GLASS 657 



and rolled into a pear-shapo, on a cast-iron plate ; the workman taking care that the 

 air blown into its cavity is surrounded with an. equal body of glass, and if he perceives 

 any side to be thicker than another, he coiTects the inequality by rolling it on the slop- 

 ing iron table called a marver (marbre). He now heats the bulb in the fire, and rolls it 

 so as to form the glass upon the end of the tube, and by a dexterous swing or two he 

 lengthens it, as shown in i, fig. 1098. To extend the neck of that pear, he next rolls it 

 over a smooth iron rod, turned round in a horizontal direction, into the shape K, fig. 1098. 

 By further expansion at the blowing furnace, he now brings it to the shape L, repre- 

 sented in fig. 1098. 



This spheroid having become cool and somewhat stiff, is next carried to the bottom- 

 ing hole (like^. 1096), to be exposed to the action of flame. A slight wall erected 

 before one half of this hole, screens the workman from the heat, but leaves room for 

 the globe to pass between it and the posterior wall. The blowing-pipe is made to 

 rest a little way from the neck of the globe, on a hook fixed in the front wall ; and 

 thus may be made easily to revolve on its axis, and by giving centrifugal force to the 

 globe r while the bottom of it, or part opposite to the pipe, is softened by the heat, it 

 soon assumes the form exhibited in is., fig. 1098. 



In thid state the flattened globe is removed from the fire, and its rod being rested 

 on the casher box covered with coal cinders, another workman now applies the end of 

 a solid iron rod tipped with melted glass, called a punto, to the nipple or prominence 

 in the middle ; and thus attaches it to the centre of the globe, while the first work- 

 man cracks off the globe by touching its tubular neck with an iron chisel dipped 

 in cold water. The workman having thereby taken possession of the globe by its 

 bottom or knobbled pole attached to its punty rod, he now carries it to another cir- 

 cular opening, where he exposes it to the action of moderate flame with regular rota- 

 tion, and thus slowly heats the thick projecting remains of the former neck, and opens 

 it slightly out, as shown at N, in fig. 1098. He next hands it to the flasher, who, resting 

 the iron rod in a hook placed near the side of the orifice &,fig. 1096, wheels it rapidly 

 round opposite to a powerful flame, till it assumes first the figure o,fig. 1098, and 

 finally that of a flat circular table. 



The flasher then walks off with the table, keeping up a slight rotation as he moves 

 along, and when it is sufficiently cool, he turns down his rod into a vertical position, 

 and lays the table flat on a dry block of fire-clay, or bed of sand, when an assistant 

 nips it off from the punto with a pair of long iron shears, or cracks it off with a touch 

 of cold iron. The loose table or plate is lastly lifted up horizontally on a double 

 pronged iron fork, introduced into the annealing arch, fig. 1097, and raised on edge ; an 

 assistant with a long-kneed fork preventing it from falling too rapidly backwards. 

 In this arch a great many tables of glass are piled up in iron frames, and slowly 

 cooled from a heat of about 600 to 100 F., which takes about 24 hours ; when they 

 are removed. A circular plate or table of about 5 feet diameter weighs on an average 

 9 pounds. 



4. Flint-glass. This kind of glass is so called because originally made with cal- 

 cined flints, as the siliceous ingredient. The materials at present employed in this 

 country for the finest flint-glass are, first, sand, calcined, sifted, and washed; second, 

 an oxide of lead, either red lead or litharge ; and third, pearlash. Sand for flint- 

 glass manufacture is obtained from the Isle of Wight, Aylesbury, the New Forest, 

 and some other localities in this country. A very beautiful sand is brought from 

 America, and some has been sent home from Australia. The pearl-ash of commerce 

 must however be purified by digesting it in a very little hot water, which dissolves 

 the carbonate of potash, and leaves the foreign salts, chiefly sulphate of potash, 

 and chlorides of potassium and sodium. The solution of the carbonate being allowed 

 to cool and become clear in lead pans, is then run off into a shallow iron boiler, and 

 evaporated to dryness. Nitre is generally added as a fourth ingredient of the body 

 of the glass ; and it serves to correct any imperfections which might arise from acci- 

 dental combustible particles, or from the lead being not duly oxidised. The above 

 four substances constitute the main articles ; to which we may add, arsenic and man- 

 ganese, introduced in very small quantities, to purify the colour and clear up the 

 transparency of the glass. The black oxide of manganese, when used in such quantity 

 only as to peroxidise the iron of the sand, simply removes the green tinge caused by 

 the protoxide of iron ; but if more manganese be added than accomplishes that 

 purpose, it will give a purple tinge to the glass. The arsenic is supposed to counter- 

 act the injury arising from excess of manganese, but is itself very apt on the other 

 hand to communicate some degree of opalescence, or at least to impair the lustre of the 

 glass. 



The raw materials of flint-glass are always mixed with about a third or a fourth 

 of their weight of broken glass of like quality ; this mixture is thrown into the pot 

 with a siovel ; and more is added whenever the preceding portions by melting subside i 



VOL. II. U U 



