All About Glass
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Glass-making is the manufacture of the smooth, transparent and usually clear substance that we know as glass begins with the collection of quantities of coarsegrained sand.
Sand is chiefly composed of silica and it is the special properties of this compound that cause sand to melt and flow like a liquid when it is heated to high temperatures. White silica sand is usually purified for use in glass-making to prevent discoloration of the glass and to ensure uniformity of texture. Until the present industrial age, glass was made by individual workers or groups of craftsmen in small workshops. A batch of carefully mixed sand was melted in a kiln and the craftsman then worked the molten glass by one of several different techniques. He could exhale through an iron pipe to blow a blob of molten glass into the required shape; alternatively, he could stretch the glass into rods or sheets or press it into moulds. The twentieth century has brought mechanisation to the ancient industry. Mass production has removed glass from the class of luxury items and allowed its widespread use.
A modern glass factory contains a huge furnace, which may be in continuous operation. A 'batch', or mixture, consisting of silica sand and required additives (fluxes) is fed into the furnace. The batch is then heated and melted; as molten glass is drawn off for use, more raw materials are delivered into the furnace. The molten glass is then treated to increase its strength. Annealing is a process in which the glass is heated and then uniformly cooled, "this is not done, the exterior and interior of the glass will cool at different rates, and the resultant product will be brittle. Glass can also be tempered by being heated and then suddenly cooled, which makes the exterior of the glass very resistant to stress and fracture. Mechanical procedures are employed to adapt glass to its final shape. Automatic assembly lines can accurately force glass into moulds. Glass sheets or rods of precise dimensions can be rolled or stretched.
Coloured glass is produced by mixing additives with the original batch. The metals copper, iron and nickel produce colours of red, yellow and purple respectively. Frosting is achieved by the addition of fluorides. Glass is generally a fragile material but modern research has produced types of glass strong enough to hammer nails or resist the passage of bullets.
The History of Glass Making
Glass to anyone who is not a chemist is a mystery. Why should opaque and gritty materials like sand, soda and lime, when mixed together in certain proportions and fired, produce the crystal clear, smooth and impervious substance called glass, which can be adapted for so many different uses? Few of us, if asked, could give a plain and factual answer, and it is the more amazing that this wonderful thing was invented as much as 5,000 or more years ago.
We do not know exactly when, or by whom. In Egypt or Mesopotamia over 3,000 years before Christ men had discovered how to make an alkaline vitreous glaze which would adhere, as a surface coating, to stone and other substances. By the middle of the third millennnium B.C. both the Sumerians in Mesopotamia and the Egyptians had discovered that this vitreous glaze could be moulded into small objects by itself without the aid of a core.
At the beginning, so far as we know, the objects made were only small ones - beads and amulets and perhaps pieces for inlay patterns on caskets and furniture - and these glass objects were rare and almost as precious as the stones and gems which they were probably intended to imitate. They were all made by moulding, except the beads, which needed, of course, a hole to string them by, and so had to be made by winding viscous glass round a metal wire, which was withdrawn when the glass cooled.
Nearly a thousand years more were to elapse before the first glass vessels were made. The date is about 1500 B.C., at which time we find vessels in use both in Egypt, under Thothmes III, the famous pharaoh of the eighteenth dynasty, and in the Asiatic countries nearby. Most of these earliest vessels were made by what we call the sand-core process, which was perhaps developed from the technique just described for making beads. It consisted of building up a core of sand, wrapped in cloth round a metal rod and shaped like the inside of the vessel it was intended to produce. Warm viscous glass was applied to this core, either by dipping it in a crucible of glass or by trailing the glass on to the core. When the glass cooled, the metal rod and the core of sand were withdrawn and the vessel was finished by adding base, rim, handles and decorations, as required. This process was useful only for flasks and jugs and other vessels with narrow mouths. Occasionally open bowl shapes were made by pressing glass into moulds; and a third process which was also probably used from the first, though not so commonly, was one that was borrowed from the makers of stone vases - grinding a vessel out of a cold block of glass, just as if it were a piece of rock-crystal or quartz.
No major change in glass-making techniques took place for another 1,500 years. But the change when it did occur, was revolutionary - none other than the discovery that glass vessels could be made by inflating a solid blob (or 'paraison') of glass with a blowpipe. It is believed that this invention took place in Syria just before or just after the birth of Christ. At that time there were flourishing glass industries of long standing both in Syria and in Egypt. The Egyptian industry, which was centred in Alexandria had by this time moved away from sand-core and was concentrating on moulding and grinding; one of its special lines was the manufacture of the celebrated mosaic bowls formed of sections of polychrome canes of glass fused together in moulds and afterwards smoothed and polished by grinding on a wheel. The Syrian industry, on the other hand, which, like the Egyptian, had longbeen making sand-core glasses, never really took to the moulding and grinding processes. It was therefore very ready to become a blowing industry once that process was invented, making vessels both by blowing them into patterned moulds and by free-blowing.
Both these centres sent out trained glass workers to Italy, once their respective homelands had been conquered by Rome in the latter half of the first century B.C.; and from the beginning of Augustus's reign great quantities of glass were made by these Syrians and Alexandrians, both in Rome itself and elsewhere in Italy. We even know the names of some of the Syrian makers of the time - e.g. Artas and Ennion - to mention only the two most prominent, for they signed their products.
The invention of glassblowing was, in fact, the last major innovation in glass-making processes, not only in antiquity, but even up to modern times. A Syrian or Alexandrian worker of the first century A.D. would have been as much at home in a modern glass-house as would a Venetian worker of, say, the sixteenth century A.D. The only things that would puzzle him would be the modern machines for mass production. There are still glass-houses in England where the industry makes fine glassware in the old way, although the great majority of modern glass is made by machines which blow the glass into moulds. The human labour force could no longer cope with the enormous quantities required for all the many uses to which glass is put today.
Among the kinds of glass the Romans made, first and most common were flasks for toilet preparations, wine bottles and ordinary table-ware of undecorated glass, mostly green. Quantities of fragments of all these are found on almost every site where Roman remains exist, and quantities of them are also found in Roman burials, for it was the ancient practice to put vessels of glass, as well as of pottery and metal, in the tombs, to contain food and drink and unguents for the use of the dead. In the two first centuries A.D., when burial by cremation was common, glass was even used to make urns to hold the ashes of the dead. Thus Roman burials are the main sources from which complete Roman glasses are derived; glasses found on dwelling sites are normally in fragments.
As well as common plain ware, we have much fine glassware of Roman date, both colourless (for glass workers in Roman times could make as good colourless glass as we can today) and coloured. Richer Roman burials, especially in Italy and certain other main centres of Roman life, such as Cologne, often contain beautiful pieces of glass bearing elaborate cut designs and painted patterns. The cut designs were made either in facet cutting on the wheel or with an engraving tool, or a mixture of both, and they comprise representations of mythological and biblical stories, or scenes of everyday life, as well as geometrical patterns. The painted designs were made usually in enamel paint, fired on, and they, too, sometimes include mythological scenes, though these were more normally confined to animal and vegetable motifs. Some very pretty and attractive glassware was also made with patterns of applied trails and blobs, especially in a variety that was produced about 200 A.D., both in Syria and in Cologne, termed 'snake thread' ware, because the trailed designs are often serpent-like.
Even in the first century A.D. glass was as common, in everyday use, both for vessels of every kind and for ornaments, as bronze and silver. But at the start the shapes were on the whole simple. It was not until late in the second century, and still more in the third and fourth centuries, that really elaborate and complicated shapes became prevalent.
What happened to the glass industry in the Dark Ages and medieval times? The answer is different in the East and in the West. In the East, both in Egypt and Syria, the tradition of fine glass-making continued without any noticeable break or decline up to the fourteenth century A.D., and beyond. In the thirteenth and fourteenth centuries some of the finest enamelled glass ever made came out of Syrian factories. In the West, on the other hand, the end of Roman power in the early fifth century A.D. brought with it a rapid decline both in the quantity and in the quality of glassware. So much was this so that within a few centuries only a limited number of glasses of any consequence were being made, even in Italy.
During Roman days the western industry had largely been in the hands of glass workers from Syria and Egypt. When Roman rule declined and the barbarian invasions overran the western provinces and Italy itself, the incentive, and even the opportunity, for further waves of Eastern workers to migrate westwards ceased, and so the industry was left to local initiative; and that was lacking. The West had to wait until the thirteenth century when the crusades re-opened contact between East and West, and made it easy once more for workers from the East to come westwards. After that, at Venice and in other centres in Europe, glass works grew up which were again able and ready to turn out the best of wares. So began the modern glass industry of Europe.
In earlier centuries in England, the making of glass was widely scattered over the countryside. 'Glasshouse Wood' is not an uncommon place name for woodland sites where the glass-makers built their furnaces with supplies of fuel to hand.