In the present chapter we propose to deal with all those processes of glass manufacture in which the first stage consists in converting the glass into a slab or plate by some process of rolling. We have already considered the general character of the rolling process, and have seen that, although hot, viscous glass lends itself readily to being rolled into sheets or slabs, these cannot be turned out with a smooth, flat surface. In practice the surface of rolled glass is always more or less dimmed by contact with the minute irregularities of table or roller, and larger irregularities of the surface arise from the buckling that occurs at a great many places in the sheet. These limitations govern the varieties of glass that can be produced by processes that involve rolling, and have led to the somewhat curious result that both the cheapest and roughest, as well as the best and most expensive kinds of flat glass, are produced by rolling processes. Ordinary rough “rolled plate,” such as that used in the skylights of workshops and of railway stations, is the extreme on the one hand, while polished plate-glass represents the other end of the scale. The apparent paradox is, however, solved when it is noted that in the production of polished plate-glass the character of the surface of the glass as it leaves the rollers is of very minor importance, since it is entirely obliterated by the subsequent processes of grinding, smoothing, and polishing. Intermediate between the rough “rolled” and the “polished” plate-glass we have a variety of glasses in which the appearance of the rolled surface is hidden or disguised to a greater or lesser extent by the application of a pattern that is impressed upon the glass during the rolling process; thus we have rolled plate having a ribbed or lozenge-patterned surface, or the well-known variety of “figured rolled” plate, sometimes known as “Muranese,” whose elaborate and deeply-imprinted patterns give a very brilliant effect.

Rolled plate-glass being practically the roughest and cheapest form of glazing, is principally employed where appearance is not considered, and its chief requirement is, therefore, cheapness, although both the colour and quality of the glass are of importance as affecting the quantity and character of the light which it admits to the building where the glass is used. On the ground of cheapness it will be obvious from what we have said above (Chapter IV.), that such glass can only be produced economically in large tank furnaces, and these are universally used for this purpose. The requirements as regards freedom from enclosed foreign bodies of small size and of enclosed air-bells are not very high in such glass, and, therefore, tanks of very simple form are generally used. No refinements for regulating the temperature of various parts of the furnace in order to ensure perfect fining of the glass are required, and the furnace generally consists simply of an oblong chamber or tank, at one end of which the raw materials are fed in, while the glass is withdrawn by means of ladles from one or two suitable apertures at the other end. For economical working, however, the furnace must be capable of working at a high temperature, because a cheap glass mixture is necessarily somewhat infusible, at all events where colour is considered. This will be obvious if we remember that the fusibility of a glass depends upon its alkali contents, and alkali is the most expensive constituent of such glasses.

The actual raw materials used in the production of rolled plate-glass are sand, limestone and salt-cake, with the requisite addition of carbon and of fluxing and purifying materials. The selection of these materials is made with a view to the greatest purity and constancy of composition which is available within the strictly-set limits of price which the low value of the finished product entails. These materials are handled in very large quantities, outputs of from 60 to 150 tons of finished glass per week from a single furnace being by no means uncommon; mechanical means of handling the raw materials and of charging them into the furnace are therefore adopted wherever possible.

The glass is withdrawn from the furnace by means of large iron ladles. These ladles are used of varying sizes in such a way as to contain the proper amount of glass to roll to the various sizes of sheets required. The sizes used are sometimes very large, and ladles holding as much as 180 to 200 lbs. of glass are used. These ladles, when filled with glass, are not carried by hand, but are suspended from slings attached to trolleys that run on an overhead rail. The ladler, whose body is protected by a felt apron and his face by a mask having view-holes glazed with green glass, takes the empty ladle from a water-trough, in which it has been cooled, carries it to the slightly inclined gangway that leads up to the opening in the front of the furnace, and there introduces the ladle into the molten glass, giving it a half-turn so as to fill it with a “solid” mass of glass. By giving the ladle two or three rapid upward jerks, the operator then detaches the glass in the ladle as far as possible from the sheets and threads of glass which would otherwise follow its withdrawal; then the part of the handle of the ladle near the bowl is placed in the hook attached to the overhead trolley, and by bearing his weight on the other end of the handle, the workman draws the whole ladle up from the molten bath in the furnace and out through the working aperture. This operation only takes a few seconds to perform, but during this time the ladler is exposed to great heat, as a more or less intense flame generally issues from the working aperture, whence it is drawn upward under the hood of the furnace. From the furnace opening, the ladler, generally aided by a boy, runs the full ladle to the rolling table and there empties the ladle upon the table just in front of the roller. In doing this, two distinctly different methods are employed. In one, only the perfectly fluid portion of the glass is poured out of the ladle by gradually tilting it, the chilled glass next to the walls of the ladle being retained there and ultimately returned to the furnace while still hot. In the other method, the chilling of the glass is minimised as far as possible, and the entire contents of the ladle are emptied upon the rolling table by the ladler, who turns the entire ladle over with a rapid jerk which is so arranged as to throw the coldest part of the glass well away from the rest. When the sheet is subsequently rolled this chilled portion is readily recognised by its darker colour, and since it lies entirely at one end of the sheet it is detached before the sheet goes any further. Neither method appears to present any preponderating advantage.

The rolling table used in the manufacture of rolled plate is essentially a cast-iron slab of sufficient size to accommodate the largest sheet which is to be rolled; over this slab moves a massive iron roller which may be actuated either by hand or by mechanical power—the latter, however, being now almost universal. The thickness of the sheet to be rolled is regulated by means of slips of iron placed at the sides of the table in such a way as to prevent the roller from descending any further towards the surface of the table: so long as the layer of glass is thicker than these slips, the entire weight of the roller comes upon the soft glass and presses it down, but as soon as the required thickness is attained, the weight of the roller is taken by the iron slips and the glass is not further reduced in thickness. The width of the sheet is regulated by means of a pair of iron guides, formed to fit the forward face of the roller and the surface of the table, in the manner indicated in Fig. 9. The roller, as it moves forward, pushes these guides before it, and the glass is confined between them. When the roller has passed over the glass, the sheet is left on the iron table in a red-hot, soft condition, and it must be allowed to cool and harden to a certain extent before it can be safely moved. In this interval, the chilled portion—if any—is partially severed by an incision made in the sheet by means of a long iron implement somewhat like a large knife, and then the sheet is loosened from the bed of the table by passing under it, with a smooth rapid stroke, a flat-bladed iron tool. The sheet is next removed to the annealing kiln or “lear,” being first drawn on to a stone slab and thence pushed into the mouth of the kiln. At this stage the chilled portion of the sheet is completely severed by a blow which causes the glass to break along the incision previously made.

The rolled-plate annealing kiln is essentially a long, low tunnel, kept hot at one end, where the freshly-rolled sheets are introduced, and cold at the other end, the temperature decreasing uniformly down the length of the tunnel. The sheets pass down this tunnel at a slow rate, and are thus gradually cooled and annealed sufficiently to undergo the necessary operations of cutting, etc. Although thus simple in principle, the proper design and working of these “lears” is by no means simple or easy, since success depends upon the correct adjustment of temperatures throughout the length of the tunnel and a proper rate of movement of the sheets, while the manner of handling and supporting the sheets is vital to their remaining flat and unbroken. The actual movement of the sheets is effected by a system of moving grids which run longitudinally down the tunnel. The sheets ordinarily lie flat upon the stone slabs that form the floor of the tunnel, and the grids are lowered into recesses cut to receive them. At regular intervals the iron grid bars are raised just sufficiently to lift the sheets from the bed of the kiln, and are then moved longitudinally a short distance, carrying the sheets forward with them and immediately afterwards again depositing them on the stone bed. The grids return to their former position while lowered into their recesses below the level of the kiln bed.

When they emerge from the annealing kiln or “lear” the sheets of rolled plate-glass are carried to the cutting and sorting room. Here the sheets are trimmed and cut to size. The edges of the sheets as they leave the rolling table are somewhat irregular, and sometimes a little “beaded,” while the ends are always very irregular. Ends and edges are therefore cut square or “trimmed” by the aid of the cutting diamond. For this purpose the sheet is laid upon a flat table, the smoothest side of the sheet being placed upwards, and long cuts are taken with a diamond—good diamonds of adequate size and skilful operators being necessary to ensure good cutting on such thick glass over long lengths. Strips of glass six or eight feet long and half an inch wide are frequently detached in the course of this operation, and the final separation is aided by slight tapping of the underside of the glass just below the cut and—if necessary—by breaking the strip off by the aid of suitable tongs.

No very elaborate “sorting” of rolled plate glass is required, except perhaps that the shade of colour in the glass may vary slightly from time to time, and it is generally preferable to keep to one shade of glass in filling any particular order. Apart from this, the rolled plate cutter has merely to cut out gross defects which would interfere too seriously with the usefulness of the glass. As we have already indicated, air-bells and minute enclosures of opaque matter are not objectionable in this kind of glass, but large pieces of opaque material must generally be cut out and rejected, not only because they are too unsightly to pass even for rough glazing purposes, but also because they entail a considerable risk of spontaneous cracking of the glass—in fact, visible cracks are nearly always seen around large “stones,” as these inclusions are called. These may arise from various causes, such as incomplete melting of the raw materials, or the contamination of the raw materials with infusible impurities, but the most fruitful source of trouble in this direction lies in the crumbling of the furnace lining, which introduces small lumps of partially melted fire-clay into the glass. In a rolled plate tank furnace which is properly constructed and worked, the percentage of sheets which have to be cut up on account of such enclosures should be very small, at all events until the furnace is old, when the linings naturally show an increasing tendency to disintegrate.

Returning now to the rolling process, it is readily seen that a very slight modification will result in the production of rolled plate-glass having a pattern impressed upon one surface; this modification consists in engraving upon the cast-iron plate of the rolling table in intaglio any pattern that is to appear upon the glass in relief. As a matter of fact only very simple patterns are produced in this way, such as close parallel longitudinal ribbing and a lozenge-pattern, the reason probably being that the cost of cutting an elaborate pattern over the large area of the bed-plate of one of these tables would be very considerable. Further, as these tables and their bed-plates are so very heavy, they are not readily interchanged or left standing idle, so that only patterns required in very great quantity could be profitably produced in this way. These disadvantages are, however, largely overcome by the double-rolling machine. In this machine, into whose rather elaborate details we cannot enter here, the glass is rolled out into a sheet of the desired size and thickness by being passed between two rollers revolving about stationary axes, the finished sheet emerging over another roller, and passing on to a stone slab that moves forward at the same rate as the sheet is fed down upon it. In this machine a pattern can be readily imprinted upon the soft sheet as it passes over the last roller by means of a fourth roller, upon which the pattern is engraved; this is pressed down upon the sheet, and leaves upon it a clear, sharp and deep impress of its pattern. The general arrangement of the rollers in this machine is shown in the diagram of Fig. 10, which represents the sectional elevation of the appliance. After leaving the rolling machine, the course of the “figured rolled plate” produced in this manner is exactly similar to that of ordinary rolled plate, except that as a somewhat softer kind of glass is generally used for “figured,” the temperature of the annealing kilns requires somewhat different adjustment. The cutting of the glass also requires rather more care, and it should be noted that such glass can only be cut with a diamond on the smooth side; the side upon which the pattern has been impressed in relief cannot be materially affected by a diamond. This is one reason why it is not feasible to produce such glass with a pattern on both sides.

Figured rolled glass, being essentially of an ornamental or decorative nature, is generally produced in either brilliantly white glass or in special tints and colours, and the mixtures used for attaining these are, of course, the trade property of the various manufacturers; the whiteness of the glass, however, is only obtainable by the use of very pure and, therefore, expensive materials. As regards the coloured plate-glasses, a general account of the principles underlying the production of coloured glass will be found in Chapter XI. The manufacture of polished plate-glass really stands somewhat by itself, almost the only feature which it has in common with the branches of manufacture just described being the initial rolling process.

The raw materials for the production of plate-glass are chosen with the greatest possible care to ensure purity and regularity; owing to the very considerable thickness of glass which is sometimes employed in plate, and also to the linear dimensions of the sheets which allow of numerous internal reflections, the colour of the glass would become unpleasantly obtrusive if the shade were at all pronounced. The actual raw materials used vary somewhat from one works to another; but, as a rule, they consist of sand, limestone, and salt-cake, with some soda-ash and the usual additions of fluxing and purifying material such as arsenic, manganese, etc. The glass is generally melted in pots, and extreme care is required to ensure perfect melting and fining, since very minute defects are readily visible in this glass when finished, and, of course, detract most seriously from its value.

The method of transferring the glass from the melting-pot to the rolling table differs somewhat in different works. In many cases the melting-pots themselves are taken bodily from the furnace and emptied upon the bed-plate of the rolling machine, while in other cases the glass is first transferred to smaller “casting” pots, where it has to be heated again until it has freed itself from the bubbles enclosed during the transference, and then these smaller pots are used for pouring the glass upon the rolling slab. The advantage of the latter more complicated method lies, no doubt, in the fact that the large melting-pots, which have to bear the brunt of the heat and chemical action during the early stages of melting, are not exposed to the great additional strain of being taken from the hot furnace and exposed for some time to the cold outside air. Apart from the mechanical risks of fracture, this treatment exposes the pots to grave risks of breakage from unequal expansion and contraction on account of the great differences of temperature involved. Where smaller special casting-pots are used, these are not exposed to such prolonged heat in the furnace, and are never exposed to the chemical action of the raw materials, so that these subsidiary pots may perhaps be made of a material better adapted to withstand sudden changes of temperature than the high-class fire-clay which must be used in the construction of melting pots. On the other hand, the transference of the glass from the melting to the casting-pots involves a laborious operation of ladling and the refining of the glass, with its attendant expenditure of time and fuel. Finally, the production of plate-glass in tank furnaces could only be attempted by the aid of such casting-pots in which the glass would have to undergo a second fining after being ladled from the tank, and this would materially lessen the economy of the tank for this purpose, while it is by no means an easy matter to produce in tank furnaces qualities of glass equal as regards colour and purity to the best products of the pot furnace.

The withdrawal of the pots containing the molten glass from the furnace is now universally carried out by powerful machinery. The pots are provided on their outer surface with projections by which they can be held in suitably-shaped tongs or cradles. A part of the furnace wall, which is constructed each time in a temporary manner, is broken down; the pot is raised from the bed or “siege” of the furnace by the aid of levers, and is then bodily lifted out by means of a powerful fork. The pot is then lifted and carried by means of cranes until it is in position above the rolling table; there the pot is tilted and the glass poured out in a steady stream upon the table, care being taken to avoid the inclusion of air-bells in the mass during the process of pouring. When empty, the pot is returned to the furnace as rapidly as possible, the glass being meanwhile rolled out into a slab by the machine. Except for the greater size and weight of both table and roller, the plate-glass rolling table is similar to that already described in connection with rolled plate. Of course, since the glass is poured direct from the pot, there is no chilled glass to be removed. Further, owing to the large size of sheets frequently required, the bed of the rolling table cannot be made of a single slab of cast-iron, a number of carefully jointed plates being, in fact, preferable, as they are less liable to warp under the action of the hot glass.

In arranging the whole of the rolling plant, the chief consideration to be kept in mind is that it is necessary to produce a flat sheet of glass of as nearly as possible equal thickness all over. The final thickness of the whole slab when ground and polished into a sheet of plate-glass must necessarily be slightly less than that of the thinnest part of the rough rolled sheet. If, therefore, there are any considerable variations of thickness, the result will be that in some parts of the sheet a considerable thickness of glass will have to be removed during the grinding process. This will arise to a still more serious extent if the sheet as a whole should be bent or warped so as to depart materially from flatness. The two cases are illustrated diagrammatically in Fig. 11, which shows sectional views of the sheets before and after grinding on an exaggerated scale.

While it is evident that careful design of the rolling table will avoid all tendency to the formation of sheets of such undesirable form, it is a much more difficult matter to avoid all distortion of the sheet during the annealing process and while the sheet is being moved from the rolling table to the annealing kiln. Owing to the great size of the slabs of glass to be dealt with, and still more to the stringent requirement of flatness, the continuous annealing kiln, in which the glass travels slowly down a tunnel from the hot to the cold end, has not been adopted for the annealing of plate-glass, and a form of annealing kiln is still used for that glass which is similar in its mode of operation to the old-fashioned kilns that were used for other kinds of glass before the continuous kiln was introduced. These kilns simply consist of chambers in which the hot glass is sealed up and allowed to cool slowly and uniformly during a more or less protracted period. In the case of plate-glass, the slabs are laid flat on the stone bed of the kiln. This stone bed is built up of carefully dressed stone, or blocks of fire-brick bedded in sand in such a way that they can expand freely laterally without causing any tendency for the floor to buckle upwards as it would do if the blocks were set firmly against one another. The whole chamber is previously heated to the requisite temperature at which the glass still shows a very slight plasticity. The hot glass slabs from the rolling table are laid upon the bed of this kiln, several being usually placed side by side in the one chamber, and the slabs in the course of the first few hours settle down to the contour of the bed of the kiln, from which shape and position they are never disturbed until they are removed when quite cold. In modern practice the cooling of a kiln is allowed to occupy from four to five days; even this rate of cooling is only permissible if care is taken to provide for the even cooling of all parts of the kiln, and for this purpose special air-passages are built into the walls of the chamber and beneath the bed upon which the glass rests, and air circulation is admitted to these in such a way as to allow the whole of the kiln to cool down at the same rate; in the absence of such special arrangements, the upper parts of the kiln would probably cool much more rapidly than the base, so that the glass would be much warmer on its under than on its upper surface.

When the slabs of plate-glass are removed from the annealing kilns they very closely resemble sheets of rolled plate in appearance, and they are quite sufficiently transparent to allow of examination and the rejection of the more grossly defective portions; the more minute defects, of course, can only be detected after the sheets have been polished, but this preliminary examination saves the laborious polishing of much useless glass.

The process of grinding and polishing plate-glass consists of three principal stages. In the first stage the surfaces of the glass are ground so as to be as perfectly flat and parallel as possible; in order to effect this object as rapidly as possible, a coarse abrasive is used which leaves the glass with a rough grey surface. In the second stage, that of smoothing, these rough grey surfaces are ground down with several grades of successively finer abrasive until finally an exceedingly smooth grey surface is left. In the third and final stage, the smooth grey surface is converted into the brilliant polished surface with which we are familiar by the action of a polishing medium.

Originally the various stages of the grinding and polishing processes were carried out by hand, but a whole series of ingenious machines has been produced for effecting the same purpose more rapidly and more perfectly than hand-labour could ever do. We cannot hope to give any detailed account of the various systems of grinding and polishing machines which are even now in use, but must content ourselves with a survey of some of the more important considerations governing the design and construction of such machinery.

In the first place, before vigorous mechanical work can be applied to the surface of a plate of glass, that plate must be firmly fixed in a definite position relatively to the rest of the machinery, and such firm fixing of a plate of glass is by no means readily attained, since the plate must be supported over its whole area if local fracture is to be avoided. While the surface of the plate is in the uneven condition in which it leaves the rolling table, such a firm setting of the glass can only be attained by bedding it in plaster, and this must be done in such a manner as to avoid the formation of air-bubbles between plaster and glass; if bubbles are allowed to form, they constitute places where the glass is unsupported. During the grinding and polishing processes these unsupported places yield to the heavy pressure that comes upon them, and irregularities in the finished polished surfaces result. The most perfect adhesion between glass and plaster is attained by spreading the paste of plaster on the up-turned surface of the slab of glass and lowering the iron bed-plate of the grinding table down upon it, the bed-plate with the adhering slab of glass being afterwards turned over and brought into position in the grinding machine. When one side of the glass has been polished, it is generally found sufficient to lay the slab down on a bed of damp cloth, to which it adheres very firmly, although sliding is entirely prevented by a few blocks fixed to the table in such a way as to abut against the edges of the sheet. In many works, however, the glass is set in plaster for the grinding and polishing of the second side as well as of the first.

The process of grinding and polishing is still regarded in many plate-glass works as consisting of three distinct processes, known as rough grinding, smoothing and polishing respectively. Formerly these three stages of the process were carried out separately; at first by hand, and later by three different machines. In the most modern practice, however, the rough and smooth grinding are done on the same machine, the only change required being the substitution of a finer grade of abrasive at each step for the coarser grade used in the previous stage. For the polishing process, however, the rubbing implements themselves must be of a different kind, for while the grinding and smoothing is generally done by means of cast-iron rubbers moving over the glass, the polishing is done with felt pads. The table of the machine, to which the glass under treatment is attached, is therefore made movable, and when the grinding and smoothing processes are complete, the table with its attached glass is moved so as to come beneath a superstructure carrying the polishing rubbers, and the whole is then elevated so as to allow the rubbers to bear on the glass.

The earliest forms of grinding machines gave a reciprocal motion to the table which carries the glass, or the grinding rubbers were moved backward and forward over the stationary table. Rotary machines, however, were introduced and rapidly asserted their superiority, until, at the present time, practically all plate-glass is ground on rotating tables, some of these attaining a diameter of over 30 ft. The grinding “rubbers” consist of heavy iron slabs, or of wood boxes shod with iron, but of much smaller diameter than the grinding table. The rubbers themselves are rotary, being caused to rotate either by the frictional drive of the rotating table below them, or by the action of independent driving mechanism, but the design of the motions must be so arranged that the relative motion of rubber and glass shall be approximately the same at all parts of the glass sheets, otherwise curved instead of plane surfaces would be formed. This condition can be met by placing the axes of the rubbers at suitable points on the diameter of the table. The abrasive is fed on to the glass in the form of a thin paste, and when each grade or “course” has done the work required of it, the whole table is washed down thoroughly with water and then the next finer grade is applied. The function of the first or coarsest grade is simply to remove the surface irregularities and to form a rough but plane surface. The abrasive ordinarily employed is sharp sand, but only comparatively light pressure can be applied, especially at the beginning of this stage, since at that period the weight of the rubber is at times borne by relatively small areas of glass that project here and there above the general level of the slab. As these are ground away, the rubbers take a larger and more uniform bearing, and greater pressure can be applied. The subsequent courses of finer abrasives are only required to remove the coarse pittings left in the surface by the action of the first rough grinding sand; the finer abrasive replaces the deep pits of the former grade by shallower pits, and this is carried on in a number of steps until a very smooth “grey” surface is attained and the smoothing process is complete. The revolving table or “platform” is now detached from the driving mechanism, and moved along suitably placed rails on wheels provided for that purpose, until it stands below the polishing mechanism. Here it is attached to a fresh driving mechanism, and it is then either raised so as to bring the glass into contact with the felt-covered polishing rubbers, or the latter are lowered down upon the glass. The polishing rubbers are large felt-covered slabs of wood or iron which are pressed against the glass with considerable force; their movement is very similar to that of the grinding rubbers, but in place of an abrasive they are supplied with a thin paste of rouge and water. The time required for the polishing process depends upon the perfection of the smoothing that has been attained; in favourable cases two or three hours are sufficient to convert the “grey” surface into a perfectly polished one; where, however, somewhat deeper pits have been left in the glass, the time required for polishing may be much longer, and the polish attained will not be so perfect. The mode of action of a polishing medium such as rouge is now recognised to be totally different in character from that of even the finest abrasive; the grains of the abrasive act by their hardness and the sharpness of their edges, chipping away tiny particles of the glass, so that the glass steadily loses weight during the grinding and smoothing processes. During the polishing process, however, there is little or no further loss of weight, the glass forming the hills or highest parts of the minutely pitted surface being dragged or smeared over the surface in such a way as to gradually fill up the pits and hollows. The part played by the polishing medium is probably partly chemical and partly physical, but it results, together with the pressure of the rubber, in giving to the surface molecules of the glass a certain amount of freedom of movement, similar to that of the molecules of a viscid liquid; the surface layers of glass are thus enabled to “flow” under the action of the polisher and to smooth out the surface to the beautiful level smoothness which is so characteristic of the surfaces of liquids at rest. This explanation of the polishing process enables us to understand why the proper consistency of the polishing paste, as well as the proper adjustment of the speed and pressure of the rubbers, plays such an important part in successful polishing; it also serves to explain the well-known fact that rapid polishing only takes place when the glass surface has begun to be perceptibly heated by the friction spent upon it.

It has been estimated that, on the average, slabs of plate-glass lose one-third of their original weight in the grinding and polishing processes, and it is obvious that the erosion of this great weight of glass must absorb a great amount of mechanical energy, while the cost of the plant and upkeep is proportionately great. Every factor that tends to diminish either the total weight of glass to be removed per square yard of finished plate, or reduces the cost of removal, must be of the utmost importance in this manufacture. The flatness of the plates as they leave the annealing kiln has already been referred to, and the reason why the processes of grinding and polishing have formed the subject for innumerable patents will now be apparent. The very large expansion of the use of plate-glass in modern building construction, together with the steady reduction in the prices of plate, are evidence of the success that has attended the efforts of inventors and manufacturers in this direction.

At the present time, plate-glass is manufactured in very large sheets, measuring up to 26 ft. in length by 14 ft. in width, and in thickness varying from 3/16th of an inch up to 1½ in., or more, for special purposes. At the same time the quality of the glass is far higher to-day than it was at earlier times. This high quality chiefly results from more careful choice of raw materials and greater freedom from the defects arising during the melting and refining processes, while a rigid process of inspection is applied to the glass as it comes from the polishing machines. For this purpose the sheets are examined in a darkened room by the aid of a lamp placed in such a way that its oblique rays reveal every minute imperfection of the glass; these imperfections are marked with chalk, and the plate is subsequently cut up so as to avoid the defects that have thus been detected.

Perhaps the most remarkable fact about the quality of modern plate-glass is its relatively high degree of homogeneity. Glass, as we have seen in Chapter I., is not a chemically homogeneous substance, but rather a mixture of a number of substances of different density and viscosity. Wherever this mixture is not sufficiently intimate, the presence of diverse constituents becomes apparent in the form of striÆ, arising from the refraction or bending of light-rays as they pass from one medium into another of different density. Except in glass that has undergone elaborate stirring processes, such striÆ are never absent, but the skill of the glass-maker consists in making them as few and as minute as possible, and causing them to assume directions and positions in which they shall be as inconspicuous as possible. In plate-glass this is generally secured in a very perfect manner, and to ordinary observation no striÆ are visible when a piece of plate-glass is looked at in the ordinary way, i.e., through its smallest thickness; if the same piece of glass be looked at transversely, the edges having first been polished in such a way as to render this possible, the glass will be seen to be full of striÆ, generally running in fine lines parallel with the polished surfaces of the glass. This uniform direction of the striÆ is partly derived from the fact that the glass has been caused to flow in this direction by the action of the roller when first formed into a slab, but this process would not obliterate any serious inequalities of density which might exist in the glass as it leaves the pot, so that successful results are only attainable if great care is taken to secure the greatest possible homogeneity in the glass during the melting process.

At the present time probably the greater bulk of plate-glass is used for the purpose of glazing windows of various kinds, principally the show windows of shops, etc. As used for this purpose the glass is finished when polished and cut to size. The only further manipulation that is sometimes required is that of bending the glass to some desired curvature, examples of bent plate-glass window-panes being very frequently seen. This bending is carried out on the finished glass, i.e., after it has been polished; the glass is carefully heated in a special furnace until softened, and is then gently made to lie against a stone or metal mould which has been provided with the desired curvature. It is obvious that during this operation there are great risks of spoiling the glass; roughening of the surface by contact with irregular surfaces on either the mould, the floor of the kiln, or the implements used in handling the glass, can only be avoided by the exercise of much skill and care, while all dust must also be excluded since any particles settling on the surface of the hot glass would be “burnt in,” and could not afterwards be detached. Small defects can, of course, be subsequently removed by local hand-polishing, and this operation is nearly always resorted to where polished glass has to undergo fire-treatment for the purpose of bending.

In addition to its use for glazing in the ordinary sense, plate-glass is employed for a number of purposes; the most important and frequent of these is in the construction of the better varieties of mirrors. For this purpose the glass is frequently bevelled at the edges, and sometimes a certain amount of cutting is also introduced on the face of the mirror. Bevelling is carried out on special grinding and polishing machines, and a great variety of these are in use at the present time. The process consists in grinding off the corners of the sheet of glass and replacing the rough perpendicular edge left by the cutting diamond by a smooth polished slope running down from the front surface to the lower edge at an angle of from 45 to 60 degrees. Since only relatively small quantities of glass have to be removed, small grinding rubbers only are used, and in some of the latest machines these take the form of rapidly-revolving emery or carborundum wheels. These grinding wheels have proved so successful in grinding even the hardest metals that it is surprising to find their use in the glass industry almost entirely restricted to the “cutting” of the better kinds of flint and “crystal” glass for table ware or other ornamental purposes. The reason probably lies in the fact that the use of such grinding wheels results in the generation of a very considerable amount of local heat, this effect being intensified on account of the low heat-conducting power of glass. If a piece of glass be held even lightly against a rapidly-revolving emery wheel it will be seen that the part in contact with the wheel is visibly red-hot. This local heating is liable to lead to chipping and cracking of the glass, and these troubles are those actually experienced when emery or carborundum grinding is attempted on larger pieces of glass. In the case of at least one modern bevel-grinding machine, however, it is claimed that the injurious effects of local heating are avoided by carrying out the entire operation under water.

For the purpose of use in mirrors, plate-glass is frequently silvered, and this process is carried on so extensively that it has come to constitute an entire industry which has no essential connection with glass manufacture itself; for that reason we do not propose to enter on the subject here, only adding that the nature and quality of the glass itself considerably affects the ease and success of the various silvering processes. Ordinary plate-glass, of course, takes the various silvering coatings very easily and uniformly, but there are numerous kinds of glass to which this does not apply, although there are probably few varieties of glass which are sufficiently stable for practical use, and to which a silvering coating cannot be satisfactorily applied, provided that the most suitable process be chosen in each case.

While there is little if any use for coloured glass in the form of polished plate, entirely opaque plate-glass, coloured both black and white, is used for certain purposes. Thus, glass fascias over shop-fronts, the counters and shelves of some shops, and even tombstones are sometimes made of black or white polished plate. From the point of view of glass manufacture, however, these varieties only differ from ordinary plate-glass in respect of certain additions to the raw materials, resulting in the production of the white or black opacity. The subsequent treatment of the glass is identical with that of ordinary plate-glass, except that these opaque varieties are rarely required to be polished on both sides, so that the operations are simplified to that extent.

Certain limitations to the use of all kinds of plate-glass, whether rough-rolled, figured or polished, were formerly set by the fact that under the influence of fire, partitions of glass were liable to crack, splinter and fall to pieces, thus causing damage beyond their own destruction and leaving a free passage for the propagation of the fire. To overcome these disadvantages, glass manufacturers have been led to introduce a network or meshing of wire into the body of such glass. Provided that the glass and wire can be made so as to unite properly, then the properties of such reinforced or “wired” glass should be extremely valuable. In the event of breakage from any cause, such as fire or a violent blow, while the glass would still crack, the fragments would be held together by the wire network, and the plates of glass as a whole would remain in place, neither causing destruction through flying fragments nor allowing fire or, for the matter of that, burglar a free passage. The utility of such a material has been readily recognised, but the difficulty lies in its production. These difficulties arise from two causes. The most serious of these is the considerable difference between the thermal expansion of the glass and of the wire to be embedded in it. The wire is necessarily introduced into red-hot glass while the latter is being rolled or cast, and therefore glass and wire have to cool down from a red heat together. During this cooling process the wire contracts much more than the glass, and breakage either results immediately, or the glass is left in a condition of severe strain and is liable to crack spontaneously afterwards. An attempt has been made to overcome this difficulty by using wire made of a nickel steel alloy, whose thermal expansion is very similar to that of glass; but, as a matter of fact, this similarity of thermal expansion is only known to hold for a short range of moderate temperatures, and probably does not hold when the steel alloy is heated to redness. In another direction, greater success is to be attained by the use of wire of a very ductile metal which should yield to the stress that comes upon it during cooling; probably copper wire would answer the purpose, but the great cost of copper is a deterrent from its use. A second difficulty is met with in introducing wire netting into glass during the rolling operation, and this lies in effecting a clean join between glass and wire. Most metals when heated give off a considerable quantity of gas, and when this gas is evolved after the wire has been embedded in glass, numerous bubbles are formed, and these not only render the glass very unsightly but also lessen the adhesion between the wire and the glass. This difficulty, however, can be overcome more readily than the first, since the surface of the metal can be kept clean and the gas expelled from the interior of the wire by preliminary heating. On the whole, however, wired glass is perhaps still to be regarded as a product whose evolution is not yet complete, and there can be no doubt that there are great possibilities open to the material when its manufacture has been more fully developed.