242 



GLASS 



departure, and introduces the gas and air at 

 a considerable height above the pots, the heat 

 being thus obtained entirely by radiation. This 

 arrangement is applicable to tanks as well as to 

 pot furnaces. Fig. 11 shows the system of heat- 

 ing by radiation as adapted to a long furnace 



Fig. 11. 



holding, say, 30 pots ; for a shorter furnace the 

 arrangement is modified, the pots and the ends 

 of the furnace being curvea. The workman, 

 having made his gathering (a, b, fig. 12), forms 

 it into a cylindrical mass of the diameter required 

 by blowing and turning it in the cavity either of a 

 solid block of wood which is sprinkled with water, 

 or of a hollow metallic block which is kept cool by 

 water passing through it. By more blowing and 

 swinging over the head, the workman brings it by 

 degrees nearer to the form of an elongated cylinder 

 (c, d, fig. 12). As it cools rapidly in this operation, 

 he from time to time places his pipe in the rest before 

 the furnace-mouth, and, gently turning it round, he 

 brings it again nearly to the melting-point ; then 

 he repeats the blowing and swinging, standing 

 over the pit, to enable him to swing it completely 

 round as it lengthens out. These operations are 

 continued until the cylinder has reached its maxi- 

 mum size i.e. until it is of equal thickness 

 throughout, and sufficiently long and broad to 

 admit of sheets of the required size being made 

 from it (e, fig. 12). Sometimes these cylinders are 

 made 60 inches in length, allowing sheets of glass 

 49 inches in length to be made from them, but the 

 Belgians make them much larger. In the Vienna 

 Exhibition they exhibited sheets 10 x 4 feet. The 

 next operation is to place the pipe in the rest, and 

 apply the thumb so as to close the opening at the 

 blowing end : the heat of the furnace soon softens 

 the glass at the closed extremity of the cylinder, 

 and, as the enclosed air is prevented escaping, as it 

 rarefies, by the thumb placed on the opening of the 

 blowpipe, it bursts at the softened part (/, fig. 12). 

 The operator then quickly turns the cylinder, still 



A 



Fig. 12. 



with its end to the fire, and the softened edges of 

 the opening, which at first are curved inwards, 

 are flashed out until they are in a straight line 

 with the sides of the cylinder (g, fig. 12). It is 

 then removed, and placed on a wooden rest or 



chevalet. Just at the shoulder near the blow- 



Eipe the workman wraps round a thread of red- 

 ot glass, which after a few seconds he withdraws ; 

 then he applies his cold shears quickly, and the 

 shoulder and neck drops off' as neatly as if cut with 

 a diamond. The removal of this neck of glass can 

 also be effected by drawing a red-hot iron rod round 

 the shoulder, and then dropping a little cold water 

 upon it. The continuous tank furnace, and the pot 

 furnaces of France and Belgium, are so arranged as 

 to serve for both melting and blowing. In England 

 the manufacturers who employ pot furnaces prefer 

 to have a separate construction, called the ' blow- 

 ing-holes,' for the reheating and manipulation of 

 the cylinder. The advantage of this method, as 

 regards pot furnaces, is that the heat of the blowing- 

 holes, being independent of the melting process, 

 can be adjusted to suit the requirements of the 

 blower. 



The finished cylinder (h, fig. 12) is split open by 

 a diamond attached to a long handle, and guided 

 by a wooden rule. This was formerly effected by a 

 red-hot iron rod. It is then taken to the flattening 

 kiln, where it is laid with the split upwards on the 

 flattening stone, which is generally covered by a 

 sheet of glass called a ' lagre,' to protect the cylin- 

 der from the irregularities of its surface. Here the 

 heat is sufficient to soften without melting the glass, 

 and theflattener, as it softens, opens the two edges 

 of the crack until by its own weight the sheet falls 

 flat on the stone ; he then takes an implement in 

 the form of a rake, made by placing a piece of 

 charred wood transversely at the end of a long 

 handle, and this is gently rubbed over the glass, 

 producing a very smooth surface. The annealing 

 kiln is immediately at the back of the flattening 

 arch, and the flattening stone mounted on a wagon 

 and carrying the sheet of now flat glass is moved 

 into the annealing chamber. Here, when cooled 

 enough to bear moving, the sheet is first placed 

 horizontally, and afterwards with others piled up- 

 right. The wagon is in this way moved from one 

 chamber to the other with successive sheets of glass 

 until the annealing oven is filled. The oven is then 

 closed up so that it may be free from draughts, and 

 allowed to cool slowly down for a period varying 

 from three to five days. The annealing may be 

 accelerated by substituting for the oven a series of 

 iron boxes on wheels, which are filled in succession 

 with the sheets on edge, and pass on when full into 

 a cooler place. There is another and more modern 

 form of lear in which the flattened sheets are passed 

 through the annealing chamber one at a time. A 

 single sheet will cool very rapidly, and at the end 

 of about half an hour will emerge thoroughly 

 annealed. 



Glass-shades are made in the same manner as 

 above described ; they are nothing more, indeed, 

 than the rounded ends of the cylinders before being 

 burst. When wanted oval or square, these forms 

 are produced by the use of boxes of wood charred 

 inside, of the size of the shades required, through 

 which the cylinder is passed, when being blown, 

 until the soft glass touches and receives its shape 

 from the inside of the box or mould ; they are after- 

 wards annealed, and cut to the lengths required. 



Plate-glass is made in a totally different manner 

 from crown or sheet. Great care is taken in the 

 selection of the materials, as they require to be of 

 a purer kind than those used for ordinary window- 

 glass. From its thickness, any impurity of colour 

 is readily noticed, and, on account of its flat surface 

 when polished, air-cavities are conspicuous defects. 

 The sand used must be as free as possible from 

 iron, the staining power of which is most usually 

 corrected in the case of plate-glass by the addition 

 of a little arsenious acid. Almost every manufac- 

 turer has his own private receipt for the mixture 



