GLASS 



2.-.OS 



GLASS 



present over one-half of one per cent of iron 

 in the sand the latter cannot be used for the 

 manufacture of colorless glass. In the seven- 

 teenth century ground flint was used in the 

 best glass, because it was purer than any sand 

 then available. Hence the name flint glass, 

 which, curiously enough, is to-day applied to 

 a variety of glass that is extraordinarily soft, 

 and in the manufacture of which no flint is 

 used. The mixing of the ingredients into what 

 the glassmaker calls the "batch" is a process 

 that often requires the services of an expert 

 chemist, and in the best modern factories it is 

 customary to prepare the batch according to 

 special chemical formulas. 



The melting furnace may be either of two 

 general types the pot furnace and the tank 

 furnace. In the former the ingredients are 

 melted in huge pots made of fire clay, arranged 

 in a circle around a central fire, at the base 

 of a huge chimney. As these vessels are very 

 difficult to make and of uncertain durability, 

 the tank furnace, heated by gas, has come 

 into general use in the glass-making industry. 

 This type of furnace is provided with a tank 

 in which the ingredients are melted and from 

 which the molten mass is drawn. In the 

 largest factories these furnaces are seventy-five 

 feet long, sixteen feet wide and five feet deep. 

 They are usually worked without interruption, 

 new material being fed into them at one end 

 as the supply of melted glass is drawn out at 

 the opposite end. 



Window Glass. Window glass was formerly 

 made entirely by hand labor, but in recent 

 years machines for various steps in the process 

 have been introduced. When performed by 

 workmen the process is essentially as follows: 

 For each part of the operation there is one 

 man especially trained for the work. The first 

 step is the gathering. The gatherer dips a 

 long iron blowpipe into the white-hot glass, 

 and skilfully forms on the end a mass of the 

 substance, weighing from twenty to forty 

 pounds. This ball of glass he turns in an 

 iron mold until it assumes the shape of a pear, 

 and then passes it to the blower, who is the 

 master workman of the factory. 



The blower, by a process of blowing, swing- 

 ing and revolving the mass, shapes it into the 

 form of a cylinder, sometimes as many inches 

 long as himself. When it has cooled some- 

 what he holds the end of the cylinder in the 

 furnace, blows into the blowpipe, and then 

 covers the mouthpiece with his thumb. The 

 air, thus imprisoned, and expanding with the 



heat, splits an opening in the end of the 

 cylinder, which the blower enlarges by revolv- 

 ing the end swiftly in the furnace. When the 

 hole is as large as the diameter of the cylinder, 

 and the mass is cooled to cherry-red heat, an 

 assistant detaches the glass from the blowpipe, 

 and the cylinder is cracked lengthwise with a 

 red-hot iron or a diamond on a long handle. 

 The opened cylinder, with the split side up, 

 is next placed on a fire-clay table which re- 

 volves in an oven. The heat soon flattens the 

 cylinder into an irregular surface, which the 

 next workman, the flattener, smooths out with 

 a tool called the polissoir (French for polisher). 

 This is an iron rod, to each end of which a 

 block of wood is fastened. The smoothed 

 sheet is next placed in the coolest part of the 

 furnace and is then removed to the cooling 

 stone. When rigid enough to be moved it is 

 carried to the annealing chamber. 



Annealing is for the purpose of preventing 

 breakage when glass is subjected to changes of 

 temperature, and all articles made from glass 

 undergo this process. This breaking is due to 

 the fact that one surface expands or otherwise 

 dilates before the other has had time to be 

 affected. The objects are slowly heated until 

 the melting point approaches, and are then 

 gradually cooled, the process taking place in a 

 chamber having various compartments of dif- 

 ferent degrees of temperature. The principle 

 is that the glass, which has been distended by 

 heat, contracts evenly throughout as it cools, 

 and the molecules have time to arrange them- 

 selves in a stable position. The glass therefore 

 is made "tough." 



Plate Glass, which is the most expensive 

 form of window glass, is made by a special 

 process. The ingredients are melted in huge, 

 open vessels, or tanks, some of which have a 

 capacity of two and one-half tons. These ves- 

 sels rest upon frames behind fire-clay doors. 

 When the melting has reached the required 

 condition the tank is drawn out by a great 

 fork mounted on a truck, and is rolled to the 

 casting table. There it is hoisted by a crane, 

 and the contents are poured over the metal 

 bed. The molten glass is spread out uniformly 

 by means of a heavy roller, and is then placed 

 in the annealing chamber, where it remains 

 for several days. It comes out in the form 

 of rough plate, and must be polished before 

 it is ready for service. Polishing is done by 

 means of grinding machines which rub the 

 surface with sand, emery and rouge, first on 

 one side and then on the other. In this process 



