History of glass
The history of glassmaking can be traced back to 3500 BC in Mesopotamia. Archaeological evidence suggests that the first true glass was made in coastal north Syria, Mesopotamia or Ancient Egypt. The earliest known glass objects, of the mid second millennium BC, were beads, perhaps initially created as accidental by-products of metal-working (slags) or during the production of faience, a pre-glass vitreous material made by a process similar to glazing.[n 1] Glass remained a luxury material, and the disasters that overtook Late Bronze Age civilizations seem to have brought glass-making to a halt.
Indigenous development of glass technology in South Asia may have begun in 1730 BC. In ancient China, though, glassmaking seems to have a late start, compared to ceramics and metal work. In the Roman Empire, glass objects have been recovered across the Roman empire in domestic, industrial and funerary contexts. Anglo-Saxon glass has been found across England during archaeological excavations of both settlement and cemetery sites. Glass in the Anglo-Saxon period was used in the manufacture of a range of objects including vessels, beads, windows and was even used in jewelry.
Naturally occurring glass, especially the volcanic glass obsidian, has been used by many Stone Age societies across the globe for the production of sharp cutting tools and, due to its limited source areas, was extensively traded. But in general, archaeological evidence suggests that the first true glass was made in coastal north Syria, Mesopotamia or Ancient Egypt. Because of Egypt's favorable environment for preservation, the majority of well-studied early glass is found there, although some of this is likely to have been imported. The earliest known glass objects, of the mid third millennium BC, were beads, perhaps initially created as accidental by-products of metal-working (slags) or during the production of faience, a pre-glass vitreous material made by a process similar to glazing.[n 1]
During the Late Bronze Age in Egypt (e.g., the Ahhotep "Treasure") and Western Asia (e.g. Megiddo) there was a rapid growth in glass-making technology. Archaeological finds from this period include colored glass ingots, vessels (often colored and shaped in imitation of highly prized hardstone carvings in semi-precious stones) and the ubiquitous beads. The alkali of Syrian and Egyptian glass was soda ash, sodium carbonate, which can be extracted from the ashes of many plants, notably halophile seashore plants: (see saltwort). The earliest vessels were 'core-formed', produced by winding a ductile rope of glass round a shaped core of sand and clay over a metal rod, then fusing it with repeated reheatings.
Threads of thin glass of different colors made with admixtures of oxides were subsequently wound around these to create patterns, which could be drawn into festoons by using metal raking tools. The vessel would then be rolled smooth ('marvered') on a slab in order to press the decorative threads into its body. Handles and feet were applied separately. The rod was subsequently allowed to cool as the glass slowly annealed and was eventually removed from the center of the vessel, after which the core material was scraped out. Glass shapes for inlays were also often created in moulds. Much early glass production, however, relied on grinding techniques borrowed from stone working. This meant that the glass was ground and carved in a cold state.
By the 15th century BC extensive glass production was occurring in Western Asia, Crete and Egypt and the Mycenaean Greek term 𐀓𐀷𐀜𐀺𐀒𐀂, ku-wa-no-wo-ko-i, meaning "workers of lapis lazuli and glass" (written in Linear b syllabic script) is attested.[n 2][n 3] It is thought the techniques and recipes required for the initial fusing of glass from raw materials was a closely guarded technological secret reserved for the large palace industries of powerful states. Glass workers in other areas therefore relied on imports of pre-formed glass, often in the form of cast ingots such as those found on the Ulu Burun shipwreck off the coast of modern Turkey.
Glass remained a luxury material, and the disasters that overtook Late Bronze Age civilizations seem to have brought glass-making to a halt. It picked up again in its former sites, in Syria and Cyprus, in the 9th century BC, when the techniques for making colorless glass were discovered. The first glassmaking "manual" dates back to ca. 650 BC. Instructions on how to make glass are contained in cuneiform tablets discovered in the library of the Assyrian king Ashurbanipal. In Egypt glass-making did not revive until it was reintroduced in Ptolemaic Alexandria. Core-formed vessels and beads were still widely produced, but other techniques came to the fore with experimentation and technological advancements. During the Hellenistic period many new techniques of glass production were introduced and glass began to be used to make larger pieces, notably table wares. Techniques developed during this period include 'slumping' viscous (but not fully molten) glass over a mould in order to form a dish and 'millefiori' (meaning 'thousand flowers') technique, where canes of multi-colored glass were sliced and the slices arranged together and fused in a mould to create a mosaic-like effect. It was also during this period that colorless or decolored glass began to be prized and methods for achieving this effect were investigated more fully.
According to Pliny the Elder, Phoenician traders were the first to stumble upon glass manufacturing techniques at the site of the Belus River. Georgius Agricola, in De re metallica, reported a traditional serendipitous "discovery" tale of familiar type:
"The tradition is that a merchant ship laden with nitrum being moored at this place, the merchants were preparing their meal on the beach, and not having stones to prop up their pots, they used lumps of nitrum from the ship, which fused and mixed with the sands of the shore, and there flowed streams of a new translucent liquid, and thus was the origin of glass."
This account is more a reflection of Roman experience of glass production, however, as white silica sand from this area was used in the production of glass within the Roman Empire due to its high purity levels. During the 1st century BC glass blowing was discovered on the Syro-Judean coast, revolutionizing the industry. Glass vessels were now inexpensive compared to pottery vessels. A growth of the use of glass products occurred throughout the Roman world. Glass became the Roman plastic, and glass containers produced in Alexandria spread throughout the Roman Empire. With the discovery of clear glass (through the introduction of manganese dioxide), by glass blowers in Alexandria circa 100 AD, the Romans began to use glass for architectural purposes. Cast glass windows, albeit with poor optical qualities, began to appear in the most important buildings in Rome and the most luxurious villas of Herculaneum and Pompeii. Over the next 1,000 years glass making and working continued and spread through southern Europe and beyond.
History by culture
Indigenous development of glass technology in South Asia may have begun in 1730 BC. Evidence of this culture includes a red-brown glass bead along with a hoard of beads dating to that period, making it the earliest attested glass from the Indus Valley locations. Glass discovered from later sites dating from 600–300 BC displays common colors.
Chalcolithic evidence of glass has been found in Hastinapur, India. Some of the texts which mention glass in India are the Shatapatha Brahmana and Vinaya Pitaka. However, the first unmistakable evidence in large quantities, dating from the 3rd century BC, has been uncovered from the archaeological site in Takshashila, ancient India.
By the 1st century AD, glass was being used for ornaments and casing in South Asia. Contact with the Greco-Roman world added newer techniques, and Indians artisans mastered several techniques of glass molding, decorating and coloring by the succeeding centuries. The Satavahana period of India also produced short cylinders of composite glass, including those displaying a lemon yellow matrix covered with green glass.
In Chinese history, glass played a peripheral role in the arts and crafts, when compared to ceramics and metal work. The limited archaeological distribution and use of glass objects are evidence of the rarity of the material. Literary sources date the first manufacture of glass to the 5th century AD. However, the earliest archaeological evidence for glass manufacture in China comes from the late Zhou Dynasty (1046 BC to 221 BC).
Chinese learned to manufacture glass comparably later than the Mesopotamians, Egyptians and Indians. Imported glass objects first reached China during the late Spring and Autumn period – early Warring States period (early 5th century BC), in the form of polychrome ‘eye beads’. These imports created the impetus for the production of indigenous glass beads.
During the Han period (206 BC to 220 AD) the use of glass diversified. The introduction of glass casting in this period encouraged the production of moulded objects, such as bi disks and other ritual objects. The Chinese glass objects from the Warring States period and Han Dynasty vary greatly in chemical composition from the imported glass objects. The glasses from this period contain high levels of barium oxide (BaO) and lead, distinguishing them from the soda-lime-silica glasses of Western Asia and Mesopotamia. At the end of the Han Dynasty (AD 220), the lead-barium glass tradition declined, with glass production only resuming during the 4th and 5th centuries AD.
Glass objects have been recovered across the Roman empire in domestic, industrial and funerary contexts. Glass was used primarily for the production of vessels, although mosaic tiles and window glass were also produced. Roman glass production developed from Hellenistic technical traditions, initially concentrating on the production of intensely colored cast glass vessels.
However, during the 1st century AD the industry underwent rapid technical growth that saw the introduction of glass blowing and the dominance of colorless or ‘aqua’ glasses. Production of raw glass was undertaken in geographically separate locations to the working of glass into finished vessels, and by the end of the 1st century AD large scale manufacturing, primarily in Alexandria, resulted in the establishment of glass as a commonly available material in the Roman world.
Anglo-Saxon glass has been found across England during archaeological excavations of both settlement and cemetery sites. Glass in the Anglo-Saxon period was used in the manufacture of a range of objects including vessels, beads, windows and was even used in jewelry. In the 5th century AD with the Roman departure from Britain, there were also considerable changes in the usage of glass. Excavation of Romano-British sites have revealed plentiful amounts of glass but, in contrast, the amount recovered from 5th century and later Anglo-Saxon sites is minuscule.
The majority of complete vessels and assemblages of beads come from the excavations of early Anglo-Saxon cemeteries, but a change in burial rites in the late 7th century affected the recovery of glass, as Christian Anglo-Saxons were buried with fewer grave goods, and glass is rarely found. From the late 7th century onwards, window glass is found more frequently. This is directly related to the introduction of Christianity and the construction of churches and monasteries. There are a few Anglo-Saxon ecclesiastical literary sources that mention the production and use of glass, although these relate to window glass used in ecclesiastical buildings. Glass was also used by the Anglo-Saxons in their jewelry, both as enamel or as cut glass insets.
The Arab poet al-Buhturi (820–897) described the clarity of such glass, "Its color hides the glass as if it is standing in it without a container." In the 8th century, the Persian chemist Jābir ibn Hayyān (Geber) described 46 recipes for producing colored glass in Kitab al-Durra al-Maknuna (The Book of the Hidden Pearl), in addition to 12 recipes inserted by al-Marrakishi in a later edition of the book. By the 11th century, clear glass mirrors were being produced in Arab Islamic Spain.
Glass objects from the 7th and 8th centuries have been found on the island of Torcello near Venice. These form an important link between Roman times and the later importance of that city in the production of the material. Around 1000 AD, an important technical breakthrough was made in Northern Europe when soda glass, produced from white pebbles and burnt vegetation was replaced by glass made from a much more readily available material: potash obtained from wood ashes. From this point on, northern glass differed significantly from that made in the Mediterranean area, where soda remained in common use.
Until the 12th century, stained glass – glass to which metallic or other impurities had been added for coloring – was not widely used, but it rapidly became an important medium for Romanesque art and especially Gothic art. Almost all survivals are in church buildings, but it was also used in grand secular buildings. The 11th century saw the emergence in Germany of new ways of making sheet glass by blowing spheres. The spheres were swung out to form cylinders and then cut while still hot, after which the sheets were flattened. This technique was perfected in 13th century Venice. The Crown glass process was used up to the mid-19th century. In this process, the glassblower would spin approximately 9 pounds (4 kg) of molten glass at the end of a rod until it flattened into a disk approximately 5 feet (1.5 m) in diameter. The disk would then be cut into panes. Domestic glass vessels in late medieval Northern Europe are known as Forest glass.
The center for luxury Italian glassmaking from the 14th century was the island of Murano, which developed many new techniques and became the center of a lucrative export trade in dinnerware, mirrors, and other items. What made Venetian Murano glass significantly different was that the local quartz pebbles were almost pure silica, and were ground into a fine clear sand that was combined with soda ash obtained from the Levant, for which the Venetians held the sole monopoly. The clearest and finest glass is tinted in two ways: firstly, a natural coloring agent is ground and melted with the glass. Many of these coloring agents still exist today; for a list of coloring agents, see below. Black glass was called obsidianus after obsidian stone. A second method is apparently to produce a black glass which, when held to the light, will show the true color that this glass will give to another glass when used as a dye.
The Venetian ability to produce this superior form of glass resulted in a trade advantage over other glass producing lands. Murano’s reputation as a center for glassmaking was born when the Venetian Republic, fearing fire might burn down the city’s mostly wood buildings, ordered glassmakers to move their foundries to Murano in 1291. Murano's glassmakers were soon the island’s most prominent citizens. Glassmakers were not allowed to leave the Republic. Many took a risk and set up glass furnaces in surrounding cities and as far afield as England and the Netherlands.
Bohemian glass, or Bohemia crystal, is a decorative glass produced in regions of Bohemia and Silesia, now in the current state of the Czech Republic, since the 13th century. Oldest archaeology excavations of glass-making sites date to around 1250 and are located in the Lusatian Mountains of Northern Bohemia. Most notable sites of glass-making throughout the ages are Skalice (German: Langenau), Kamenický Šenov (German: Steinschönau) and Nový Bor (German: Haida). Both Nový Bor and Kamenický Šenov have their own Glass Museums with many items dating since around 1600. It was especially outstanding in its manufacture of glass in high Baroque style from 1685 to 1750. In the 17th century, Caspar Lehmann, gem cutter to Emperor Rudolf II in Prague, adapted to glass the technique of gem engraving with copper and bronze wheels.
Modern glass production
A very important advance in glass manufacture was the technique of adding lead oxide to the molten glass; this improved the appearance of the glass and made it easier to melt using sea-coal as a furnace fuel. This technique also increased the "working period" of the glass, making it easier to manipulate. The process was first discovered by George Ravenscroft in 1674, who was the first to produce clear lead crystal glassware on an industrial scale. Ravenscroft had the cultural and financial resources necessary to revolutionise the glass trade, allowing England to overtake Venice as the centre of the glass industry in the eighteenth and nineteenth centuries. Seeking to find an alternative to Venetian cristallo, he used flint as a silica source, but his glasses tended to crizzle, developing a network of small cracks destroying its transparency. This was eventually overcome by replacing some of the potash flux with lead oxide to the melt.
By 1696, after the patent expired, twenty-seven glasshouses in England were producing flint glass and were exporting all over Europe with such success that, in 1746, the British Government imposed a lucrative tax on it. Rather than drastically reduce the lead content of their glass, manufacturers responded by creating highly decorated, smaller, more delicate forms, often with hollow stems, known to collectors today as Excise glasses. The British glass making industry was able to take off with the repeal of the tax in 1845.
Evidence of the use of the blown plate glass method dates back to 1620 in London and was used for mirrors and coach plates. Louis Lucas de Nehou and A. Thevart perfected the process of casting Polished plate glass in 1688 in France. Prior to this invention, mirror plates, made from blown "sheet" glass, had been limited in size. De Nehou's process of rolling molten glass poured on an iron table rendered the manufacture of very large plates possible. This method of production was adopted by the English in 1773 at Ravenhead. The polishing process was industrialized around 1800 with the adoption of a steam engine to carry out the grinding and polishing of the cast glass.
The use of glass as a building material was heralded by The Crystal Palace of 1851, built by Joseph Paxton to house the Great Exhibition. Paxton's revolutionary new building inspired the public use of glass as a material for domestic and horticultural architecture. The requisite technology for the construction of plate-glass had been made possible just a few years earlier by James Hartley, working for the firm Chance Brothers. In 1832, Chance Brothers became the first company to adopt the cylinder method to produce sheet glass with the expertise of Georges Bontemps, a famous French glassmaker.[n 4] The glass was taken from the furnace in large iron ladles and thrown upon a cast-iron bed of a rolling-table, where it is rolled into a sheet with an iron roller. The sheet, still soft, was pushed into the open mouth of an annealing tunnel or temperature-controlled oven called a lehr, down which it was carried by a system of rollers. Hartley introduced the Rolled Plate method in 1847. This allowed a ribbed finish and was often used for extensive glass roofs such as within railway stations.
An early advance in automating glass manufacturing was patented in 1848 by the engineer Henry Bessemer. His system produced a continuous ribbon of flat glass by forming the ribbon between rollers. This was an expensive process, as the surfaces of the glass needed polishing. Bessemer also introduced an early form of "Float Glass" in 1843, which involved pouring glass onto liquid tin.
The mass production of glass was developed in 1887 by the firm Ashley in Castleford, Yorkshire. This semi-automatic process used machines that were capable of producing 200 standardized bottles per hour, many times quicker than the traditional methods of manufacture. Chance Brothers also introduced the machine rolled patterned glass method in 1888.
In 1898, Pilkington invented Wired Cast glass, where the glass incorporates a strong steel-wire mesh for safety and security. This was commonly given the misnomer "Georgian Wired Glass" but it greatly post-dates the Georgian era. The Machine Drawn Cylinder technique was invented in the USA and was the first mechanical method for the drawing of window glass. It was manufactured under licence in the UK by Pilkington from 1910 onwards.
The polished plate process was improved by Pilkington in 1938 and incorporated a double grinding process to give an improved quality to the finish. Between 1953 and 1957, Sir Alastair Pilkington and Kenneth Bickerstaff of the UK's Pilkington Brothers developed the revolutionary float glass process, the first successful commercial application for forming a continuous ribbon of glass using a molten tin bath on which the molten glass flows unhindered under the influence of gravity. This method gave the sheet uniform thickness and very flat surfaces. Modern windows are made from float glass. Most float glass is soda-lime glass, but relatively minor quantities of specialty borosilicate and flat panel display glass are also produced using the float glass process. The success of this process lay in the careful balance of the volume of glass fed onto the bath, where it was flattened by its own weight. Full scale profitable sales of float glass were first achieved in 1960.
Notes and references
- True glazing over a ceramic body was not used until many centuries after the production of the first glass.
- Found on the MY Oi 701, MY Oi 702, MY Oi 703 and MY Oi 704 tablets; the least damaged, as far as this word is concerned, is MY Oi 703.
- Cf. κύανος.
- This process was used extensively until early in the 20th Century to make window glass.
- "Glass Online: The History of Glass". Retrieved 2007-10-29.
- Gowlett, J.A.J. (1997). High Definition Archaeology: Threads Through the Past. Routledge. ISBN 0-415-18429-0.
- These early examples are drawn from Christine Lilyquist (1993). "Granulation and Glass: Chronological and Stylistic Investigations at Selected Sites, ca. 2500-1400 B.C.E.". Bulletin of the American Schools of Oriental Research. 290/291 (290): 29–94. JSTOR 1357319.
- Wilde, H. "Technologische Innovationen im 2. Jahrtausend v. Chr. Zur Verwendung und Verbreitung neuer Werkstoffe im ostmediterranen Raum". GOF IV, Bd 44, Wiesbaden 2003, 25–26.
- "The Linear B word ku-wa-no-wo-ko". Palaeolexicon. Word study tool for ancient languages.
- Wilde, H. "Technologische Innovationen im 2. Jahrtausend v. Chr. Zur Verwendung und Verbreitung neuer Werkstoffe im ostmediterranen Raum". GOF IV, Bd 44, Wiesbaden 2003, 25–26 ISBN 3-447-04781-X
- McCray, W. Patrick (2007) Prehistory and history of glassmaking technology, American Ceramic Society, ISBN 1-57498-041-6
- "MY Oi 701 (63)". "MY Oi 702 (64)". "MY Oi 703 (64)". "MY Oi 704 (64)". DĀMOS: Database of Mycenaean at Oslo. University of Oslo.
- κύανος. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project.
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- Agricola, Georgius, De re metallica, translated by Herbert Clark Hoover and Lou Henry Hoover, Dover Publishing. De Re Metallica Trans. by Hoover Online Version Page 586. Retrieved September 12, 2007
- Ghosh, Amalananda (1990). An Encyclopaedia of Indian Archaeology. BRILL. ISBN 90-04-09262-5.
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- Braghin, C. (2002) "Introduction" pp. XI-XIV in Braghin, C. (ed) Chinese Glass. Archaeological studies on the uses and social contest of glass artefacts from the Warring States to the Northern Song Period (fifth century B.C. to twelfth century A.D.). ISBN 8822251628.
- Jenyns, R. (1981) Chinese Art III: Textiles, Glass and Painting on Glass. Phaidon Press
- Pinder-Wilson, R. (1991) "The Islamic lands and China" p. 140 in Tait, H. (ed) Five thousand years of glass. University of Pennsylvania Press.
- Braghin, C. (2002) "Polycrome and monochrome glass of the Warring States and Han periods" p. 6 in Braghin, C. (ed) Chinese Glass. Archaeological studies on the uses and social contest of glass artefacts from the Warring States to the Northern Song Period (fifth century B.C. to twelfth century A.D.). ISBN 8822251628
- Kerr, R. and Wood, N. (2004) "Part XII: Ceramic technology" pp. 474–477 in Science and Civilisation in China. Volume 5, Chemistry and Chemical Technology. Cambridge University Press. ISBN 0521838339
- An Jiayao (2002) "Polycrome and monochrome glass of the Warring States and Han periods" pp. 45–46 in Braghin, C. (ed) Chinese Glass. Archaeological studies on the uses and social contest of glass artefacts from the Warring States to the Northern Song Period (fifth century B.C. to twelfth century A.D.). ISBN 8822251628.
- Fleming, S. J., 1999. Roman Glass; reflections on cultural change. Philadelphia, University of Pennsylvania Museum of Archaeology and Anthropology.
- Stern, E. M. (1999). "Roman Glassblowing in a Cultural Context". American Journal of Archaeology 103 (3): 441–484. doi:10.2307/506970. JSTOR 506970.
- Toner, J. P. (2009) Popular culture in ancient Rome. ISBN 0-7456-4310-8. p. 19
- Bayley, J. (2000). "Glass-working in Early Medieval England" pp. 137–142 in Price, J. Glass in Britain and Ireland AD 350–1100. London: British Museum Occasional paper 127. ISBN 0861591275
- Evison, V. I. (2000). "Glass vessels in England, 400–1100 CE" pp. 47–104 in Price, J. Glass in Britain and Ireland AD 350–1100. London: British Museum Occasional paper 127. ISBN 0861591275
- Heyworth, M. (1992) "Evidence for early medieval glass-working in north-western Europe" pp. 169–174 in S. Jennings and A. Vince (eds) Medieval Europe 1992: Volume 3 Technology and Innovation. York: Medieval Europe 1992
- Ecclesiastical: Of or relating to a church or to an established religion.
- Harden, D. B. (1978). "Anglo-Saxon and later Medieval glass in Britain: Some recent developments" (PDF). Medieval Archaeology 22: 1–24.
- Bimson M. and Freestone, I.C. (2000). "Analysis of some glass from Anglo-Saxon Jewellery" pp. 137–142 in Price, J. Glass in Britain and Ireland AD 350–1100. London: British Museum Occasional paper 127. ISBN 0861591275
- Bimson, M. (1978) "Coloured glass and millefiori in the Sutton Hoo Ship Burial". In Annales du 7e congrès international d'etude historique du verre: Berlin, Leipzig, 15–21 August 1977: Liège: Editions du Secretariat Général.
- Hassan, Ahmad Y, Assessment of Kitab al-Durra al-Maknuna, History of Science and Technology in Islam.
- Hassan, Ahmad Y. The Manufacture of Coloured Glass, History of Science and Technology in Islam.
- Donny L. Hamilton. "Glass Conservation". Conservation Research Laboratory, Texas A&M University. Retrieved 2007-03-21.
- Georg Agricola De Natura Fossilium, Textbook of Mineralogy, M.C. Bandy, J. Bandy, Mineralogical Society of America, 1955, p. 111 Section on Murano Glass, De Natura Fossilium. Retrieved 2007-09-12.
- Newton, Roy G.; Sandra Davison (1989). Conservation of Glass. Butterworth – Heinemann Series in Conservation and Museology. London: Butterworths. ISBN 0-408-10623-9.
- MacLeod, Christine (1987). "Accident or Design? George Ravenscroft's Patent and the Invention of Lead-Crystal Glass". Technology and Culture 28 (4): 776–803. doi:10.2307/3105182. JSTOR 3105182.
- Hurst-Vose, Ruth (1980). Glass. Collins Archaeology. London: Collins. ISBN 0-00-211379-1.
- Encyclopædia Britannica 11th edition (1911)
- Bontemps on Glassmaking: the Guide du Verrier of Georges Bontemps, translated by Michael Cable (2008). Society of Glass Technology. ISBN 0900682604
- Buch Polak, Ada (1975). Glass: its tradition and its makers. Putnam. p. 169.
- "Chance Brothers and Co". Retrieved 2012-12-17.
- Practical Building Conservation: Glass and glazing. Ashgate Publishing. 2011. p. 468. ISBN 9780754645573.
- Pilkington, L. A. B. (1969). "Review Lecture. The Float Glass Process". Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences (The Royal Society) 314 (1516): 1–25. Bibcode:1969RSPSA.314....1P. doi:10.1098/rspa.1969.0212. JSTOR 2416528.
- "Borosilikatglas BOROFLOAT®". SCHOTT AG.
- Bickerstaff, Kenneth and Pilkington, Lionel A B U.S. Patent 2,911,759 "Manufacture of flat glass". Priority date December 10, 1953