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… and so we must now proceed to explain also the nature of glass …

Pliny the Elder (AD 23–79)

The history of lighthouses is intimately associated with the history, composition and manufacture of glass – thus it is essential that we take a quick survey of the development of this most versatile of materials.

Glass in its simplest form is a noncrystalline substance made from a fused mixture in the approximate proportions of 55 per cent silica sand, 25 per cent soda ash (in the early days potash) and 20 per cent lime. The solid translucent material is formed from the molten state by rapid cooling that prevents the formation of crystals. The colour of glass made from naturally occurring sand is green to bluish green, which is caused by iron and other impurities. Glassmakers learned to make coloured glass by adding metallic compounds and mineral oxides to produce brilliant hues of red, green and blue. Though coloured glass was used for decoration, for example stained-glass windows in churches, lighthouse lens makers quickly realized that coloured glass could be used to give a red or green characteristic to a white light, by placing a sheet of coloured glass in front of a clear lens, a technique that is still in use.

Glass has existed in one form or another since prehistory, not always man-made, but also formed naturally by volcanic action, when the erupting lava flow cooled too rapidly for there to have been time for crystals to form. This form of glass is called obsidian, and is usually black due to impurities. Stone Age people are known to have used it to make sharp knives and spear points as the noncrystalline structure allowed it to be honed to a sharp edge, and it is still used today for surgical scalpels. Early glass was a valuable commodity known to have been used as early as 3000 BC for beads, seals and decorations. While a date for the invention of man-made glass has never been established, there is a tradition that, around 1000 BC, the crew of a Phoenician ship carrying a cargo of nitrum – a type of hard, dense rock salt – dropped anchor off a sandy beach and, finding no stones, used lumps of their nitrium cargo to support the cooking pot. The heat of the fire caused the nitrium to fuse with the sand to form a translucent liquid. In Egypt, small bottles were made by winding a glass thread round a bag of sand and heating it at the same time to fuse the threads together. When the desired thickness and shape had been formed the bag was cut and the sand removed. The first cast glass was also ancient Egyptian in origin, made by pouring molten glass into a mould. By the first century BC, the technique had travelled to Greece and other trading centres around the Mediterranean.

At the start of the Christian era, methods of glass blowing were discovered as the craftsmen of the Roman Empire developed many new ways of working with glass. The Roman conquests and the influence of Roman culture spread the use of glass objects and techniques as far as Britain and Northern Europe. The major centres of glassmaking were now at Alexandria, Egypt, Byzantium (Constantinople, now Istanbul) and in the German Rhine Valley. New processes, such as staining, gilding and enamelling were developed as were new products, such as translucent and stained-glass windows, pipes and vases. In Europe during the Dark Ages, the use of glass declined, with many methods of manufacture and finishing being forgotten. However, glass was not entirely abandoned in Northern Europe, as it was known to have existed in Anglo-Saxon times. There was a revival in the 7th century but the ‘Golden Age of Islam’ led to the next major development, the first clear, colourless, high-purity glass.

The Muslim chemist Abbas Ibn Firnas (AD 810–87) perfected the technique of producing clear glass, his glass described by the Baghdad poet Al Buhturi as follows: ‘Its colour hides the glass as if it is standing in it without a container.’The coloured or stained glass so much used in Christian church architecture was made from nearly 50 recipes for different colours published by Jabir ibn Hayyan in the 8th century. By the 10th century, the sun was being harnessed as a heat source, with Ibn Sahl first describing the refracting parabolic mirror made from glass to intensify and concentrate the sun’s rays into a crucible furnace. As William the Conqueror was planning to invade England in 1066, glass makers in Northern Europe were moving forward from the Mediterranean soda glass produced by heating pebbles and charcoal to a mixture of lime, sand and potash. European glass was now significantly different from glass produced in Mediterranean countries. In Germany, a method of making sheet glass was invented, a method that was, with refinements, to remain in use until the mid-19th century. Molten glass was blown into spheres, which, while still hot, were swung to form cylinders that were cut and flattened before they cooled.

A variation of this method, developed by the Venetians, was called crown glass. This was used for glazing windows. To make the panes, the glass was first blown into a hollow sphere that was then reheated and spun to form a large disc about 5ft (1.5m) in diameter. This disc was then cut into panes, the clearest and thinnest glass cut from the edge of the disc, progressively getting thicker and more opaque towards the bullion or bull’s eye. Large good-quality windows were made by setting the small diamond shapes taken from the edge of the glass into a lead lattice frame. The bull’s eye and centre glass were used up on smaller windows. These lead-framed windows are still found in churches and older houses throughout Europe and North America.

In the manufacture of glass, two ingredients – sand and furnace fuel – have to be sourced as near as possible to the glassmaker’s furnace to make the product economically viable. In England, the first furnaces were located in Sussex, near to the major markets of London and Paris, and where they were able to utilize local timber and silica sand. However, the huge amount of timber consumed led to King James I banning the use of timber, forcing the glassmakers to move near to another source of fuel: the coal-mining districts in the North East, West Midlands, Lancashire and Bristol areas, where suitable sand and limestone were available within a cost-effective distance.

Though sand can be fused at very high temperatures, in the region of 2,000ºC (3,632ºF), in order to form glass it was found that the addition of potash and limestone not only improved the quality of the glass, but also lowered the temperature needed for fusion. Potash occurs in some parts of the world naturally, where it is mined, but most supplies in northern Europe were sourced from a cottage industry that burned broad-leaved timber or kelp. In the United States in 1790, Samuel Hopkins patented an industrialized method of making potash, but hitherto burning wood enclosed in barrels and leaching water through the resulting ashes had been the usual method. In coastal communities, notably the west coast of Scotland and Spain, seaweed was substituted for wood. By the 1800s, potash supplies, both in quality and quantity, were not keeping abreast of demand. Nicolas Leblanc, a French chemist, invented a process to manufacture soda ash by heating a mixture of salt, limestone and sulphuric acid. This process was highly toxic, and along with the soda ash ‘cake’, it left a residue of hydrochloric acid and hydrogen chloride gas. The former polluted watercourses in the neighbourhood of the manufacturing plant while the latter destroyed the vegetation. By the 1860s, glassmakers had changed to a more environmentally friendly process invented in Belgium by Ernest Solvay.