Glass plate

“ In forming glass-plates by the extension of a cylinder into a plane, the workman first blows the lump of glass into the shape of an oblong pear, the length of which must be nearly equal to the length of the intended plate, and its diameter such, that the circumference when developed, will be equal to the breadth of the plate. He now rests the blowing-iron on a stool or iron bar, while an assistant with a pointed iron, pierces a hole into the extreme end of the pear, in the line of the blowing-pipe. ”

“ The more elaborate patterns found on what is known as “figure rolled plate” are produced in a somewhat different manner; the glass used for this purpose is considerably whiter in colour and much softer than ordinary rolled plate, and instead of being rolled out on a table it is produced by rolling between two moving rollers from which the sheet issues. The pattern is impressed upon the soft sheet by a printing roller which is brought down upon the glass as it leaves the main rolls. This glass shows a pattern in high relief and gives a very brilliant effect. ”

“ The glass to be used for the production of plate is universally melted in pots or crucibles and not in open tank furnaces. When the glass is completely melted and “fine,” i.e. free from bubbles, it is allowed to cool down to a certain extent so as to become viscous or pasty. The whole pot, with its contents of viscous glass, is then removed bodily from the furnace by means of huge tongs and is transported to a crane, which grips the pot, raises it, and ultimately tips it over so as to pour the glass upon the slab of the rolling-table. ”

“ 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 rest138 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. ”

“ The manufacture of plate glass is confined to very few hands, and great reluctance is manifested by the proprietors to permit visitors to inspect their works. The late Mr. Parkes, however, was permitted to visit the works of the British Plate Glass Company, at Ravenhead, and has recorded his observations in one of his valuable chemical essays, from which the following details are taken.
In the preparation of plate glass the materials are selected with greater care than in any other branch of the glass manufacture. The materials employed are sand of the finest and whitest kind, soda, and lime. ”

“ PRESSED GLASS Many articles are made by pressing glass into molds, that is, the molten glass is poured into molds and is pressed against the sides of the mold by means of a plunger. Imitation cut glass is pressed in this way.
PLATE GLASS The large sheets of plate glass used in store windows are not blown, but rolled. The molten glass is poured from the fire clay pots upon a cast-iron table and is rolled flat by means of a large iron roller (Fig. 24) . The glass is then in the shape of plate glass, but is rough on both sides. ”

“ When the glass is being blown in the mould the blowing iron is twisted round and round so that the finished bulb may not be marked by the joint of the mould.
III. Mechanically Pressed Glass. (A) Plate-glass.—The glass popularly known as “plate-glass” is made by casting and rolling. The following are typical analyses:
The raw materials for the production of plate-glass are chosen with great care so as to secure a product as free from colour as possible, since the relatively great thickness of the sheets 96 would render even a faint tint conspicuous. ”

“ Don't you think it's worth while spending half-an-hour to paint false lead lines on the back of the plate? A ha'p'orth of lamp-black from the oil-shop, with a little water and treacle and a long-haired brush, like a coach-painter's, will do it for you (see Plate XIII.) .
Another thing: when the window is in its place, each light will be surrounded with stone or brick, which, although not so black as the lead lines, will tell as a strong dark against the glass. See therefore that while you are painting, your glass is surrounded by dark, or at any rate not by clear, glittering light. ”

“ Zinc plates are also employed as provisory supports instead of glass, opal or porcelain plates. The modus operandi is exactly the same.30 The plates should be well planed, free from scratches, etc., and well polished to obtain glossy pictures without one having recourse to a film of collodion. For matt pictures, i.e., without gloss whatever, the plate should be finely granulated, and when waxing a very light pressure should be exerted to remove the excess of wax, else it might be quite impossible to strip off the picture in transferring on paper. ”

“ The lower end of the glass plate heated upon the hot steel plate cools when the lamp is taken away quicker than the upper one, to which heat is also communicated by con- duction. After some time therefore the centre of the plate becomes its warmest part. As the lower end, standing upon the rapidly cooled conductor of heat, becomes still cooler, the warmer spot moves upwards until finally the upper angle becomes the warmest. ”

“ The glass must now be ground with coarse emery upon its appropriate mould, whether convex or concave, the emery being all the time kept moist with water. To prevent the heat of the hand from affecting the glass, a rod for holding the brass plate is screwed to its back. For every six turns of circular motion, it must receive two or three rubs across the diameter in different directions, and so on alternately. The middle point of the glass must never pass beyond the edge of the mould; nor should strong pressure be at any time applied. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ It goes into the annealing ovens right from this table and comes out all irregular—full of pits and imperfections. No matter how flat the casting table is, or how much care is taken, the surface of the glass after annealing is always bad. If it is to be made into polished plate it must be ground down first with sand and water; then ground smoother still with a coarse kind of emery stone and water; next ground again with water and powdered emery stone. After that comes the smoothing process done with a finer sort of emery and water. ”

“ I then changed the roller so as to come lower down to the iron plate, and by this method commenced turning out sheets of plate glass two feet square and about one quarter of an inch thick. My task was done. I had all the glass I should ever want as long as I should live; enough for the side lights of my boat, and also for windows to the Hermitage. ”

“ 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. ”

“ 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. ”

“ The smooth surface of the rough plate-glass, not being perfectly flat, produces a slight distortion, which might, if it were worth while, be cured by cementing another disc of glass upon that side. In case the ground glass itself happens to be plate-glass, the image of external objects is perfect. ”

“ To grind glass. Rough edges of glass can be ground smooth by means of emery paper. For example, to smooth the edges of the glass bottle you have just cut in two, use the file for the rough work, then lay a piece of emery paper on a plate of glass, emery side up, pour a little kerosene on it and rub the rough surface on the emery with a rotary motion (Fig. 91) . ”

“ Large plate glass for looking-glasses is made by suffering the mass, in a state of complete fusion, to flow upon a casting table, with iron ledges. These confine the melted matter, and, as it cools, a metallic roller is passed over it, to reduce it to an uniform thickness. ”

“ 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. ”

“ In another direction sheet-glass has the great advantage over plate-glass that it is very much lighter, or can at least be produced of much smaller weight and thickness, although this advantage entails the150 consequent disadvantage that sheet-glass is usually much weaker than plate, and can only be used in much smaller sizes. In recent times the production of relatively thin plate-glass has, however, made such strides that it is now possible to obtain polished plate-glass thin enough and light enough for almost every architectural purpose. ”

“ As soon as the glass has cooled to a proper consistence it is examined; and if any bubbles or flaws are found, it is broken up and returned to the melting pot: but if it has a sound appearance, the table is rolled to the mouth of the annealing furnace, and the plate is carefully deposited there. The heat of this furnace is at first very great, but it is diminished every day for a fortnight, by which time the glass is sufficiently annealed. This process renders the glass less brittle; for, if suddenly cooled, my uncle says, it would fly into pieces when touched. ”

“ The glass-pots employed in this branch of the manufacture are covered, so that the flames of the furnace do not come in contact with the materials, the object in thus isolating them from direct contact with the flame being to prevent the entrance of impurities, by which the colour might be injured. On account of the pots being covered, the materials take a much longer time to get hot, and require quite double the time in founding that sheet or plate glass does; the presence of oxide of lead materially assisting the rapidity of the fusion. ”

“ All but the very commonest mirrors are now made of plate-glass; which is also used to a great extent for window-panes, and is manufactured by casting, rolling and polishing. The enormous plates of glass which are seen in many of the large shops of this city are well-calculated to excite the astonishment of those who are not yet aware of the late improvements in this branch of manufacture. An idea of what may be accomplished by blowing was given in 1845, at the Exhibition at Vienna, where a blown glass 7 feet in length and 31⁄2 in breadth was exhibited ”

“ A circular plate of glass, cooled with its centre resting on a piece of cold iron, or a cylindrical rod of glass cooled in the open air, when examined by polarized light in the direction of its axis, each present the same appearance of a black cross through their centre, and concentric fringes parallel to their circumference. ”

“ England has now large plate glass factories in different parts of the country, and these together yield as their weekly production at least 140,000 superficial feet of the best polished plate, or seven and a quarter millions of feet yearly. The value of plate glass made in England annually, including the rough kinds used for glazing roofs, &c., is estimated at 1,000,000l. France still stands very high, and her plates are extremely perfect in manufacture. ”

“ We here see that lead-glass or flint-glass has little greater refractive power on light than rock crystal or the ordinary plate and crown glass of commerce which belongs to the same family as the cristallo of the Venetians. In dispersive power, on the other hand, it stands apart from both these substances and rivals the diamond in scattering the component rays of white light. ”

“ How easy it would be, if things were thought of from the beginning and the place built for the work, to have such width of bench and space of window that, along the latter, easily and comfortably within reach, should run stages, tier above tier, of strong sheet or thin plate glass, sloping at such an angle that the cuttings might lie along them against the light, with a fillet to stop them from falling off. ”