GLASS-MAKING. 605 



nary heating. When the time conies, this is done very cautiously 

 in a little furnace specially constructed for the purpose. Here the 

 temperature is gradually raised to that of the melting furnace. 

 The transfer from the one to the other is accomplished as rapidly 

 as possible. The interior of the crucible is then glazed with a lit- 

 tle molten glass, and the vessel is ready to lend itself to the trans- 

 formation of the oj^aque into the transparent. After a variable 

 term of servitude, whose length is totally unpredictable, the cruci- 

 ble finally succumbs to the combined attacks of heat and chemical 

 action, and must be replaced by a fresh one. 



When gas is used as the fuel, the melting furnace is a very 

 simple affair. It consists of a plain rectangular floor or hearth, 

 which supports from eight to ten crucibles, two abreast. On each 

 side of the furnace there is a series of round openings giving ac- 

 cess to each pot. Arches at the end permit the admission of the 

 fresh crucibles and the removal of the exhausted ones. The chim- 

 ney is placed in the center, the gas being admitted at each end. 

 The air necessary for combustion is first heated by passing through 

 chambers in the base of the furnace. It will not be necessary to 

 go into any further details of construction, for if one will simply 

 imagine a white-hot apartment, perhaps forty feet long, eight feet 

 wide, and six feet high, with ten crucibles of molten glass stand- 

 ing two abreast on the floor, and half as many openings on each 

 side, he will have a sufficiently vivid picture of the melting fur- 

 nace of a glass-factory. The batch is introduced into the cruci- 

 bles in small quantities at a time, and then patiently coaxed into 

 a proper degree of fluidity. When the last portion is added, a 

 decolorizing agent goes with it, for, however pure the crude ma- 

 terials may be, there is always sufficient iron present to give the 

 glass a greenish cast. Arsenic is a favorite bleaching agent. It 

 acts by converting the iron into a higher oxide. In some estab- 

 lishments the peroxide of manganese is used for this purpose, but 

 the least excess gives the glass a pinkish color, and it is also 

 thought to make its transparency less durable. 



Style dominates even so apparently an unmodifiable thing as 

 window glass. Some years ago a slight excess of manganese was 

 employed intentionally. It was thought that the mistress of the 

 house — or her daughters — looked the prettier when seen through 

 rose-colored window-panes. This decidedly pink glass may still 

 be seen in not a few of the older houses in our Eastern cities. Its 

 use is occasionally revived by some emergency. 



This completes the chemistry of the process ; the remaining 

 operations are purely physical. 



After the contents of the crucibles have become thoroughly 

 fused, the temperature of the melting furnace is gradually re- 

 duced, so that the molten glass shall become less liquid, and thus 



