476 



GLASS. 



employe*!, it is combined with about a fourth part by 

 iiiraMir*- of sea or river sand, the proportion will de- 

 pond on the quantity of alkali in the waste. A cheap 

 mixture for this kind of glass may be made of com- 

 mon sand and lime, with a little blue clay and sea salt. 

 The soap waste is calcined, being kept at a red heat 

 for about 30 hours, in an arch of the furnace kept for 

 that purpose. It is then withdrawn and bruised 

 along with the proper proportion of sand, and the 

 mixture being put into another arch is calcined for 

 about 12 hours. The crucibles are now filled with 

 this red hot calcined frit, which being kept at a great 

 heat for twelve or fifteen hours, glass is formed fitted 

 for being fashioned into the required forms. The 

 manipulations of tin glass blower in fashioning bottle 

 glass into various forms, are in general the same as 

 those performed by the flint glass blower, which we 

 will describe afterwards. W ine and beer bottles, which 

 are required to be all of a certain capacity, are blown 

 in moulds, so that their containing portion may be as 

 nearly as possible of the requisite size. The articles 

 being made, are carried to .the annealing furnace. 

 It is worthy of remark that green bottle glass is pre- 

 ferable to all other kinds for vessels required to con- 

 tain corrosive substances ; it is also less fusible than 

 flint glass, and therefore better calculated for many 

 chemical purposes. 



Broad glass is the coarsest description of window 

 glass. This glass is usually formed by a mixture of 

 two parts by measure of soap-boilers' waste, one of 

 kelp, and one of cleaned sand. When these materi- 

 als have been calcined for about twenty-four hours, 

 the frit is removed, while red hot, on iron shovels, 

 to the crucibles of the working furnace, where being 

 kept under the action of a great heat for twelve 

 hours, it becomes fused and fit for being blown. 

 The melted glass is taken out of the crucible upon 

 the end of an iron tube about five feet in length. 

 The end of the tube being heated, is dipped into the 

 pot, and then slightly turned upon its axis, by which 

 means a quantity of the liquid glass collects upon it. 

 The tube is then withdrawn, and should there not be 

 sufficient, it is exposed for a little to the cold air, 

 until its surface harden, when it is returned to the 

 pot, and an additional quantity of glass taken on. 

 The workman then blows in at the other end of the 

 tube, and the glass assumes the form of a globe usually 

 about a foot in diameter.* The globe is then pre- 

 sented to the mouth of the furnace ; a longitudinal 

 crack is produced by touching it with an iron bar 

 dipped in cold water ; and on the other end of the tube 

 being stopped, the expansion of the air causes the 

 globe to burst in its weakest part, when it is cut 

 open by a pair of shears ; and being laid open on a 

 flat iron table, forming a sheet of window glass, 

 which is carried to the annealing oven. 



Crown, or the best window glass, is differently 

 formed by different manufacturers. According to 

 Dr Ure, good crown glass may be made of twenty 

 parts by weight of fine purified sand, together with 

 thirty-three of the best ground kelp ; but the quality 

 will be much improved by the addition of l of slaked 



* A section of an improved tube is shown in fig. 7. It con- 

 sists of a tin plate tube, an inch and half in diameter, and 

 fifteen inches long. One end of the tube is terminated in 

 a c.ip which may be fitted on to one end of the long iron 

 tube, on whose other end the liquid glass is lifted. The in- 

 terior of the small tube is furnished with a spiral spring, 

 as chown in the figure, which spring acts upon a solid air- 

 tight piston, tending to force it to the end of the tube, but 

 prevented from coming out by a latch. If the piston be 

 pressed inwards, the air within the tube will be com- 

 pressed, and the requisite degree of expansion given to the 

 class at the other end. The great advantage of this simple 

 instrument consists in the means it affords of regulating 

 the degree of expansion given to the globe of glass, and of 

 rendering blowing wi.h the lungs unnecessary. 



lime. Very superior crown glass is made from 

 120 parts by weight of white sand, sixty purified 

 pearl ash, thirty of saltpetre, two borax, and one 

 arsenic, together with a small quantity of manganese. 

 The sand, after being thoroughly washed, is calcined 

 in a reverberatory furnace for eighteen or twenty 

 hours, when it is taken out and immediately plunged 

 into cold water. The sand is then mixed with the 

 kelp in a state of very fine powder, and the mixture 

 is carried to the fritting furnace, from whence it is 

 taken to the crucibles in the working furnace, and 

 there mixed with about an eighth part of its weight 

 of broken crown glass, called by the workmen cullet. 

 The furnace is heated to a very high temperature, 

 the crucibles being supplied with more frit as that 

 already in melts, until the pots are full. In about 

 thirty hours the whole is converted into liquid glass. 

 The furnace being now reduced in temperature, the 

 glass is allowed to cool a little for about two hours, 

 when the crude matter on the top of the crucible is 

 skimmed off. The workman then introduces the end 

 of the iron tube formerly spoken of, and collects some 

 glass, withdrawing and returning the tube twice or 

 thrice, until he has collected a sufficient quantity, 

 which for window glass is usually about ten pounds. 

 The tube is now held perpendicularly, for a few se- 

 conds, with the loaded end down, after which aregular 

 shape is given to the glass, by rolling it on a smooth 

 iron table, and the operator, by blowing into the tube, 

 causes the glass to swell into a pear shaped hollow 

 vessel. It is again heated by being held a little at 

 the mouth of the furnace, and blown still larger, the 

 end being pressed against an iron bar, which process 

 being repeated several times, the glass assumes a 

 spherical form. This globe is again heated, and the 

 back opposite the end of the rod pressed against a 

 flat surface, and made to assume the form of an ob- 

 late spheroid. It is then held over a box, the tube 

 being in an horizontal direction, and an iron rod, cal- 

 led the punty, the end of which having been previ- 

 ously dipped in melted glass, is applied to the centre 

 of the flat side opposite the tube. The punty is then 

 detached by the application of a piece of cold iron, 

 leaving a hole in the spheroid of about two inches in 

 diameter. The glass is now held to the mouth of the 

 furnace, and the tube turned on its axis, slowly at 

 first, but gradually increasing as the glass expands, 

 until it at last assumes the form of a thin circular 

 plate of from three to four feet diameter, and uniform 

 in thickness, with the exception of a thick part in 

 the centre where the tube is attached, called the bull's 

 eye. The tube is detached by the touch of a piece 

 of wetted iron, and a slight stroke, the plate being 

 laid on ashes ; it is then conveyed to the anneal- 

 ing furnace. The crown glass house is shown in 

 fig. 1. 



Flint glass, or crystal. Powdered flint was for- 

 merly employed in the manufacture of this species of 

 glass ; but from its great expense, fine white sand 

 has for many years been substituted. Dr Ure states 

 it to be commonly composed of purified linn sand 

 109 parts, of litharge, or, what is better, red lead, 

 sixty parts, and of purified pearl salt, thirty parts. In 

 order to correct the green colour given by the oxide of 

 iron which the sand contains, a small quantity of black 

 oxide of manganese, with a little arsenic, is added ; 

 and sometimes a little nitre is employed in thecomposi- 

 tion. The sand is thoroughly washed, then calcined, 

 and afterwards passed through a fine sieve of forty or 

 fifty passes in the inch. The materials of which the 

 glass is to be formed are now intimately mixed and 

 pounded, and without being fritted, they are at once 

 carried to the crucibles of the working furnace, the 

 crucibles having been previously brought to a white 

 heat ; and as the mixture fuses, more of the ingredi- 



