928 PRINCIPLES OF CHEMISTRY 



poses contains impurities. Chemically pure iron may he obtained by 

 precipitating iron from a solution (a mixture of ferrous sulphate with 



of cast iron, pressing the oxides into the molten iron. This resembles kneading dough, 

 and the process introduced in England became known as puddling. It ia evident that 

 the puddled mass, or bloom, is a heterogeneous substance obtained by mixing, and 

 hence one part of the mass will still be rich in carbon, another will be poor, some parts 

 will contain oxide not reduced, &c. The further treatment of the puddled mass consists 

 in hammering and drawing it out into flat pieces, which on being hammered become 

 more homogeneous, and when several pieces are welded together and again hammered 

 out a still more homogeneous mass is obtained. The quality of the steel and iron thug 

 formed depends principally on their uniformity. The want of uniformity depends on 

 the oxides remaining inside the mass, and on the variable distribution of the carbon 

 throughout the mass. In order to obtain a more homogeneous metal for manufac- 

 turing articles out of steel, it is drawn into thin rods, which are tied together m 

 bundles and then again hammered out. As an example of what may be attained in this 

 direction, imitation Damascus steel may be cited ; it consists of twisted and plaited 

 wire, which is then hammered into a dense mass. (Real damascened wootz steel 

 inay be made by melting a mixture of the best iron with graphite (fa) and iron rust; 

 the article is then corroded with acid, and the carbon remains in the form of a pattern.) 

 Steel and wrought iron are manufactured from cast iron by puddling. They are, how- 

 ever, obtained not only by this method but also by the bloomery process, which is carried 

 out in a fire similar to a blacksmith's forge, fed with charcoal and provided with a blast j 

 a pig of cast iron is gradually pushed into the fire, and portions of it melt and fall to the 

 bottom of the hearth, coming into contact with an air blast, and are thus oxidised. Tho 

 bloom thus formed is then squeezed and hammered. It is evident that this process is 

 only available when the charcoal used in the fire does not contain any foreign matter 

 which might injure t"he quality of the iron or steel for instance, sulphur or phosphorus 

 and therefore only wood charcoal may be used with impunity, from which it follows 

 that this process can only be carried on where the manufacture of iron can be conducted 

 With this fuel. Coal and coke contain the above-mentioned impurities, and would 

 therefore produce iron of a brittle nature, and thus it would be necessary to have 

 recourse to puddling, where the fuel is burnt on a special hearth, separate from the 

 cast iron, whereby the impurities of the fuel do not come into contact with it. The 

 manufacture of steel from cast iron may also be conducted m fires ; but, in addition to 

 this, it is also now prepared by many other methods. One of the long-known processes 

 is called cementation, by which steel is prepared from wrought iron but not from cast 

 iron. For this process strips of iron are heated red-hot for a considerable time whilst 

 immersed, in powdered charcoal ; during this operation the iron at the surface combines 

 with the charcoal, which however does not penetrate ; after this the iron strips are 

 re- forged, drawn out again, and cemented anew, repeating this process until a steel of the 

 desired quality is formed that is, containing the requisite proportion of carbon. The 

 JBessemer process occupies the front rank among the newer methods (since 1856) ; it 

 is so called from the name of its inventor. This process consists in running melted 

 cast iron into converters (holding about 6 tons of cast iron) that is, egg-shaped 

 receivers, fig. 94, capable of revolving on trunnions (in order to charge in the 

 cast iron and discharge the steel), and forcing a stream of air through small apertures 

 at a considerable pressure. Combustion of the iron and carbon at an elevated tempera- 

 ture then takes place, resulting from the bubbles of oxygen thus penetrating the mass 

 of the cast iron. The carbon, however, burns to a greater extent than the iron, and 

 therefore a mass is obtained which is much poorer in carbon than cast iron. As the 

 combustion proceeds very rapidly in the mass of metal, the temperature rises to such an 

 extent that even the wrought iron which may be formed remains in a molten condition, 

 whilst the steel, being more fusible than the wrought iron, remains very liquid. In 

 half an hour the mass is ready. The purest possible cast iron is used in the Bessemer 



