Glass In all its Glory: Part 1

We look through it and maybe overlook it, but glass is a vital 200m t/y material for the world today. Martin Pitt looks at its development and impact on society

GLASS is an amorphous state of matter, formed when a liquid is unable to form crystals as it cools from its liquid state. It occurs in nature around volcanoes when lava forms a mineral called obsidian. Early humans used fragments of it as cutting tools and arrowheads a million years ago, and it is still used by some surgeons today in scalpels with a cutting edge ten times smaller than a metal blade. Black obsidian could be polished to make the best mirrors until silvered glass ones came into production in the 14th century.

Human-made glass is the oldest chemically manufactured product. Unlike pottery, which can be made from a single clay, glass requires two separate minerals mixed then heated very strongly.

It is made from sand, mainly quartz (SiO₂), which melts at about 1,700°C, too high for early furnaces, but will dissolve in sodium carbonate (Na₂CO₃, melting point 851°C), known as soda. Decomposition gives off CO2 and the strong base Na₂O reacts, forming ionic bonds which prevent the formation of silica crystals when it cools. 

This is soda glass. Potassium carbonate (K₂CO₃), known as potash, can also be used.

The addition of basic calcium compounds (lime) gives soda-lime glass, which is more resistant to water, and is 90% of glass production, used for jars, bottles and windowpanes. The calcium (Ca) may come from chalk (CaCO₃), or dolomite CaMg(CO₃)₂ and Mg (magnesium) has a similar effect to calcium. Sea sands often contain this from shells. Alumina (Al₂O₃) may be found in the sand or added, the combinations and ratios affecting the properties of the glass for specific purposes. Boric oxide (B₂O₃) partially replacing sodium carbonate gives borosilicate glass. Lead as PbO gives a heavy high refractive index glass, particularly valued for cut crystal items.

Sand usually contains iron, which gives a green colour due to FeO, but the colour can be substantially reduced by an oxidant converting it to yellow Fe₂O₃. The addition of other compounds (naturally or deliberately) can give a range of other colours. Beads of coloured glass were almost certainly first discovered from accidental mixing of minerals in a metalworker or potter’s furnace and were valued as gems.

Early history

The first example of artificial glass, a green bead made from silica, lime and soda, dates from Egypt in about 3,500 BC. For thousands of years bead necklaces were an important treasure for the rich in many places across north and central Africa and the Middle East. In 1609, the very first factory in North America made coloured bead necklaces to trade with native people.

The first recipe for glass is on an Assyrian cuneiform tablet from about 650 BC, giving 60 parts sand, 180 parts dried seaweed (which combusts to soda ash) and five parts chalk.

The Egyptians also made small items and later vases by dribbling or moulding glass around a clay shape, then carefully scraping out the clay.

However, for mass production of glass unrivalled until the 18th century, we have to credit the Romans. An Egyptian luxury became a commonplace item: bottles, dishes, jugs, plates, spoons, covers for seedlings, even drainpipes. Plates and bowls were made by slumping hot glass over a mould. From the first century glassblowing was used. Blowing into a mould produced standard items more quickly, including square bottles for efficient storage. Powdered glass could be melted on ceramic to give glazed tiles and pots. The Romans also made clear glass (using arsenic, antimony or manganese oxides to decolorise the iron) which underpins most of its modern uses. In the many wine bars, patrons could admire the colour before drinking.

After the fall of the empire, glass production faltered, starting again in the 8th century Islamic world, firstly as fine artistic products. Then between the 9th and 12th centuries a key foundation of chemical engineering occurred with clear glass equipment being used by Arab alchemists. This largely impervious material allowed different shapes to be made in which level and colour could be seen for reactions, distillations, and other processes. This was the precursor of modern laboratory glassware which enabled 19th and 20th century chemists to develop our processes.

The alkali problem

Sand was plentiful. The limit to glass production was generally the availability of Na₂CO₃ (soda) or K₂CO₃ (potash). Egypt had natural deposits of soda, and from 30 BC it belonged to the Roman empire who exploited this, in an early form of globalisation. Soda could be taken to a source of good sand, lumps made and shipped to product manufacturing sites. Sites in the Middle East continued to supply glass in this way during the Islamic empire.

Venice had a sandbank nearby of high-quality sand and could import wood for the furnaces from the north, and soda from Egypt, and was ideally placed for export so became the centre for fine glass production from the Middle Ages to the Renaissance.

Elsewhere, potash was made by burning wood, leaching the ashes with water then boiling down to a white solid. During the Middle Ages, small family businesses throughout Europe made “forest glass”.

Making the glass

For almost all of glassmaking history the ingredients have been melted in a ceramic pot inside a brick furnace using a wood fire. Making pots which withstood the conditions without cracking was a carefully guarded art. It was also a matter of art to minimise the number of bubbles in the final glass, a process called fining. Slag or scum on the top could also need to be removed. Once the item had been made it was necessary to allow the object to cool slowly, known as annealing, which was often done using waste heat from the melting chamber. Later ones had a series of chambers called a lehr, with objects being transferred over hours or days for the largest ones. Coal was substituted for wood in England in the 17th century in conical glass buildings up to 40 m tall, in which the heat was economised in several chambers and working stages inside, with the top being used to dry pots before preheating.

Glassware

After purely decorative items, the commonest use of glass in Eurasia was for bottles and drinking cups. Highly decorated drinking glasses and tiny bottles of cosmetics, perfume or mourners’ tears were part of the treasure in Egyptian tombs. The Romans made bottles by the hundred. China’s mastery of porcelain from 200 BC meant that glass for this purpose only appeared with European imports in the 13th century, though they produced some of their own from the 16th century.

English courtier and privateer (a pirate licensed to rob enemy ships) Sir Kenelm Digby (1603–1665) owned a glassworks in which he made bottles known as “glass onions” due to their shape. Fuelled by coal and given a much greater draft than wood, it burned hotter and produced a much stronger glass, because small bubbles had been let out by the lower viscosity at higher temperatures. However, the glass was darkened by sulfur fumes reacting with iron in the glass to make black ferrous sulfide (FeS). Fortuitously, wine makers were pleased with the protection from light and the toughness enabled transport, revolutionised the wine trade and made English dark glass wine bottles a world-leading export. It was the great strength of this glass which made champagne possible, since the fermentation takes the wine to a pressure of eight atmospheres.

Windows

The English word comes from wind-door, a hole for ventilation and vision. Some light could be let through with waxed linen, or an oiled animal skin. In the first century AD, the Romans were the first to use small glass panes to let in light by pouring, hammering (with a wooden mallet), and stretching glass in small increments with frequent reheating. In fifth-century Constantinople (Istanbul) they blew a spherical bubble and flattened it on an iron plate to get a circular piece of glass with the ripples of the pipe in the centre to give light, though not good vision.

From about 1300 in France this method was improved by reheating and spinning to get a disc up to 1.5 m across. After square panes had been cut, the rest was made into triangular or diamond shapes, which could be joined together with lead to make larger panes. The part around the blowpipe rounded ripple pattern was known as a bullseye. This was known as crown glass and was the best general window glass up until the 19th century.

An alternative developed in England at the same time was by blowing a glass bubble into a long balloon about 150 cm long and 30 cm wide. The ends were cut off and the tube slit and uncurled while hot, giving a wavy pane called broad sheet glass, or cylinder glass. This gave larger sheets of lower quality.

Stained (painted or coloured) glass appeared in churches about 1000, not just to let light in, but to awe. In Europe it dominated the glass industry from the 12th century, and when building a cathedral, a glassworks was often created on the grounds.

In 1688 French glassmaker Louis Lucas de Néhou (1650–1728) developed a process to make flat glass for mirrors. Glass was poured onto an iron plate, rolled, then ground and polished to give a smooth and very flat surface. This plate glass was much later used for windows and remained the best for this purpose until the middle of the 20th century.

It is notable that India, China, and Japan had early glass technology, but being hot (and in many cases having homes of flimsy construction) had no great need for clear glass windows so did not develop this large-scale glass manufacture like Europe. Wealthy people in the Islamic empire had dwellings around a central courtyard with no windows to the street.

The lens

Simple lenses are referred to in the ancient world, and Muslim scholars explored their effect on light, but in Italy in the late 13th century they began to change the world. The first was from spectacles, which enabled scholars and scientists to have an extra 20 or 30 years of productive life and are one of the most significant inventions in history. Mass production was based in Venice. Combined with the invention of printing, they assisted the rapid developments known as the Renaissance. Lens production also gave us the microscope from 1590 which led to a true understanding of biology, and the telescope from 1609 enabled new astronomy, which in turn promoted mathematics and the art of navigation. Galileo Galilei (1564–1642) used ordinary soda-lime glass, but in 1674 the British inventor George Ravenscroft (1632–1683) produced glass of superior refractive index and transparency by the addition of lead and potassium. Called flint glass due to his source of high purity quartz, crushed flint nodules, the term is still used today for some hard transparent glasses. Ravenscroft mainly used it for decorative cut crystal glassware. Isaac Newton (1642–1726) replaced lead with zinc in 1681 for his optics experiments and telescopes.

In 1730, English amateur optical scientist Chester Moore Hall (1703–1771) produced the first achromatic lens (which did not spread the colours). A convex and a concave lens of different refractive indexes effectively cancelled each other out, really important for telescopes etc. To keep his secret, he ordered the lenses from different companies, not knowing they used the same lens grinder, George Bass (1700–1770), who realised the purpose and mentioned it to English optician John Dolland (1706–1761). Dolland patented it, and with his son built a large company, which was absorbed into the Boots company as recently as 2009.

Recycling

Broken glass (cullet) was recycled by the Romans and the Persians, and even carried by ship back to producers. Towards the end of the Roman empire, glassmakers in Britain increasingly used cullet, and in later centuries virtually all glass was made from earlier glass. An 11th century Mediterranean shipwreck with three tonnes of cullet reveals trade in the Middle Ages. As well as reducing costs, melted cullet makes a bath in which the other components can dissolve, and has remained a major part of the glass industry through to today.

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In the next part I will look at the way in which new techniques changed the world from the 19th century.

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Martin Pitt CEng FIChemE is a regular contributor. Read other articles in his history series: https://www.thechemicalengineer.com/tags/chemicalengineering-history