IRON. 



The furnace in which the procefs of cementing bar iron is 

 performed is called a converting furnace. 



The pots are made of a peculi?r ftone called fire-ftone, 

 from its joint property of not being liable to crack by the 

 heat, and its little diipofition to enter into fulion. It is a 

 fine grit, and occurs abundantly in the neighbourhood of 

 Sheffield. The interior of thefe pots is in dimenCons from 

 12 to 1) feet long, and from 2 feet to 30 inches fquare. 

 Every furnace contains two of them ; and they generally 

 hold about 5 tons of iron each. The metal is [Iratitied with 

 the charcoal dud, in fucli a way that each bar may be com- 

 pletely covered; and the laft ftratum, which fliould be 

 thicker than the reft, is kept clofe with a mixture of clay 

 and fand, fo as to prevent the charcoal beneath it from en- 

 tering into combuttion with the outer air. 



The fire is then gradually applied, and the exterior fur- 

 face of the pots conftantly enveloped with flame, till the 

 whole mafs has become of the heat required. This heat is 

 kept up for a confiderable time, fo that from the commence- 

 ment of firing to the maximum is about feven days. The 

 fame fpace is afterwards required to allow the mafs to cool. 

 This rule, however, is not fufficient to tell when the ce- 

 mentation is pcrfed. A hole is generally left in the front 

 of the furnace, paffing through the wall to the interior of 

 the pot. One or two bars are laid with their ends pro- 

 jefting into the opening, which is loofely filled with pow- 

 dered charcoal. When the procefs is fuppofed to have 

 gone on long enough, one of thefe bars is drawn out and 

 examined. 



All bar iron miift, owing to the manner in which it is 

 made from the pig. contain a fmall portion of oxyd of iron, 

 as well as fome remains of iron not completely deprived of 

 its carbon. The long continued heat which this procefs re- 

 quires, cannot fail to caufe an union between the oxygen 

 and carbon, accidentally cxifting in the bars ; and we may 

 expect, in confequence, that an elaltic fluid will be formed, 

 which muil be either carbonic acid or the carbonic oxyd ; 

 but fmce the oxyd is in the ftate of finery cinder, we fliould 

 ratlicr conceive it to be the latter ; and we believe experience 

 warrants the conchifion. 



This elaftic fluid, when the metal is fo near the fufing 

 point, caufes the furface of the bars to be covered with 

 bUfters, which are a fure fign that the cementation is com- 

 plete : and it is from the number and fize of thefe bhflers, 

 that the v\orkmen know when to ceafe adding fuel to the 

 furnace. 



Steel is made of different degrees of hardnefs, by giving 

 it more or lefs carbon ; and this is effected by keepmg up 

 the heat a longer time, ftill having regard to the quantity 

 and fize of the blifters. 



The flieel ufed for coach-fprings contains the fmalleft 

 quantity of carbon ; a fomewliat greater proportion is re- 

 quired for table knives, fork.-;, carpenters' tools, and agri- 

 cultural implements ; and the largcft dofe of ;;11 is wanted 

 for tiles, which can fcarcely be too hard, if the Heel be 

 fufTiciently malleable to work. 



Steel, in the Rate it comes from the cementing furnace, is 

 called hlijfereil Jieel, from the appearance we have juft de- 

 fcribed. It is not ufed but for common purpofes, although 

 formerly we had no other kind. The bars of iron being 

 formed under the forge hammer, and produced in a country 

 not remarkable for the excelleuce of its machinery, the 

 metal is found to abound with numerous feams and Ihells, 

 which good workmanfliip might in a great meafurc avoid. 



This evil is, however, very happily remedied, by making 



the bliftered-fteel into what is called f:iar-Jlcel, and cajl- 

 Jleel. 



Shear-fteel has derived its name from the advantage with 

 which it has been appLed in the manufaclure of flicep-lhears. 

 Its admirable property of welding to iron, without the other 

 quahtiesit poffeffes being injured, renders itof great importance 

 in all cafes where the body of the edge-tool is conftruded of 

 iron, and the edge m.ereiy fteel. 



From having been firil made at Newcaille-upon-Tyne, it 

 has alfo been called Kcxcajlle Jleel. The apparatus ufed for 

 its manufadure confilis of a pair of bellows, or other blow- 

 in^r machine, and a fire place fin.ilar to that of a fmith's 

 hearth, but upon a larger fcale. The hammer for drawing 

 the bars is larger than a tilting hammer, but fmaller tlian the 

 one employed at an iron forge. 



In order to make fliear-fteel, a number of bars of the bed 

 bliftered-fteel are laid together, and temporally faftened. In 

 this ftate they are introduced inlo the fire, and heated to a 

 welding temperature ; after v. bich they are firmly united by- 

 means of the hammer, and drawn. At a fecoud heat thefe 

 raaffes arc beaten down into bars about i^ inch broad 

 and iths of an inch thick. By this procefs the loofe parts 

 and feam.s of the bars are clofed together, and the fteel is 

 rendered fufceptible of a pohfli, of which before it was not 

 capnble. But thefe are not the only advantages. The ad- 

 ditional hammering fo far improves its mallcabihty, that the 

 tenacity it enjoys is much greater, while its hardnefs is very 

 httl- lefs. 



This fteel is particular'y adapted for fprings of every de. 

 fcription,'ahd for all edge-tools requiring great tenacity ra- 

 ther than hardnefs. 



Caft-fteel is entirely free from the mechanical defefts which 

 belong to bliftered-ileel, a:id even, in fome degree, to fliear- 

 fteel ; fince it confifts of the latter variety completely fufed, 

 and caft into ingots. 



The furnaces employed for this purpofe fliould poffefa all 

 the advantages which can be given to air-furnace.^, on account 

 of the great heat requifite for the procefs. (See FuRXACE.) 

 The crucibles, in which the fteel is melted, are made of 

 Stourbridge clay, mixed with a fmall quantity of powdered 

 coke, which makes them lefs liable to crack in the heating 

 or cooling ; and, at the fame time, gives to them a confider- 

 able degree of ftiffnefs in the fire, when raifed to the very 

 great heat required. Thefe crucibles are fumiftied with 

 covers, which are of rather more fufible clay than the body 

 of the veffel, and, on that account, are foon partially vitri- 

 fied ; by which means they become clofely luted at the time 

 the fteel is at a temperature fuificiently high to be deftroyed 

 by the oxygen of the atmofphere. 



The fuel employed for melting fteel, confifts of the hardeft 

 cokes, and in all cafes where long continued and high tem- 

 peratures are neceffary, cokes of this defcription fliould aU 

 ways be employed. Two advantages attend the ufe of ihem ; 

 for although the foft cokes produce a very great heat during 

 a fiiort fpace, yet the length of time required for melting 

 fteel would oblige the workman to charge the furnace too 

 often ; fo that he would ultimately get a greater heat by^the 

 permanence of the hard coke ; and the fpecific gravity of 

 the latter is fo much greater than the foft, that the fuel is 

 more condenfed, and hence, allowing for the difference of 

 cohefion, will give more concentrated heat, if the fupply of 

 air be fufficient. 



The crucibles are of a fi^e fufficient to hold about ^olbs. 

 of fteel ; and, in general, each will bear charging three 



times, or even foil 



ome woui( 



found ftiU 1 



onger 



than that, if the fires were continued ; but they are feldom 



kept 



