IRON ORE?. 255 



ned to the top of the furnace, often by a railway, and thrown in at inter- 

 vals of an half hour or so, as the coal sinks, so that the furnace is kept 

 full. The charge at the top of the furnace is two days or more in de- 

 scending to where it comes within the direct action of the blast. The 

 fusion of the ore finally takes place a short distance above the twiers, and 

 its reduction is completed at the same time by the burning coal and flux 

 in a few hours the hearth fills with metal and slag, and as it accumulates, 

 the fused iron displaces the slag which is continually running over and 

 conveyed off by the workmen : the metal being let out below by remov- 

 ing a luting of clay, is run into moulds of sand, to form pigs — oblong 

 masses of about 180 pounds each. The slag in this process serves to 

 protect the metal from combustion as it is reduced. Its color and condi- 

 tion indicate the success of the reduction. If of a dark color and heavy, 

 it shows that all the ore is not reduced, and much metal lost; probably 

 owing to too little coal or too rapid working. If dark vitreous, with 

 streaks of green, there is some oxyd of iron carried off by the silica, 

 which may probably be remedied by adding more lime to take up the 

 silica. If light colored, all is going on well.* 



The proportion of fluxes depends on the ore and its condition, and 

 no general rule can be given. With the argillaceous carbonate of iron 

 of Staffordshire, limestone alone is used, 10 to 12 per cent, being em- 

 ploye d for 45 per cent, of ore, and 45 of coke. Even this addition is 

 unnecessary when the ore is associated with much lime. For the ordi- 

 nary argillaceous ores, the weight of limestone used is about one-fourth 

 the weight of the ore, or from one-third to one-sixth. When there is 

 no silica in the ore, it is added in nearly equal proportions with the 

 lime and other earthy ingredients present. Previous assays must de- 

 termine what is required for each variety of ore. The brown hematite 

 is easily reduced, and requires much coal with a slow process, or only 

 a white iron is produced ; 8 to 12 per cent, of limestone is added to a 

 charge as a flux. 



Good metal is strong of a dark gray color, with a granular texture, 

 and runs fluid when melted ; while the bad metal is light colored and 

 brittle, and runs thick and sluggish. There are numbers 1, 2, 3, 4, in 

 market, including the two kinds just described and two intermediate 

 grades. Number 1 is best fitted for castings, as it 'contains the most 

 carbon and is more fusible than the others. Cast iron sometimes con- 

 tains a trace of silicium without injury, and according to Berzelius, the 

 best Swedish iron contains after it is made into wrought iron 1-20 per 

 cent, of silicium. Sulphur and • phosphorus .are highly deleterious, ex- 

 cept when a fusible metal is desired with the strength comparatively 

 unessential. 



Wrought or malleable iron. As cast iron owes its fusibility princi- 

 pally to the carbon present, the change of cast to wrought iron, called 



W'hat is said of the slag 1 Oi. what does the proportion of fluxes 

 epend ? 



* The slag from Merthyr Tydvil, in South Wales, afforded Berthier on 

 nalysis, silica 404, lime 384, magnesia 52^ alumina 112, protoxyd 

 of iron 3"8, and a trace of sulphur. 



