152 BULLETIN 42, UNITED STATES NATIONAL MUSEUM. 



just below the top are openings for drawing off the hot gases, which 

 are used for varions purposes about the furnace. 



The general arrangement and construction of a typical blast fur- 

 nace is shown in pl. xvii. Pls. xviii and xix show two principal 

 views of a charcoal furnace. The stock floor, or where the material 

 is assembled preparatory to charging into the furnace, is shown in PL. 

 xvui, and the bottom of the furnace where the pig-iron and slag are 

 tapped out, just after a cast has been made and while the iron is 

 still very hot, in pl. xix. Pls. xx, xxi, and xxii show typical 

 scenes about a coke furnace. The top of the furnace where the materi- 

 als are charged is shown in pl. xx ; the hot-blast stoves in pl. xxi, 

 and the general arrangements and surroundings of a furnace in pl. 



XXII. 



As the charge descends gradually in the furnace it is first thoroughly 

 dried, then any volatile constituents are driven oft; and finally the iron 

 is retluced to metal from the ore. This passes on through the furnace 

 and absorbs carbon, and when it comes to the zone of fusion near the 

 tuyeres the whole charge melts. The iron, by its greater specific grav- 

 ity, settles to the bottom, while the slag floats on top, and both are 

 periodically tapped from the furnace. 



The reduction of the iron is accompanied by the reduction of other 

 constituents of the ore, and the product always contains, besides car- 

 bon, which is both combined with the iron and separate as graphite, 

 silicon, manganese, phosphorus, and sulphur. The amount of silicon 

 depends upon the manner of running the furnace; the amount of man- 

 ganese and sulphur dei>ends partly on this and partly on the amounts 

 hi the charge, while all the phosphorus m the charge will be found -in 

 the pig metal. Occasionally other things occur in small amounts in the 



metal. 



A few ores contain Just the right amount and kind of earthy matter 

 to furnish sufficient slag to cover the melted pig-iron in tlie bottom of 

 the furnace and protect it from the direct action of the blast, which 

 would otherwise oxidize it again and thus defeat the object of the 

 smelting. It is more usual, however, for the ores to contain an excess 

 of acid constituents which would form a corrosive slag that would soon 

 destroy the lining of the furnace. It is therefore usual to add a basic 

 flux, which is generally limestone. These ores form more slag than is 

 required to cover the iron. 



Charcoal, anthracite coal, coke, and raw bituminous coal are used as 

 fuel, and sometimes two or more of these are used together. 



CROWN POINT FURNACE. 



These works consist of two blast furnaces, 70 feet by 18 feet and GO feet 

 bylOi feet, built on the banks of Lake Champlain, at Crown Point, Essex 

 Connty, New York. The ores used are from Crown Point Mine, near by, 

 and from the Chateaugay Mine. The product is Bessemer pigirou, 



