IRON. 



547 



been calcined, to expel carbonic acid and water about 40 per 

 cent.* 



SMELTING CLAY IRON-STONE. 



The blast furnace, in which 

 the ore is reduced, is of the 

 form represented in the 

 margin, 55 to 60 feet in 

 height, with an interior dia- 

 meter of from 14 to 17 

 feet at the widest part. The 

 cavity of the furnace is 

 entirely filled with fuel, and 

 the other materials, which 

 are continuously supplied 

 from an opening near the top ; 

 and the combustion main- 

 tained by air thrown in at 

 two or more openings, 

 called twyeres near the bot- 

 tom, under a pressure of 

 about 6 inches of mercury, 

 from a blowing apparatus, 

 so as to maintain the whole 

 contents of the furnace in a 



state of intense ignition. When the air to support the combus- 

 tion has attained a temperature of 600 or 700, by passing 

 through heated iron tubes, before it is thrown into the furnace, 

 raw coal may be used as the fuel ; but with cold air, the coal 

 must be previously charred to expel its volatile matter, and con- 

 verted into coke, otherwise the heat produced by its combus- 

 tion is insufficient. With the ore and fuel a third substance is 

 added, generally limestone, the object of which is to form a 

 fusible compound with the earthy matter of the ore ; it is, there- 

 fore, called ^ flux. Two liquid products accumulate at the bot- 

 tom of the furnace, namely a glass composed of the flux in 

 combination with the earthy impurities of the ore, which when 



* Accurate analyses of several Scotch varieties, of tins ore have been pub- 

 lished byDr. H. Colquhoun. Brewstcr's Journal, vii, 234 ; or Dr. Thom- 

 son's Outlines of Mineralogy and Geology, i, 446 ; and of the French ores 

 by M. Berthier, in his Traite des Essais par la Voie Seche, ii, 252, a work 

 which is quite invaluable for the metallurgic student. 



