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The numerous forms and types of glass are produced by a specialized and often spectacular process from a common basis of sand fused with an alkaline flux and a metallic oxide. Some of the machinery used is remarkably ingenious


A SHEET OF MOLTEN GLASS emerging from the roller

A SHEET OF MOLTEN GLASS emerging from the roller. In the manufacture of plate glass the molten material is poured on to a table and squeezed flat by a heavy roller with a diameter of about 30 in. Guillotines cut the sheet into suitable lengths which then pass into a lehr for the annealing process. Annealing is necessary because glass, on leaving the furnace, is in a state of tension.

MODERN engineering has provided us with many spectacular displays in its application to the requirements of industry. The fiery blast of the converter used, in making steel, the ruddy glow of red-hot coke forced hydraulically from gas retorts, the blinding glare of the welder’s electric arc - these and many similar processes lend light and colour to the often drab surroundings in which they are carried out.

Among such processes few are more beautiful than that of glass making, in which molten rivers of red-hot glass slowly change to streams of gleaming crystal, great cylinders of shining glass rise majestically from tanks of molten material and tower in shimmering beauty forty and fifty feet above the dark floor of the glass house. Again, the amazing machines that produce glass bottles at the rate of many thousands a day are in themselves marvels of ingenuity. Few substances beside glass can exist in so many different forms with correspondingly differing properties. Glass can be brittle or tough, flexible or brilliantly hard, transparent, translucent or opaque - but the basis of glass is sand.

Sand, in common with flint and quartz, is a form of silica, and this substance, when fused with an alkaline flux and a metallic oxide, forms glass. The silica is dissolved in the fused alkali and oxide. The sand used for making the best glass is of the fine white variety found in Great Britain and on the continent of Europe. Commoner forms of sand are used for lower grade glass.

Among the alkalis used in the making of glass are many substances in everyday use, such as potash (potassium carbonate) and soda ash (carbonate of soda), which is manufactured from common salt or sodium chloride. Saltpetre, Chile nitre, boric acid and borax are further substances used in glass manufacture to improve colour and quality.

The metallic oxides used in the preparation of glass include those of zinc, tin, antimony and lead. The lead oxide is the red lead of commerce and is much used for table glassware and heavy optical glass. None of the silicates used in glass making is entirely free from colour, and various decolorizers such as manganese, nickel and arsenic are used to give a crystal effect.

Manufacture of glass

THE ANNEALING OR TEMPERING PROCESS in the manufacture of glass is carried out in machines know as lehrs. A modern lehr comprises a heated tunnel through which glassware is passed on an endless metal belt driven by an electric motor. The lehr illustrated comprises sections with varying degrees of insulation in which the glass is gradually annealed and cooled.

ELECTRICALLY OPERATED BOTTLE-MAKING MACHINE, comprising two turntables geared together, each carrying six moulds. This machine takes molten glass directly from the furnaces into the moulds. Compressed air blows the bottles, once with the bottle inverted on on turntable and once with the neck upwards on the other turntable.

The underlying principle of glass making is the fusion of silicates with alkalis and oxides. The various combinations of substances produce different kinds of glass, but the properties of the finished material are dependent also on the heat treatment during and after the fusion process. When glass is subjected to a gradually increasing temperature it becomes plastic and can be cut, drawn, welded or pressed into any desired form. It can be spun as a fine and flexible thread, blown into flasks and bottles, rolled or cast.

The first process in the manufacture of glass is the weighing of the materials that comprise the mixture or “batch”. The various materials, as they are weighed, are put into a wooden “arbour” or box. The batch is then tipped into an electrically driven machine, where the materials are thoroughly mixed. The materials are all crushed to size not coarser than granulated sugar. From the mixing room the batch is taken in tracks to the “glass house” containing the furnace, where it is tipped into another box. A proportion of broken glass or “cullet” is added to the batch and it is then ready for the process of fusion in the furnace.

The furnaces used in the manufacture of glass are of special interest, and centuries of research have developed the highly efficient plant now in use throughout the world. Two engineers, famous in other directions, did much to improve glass furnaces. The engineers were Sir Henry Bessemer and the Hon. Sir Charles A. Parsons. The melting of the batch is done in large tanks or in “pots” which, with the furnace itself, are made of highly refractory fireclay. The mixture from the batch is shovelled into a series of hot pots in the furnace and the lids are replaced.

Pot and Tank Furnaces

There are many different kinds of glass house furnaces, one of the earliest of which is the English circular type containing six to twelve pots standing in a circle on the floor or “siege” of the furnace. In the centre of the siege is the “eye” through which flames issue from a fire below. The cylindrical firebox, about 4 feet to 5 feet in diameter, is supported on a number of horizontal iron bars. Beneath the firebox is a tunnel known as the “cave”, through which air passes to the fire.

Coal is fed to the fire through a chute running under the siege. The fire is stirred by a fireman, or “teaser”, who, for the purpose, uses a long, hooked iron bar from the cave. Above the siege is the furnace “crown”, against which beat the flames from the fire. The intense heat is deflected downwards on to the pots and the flames pass through holes in the siege to flues carried in numbers of vertical pillars that support the crown.

English glass furnaces are sometimes fired by mechanical stokers. Old-type English furnaces may contain twelve pots, of 38 in. diameter, each with a capacity of 15 cwt. of glass. Modern furnaces work on the regenerative or recuperative principle to ensure economy, and the fuel used is often producer gas generated either within the furnace itself or in an independent gas producer. The pots used in these furnaces vary in capacity from 5 cwt. to 1 ton.

The pot type of furnace is still used for certain kinds of glassware, but for the mass production of glass in large quantities tank furnaces are used, and these are generally fired by gas. A tank furnace comprises a huge reservoir, longer than it is wide. Materials are fed into the tank at one end and are melted and refined during their slow progress to the working end containing the molten glass. Some tank furnaces are oil-fired.

Glass founding involves high temperatures, often up to 1,400 degrees centigrade, and the delicate heat control is assisted by the use of pyrometers. One type of these interesting instruments resembles a telescope inside which is an electrically heated filament. The “telescope” is focused on to a glowing red-hot wall of the furnace. Current is switched on to the filament, and this also glows red hot. Seen through the eyepiece, the filament shows dark red against the bright red of the furnace wall. Current is then increased until the filament assumes the same colour as the wall and “disappears”. The amount of current required to heat the filament to this degree is known, and by the use of a calibrated dial the temperature of the furnace can be read off in degrees centigrade.

POLISHING PLATE GLASS in an American glassworks

POLISHING PLATE GLASS in an American glassworks at Creighton, near Pittsburgh, Pennsylvania. Large circular pads of felt polish the surface of the glass, a mixture of rouge and water being used as the polishing medium. This is the final process. Other abrasives for grinding and polishing plate glass range from coarse sand to fine emery.

Glass from the furnace, on first cooling, is in a state of internal tension that necessitates annealing. This tempering process is carried out in a “lehr”, where cooling of the glassware is accomplished slowly to relieve the hidden strains and stresses in the glass structure. Early types of lehr comprised an arched tunnel fired directly by a furnace, leading from the glass house to the cooler warehouse. A train of iron trays, hooked together and fitted with wheels, carried the glassware down the tunnel on rails. At the cool end of the lehr the glass was removed, and the trays were unhooked for further use at the hot glass house end of the tunnel. The annealing time varied from three to ten days.

Modern lehrs comprise a heated tunnel through which the glassware is passed on an endless metal belt driven by an electric motor. The speed of the belt is varied in accordance with the type of glassware to be annealed. Heavy glasses require slower annealing than light glassware. The time taken for annealing now varies between one and five hours. There is one type of lehr, known as the Charlton lehr, that makes use of the heat in the work itself to accomplish the annealing, and it is specially useful in the mass production of glassware. In a lehr of this “heatless” type it is possible to anneal between 20 and 30 tons of bottles in twenty-four hours, and no fuel is used for heating the lehr as long as the production is maintained. The whole of the necessary heat comes from the bottles.

Pyrometers and heat-recording instruments are used for heat control during annealing. Another instrument that assists the manufacturer in turning out perfectly annealed glass is the polariscope or “strain-viewer”, which reveals any stresses in the glass structure. The annealing process, however, is for finished glassware. Between furnace and lehr are the amazing machines that fashion the glass into any desired form, from wine glasses to window panes. The best wine glasses, are made by hand, and in general the finest glassware is produced by the skilled craftsman.

The principal tool of the glass maker is the blow-iron, a tube that may have a diameter of from ½ in. to 1¼ in. and a length of about 5 feet. At one end is a mouthpiece, the other end being thickened and pear-shaped for gathering up the molten glass from a pot. The gathered glass is manipulated on an iron slab, or “marver”, with a polished face measuring about 12 in. by 18 in. Wooden marvers are sometimes used in which shaped hollows are formed to assist the marvering process. Next, a hollow globe is formed by blowing through the iron tube. The blown globe is removed by a rod or “pontil”, and after reheating can be worked into any desired shape. Glass can be “welded” while hot. A wine glass, for example, consists of three parts, the bowl, leg and foot, welded together.

CONTINUOUS GRINDING AND POLISHING PROCESS used in the manufacture of plate glassIt is in the making of bottles in vast quantities and in forming huge sheets of plate glass that the art of the glass maker is seen in its most spectacular aspect. One of the most remarkable machines in the world is the bottle-making machine invented in the United States by Michael J. Owens. In 1912, Owens established a glassworks in Manchester to demonstrate his invention, the first automatic bottle-making machine. This wonderful and complicated device comprises essentially a series of arms which revolve round a central shaft. Each arm is in itself a complete bottle-making unit that first dips (in its turn) into a pot of molten glass, sucks up sufficient “metal” into a mould to form a bottle, blows it to shape by compressed air and finally ejects the finished product. This machine is capable of turning out 8,000 five-gallons bottles a day, in striking contrast with the 200 bottles of the skilled glass blower.

CONTINUOUS GRINDING AND POLISHING PROCESS used in the manufacture of plate glass. Glass from the lehrs is passed on to a line of tables where the glass is first ground by cast-iron disks. Abrasive material is fed to the disks by streams of water. The glass is then polished by felts, rouge and water being used.

Another type of glass bottle-making machine comprises two turntables, geared together, which carry six or eight moulds each. Molten glass is fed to the machine and the flow is controlled by a plunger. The hot glass is cut off by shears and the pieces drop into moulds, where the blowing is accomplished by compressed air in two operations, once with the bottle inverted on one turntable and then with the neck upwards on the other. The machine stops for an instant while the transfer of the bottle is made.

Window glass was at one time made by blowing a globe and attaching it, on the side opposite to the blow hole, to the end of a pontil. The pontil, with glass attached, was revolved rapidly, and the bowl opened out into a disk that was finally cut to shape. It is in this manner that the crown-glass panes of the country inn were made. The bullseye in the middle of the window was the “blob” by which the globe of half-molten glass was attached to the pontil. Later, windows were made by blowing glass cylinders. The cylinder ends were cut off while hot and the resulting tube was slit lengthways. On reheating, the cylinder fell apart into a flat sheet.

Later, however, the machine took charge of the cylinder process. Ladles take the molten glass from the furnace and pour it into shallow gas-heated crucibles on the glass house floor. In place of the simple blow-iron, a flexible metal pipe fitted with a “bait” is lowered into the crucible. The bait resembles a dish with an infolded lip, and it is sometimes heated by electricity. The molten glass adheres to the bait, and it is drawn slowly upwards by a special type of crane. At the same time a stream of compressed air is forced through the pipe, and the rising cylinder of glass is distended to the required diameter. The operation is under the control of a man seated in a cabin above the glass house floor.

At the beginning of the draw the lifting is slow and a small air pressure is used through the pipe. Speed of drawing and air pressure are gradually increased as the crystal cylinder slowly rises. The time taken in drawing, which determines the thickness of the glass, varies between fifteen and eighteen minutes, and by this process cylinders have been drawn about 45 feet high and with a diameter of 40 in.

At the end of the draw the action is suddenly expedited so that the resulting thin wall of glass is broken away easily from the crucible. When finally drawn, the glass cylinder is lowered on to a “horse” or carrier for removal of the capped top and the thinned lower end. The huge cylinder is then divided into a number of short sections, each of which is split into two parts longitudinally.

140 Feet of Glass an Hour

For the dividing process, use is made of a natural property of glass - that of cracking in any sudden change in temperature. The cylinder is accordingly encircled at the desired line of fracture by a length of wire which is heated electrically. The cut curved sections of glass are heated on a smooth stone surface in a special oven, and the final flattening is then done by the use of a flat block of wood attached to a pole. Sheets of glass made by this process are, however, never perfectly flat, and other means are now more commonly adopted in the production of the best sheet glass.

FEEDING END OF A LEHR used for annealing glass jarsA great improvement in the making of sheet glass was introduced in 1904 by a Belgian, Emil Fourcault. In the Fourcault process a debiteuse, a box-shaped block of refractory material with a slot in the bottom, is placed in a tank of molten glass. The slotted box floats on the surface of the molten glass and, when pressure is applied, it becomes partly immersed. Glass is thus forced upwards through the slot.

FEEDING END OF A LEHR used for annealing glass jars, with the side panel removed to show the automatic stacker. A conveyer brings a line of jars into the lehr. As soon as a complete row has been assembled a stacker bar descends and gently pushes the line of jars on to the conveyer which runs through the lehr.

A bait of wire mesh, with a lower comb of clean iron nails, is lowered vertically into contact with the glass in the slot of the debiteuse. The bait is then drawn slowly upwards between long, flat iron boxes placed immediately above the slot on both sides of the debiteuse. A stream of water is passed through the iron boxes, and this cools and solidifies the glass sufficiently to permit of its being gripped by the first of a series of asbestos-covered rollers. These rollers may be about 90 in. long, with a diameter of 6 in., and tapered at the ends. They are arranged in pairs in a large vertical iron casing. The sheet of glass is drawn continuously upwards between the rollers, which are geared together and driven electrically. All the rollers on one side of the glass sheet run on fixed centres, but those on the other side are permitted a slight movement to accommodate different thicknesses. The movable rollers are held in contact with the glass by counterweights.

The vertical box is lined with asbestos and is fitted with inspection doors and with instruments that record the temperature. The box is, in effect, an annealing lehr as well as a container for the drawing rollers. Thickness of the sheet is determined by the rate of drawing, and a typical Fourcault machine will produce glass 1/12 in. thick, weighing 18½ oz. to the square foot, at the rate of 140 feet an hour. At the top of the vertical cooling chamber the emerging glass is cut off and divided up into standard-sized sheets.

This description applies to a single machine only, but in practice a dozen or more of these units are grouped together. They are supplied by one enormous tank furnace, holding some 1,500 tons of molten glass.

In addition to the Fourcault process there is another process in common use for making sheet glass. This is the process invented in the United States by Irving W. Colburn in 1905. It was not until 1916, however, that Colburn’s idea brought success, and in that year the Libbey Owens Sheet Glass Company was formed to exploit the method.

Non-Stop Machines

The Libbey Owens process, as it is known, dispenses with the debiteuse, and a bait of flat iron bar about 6 feet long and 6 in. wide is lowered directly into a tank of molten glass. The bait is attached to flexible metal strips. It is slowly raised, with the glass adhering to it, between water-cooled screens that “set” the sheet, which is then reheated by a gas burner for the next operation. This consists in bending the glass sheet over a hollow air-cooled roller. The sheet is then carried along horizontally for a distance of about 10 feet, when the bait is cracked off. Knurled water-cooled rollers then grip the glass on either side and propel it forward. The margins, about 2 in. wide, of the glass marked by the rollers are later cut off the sheet.

The glass next enters the lehr, or annealing chamber, which is heated by gas and provided with elaborate devices for regulating the temperature. The lehr is 200 feet long, and the glass slides forward on a conveyer that consists of 200 asbestos-covered rollers. The drawing speed which, in this instance also, controls the thickness of the glass sheet, varies from 25 in. to 100 in. a minute for glass that ranges from 5/16 in. to as thin a sheet as 1/24 in. The Libbey Owens machines run continuously day and night, seven days a week, and their individual output of window glass is from 2,500 to 2,750 square feet in twenty-four hours. These flat-drawn processes have completely superseded the cylinder process.

Plate glass,FINISHED GLASSWARE, having emerged from the lehr with its wonderfully true and polished surface, is made by a different process. The general principles of casting plate glass have changed little over a long period of time, but the process introduced into France by Lucas de Nehou in 1688 has been improved in detail.

FINISHED GLASSWARE, having emerged from the lehr thoroughly cooled and annealed, is assembled for sorting. In a modern “heatless” type of lehr it is possible to anneal 22 tons of pint milk bottles in twenty-four hours.

In the making of plate glass about 2,000 lb. of molten glass, enough for 300 square feet, are poured on to a steel table measuring 14 feet by 24 feet and about 8 in. thick. The mass of plastic glass is then squeezed flat by a heavy grooved roller of 30 in. diameter. The rolled plate is then pushed on rails through an annealing lehr. The first five compartments of the lehr are arranged in a zigzag and their function is to “set” the glass. Then the plate enters the straight portion of the lehr, about 500 feet long, for final annealing. At this stage the glass is rough and merely translucent, and it has to be ground and polished by the use of abrasives ranging from coarse sand to the finest emery and rouge to make it transparent.

The circular grinding and polishing tables used in the manufacture of plate glass vary in diameter between 24 feet and 36 feet, and weigh up to 80 tons each. The table is mounted on a platform that rotates on rails, and above it are two large cast-iron disks, each half the diameter of the table. These disks perform the grinding operation on the plate of glass, which is mounted on the table in a setting of plaster of Paris. The abrasive material is fed to the disks by streams of water, and the power supply is generally electric. When the glass has been ground on one surface it is turned over by a pneumatic suction device and the reverse side is treated. In the final polishing a similar table is used, but the iron blocks give place to large pads of felt, rouge and water being used as the polishing medium. Every square foot of glass requires about 10 lb. of sand for grinding. A large glassworks may use over 150,000 tons of abrasive in a year. The grinding and polishing will absorb about 80 per cent of the total power used for manufacturing purposes. The reduction of necessary grinding to a minimum is therefore of vital importance in making plate glass, and two casting processes, the Ford and the Bicheroux, both fulfil this requirement.

Tanks of Molten Glass

The Ford process, which is continuous, is used at the River Rouge plant of the Ford Motor Company. In this enormous glassworks four complete units operate in parallel. Each tank furnace at the works is 20 feet wide and 56 feet long, with a capacity of 400 tons of molten glass. The molten glass flows out of the furnace through a fireclay gate and passes between two water-cooled rollers, the upper of 9 in., the lower of 48 in. Diameter.

As the glass leaves the rollers it passes to a flat travelling belt on to rollers which carry it in a continuous sheet through an annealing lehr 440 feet long. Grinding also is continuous, and there are two lines of tables which are rectangular in shape.

They are arranged to impart an increasingly fine degree of grinding and polishing. The glass passes down one line of tables and is then reversed for finishing on the opposite side by the second set of tables. Either line of tables is about 400 feet long.

The Bicheroux process, in contrast to the Ford process, bears more resemblance to the crucible-and-table method of making plate glass, but several important improvements have been introduced. The molten glass is poured on to an inclined plate that is gradually raised so that the material is led down between two rollers, of equal diameter, and thence down another inclined plate to a flat bed. Here two powerful knives descend from either side at intervals and cut the wide strip of plastic glass into sheets ready for the annealing furnace, which sometimes attains a length of 700 feet. The rolled glass is only slightly thicker than its finished size after grinding.

The making of mirrors from sheets of polished plate glass is fairly simple. The glass is placed with one side exposed in a solution of silver nitrate and a reducing agent. The reducing agent precipitates the silver from the nitrate solution and the silver deposit clings to the glass. The silver deposit is finally given a coat of protective paint or varnish.


GLASS FURNACE of the circular “pot” type. Twelve pots, in which the materials are melted, are arranged on a circular “siege” or base. The pots vary in capacity from 5 cwt. to 1 ton. Modern furnaces work on the regenerative or recuperative principle; the fuel used is often producer gas, generated either within the furnace itself or in an independent gas producer.

[From part 21, published 20 July 1937]

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Manufacture of Glass