216 On the Principles Involved in the 



fratz (Siderotechnie i, p. 57,) supposes the color of these irons to be 

 owing to a greater or less separation of the carburet of iron during 

 the cooling. He supposes the particles of gray iron, and also those 

 of the dark parts of the mottled irons, to be enveloped by a coating of 

 plumbago. On cooling the gray and mottled irons rapidly, a white 

 iron is produced, and by slow cooling, plumbago in plates, or kish, as 

 it is called by the workmen, collects upon its surface. Irons that by 

 refining, would make hotshort, or coldshort malleable irons, are best 

 employed in common castings. 



Gray irons requiring much labor to refine them, if they make 

 good iron, are well adapted for making steel, or when the iron is very 

 tough, it is employed in the casting of cannon and machinery. White 

 iron is rarely used for castings, except for anvils and heavy hammers, 

 but it is much used for refining to make bar iron. 



4. Ductile Iron. 



Of these, there are four principal varieties, viz. soft iron, coldshort, 

 hotshort, and brittle iron. 



The soft iron is ductile, malleable under the hammer, capable of 



being bent back and forth without breaking, and without being elastic, 

 is difficult to melt, does not acquire hardness by tempering, is of a 

 clear gray color when filed, leaves no black spot when an acid is 

 touched to a bright surface, rusts slowly and uniformly by exposure 

 to air and water, burns easily when exposed in a heated state to the 

 air, becomes strongly magnetic under the influence of a magnet, and 

 loses its magnetism when removed from magnetic influence. Its 

 structure is fine grained or fibrous. 



Coldshort iron is distinguished by its brittleness, when cold. Its 

 fracture developes a granular and laminated structure, and the iron 

 is more defective, as this structure becomes more evident. It forges 

 very easily when hot, and for this reason is much employed in forg- 

 ing small articles. 



Hotshort irons are those that crack, break, or fly in pieces when 

 forged hot. There are two kinds of defects in these irons. 



1. Some of these irons fly in pieces when forged at a particular 

 temperature, but forge well enough at a higher or lower heat. 



2. Some of them break in bending, but otherwise forge well. 

 Little hotshort iron is found in commerce, because it is difficult to 



get it into the form of bars, and the cracks and flaws upon the edges 

 would betray it. This iron is applicable to such purposes as require 

 great strength, when it can be wrought cold into the forms required* 



