240 



GLASS 



and a cross section (fig. 8) the latter showing the 

 brick regenerators of that form of it called the 

 continuous tank furnace in which no pots are used 

 a form which is now largely employed in making 

 bottle-glass, rolled plate, and sheet-glass. For 

 most kinds of glass pots are still used in the 



Fig. 7. 



Siemens as well as in the older kinds of furnaces ; 

 where pots are preferred, the chief difference is 

 that a flat platform is prepared for their reception, 

 instead of the bed of the furnace being in the 

 shape of a tank or cistern. 



The figures are to some extent rather diagrams 

 than exact representations of the 

 furnace, since, otherwise, more wood- 

 cuts than we can find room for 

 would be required to explain it. In 

 the plan (fig. 7), RM is the com- 

 partment into which the raw 

 materials are fed by the doors, D. 

 When the glass is partially melted, 

 it passes under the first floating 

 bridge of fireclay, B, which keeps 

 back floating impurities. In com- 

 partment P the glass is completely 

 melted, and it then passes in a pure 

 state under the second floating 

 bridge, B', into the compartment 

 W, where it is ready for use; h, k, 

 h being the working holes. The 

 space under RM in fig. 8 is an air- 

 flue for the purpose of keeping the 

 tank cool. In the section (fig. 8), 

 A and G represent the air and gas 

 regenerators on the left, and A' and 

 G' the corresponding ones on the 

 right. The gas-producers are not 

 shown, but, as explained under IRON, 

 the air and gas are fed for a certain 

 time through A and G respectively to the bed of 

 the furnace ; and, while this is the case, the pro- 

 ducts of combustion descend through A' and G' on 

 the right, by which the piles of open brickwork 



become in time highly heated. By a proper 

 arrangement of flues and valves, this process is 

 then reversed, so that the gas and air now enter 

 the furnace on the right, robbing in their course 

 the hot bricks in A and G of their heat, and carry- 

 ing it back to the bed of the furnace. This time, 

 of course, the products of combustion escape through 

 A and G on the left, by which these regenerators 

 become in turn heated, thus saving heat which is 

 lost by escaping up the chimney in ordinary fur- 

 naces. In a subsequent specification Messrs Sie- 

 mens replaced the fixed partitions by bars or girders 

 of fireclay or other refractory material, which float 

 transversely on the surface of the molten matter, 

 the upper stratum of which they divide into com- 

 partments. The partially melted material is thus 

 kept at the supply end of the tank, and only the 

 more thoroughly melted and purer matter is per- 

 mitted to flow towards the working end under 

 these floating bridges. More recently partitions, 

 whether fixed or floating, have been for the most 

 part dispensed with, and the tank forms one huge, 

 long basin. Floating vessels made of pot-clay, 

 divided into three compartments, or two compart- 

 ments and a floating ring, do the work of separating 

 the refined from the cruder matter. 



Bottle-glass. The tank furnace, without bridges, 

 to which reference has just been made, is admirably 

 adapted for the manufacture of bottle-glass, and 

 has superseded the system of melting in pots. In 

 the composition of this glass a great variety of 

 materials is admissible in conjunction with sand, 

 which forms the basis of this as well as of all other 

 kinds of glass. The residual alkaline and calcic 

 salts from gas, soap, and alkali works, sulphate of 

 soda, clay, common salt, chalk, basalt, and other 

 rocks containing felspar, and lastly the slag from 

 iron blast-furnaces are the materials chiefly in use. 

 When the glass is properly melted and skimmed, a 

 workman dips a long iron tube called a blowpipe 

 into a pot or tank and takes up (on repeating the 

 operation) a 'gathering,' or sufficient metal to 

 make a bottle (a, fig. 9). Another workman 

 brings this into a pear-shape (b, fig. 9) by slightly 

 blowing and turning it on a stone or iron table, 

 called a marver. Formerly the further manipula- 

 tion of the bottle was done by hand, but moulds 

 are now used. These are usually of cast-iron or 



Fig. 8. Cross Section on YY, fig. 7. 



brass, or sometimes of clay, and open or close by 

 the pressure of the foot on a spring. Into such a 

 mould the partially-distended glass is inserted, and 

 made to fill it by blowing down the tube, the 



