transliterations of the Arabic word meaning to glitter or shine.

It seems probable that real borax was known to and used by craftsmen, scholars, and alchemists of the great Islamic civilization before 800 AD, and it is possible that Harun-al-Rashid's traders transported borate to China around that time; however if so, its origin is unknown. It wasn't until the Middle Ages that borax from Tibet was regularly imported into Europe. It was very expensive, and this limited it principally to the precious metal trade. Goldsmiths used it as a soldering agent and in the refining of metals and assaying of ores. The quantities traded were small, its method of production was secret, and its source remained a mystery until the second half of 18th century.

By the early 1500s, glass making was widely practiced in Europe, however there are no references to the use of borax. In trying to fix the first use of borax in glass, it needs to be remembered that prior to the 19th century, many accounts of glass were written by observers who were not themselves involved in the art. Technical secrets were passed on by word of mouth and practical instruction, and those who knew most were not given to writing for the benefit of others.

The earliest reference to borosilicate glass comes from China, where Zhao Rukuo described glassmaking by Arabs and others in 1225: "Borax is added so that the glass endures the most severe thermal extremes and will not crack". The earliest European mention of borax in glass occurs in a German work by Johann Kunckel in 1679, giving recipes for artificial gems.

In 1739, another German, Johann Cramer, recommended for crystal glass three parts of prepared flints (silica), one part of purest alkaline salt (potash), and one part burnt borace (borax). In 1758, Robert Dossie reported the best looking glass plates were ones containing 56 percent white sand, 23.5 percent pearl ashes (potash), 14 percent saltpeter, and 6.5 percent borax. He also notes that borax helps glass to receive certain colors.

Early use of borax in enamels and ceramics

The art of enameling began to take form in the early Byzantine era, but borax was not used in the frits applied to metals until the middle of the 18th century. The early borate-containing frits were colored ground glass used almost entirely for decorative purposes, and then in small quantities. The main increase in the use of borax did not come about until the enameling of iron created a new industry in the 19th century.

Enamel was first applied to sheet iron and steel in Austria and Germany about 1850. Cast-iron shapes such as cooking pots were pre-heated in furnaces. Frit was dusted on to the hot metal as a dry powder which sintered and stuck to the iron. The article was then returned to the furnace where the enamel melted to a smooth glaze. In this way, several coats of glass were normally added one by one to achieve the desired color and finish. The enamel frits had to be easily fusible, and borax became an important ingredient. By the end of the century, a worldwide trade had developed in all kinds of household goods, as well as durable advertisement displays, street names, and signs of all kinds, as enamel frit became the largest single use of borax.

Allied to enameling is the glazing and decoration of ceramics. The history of glazing, starting with the ancient Egyptian, Chinese, Babylonian, and Greek civilizations, is lengthy and complex. The earliest evidence of B2O3 use comes from China during the Liao dynasty (916 to 1125 AD). In recently discovered green shards, the glaze contains 13 percent boric oxide. The next examples come half a millennium later in the reign of Kangxi in China (1662 to 1722) and in Japan in 1699.

During the 18th century, potters and glassmakers in many parts of the world began to gain knowledge about the glazing properties of borax, but its price - £750 a ton in London in 1750 - remained far too high for general application. Then in the 19th century, technical developments in the ceramics industries coincided with new borate discoveries first in Italy, later in Turkey and the Americas which led to substantial reductions in price - less than £100 a ton in 1850, less than £20 by the 1890s. For the first time in history, borax became viable for modestly priced, mass-produced goods. Whereas Josiah Wedgwood's Etruria Works used no borax during his lifetime, less than a century later Staffordshire earthenware and soft porcelain glazes often contained between 12 and 25 percent borax.

For glazes and enamels, B2O3 is unique in that it acts simultaneously as a glass former, as a flux, and as a viscosity stabilizer that prevents the glaze from running too much while it is being fired. Perhaps most important of all, B2O3 reduces the thermal expansion of the glaze so that it can be matched to the expansion of the underlying ceramic or metallic body. At the same time, it improves aqueous and chemical durability while adding to the brilliance of the glaze.

Glass science - the borosilicate breakthrough

A scientific understanding of the way in which B2O3 could enhance the quality and performance of glass itself first began when Otto Schott persuaded Ernst Abbé from the University of Jena to join him and Karl Zeiss in forming the Jena Glassworks of Schott and Sons in Germany in 1884. Up to this time, there were only six elements commonly used in glass: silicon, oxygen, sodium, potassium, calcium, and lead. Schott and Abbé systematically investigated the introduction of a wide range of chemical elements into glass compositions. From this work they found that in the visible spectrum, glass containing B2O3 affected short wave length dispersion, so that it became a valuable constituent of optical glasses.

Schott and Abbé succeeded where others had failed in improving the optical properties of glass and at the same time increasing its resistance to water and chemical attack. They also discovered that glasses containing B2O3 could be formulated to withstand sudden changes in temperature. This led to the commercial manufacture of many products based on borosilicate glasses.

Beginning in the early 1900s, they invented a group of borosilicate glasses that were commercialized under the name of Jena Apparatus Glass. This was resistant to thermal shock and suitable for the chemical laboratory. By lowering the alkali content and increasing alumina, another group of glasses was developed which became known under the name Supremax of Schott and Genossen.

Low expansion borosilicate glass was first produced in the United States by Corning around the turn of the century for use by the railroad in signal lanterns. With the outbreak of war in 1914, supplies of laboratory glassware from Germany were no longer available, and Corning Glass Works became a new center for the production of borosilicate glasses. This led to a patent (by Sullivan and Taylor in 1915) for a low expansion borosilicate that was soon to revolutionize laboratory glassware and bring glass into the kitchen as cooking ware - Pyrex. With that the modern era of borosilicate glass technology can be said to have begun.

Dr. Smith is senior scientist at U.S. Borax Inc.