Glass Manufacture

by Walter Rosenhain

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.