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bustible body is present, it is usual in some manu- glass is termed blowing, from its being in a great factures to add a little white oxyd of arsenic. This ineasure performed by the operator blowing through supplying osvgen, the combustible is burnt, and an iron tube, and by that means inflating a piece fies off; while the revived arsenic is at the same of glass which is heated so as to become soft and time volatilized.
exceedingly pliable. By a series of the inost simThere are several kinds of glass adapted to dif. ple and dextrous operations, this beautiful materiat ferent uses. The best and most beautiful are the is wrought into the various utensils of elegance fliot and the plate glass. These, when well made, and utility, by methods which require but very are perfectly transparent and colourless, heavy few tools, and those of the most simple construcand brilliant. They are composed of fixed alkali, tion. pure silicious sand, calcined flints, and litharge, in The glass-blowers' furnace is of a circular form, different proportions. · The flint-glass contains a as shewn in the plan, fig. 2, Plate 82. It consists of large quantity of oxyd of lead, which by certain three distinct parts. The lowest is a large arch, processes is easily separated. The plate-glass is which is carried beneath the centre of the furnace: poured in the melted state upon a table covered in the plan, tig. 2, this is represented by the with copper. The plate is cast half an inch thick, dotted lines AA: in the section, fig. d, nothing of er more, and is ground down to a proper degree this arch is seen, except part of its upright sides of thindess, and then polisbed.
AA. In the centre of the furnace the covering of Crown-glass, that used for windows, is made this arch is wanting, and its place is supplied by without lead, chiety of fixed alkali fused with si- a grate, (represented in the plan) upon which the hcions sand, to which is added some black oxyd fire is made. The arch AA, which is called the of manganese, which is apt to give the glass a draught arcb, is intended to bring a constant suplinge of purple.
ply of fresh air to the furnace. The second part Bottle-glass is the coarsest and cheapest kind: of the furnace is a circular wall KK, of masonry or into this little or no fixed alkali enters the compo- brick work, strengthened by nine ribs or piers BBB, sition. It consists of an alkaline earth combined which extend from the foundation to the top of the with alumina and silica. In this country it is furnace, (as shewn in the section). Within the cornposed of sand and the refuse of the soap-boiler, circular wall or waist of che furnace, the crucibles which consists of the lime employed in rendering or pots to contain the glass are placed; these are his alkali caustic, and of the earthy matters with nine in number, and are situated bebind the spaces which the alkali was contaminated. The most between each pier. The fire is made upon the fasible is fint-glass, and the least fusible is bottle- grate in the centre of the furnace, and its Names glass.
are reverberated down upon the pots by a dome Flint-glass inelts at the temperature of 10° DD, fig. 1, called the vault, constructed of fireWedgewoord; crown-glass at 30°; and bottle- bricks. The vault, and indeed the whole superglass at 47o. The specific gravity varies between structure of the furnace, is supported only by the 2:48 and 3.33.
nine piers B: by this means nine apertures are left Gla-s is often tinged of various colours by mix- beneath the vault which are the mouths of the ing with it wbile in fusion soine one or other of the furnace. metallic oxyds; and on this process, well conduct- The vignette at the top of Plate 81, is a view of el, depends the formation of pastes or factitious the interior of a glass-louse, with workmen pergems.
forming the various operations. In this figure, Blue glass is formed by means of oxyd of co- the nine mouths of the furnaces are represented balt.
as partially closed by a sereen of fire-bricks, in Green, by the oxyd of iron or of copper. which are three apertures to give the workinan Violet, by oxyd of manganese.
access to the pots; the use of the screen is to deRed, by a mixture of the oxyds of copper and fend the workman as much as possible from the iron.
heat of the furnace; and the apertures are there. Purple, by the purple oxyd of gold.
fore proportioned to the size of the work to be White, by the oxyd of arsenic and of zinc. performed. The nine puts are placed exactly beYellow, by tbe oxyd of silver and by combus- neath the mouths of the furnace, and are arranged tible bodies.
round the furnace upon a circular course of brick. Opticians, who employ glass for optical instru- work (EE in the clevation), so that the current of ments, ofteo complain of the many defects under tame reflected from the vault DD, strikes directly which it labours. The chief of these are the fold upon them. The flame and heated air are carrival lowing:
off from the furnace by nine flues, five of which Streaks.-These are waved lines, often visible FFF, are seen in fig. 1, Pl. 82, into an upper in glass, which interrupt distirct vision. They dume GG, which is the third part. It has a cyare probably owing sometimes to want of com- lindrie chimney HH, erected on the top of it, and plete fusion, which prevents the different materials carried up some height, to cause sufficient draught from combining sufficiently; but in some cases also for the tire. they may be produced by the workmen lifting up, The implements used by a glass-blower are nei. at two difierent times, the glass which is to go to ther vumerous nor expensive: the principal of the formation of one vessel or instrument.
them are shewn in tig. 2, Pl. 81. A is the blowing Tears. These are white specks or knots, occa- pipe, an iron tube about three feet six inches simed by the ritrified clay of the furnaces, or by long, and covered at one end with yarn, to prevent the presence of some foreign salt.
it burning the workman's hand. Bis an iron Bubbles.--These are air-bubbles which have not rod, of which the workman has several. Dare been allowed to escape. They indicate want of the pliers, with which the glass is worked: they complete fusion, either from too little alkali, or are made of steel, and the circular part being rethe application of too little heat.
duced very thin, acts both as a spring and a joint Coris -- These are asperities on the surface of to the blades. E are shears used in cutting the the glass, in consequence of too little heat. glass while in a soft and pliable stałe. Fare cali
CLASS-BLOWING. The art of forming vessels of pers used for incasuring the work occasionally.