252 METHODS OF GLASS MANUFACTUKHSTG. 



it is then screened, mixed with a certain proportion of 

 broken glass, and is then ready to be melted. 



It is impossible to state what were the exact methods 

 employed by the ancients in melting the batch. The 

 first descriptions of the art and the plans of furnaces 

 and methods of working lead us to think that the 

 general style of pot furnaces now in use is only a slight 

 modification of those used many hundred years ago. 

 These furnaces are usually built in the form of a 

 truncated cone, from 40 to 80 ft, in height and 25 to 50 ft. 

 diam. at the base. In the center of the area is situated the 

 melting furnace, which may contain, according to its 

 sizes, from five to fifteen pots or crucibles, in which the 

 materials are melted, These pots are supported on benches 

 on each side of the fire pot. The grate of the furnace is 

 nearly on a level with the floor of the glass house, and 

 the ash pit or cave is built under ground and extends 

 from one side of the building to the other, opening on 

 the outside, so that a draft may be secured from as 

 many quarters as possible. Slack coal is usually em- 

 ployed as the fuel. On each side of the pots or crucibles 

 there is a flue, which connects the inside of the furnace 

 with the stack. The arch or dome is built as low as 

 possible, consistent with strength and durability. 

 Owing to the location of the flues, the pots are nearly 

 surrounded by the flame, thus heating the charges con- 

 tained. Between the flues is an opening called the 

 working-hole, which is used for the introduction of the 

 batch into the pots, also for working out the glass when 

 it is prepared for use. The operation of working out 

 the glass in this style of furnace is carried on in the day 

 time, while the batch is melted at night. Up to 1860 

 this style of furnace was in general use in this industry. 

 In T860 glass making made rapid advancement, when the 

 Siemens regenerative principal was applied to this in- 

 dustry. The source of heat is not by burning coal 



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