IRON ever, the heating is continued for some time after the whole of the carbon originally present has been removed, the articles become brittle, owing to the formation of oxide of iron disseminated through the mass, just as copper, bronze, and analogous substances are rendered brittle through a similar cause. This circumstance, together with the known character of the chemical actions of carbon dioxide on iron and carbon at a red heat, indicates the nature of the processes taking place during the decarbonization : the ferric oxide and the heated air in contact with it first oxidize the carbon in the outermost film to carbon dioxide ; this then passes inwards by the process of "occlusion" (gradual solution of gases in solids), and reacts upon the carbon of the next layers in accordance with the equation the carbon oxide thus formed first becoming dissolved in the iron, and subsequently when the iron is saturated therewith gradually diffusing outwards, becoming converted into carbon dioxide as soon as it comes in contact with either the ferric oxide of the packing or the partially oxidized iron of the outer film, which, when free from carbon, reacts on the carbon dioxide, thus 7/C0 2 + a-Fe = Fe^O,, + yCO. In the outermost layers, accordingly, there is always a tendency to the formation of iron oxide in virtue of this reaction, and simul taneously a tendency to the reduction of this oxide by the agency of the carbon oxide which is being formed in the interior layers anil travelling outwards ; as long as this latter action keeps the former in check, the accumulation of iron oxide in the outer layers does not take place to such an extent as to deteriorate materially the tenacity of the malleable iron skin ; but, when the carbon of the core has been so completely removed that the supply of carbon oxide from the interior almost ceases, the formation and accumulation of iron oxide in the outer layers goes on, rendering them more or less brittle. In the inner layers the removal of carbon by the penetration of the dis solved carbon dioxide and its reaction on the carbon "is continually progressing, the decarbonization gradually creeping inwards, as it were, until finally the innermost central part becomes decarbonized also. The non-removal of silicon, sulphur, and phosphorus during the process is due simply to the fact that these elements are not acted upon by the occluded carbon dioxide as the carbon is, and consequently not being oxidized cannot be eliminated. The iron oxide used becomes partially reduced during the operation ; in order to make it fit for use over again, it is moistened with a solution of sal-ammoniac and exposed to the air in order to rust and so re- oxidize it. The whole process is in effect an exact inversion of the chemical changes taking place during the manufacture of blister steel from malleable iron by the process of cementation (see 32), and differs from the ordinary puddling method for the purification of cast iron in this salient respect that in the latter case the for mation of oxide of iron by the effect of heated air, and its direct addition in the form of "fettling," give rise to the production of a lluxed mass, in which is incorporated a notably larger amount of oxide of iron, which reacts on the carbon, sulphur, silicon, and phosphorus, oxidizing them and converting them into products which are either gaseous and escape (carbon and sulphur dioxides), or are non-metallic and fusible, and hence separate from the iron as a fused slag or cinder. 23. Refining, Fining, and Puddling of Cast Iron. In order to convert large masses of pig iron into wrought iron, a large variety of methods have been and to some extent are still employed, differing from one another in certain details ; they may, however, be classified under two chief heads, viz., those in which the iron is more or less completely fluxed by heat in contact with the solid fuel used, by means of a blast of air on much the same principle as an ordinary smith s forge, and those in which the treat ment of the iron is effected in a chamber separate from that in which the fuel is burnt when solid fuel is employed, or in which gaseous fuel is used in the first instance. Charcoal Finery. Prior to the invention of puddling, the conversion of cast into wrought iron was uniformly effected by a process which, though differing markedly in certain details in different countries, yet in all cases essen tially consisted of exposure to an oxidizing atmosphere and agitation until practically all the carbon and silicon, &c., is removed. As the iron becomes purer its fusibility lessens, so that ultimately it collects into pasty semi-solid masses which when united together form a "ball," which is taken out and forged into a " bloom." Of the different kinds of finery in use some fourteen principal modifications have been enumerated by Tiinner, divisible into the three classes of " Einmalschmelzerei (single fusion process), " Wallonschmiede " (Walloon process), and " Aufbrechschmiede " or " Deutscheschmiede " (breaking up process, or German process). Of these most have been virtually out of date for years past ; a few, however, are still in use, but like the puddling process are rapidly giving way to modern soft steel or ingot iron making processes, the use of malleable weld iron being on the whole rapidly decreasing, at least relatively to that of " steel " and fused iron. A single illustration of one of these processes (termed by Tiinner the English Walloon process) will suffice : as carried out in Sweden in what is somewhat inappropriately termed the Lancashire hearth, this differs slightly from the method as used in South AValcs, the chief difference being that in the former case the pigs are melted down and the whole operation finished in the same furnace (saving that the rough blooms produced are reheated in a second furnace for further hammering), whilst in the latter the pig is melted in a separate hearth, in fact is passed through a sort of "Tunning out" fire or refinery before it reaches the finery proper ; the bed of this latter is " brasqued" or lined with charcoal powder moistened and rammed in, and so forcibly compressed. The tuyeres are directed downwards, so that the blast impinges more or less directly upon the fused metal. The effect of the blast upon the metal broken up and stirred amongst charcoal heaped over it is gradually to cause the formation of iron oxide and silica, with oxidation of the carbon and other impurities, so that finally the metal used becomes changed into two separate substances, viz., a pasty mass of spongy residual purified metal, and a bath of fluid cinder mainly composed of fer rous silicate ; the former is ultimately removed as a ball and hammered into rough slabs, and finally after reheating forged into bars, &c. ; in the production of "charcoal plates" (for tinplate making), the first rough forged slabs are cut into pieces termed " stamps," which are then reheated in a reheating furnace termed a " hollow fire " on a mass of the same kind of metal forged into a shovel shape, the blades of the shovel and the mass of stamps piled on it being then forged into a slab, which is virtually a much bigger blade ; this is doubled upon itself to ensure equality of the two sides, welded, cut oil from the shank, and rolled into oars and plates, &c. In Sweden the metal is usually forged by hammering throughout and not rolled at all. For inferior iron plates this pro cess has been used with the substitution of coke for charcoal and of less pure pig for the better qualities used for the finer plates ; but puddled iron has for the most part long superseded that made in a coke-fired finery for general purposes. The following analyses illustrate the character of Swedish irons produced in the charcoal finery : Brand Dannemora Soft Iron. Pannemora Steely Iron. Hoop L. Hoop C. L. Analyst Henry. II. S. Bell. Schafhautl. Pattinson and Stead. Iron 99-863 0-054 0-028 0-055 trace 99-471 0-352 0-050 0-027 C 025 0-075 98-78 0-84 0-12
- 0-05
99-6GO 0-220 0-052 0-016 0-008 trace 0-044 99-298 0-470 0-037 0-035 0-032 0-008 0-120 Carbon (total) Silicon Sulphur Phosphorus Copper trace 100-000 100-000 99-79 100-000 100-000 Refinery. The term " refining," although in strictness applicable to all methods by which impure iron is purified, Fro. 30. Refinery Elevation. is in practice restricted to one particular operation practised as a preliminary stage in the puddling process, viz., melting pig iron on a hearth such as that shown in figs. 30, 31 (taken
