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Sect. XII.

GLASS.

1868. Glass is a combination of silex with fixed alkali, generally soda. The mixture when calcined receives the name of frit, which after the removal of all its impurities, is conveyed to the furnace and melted in large pots or crucibles till the whole mass becomes beautifully clear, and the dross rises to the top. After being formed into the figures required, it is annealed or tempered by being placed in an appropriate furnace. The fineness depends on the purity and proportion of the ingredients. An extremely fine crystal glass is obtained from 16 parts of quartz, 8 of pure potash, 6 of calcined borax, 3 of flake white, and 1 of nitre. The specific gravity of glass is about 2600; of French plates, 2840; of English fint glass, 3320. Glass is extremely elastic, and less dilatable by heat than metallic substances.

1868a. Four pieces of the common sort of glass being cut from one strip, each piece was 5 inches wide, 6 inches long, and 4 inches thick. In the trial of strength they were calcu. lated out at a standard size, and gave 17,208 lbs., 15,435 lbs., 14,931 lbs., and 11,385 lbs. ; the mean being 14,931 lbs. This great difference is the more singular from the circumstance of all the pieces being cut from the same plate. The weight of the glass at a size of 9.0 x 4.5 x 3, all in inches, would be 11•12 lbs. Sheet glass is stated to be stronger than plute or crown glass, but less flexible. The compressive strength of glass is about 124 tons per square inch. The resistance of glass to a crushing force is about 12 times its resistance to extension.

1869. Pliny gives the following account of the discovery of manufacturing glass, which was well known in Aristotle's time, 350 B.C. "A merchant vessel, laden with nitre or fossil alkali, being driven on the coast of Palestine, near the river Belus, the crew accidentally supported the kettles on which they dried their provisions on pieces of the fossil alkali; the sand about it was vitrified by its union with the alkali, and produced glass.” Though, according to Bede, artificers skilled in making glass were brought into England in 674, glass windows were not generally used here till 1180, and were for a considerable time esteemed marks of great magnificence.

1870. The manufacture of window glass during the last thirty years has undergone entire alteration, especially since the abolition of the excise duty in 1845. There are now three special kinds of glass used for glazing purposes, and several varieties of them :

1870a. 1. Crown glass, which is blown into large globes and opened out into circular flat tables. II. Skeet glass, which is blown into long cylinders or muffs; then split down and flattened. III. Plute glass, which is either cast on iron tables for large purposes and polished; or for smaller squares, blown into a cylinder and polished.

1871. Crown glass, the commonest window glass, differs from flint glass in its containing no lead or any metallic oxide except manganese, and sometimes oxide of cobalt, in minute portions, for correcting the colour, and not as a flux. It is compounded of sand, alkali, either potash or soda, the vegetable ashes that contain the alkali, and generally a small portion of lime. To facilitate fusion, a small dose of arsenic is frequently added. Zaffre or oxide of cobalt, in the proportion of 1 ounce for 1000 pounds, is added, to correct the colour ; but when the sand, alkali, and lime are very fine, and no other ingredients are used, zaffre is not required. Its manufacture is conducted differently from that of Aintglass articles, the object being to produce a large flat thin plate, which is afterwards by the glazier's diamond cut into the requisite shape. It is blown in circular plates, varying from 3 feet 6 inches to 4 and 5 feet diameter: the process is as follows:- The workman, having a sufficient mass of melted metal on his blowpipe, rolls it on an iron plate, and then, swinging it backwards and forwards, causes it by its own gravity to form into a globe, which is made and brought to the required thinness by blowing with a fan of breath, which persons accustomed to the work know how to manage. The hollow globe is then opened by holding it to the fire, which expanding the air confined within it (the hole of the blowpipe being stopped), bursts it at the weakest part, and while still soft it is opened out into a flat plate by centrifugal force; and being disengaged from the rod, a thick kuob is left in its centre. It is then placed in a furnace, or in a certain part of the furnace to undergo the process of annealing. When the table is cut for use, the centre part in which the knob remains is called knob-glass, and is used only for the very commonest purposes.

Tables are now made of such a size that squares may be procured 38 inches by 24 inches as extra sizes.

187la. The qualities of crown glass in comnion use are called best, seconds, thirds, and fourths or coarse ; with two still coarser. The last is of a very green huc, and only used for inferior buildings. They were sold by the crate, at the same price, the difference being made up by varying the number of the tables contained in it. Thus a crate of best crown glass contained twelve tables; of seconds, a crate contained fifteen; and of thirds, eighteen tables. They are now sold (by Messrs. Hartley) in crates of eighteen tables of the usual

;

as

00

90

65

50

99

thickness averaging 53 inches ; and in crates of twelve tables of extra thickness averaging 52 inches. Flattened slabs of the same qualities are sold in crates of thirty-six slabs of the usual thickness, and in crates of twenty-four slabs of extra thickness, each averaging 24 inches, 22 inches, and 214 inches. The flattened slab is also made as obscured 'glass. The sizes of both qualities vary from quarries '; under 9 by 7; up to, above 4, feet, and not above 5 feet superficial. Taking the usual thickness of Best

extra thickness 150 Seconds

135 Thirds

110
Fourths

85
CC and CCC
43 and 40

63 and 50 (Adcock). 1872. Sheet glass has been manufactured in England with great improvements since 1832 to 1838 by Messrs. Chance and Hartley, with the co-operation of M. Bontemps, of Paris. Though inferior in colour, this glass is in other points generally superior to that of the foreign manufacture. It is composed of the same or similar materials to the above, in well ascertained proportions, and with sulphate of soda to give whiteness. In the manufacture of sheet glass a sufficient quantity of the metal is collected at the extremity of a blow. pipe, and then lengthened by swinging and blowing, till it acquires the form of a hollow cylinder

, which is then detached, the neck being cut off with a thread of hot glass ; and one side of the cylinder is cut down lengthwise with a heated iron or diamond. It is then taken to the flattening kiln, where the heat causes it gradually to open nearly flat on a bed called largre, where it is rubbed down by means of a block of wood called a polissoir, and then becomes flattened sheet. After this operation it is placed in the annealing oven to cool gradually. This operation is referred to by the monk Theophilus, who wrote about the end of the twelfth century or later, as in use in his time. The method was also employed by the Venetians especially for coloured glass, as it secured uniformity. But on the cessation for its demand, the employment of the cylinders was entirely superseded in France, England, and the North of Germany, for the rotary principle.

1872a. The great advantage of sheet glass is that of affording plates of larger dimensions, and not only of avoiding the waste arising from the circular form of the crown tables, but also from the knob or bull's eye in the centre. The surface, however, is much less bril. liant than that of crown glass, and is more wavy and undulated. Messrs. Chance, in 1838, introduced a thicker quality of sheet glass, which was at the same time of a better surface, and since then its use has become general.

18726. In 1840 the same firm introduced a new variety of window glass under the name of patent plate, which they obtained from a thick sheet glass by a new process of grinding and polishing. They made plates of several degrees of thickness, and of sizes containing from 8 to 12 feet superficial. The surface of the glass obtained by this process, though not perfectly true, is very nearly so; and in brilliancy it is unsurpassed even by cast plate. For glazing sashes it has nearly superseded crown and sheet glass. But for squares of somewhat large dimensions, it may be calculated whether plate glass will not be as cheap or cheaper.

1872c. As will be perceived by the above short account of the mode of manufacture of sheet glass, its size is almost only limited by the strength of the workman. It is chiefly sold in crates as manufactured, in sheets of not less in width than 28 inches, and not less than 9 feet superficial area ; with a limit of width not exceeding 45 inches, and a limit of length not exceeding 75 inches; but these extremes of width and length cannot be combined in the same sheet. Thus in glass of 15 ounces to the foot, the dimensions 55 by 36 inches, or 12 feet in area, is the largest plate. In 21 ounce glass, 75 by 45 inches, or 18 feet area : in 26 ounce glass, 75 by 45 inches, or 17 feet area : in 32 ounce glass, 65 by 44 inches, or 15 feet area : in 36 ounce glass, 60 by 42 inches, or 12 feet area : and in 43 ounce glass, 55 by 38 inches, or 11 feet area. The four first weights are made in qualities of best, seconds, thirds, and fourths ; and the two first have two qualities A and B for pictures. There is no fourth quality to the two last named weights. All these sorts are cut into squares for glazing.

1872d. Fluted sheet glass of 15 ounce and 21 ounce is usually supplied in crates not above 43 inches long; but it is made up to 50 inches in length. Obscured sheet glass is supplied in all substances.

1873. Patent plate glass, already described (par. 1872b.) is made in three qualities, B or best, C or second, and CC or third, quality. Each of these are of four kinds, known as No. 1, which is of an average thickness of th of an inch, and is of an average weight of 13 ounces to the foot; No. 2 is lyth thick, and 17 ounces; No. 3 is jóth, and 21 ounces; and No. 4 is fth, or 24 ounces to the foot. No. 4 B is thus the very best quality made; the prices for the size required vary but about one or two pennies per foot in each kind; and from threepence to sevenpence in each quality. They are manufactured in sizes from 4 to 13 feet in area, not above 50 inches long, or 36 inches wide.

1874. German sheet, or Belgian sheet glass, as it is sometimes called, was formerly in much demand in England; and is still used for cheapness. Its appearance is more wavy

and speckled than the English manufacture. Crystal white sheet glass, for glazing pictures and prints, is imported from Florence in cases of 100, 200 and 300 feet, in first, second and third qualities, and appears superior to other glass in whiteness, but it has the defect of sweating.' Similar named glass for such purposes made by Messrs. Chance, appears to us to be very green, and therefore detrimental to prints and pictures; but on the other hand it does not sweat.

1875. Plate glass is so called from its being cast in large sheets or plates. Its constituent parts are white sand, cleansed with purified pearl-ashes, and borax. If the metal should appear yellow, it is rendered pellucid by the addition, in equal small quantities, of manganese and arsenic. It is cast on a large horizontal table, and all excrescences are pressed out by passing a large roller over the metal. To polish it, it is laid on a large hori. zontal block of freestone, perfectly smooth, and then a smaller piece of glass, fastened to a plank of wood, is passed over the other till it has received a due degree of polish. For the purpose of facilitating the process, water and sand are used, as in the polishing of marble; and lastly, Tripoli, smalt, emery, and putty, to give it lustre; but to afford the finishing polish the powder of smalt is used. Except in the very largest plates, the workmen polish their glass by means of a plank laving four wooden handles to move it, and to this plank a plate of glass is cemented.

1876. In the GLOSSARY, S. v. Glass, plate, will be found the tariff of the Thames Plate Glass Company in London, for unsilvered plates for mirrors, of which there are two qualities, second and best. The Paris factory supplied in 1865 two looking glasses for the Mayor's room in the Town Hall at Liverpool, each 15 feet by 10 feet. Polished plate glass is manufactured for general glazing purposes up to about 80 feet superficial, of two qualities, usual and best. The usual thickness is a quarter of an inch : higher prices are charged for glass selected to be cut above ja ths., faths., and sths. thick ; while for above {ths, thick, special prices are charged. The best quality is declared to be of the very purest colour, free from specks, and not subject to dampness or sweating.

1877. Rough plate glass, cast, is used for roofing, in skylights, windows, &c., in plates from not above 20 inches long, to above 120 inches long, in thicknesses of 1, 3, 1, 1, inch, 14, and 14inch ; but these thicknesses have certain limited lengths. The widths are the same as for plate glass. This glass is not ground or polished, but rough from the table, and showing the table marks on its underside.

1878. The patent rough plate glass, which is also cast, must not be confounded with the above. It is extensively used for ridge and furrow roofs, conservatories, manufactories, skylights, workshops, and other places where “obscured” glass is required to intercept the vision without diminishing the light. Blinds are unnecessary, and when it is used in greenhouses, no scorching of the plants occurs. The quality known as Ath. of an inch thick, weighing about 2 lbs , or 32 ounces to the foot, is usually provided for these purposes, and is no more, weight for weight, than common crown glass. When greater strength is required, jaths, and inch thick is said to be cheaper and of a finer quality than the common rough plate ; but we demur to this statement, as of late years the manufacture appears to have decreased in strength from the greater use of sand for cheapness; in moveable window frames in warehouses, a lamentable quantity of broken squares is to be seen almost before the floors are occupied.

1878a. This glass is made of two kinds; I. Plain, which is merely marked by the fine grain of the casting table, and is that above noticed; and II. Fluted, of two sorts, No. 1, large pattern, having 34 Autes to the inch; and No. 2, small pattern, having 12 Autes to the inch, Both the plain and the fluted kinds are made ġ th. iths. *, įths. and 4 inch in thickness. The width is about 3 feet, and the length usually not above 70 inches; but 75, 90, and 100 inches long, are also made. When a clear glass and much non-transparency are required, No. 2 futed is the best.

1879. Quarry glass is also made in this material ; No. 1 being 6 inches by 4fth inches from point to point ; No 2 being 3 inches by 276 inch. A stained ornamented patent quarry rough plate is made for churches, chapels, schools, &c. A patent diamond rough plate glass is also manufactured. A patent rough plate, and sheet, perforated glass, polished or unpolished, for ventilation, can be obtained in sizes, which require consideration in arranging, on account of the length of the slits or perforations. It is usually made in columns 11 inches wide, and 24 inches apart; the space between each slit vertically being if inches. Larger sizes, or the columns wider apart, can be obtained from various manufacturers, or to order.

1880. Many other applications of glass will be noticed in the ensuing chapter. We must bere state that the details given in this section are founded upon the price list issued by Messrs. Hartley, of Sunderland, and would state our regret that the manufacturers have not deemed it advisable for their own interests, to provide some place in London, and in other large towns, where the architect can call and compare the qualities of glass supplied under his specification with standards there placed. It was comparatively easy in former years to judge of good glass ; now it is almost impossible.

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CHAP. III.

USE OF MATERIALS, OR PRACTICAL BUILDING.

SECT. I.

FOUNDATIONS AND DRAINS.

1881. In the previous chapter, we have enumerated the principal materials used in tuilding; we shall now proceed to show how those materials may be most advantageously employed; but we shall not, in the various branches of the practice, again touch on the materials themselves, which have been, we conceive, already sufficiently described. But previous to entering upon the different branches of practical building, we think it right to submit to the reader a few observations on that most important of all considerations--a due regard to the security of the foundations on which a building is to stand, as a preliminary to the works of the bricklayer and mason, as the case may be. No advance or improvement has been made in this branch of architecture, as a science, since the time of the ancients. The advice of Vitruvius may still be followed with safety. In England, the recent introduction of concrete (no modern invention) has superseded the use of wood under walls in the earth; and piles are now quite exploded, except for the piers of bridges and other situations in which they can constantly be kept wet.

1882. The best soils for receiving the foundations of a building are rock, gravel, or close-pressed strong sandy earth; "but,” says L. B. Alberti, “ we must never trust too hastily to any ground, though it may resist the pick-axe, for it may be in a plain, and be infirm, the consequence of which might be the ruin of the whole work. I have seen a tower at Mestre, a place belonging to the Venetians, which, in a few years after it was built, made its way through the ground it stood upon, which, as the fact evinced, was a loose weak soil, and buried itself in earth up to the very battlements. For this reason, they are very much to be blamed who, not being provided by nature with a soil fit to support the weight of an edifice, and lighting upon the ruins or remains of some old structure, do not take the pains to examine the goodness of the foundation, but inconsiderately raise great piles of building upon it, and out of the avarice of saving a little expense, throw away all the money they lay out in the work. It is, therefore, excellent advice, the first thing you do, to dig vells, for several reasons, and especially in order to get acquainted with the strata of the earth, whether sound enough to bear the superstructure, or likely to give way.” It is important, previous to laying the foundations, to drain them completely, if possible, not only from the rain and other water that would lie about, but from the land water which is, as it were, pent up in the surrounding soil. In soft, loose, and boggy ground, the use of concrete will be found very great ; and in these soils, moreover, the width and depth it should be thrown in, should, as well as the lower courses of the foundation, be proportioned inversely to the badness of the soil. Clay of the plastic kind is a bad foundation, on account of the continual changes, from heat and moisture, to which it is subject, and which often cause it so to expand and contract as to produce very alarming settlements in a building. The best remedy agai this inconvenience is to tie the walls together by the means of chain plates, buried in the centre of the footings, and on the top of the landings that rest on the concrete; these plates to be, of course, connected at the returning angles, so as to encompass the whole building. In these cases, the clay must be excavated to make room for the concrete. This will be found an effectual remedy in clay soils.

1883. If the soil be a sound gravel, it will want little more than ramming with heavy rammers; and if the building be not very heavy, not even that.

1884. Where vaults and cellars are practised, the whole of the soil must, of course, be excavated; but where they are not required, trenches are dug to receive the walls, which, in both cases, must be proportioned in strength to the weight of the intended superstructure and its height. In general terms, we may direct the depth of foundations to be a sixth part of the height of the building, and the thickness of the walls twice that of those that are raised upon them. Care must be taken that that which is to receive the footings of the walls be equable; otherwise, where external and internal walls are connected together, the former, being the heaviest, may settle more than the latter, thereby causing fractures, which, though not, perhaps, dangerous, are extremely disagreeable in appearance. The lower courses, which are called the footings of the wall, are often laid dry; and, perhaps, at all events, a sparing use of mortar in a spot loaded with the greatest pressure should be preferred. If the footings be of stone, very particular attention should be bestowed on

Fig. 615.

placing the stone in the courses in the same direction or bed as it lay in the quarry, to prevent its splitting.

1885. In foundations where, from columns or small piers pressing upon particular parts, there would be a liability, from uneven bearing, to partial failure, it has been the practice fro:n a very early period, to turn inverted arches (see fig. 615.) to catch on their springing the weight to be provided against, by which means such weight is equally distributed throughout the length of the foundation. “ Standing thus,” says our master Alberti, “they (the columns or weights) will be less apt to force their way into the earth in any one place, the weight being counterpoised and thrown equally on both sides on the props of the arches. And how apt columns are to drive into the ground. by means of the great pressure of the weight laid on them, is manifest from that corner of the noble temple of Vespasian that stands to the north-west; for, being desirous to leave the public way, which was interrupted by that angle, a free and open passage underneath, they broke the area of their platform, and turned an arch against the wall, leaving that corner as a sort of pilaster on the other side of the passage, and fortifying it as well as possible with stout work, and with the assistance of a buttress. Yet this, at last, by the vast weight of so great a building, and the giving way of the carth, became ruinous."

1885a. A method of forming foundations has lately come into vogue for bridges and other hydraulic constructions by the use of cylinders or other shaped air-tight cases. In India the system of founding large masses of masonry on cylindrical piers built in the interior of wooden curbs, has prevailed for a long period. The method of constructing the piers is the same as that used in England in sinking the steining for ordinary wells ; and when sunk the interior is filled up with concrete or rubble masonry. Some of the iron bridges lately erected over the River Thames and elsewhere have been placed on foundations formed by cast iron cylinders filled in with concrete. Further details must be sought in works devoted to Civil Engineering, as the system will seldom be applicable in strictly architectural constructions.

1886. It is most important, when the walls are raised on the foundations, and brought up a little above the level of the earth, to take care that the earth, most especially if moist, should not lie against them; for if walls, before they are dry and settler, imbibe moisture, they rarely ever part with it, and thence gradually impart rot to the timbers throughout the house. In all buildings, it is an object to have a second thin wall outside the basement walls, so as to leave between it and them a cavity for the circulation of the air, such cavity being technically called an air-drain. This is in all cases desirable, but in moist and loose soils it is essentially necessary for the durability of the building, as well as for the health of those who are to dwell in it.

1886a. It is important that the air.drain or dry area should commence at least as low as the foundations of the building; in very wet situations it should be provided with pipes connected with the main drains of the building, to carry off the superabundant moisture. Even when provided, the usual precautions to prevent damp arising in the main walls must not be neglected. The air.drain, which should never be less than 8 inches wide, or more if possible, is cominonly covered with a half-brick arch, or with stone, slate, or tile, below the surface of the ground. This entirely does away with the benefit anticipated by its formation, because the surface drainage descends and injures the main wall even when cemented above the covering; this should be placed some inches above ground. It also often degenerates into a hole for dirt and vermin. The best arrangement is to make a dry area, or a space wide enough to be easily cleared out, and to which a cat or dog can have access, and to cover it with stone with moveable gratings at convenient distances ; the expense will not be much greater, while the result will be very effective. The most secure arrangement, however, is to form an open area all round the building. The want of such a precaution in the houses in the suburbs of London renders a large majority of those having basements nearly uninhabitable from the disagreeable consequences of damp walls.

18866. Damp courses. This simple provision prevent wet which is likely to get into walls, from rising in them by capillary at'raction, is too often neglected, especially in cheap work, for the present saving of a pound or two; but at the ultimate expenditure of many pounds. The simplest plan has generally been to work three courses of the brickwork above the footings and below the ground floor, in cement. Messrs. Smith of Darnick state that a coating of cement, done in a very substantial manner, did not appear to have the smallest effect, as the wall was as damp above it as below. For small cottages they found an effective plan was to build all the parts of the wall underground quite dry, and

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