MECHANICS AND USEFUL ARTS. 39 



fibrous iron, were taken up. They had served as tie bars to keep the 

 retaining walls from spreading. Screwed up tight, they had been 

 under ground about twenty-five years, embedded in clay. The out- 

 side rust, firmly combined with clay and sand, appeared to have 

 formed a protective coat. At any rate, the strength of the iron had 

 not suffered at all from oxidation ; its quality was as good as any pud- 

 dled bar manufactured at the present day. 



Last year, while removing the old St. Clair Street Bridge over the 

 Alleghany River at Pittsburgh, to make room for a new suspension 

 bridge, since completed, I examined the old iron with considerable 

 interest and care. All this iron had been manufactured about forty- 

 one years ago, and had been the result of the first attempt at pud- 

 dling ever made west of the Alleghany Mountains. The manufac- 

 turer, who is still living, informed me that in those days puddling was 

 not well understood, and that, although the stock was good cold- 

 blast charcoal pig, the iron turned out a highly crystalline texture. 

 It proved so on its fracture, but of a good color ; the texture was 

 uniform and not coarse. On being heated and drawn down to 

 half its size, it made a strong fibrous iron ; all it wanted was work. 

 There was not one fibrous bar in the whole lot of suspension bars ; 

 they were all alike crystalline and brittle in texture. This iron 

 had, from the manufacturer's own testimony, undergone no change ; 

 it was as crystalline on the last day as it was on the first. But there 

 was another quality of iron in the same structure. The straps and 

 bolts which connected the chords with the posts and braces had 

 been manufactured of a good quality of hammered charcoal iron, 

 and a most capital iron it proved, after forty years' service. 



All irons form alloys of pure iron, mixed with carbon and other 

 impurities. A certain amount of impurities in the shape of good 

 cinder appears to be necessary to impart strength and cohesion to 

 this metal, and also to make it malleable, and to give it welding 

 properties. The purer the iron is, the higher the heat at which it 

 will weld. Compare, for instance, good Swedish iron with common 

 puddled bar. While the latter will weld at a low heat, the former 

 requires a much higher heat. Compare their fracture and color. 

 The good Swedish bar will exhibit either a fine granular appearance 

 or fibre, accompanied by a silvery lustre, showing comparative purity ; 

 the puddled bar will be of a dark color, with a graphite lustre, and 

 will show a coarse texture or loose fibre. 



During the process of puddling, as well as of blooming, the melted 

 pig-iron is mixed with cinder, and this mixture, which will adhere 

 by cohesion, prevents the formation of large crystals, which is the 

 tendency of pure iron in a molten state. Now by working (bring- 

 ing to nature, as the puddler calls it), this mixing and crystalliza- 

 tion is promoted. The subsequent squeezing and rolling of the pud- 

 dled ball, or the hammering and shingling of the bloom, will have the 

 effect of condensing, laminating, reducing, and drawing out these 

 crystals, at the same time removing and squeezing out the superabun- 

 dant cinder from between the metallic crystals. Thus the drawn-out 

 fibre is composed of an aggregate of pure iron threads and leaves, 

 enveloped in cinder. 



Pure iron, as well as very impure iron, is weak ; the maximum 



