GLASS. 



477 



entsmust be added to fill the crucible. The furnace 

 is kept at a very high temperature ; and the whole 

 ol' the materials being fused, a white scum collects 

 upon the top, called glass gall or sandever, which 

 must be removed as it forms, and is useful to re- 

 finers of metals as a flux. Another kind of sand- 

 ever is found in the crucibles after the glass 

 lias been withdrawn. When the glass becomes 

 translucent, in order to render the glass work- 

 able the temperature of the. furnace is dimin- 

 ished until the glass becomes a tenacious mass, so 

 that it may be drawn out into a filamentous thread. 

 The bringing of the glass to this state from the first 

 changing of the crucibles occupies about two days. 

 The iron tube is now put into the crucible, and the 

 required quantity of glass lifted out upon its end. 

 The tube is then held perpendicularly with the 

 loaded end nearest the ground, and held for a few 

 seconds, so that the glass extends beyond the end of 

 the tube, after which it is rolled into a cylindrical form 

 on a smooth iron table called the merver. The work- 

 man then gives the glass the form of a hollow globe by 

 blowing through the tube, the process being repeated 

 several times by rehtating at the furnace, and blow- 

 ing until the globe become of the required thinness. 

 The globe may be elongated by dexterously whirling 

 it in the air. The punty formerly described is now 

 applied to the end of the glass farthest from the tube, 

 and the tube is detached by touching the glass with 

 a piece of cold wetted iron, and then giving it a 

 smart stroke. The workman now heats the glass on 

 the punty, and sitting down upon a chair with 

 smooth amis, he lays the punty upon them in an hori- 

 zontal direction, and rolling the punty with his left 

 hand backwards and forwards, he gives the glass a 

 rotatory motion, while with an instrument in his 

 right hand, somewhat resembling a pair of sugar tongs, 

 he enlarges or contracts the different parts of the 

 vessel, until it assumes the requisite shape. The 

 workman is also provided with a pair of shears, a 

 scale of inches, and a pair of compasses, so that he 

 may model the material with the greatest accuracy, 

 and if the vessel be complex in form, the glass must 

 be occasionally re-heated during the manipulation. 

 The article is then detached from the punty, and 

 carried to the annealing furnace. 



Many of the articles after coming from the annealing 

 furnace are sent to the cutter or grinder. The opera- 

 tion is performed by putting in motion wheels of vari 

 ous diameter, and of various edges, some of which 

 are made of iron, others of stone, and some of wood. 

 The operation is commenced at the iron wheel, whose 

 rim is supplied with sand and water from above, and 

 the utensil being applied to the edge of the wheel, is 

 ground to the required form, and then smoothed on 

 the stone wheel, when it is taken to a wheel where 

 a finer substance is employed for grinding, such 

 as rotten or pumice stone, and at last polished 

 on a wooden wheel, with putty or crocus as a 

 polisher. 



Various ornamental forms are given to the surface 

 of glass vessels by metallic moulds. The mould i 

 usually of copper, with the figure cut on its inside, 

 and opens with hinges to permit the glass to be 

 taken out. The mould is filled by a workman, who 

 blows fluid glass into its top. The chilling of the 

 glass, when it comes in contact with the mould, 

 impairs its ductility, and prevents the impression oi 

 the figure from being sharp. Some moulds, however, 

 are made in parts, which can be suddenly broughi 

 together on the inside and outside of the glass vessel, 

 and produce specimens nearly equal to cut glass. 

 Plate glass is of two descriptions. The first which 

 we shall notice, is made of similar materials with 

 crown glass, and used for windows or for frames of 



pictures, and called British sheet glass. The materi- 

 al being prepared as described in crown glass manu- 

 facture, the workman collects the metal out of the 

 crucible on the end of his tube, he places it horizon- 

 tally upon a block of wood, formed so that by turning 

 the tube, the glass is made to assume a cylindrical 

 form while the attendant by means of a wetted 

 sponge throws a fine stream of water upon the block. 

 This oeing done, the workman by blowing into the 

 tube, and still continuing to turn it, increases its 

 diameter to about ten inches, and then by re-heating 

 it, and swinging it in the air, he also increases its 

 length to about forty inches. Its end being now 

 heated and the end of the tube stopped, the end 

 nearest the fire bursts, and by the rapid turning of the 

 tube on its axis, the end expands, and the cylinder's 

 side becomes straight. The glass is now separated 

 from the tube, and being allowed to cool a little is 

 split lengthwise by drawing a red hot iron rod along 

 its inner surface. The cylinder is now placed in a 

 highly heated furnace, upon a smooth stone, with its 

 split side upmost, and falls out by its own weight into 

 a flat plate or sheet, which being nibbed smooth with 

 a piece of charred wood is carried to the annealing 

 furnace. In crown glass, made in the common way, 

 it is difficult to procure a sheet of 548 square inches 

 in area, but the sheet glass now described, may be 

 made of 1800 square inches in area. 



The composition of plate glass commonly so called, 

 is stated by Parkes to be linn sand, 720 Ibs., alkaline 

 salt containing 40 per cent, of soda 450, slaked 

 lime sifted 80, saltpetre 25, and broken glass 425 ; 

 a mixture calculated to make 1200 Ibs. of plate 

 glass. Another good composition has been stated to 

 be, white sand 300 Ibs., soda 200, lime 30, oxide 

 of manganese 2, oxide of cobalt 3 ounces, and 

 fragments of glass equal to the weight of sand. The 

 sand, lime, soda, and manganese are intimately mixed, 

 and fritted for about six hours, and before the pro- 

 cess is finished, the other ingredients are added. The 

 large crucibles in the working furnace are now filled, 

 and smaller ones are placed beside them in the fur- 

 nace, which is then to be raised to the greatest pos- 

 sible heat, when the materials in the large crucibles 

 are thoroughly liquefied and skimmed, the small cru- 

 cibles are filled with the liquid glass by means of a 

 copper ladle. These small crucibles are not taken 

 out of the furnace for three or four hours. When 

 the glass in the cuvette or small crucible is deemed 

 fit for casting, it is taken out of the furnace by means 

 of a crane mounted on a low carriage, and moved to a 

 smooth plane horizontal table of iron, in which the 

 plate is to be cast. The surface of the glass pre- 

 viously to being cast is skimmed, and the outside of 

 the crucible as well as the casting table cleaned. 

 The table has ribs of metal along the sides of depth 

 equal to the intended thickness of the plate, and a 

 temporary rib is placed at the further end of the 

 table. The contents of the crucible are now poured 

 upon the table, and a large copper cylinder moved 

 along upon the ribs spreads the glass into a broad uni- 

 form sheet. Twenty men are required to conduct 

 this operation, during which the room is kept quiet, 

 and as much as possible freed from currents of air, 

 which might injure the uniformity of the plate. On 

 this department of the process, Mr Parkes observes, 

 that " the variety of colours which the plate exhibits 

 immediately after the roller has passed over it, renders 

 this an operation, far more splendid and interesting 

 than can well be described." When the plate has set, 

 it is examined, and should any bubbles or flaws ap- 

 pear, it is divided by cutting through them, and when 

 fairly fixed, it is carried to the annealing furnace, 

 where it remains for about a fortnight. The table, 

 crane, and roller are shown in fig. 4, The plate 



