TILTING OF STEEL. 



and being thus rendered of lefs fpecific gravity than the ex- 

 ternal air, it rifes up in the cone, and a frefh fupply rufhes 

 in through the bars of the grate, to rcflore the equilibrium. 

 By going through the iirc, this air parts with its oxygen, 

 and excites the combuftion, and becoming heated, rifes up 

 the chimney, and caufes a very ftrong draught of air to 

 enter the fire. 



At firft kindling, the fuel is fupplied in fmall quantities, 

 that the heat in the furnace may be gradually increafed, and 

 not endanger the cracking of the ftoncs : in a few hours time 

 the quantity of fuel is increafed, fo as to produce the fuU 

 heat, which is to be maintained as equally as poflible through- 

 out the whole procefs. The fuel, which is pit-coal, is intro- 

 duced at both ends of the grate, through fmall arches in the 

 wall, which are in a line and on a level with the fire-grate, 

 a quantity of coals being always left before the end of the 

 arch to ftop it up, and prevent any air getting into the 

 furnace, vrithout palling through the grate. Part of thefe 

 coals is forced into the furnace, as before mentioned, when 

 it requires a fupply of fuel, which is generally at intervals 

 of about half an hour each. The fireman frequently ex- 

 amines the appearance of the under fide of the fire-grate, 

 and judges from it the ftate of the fire: he improves it where 

 neceffary, as before defcribed, by thrufting a hook up 

 between the bars to make way for the air. 



The flame arifing from the ignited fuel upon the grate 

 partly proceeds upwards between the pots, and heats them 

 by that means ; it then ftrikes the roof of the vault, and is 

 reverberated down upon the pots, and cfcapes through the 

 fix flues or chimnies in the vault. The draught alfo draws 

 the flame from the grate under the pots, and round the out- 

 fide and ends. The principal objeft in this ftage of the pro- 

 cefs, is to maintain the fame degree of heat in every part of 

 the pot, that every bar may be equally converted in the fame 

 fpaCe of time. The roof of the vault muft be built of very 

 good ftone (none being better than from Roches quarry, in 

 Aihover), to withftand the great heat exerted upon it : it 

 is cuftomary to build them very thin, and cover the outfide 

 with a fmall thicknefs of dry fand to keep them tight, in cafe 

 of a ftone cracking. 



In this way the fire is kept up in as equable a manner 

 as poflible, until the iron is fuppofed to have imbibed a 

 fufficient portion of carbon from the charcoal to render it 

 jit for its intended purpofe : in this circumftance, the manu- 

 fafturer regulates his judgment by his experience of former 

 procelTes. About the time that he fuppofes the converfion 

 to be fufficiently advanced, one of the trial-bars is drawn out 

 from the pot, and by comparing the fize of the blifters raifed 

 upon its furface with another bar which is known to be fuffi- 

 ciently carbonated, an idea is formed of the ftate of the furnace, 

 and accordingly the fire is, at the proper time, difcontinued, 

 and the furnace is fuffered to cool. Some manufafturers pro- 

 ceed to make experiment of the trial-bar by hardening and 

 tempering it, fo as to prove to a certainty the degree of its 

 converfion, the blifters being found in fome degree fallacious ; 

 for their fize depends as much upon the degree of heat to 

 which the bar has been expofed, as upon its carbonization, 

 and (hew the rapidity with which the converfion has been car- 

 ried on, rather than its adlual ftate. 



The time which the iron is required to be in the procefs 



of cementation depends upon a variety of concurring cir- 



cumftances, i. The degree of carbonization requiredto form 



a fteel of the proper quality ; this varies with the ufe the fteel 



is to be applied to. 2. The heat it is fubjefted to. 3. The 



nature of the iron employed in the procefs. The combinations 



of thefe circumftancps arc fo numerous, that nothing but lonsr 



... 00 



experience can dctcrmme the proper duration ot the procefs, 



Vol. XXXV. 



In general terms it may be obferved, that a (hort period 

 will produce a fteel very foft and tenacious, which, when 

 properly treated, will poflefs elafticity as its moft ftriking 

 property, and is therefore very proper for fprings, wire- 

 drav/ing, and other purpofes requiring duftility, but without 

 the hardnefs requifite for edge-tools. The period of cement- 

 ation for fuch fteel varies in different manufaftories, from 

 four to fix days and nights. 



Steel which requires more hardnefs, but at the fame time 

 fufficient tenacity to refill fudden fhocks, fuch asthe edge-tools 

 for working wood are fubjeft to, muft be cemented a longer 

 time. This, which is moftly tilted into fhear fteel, is cemented 

 fix, feven, or eight days, according to the heat and tlie qua- 

 lity of iron employed. Tlie fteel employed for fabricating 

 tools for cutting metals and hard fiihftances being but fmall 

 in demand compared with the others, is not cemented a 

 longer time, but is returned into the furnace at the next 

 cliarge, along with a charge of iron, and cemented again 

 with frefh charcoal : this is termed double converted fteel. 

 But for fome few purpofes, fuch as the turning and boring 

 of caft-iron, the fteel is converted three times : in this ftate it 

 becomes fo hard and brittle as to be totally unfit for any 

 purpofe requiring tenacity, or for any cutting edge which is 

 lefs than an angle of 70 degrees, or it would be continually 

 breaking. 



The heat which is requifite for the procefs, muft be as 

 great as to give the iron nearly a welding heat, but if car- 

 ried farther, will endanger melting the bars when the procefs 

 has proceeded fome time ; an accident which has frequently 

 occurred througli the inattention of the fireman. It is ob-i 

 ferved by manufaifturers, that the carbonization proceeds 

 quicker when the heat is greateft, and for this reafon the 

 duration of the procefs varies in different furnaces, in forr.c 

 meafure from their conftruftion, in urging a greater heat, 

 and this depends chiefly upon the height of the chimney, 

 and the draught it occafions. 



When the converfion is fuppofed to be complete, the fur- 

 nace is fuffered to cool, until a man can conveniently enter 

 the furnace, to take out the bars and remaining charcoal, and 

 prepare the furnace for a new charge. The bars which are 

 brought out are (from being covered with bUfters upon the 

 furface) termed bliftered fteel. 



On examination of the frafture of a bliftered bar, it is 

 found full of internal cracks, which are generally parallel to 

 the flat fide of the bair : fome of them are larger than others, 

 and extend the parts of the bar fufficiently to raife numerous 

 protuberances or blifters upon its furface. Thefe cracks have 

 every appearance of being opened by the expanfive force of 

 fome gas generated in the iron during the procefs, but what 

 the nature of this gas is, ftill remains to be inveftigated. It 

 feems to arife from the body of the iron itfelf, by the crack 

 being within the fohd fubftance of the bar. The fracture 

 of the bliftered fteel is exceedingly irregular, of a white 

 colour, like frofted filver, and appears like an irregular cryf- 

 tallization ; but the facets exhibited are larger in proportion as 

 the cementation has been longer continued, and from this 

 reafon they are larger towards the furface of the bar than 

 in its centre. 



The furnace above defcribed is of that kind which is 

 efteemed the beft for the procefs, and is moft generally em- 

 ployed in and about the neighbourhood of Sheffield in York- 

 fhire, where the manufaflure of fteel is carried on in a larger 

 fcale than in any other part of England. The furnaces ufed 

 at Newcaftle, which is another feat of this trade, are very 

 fimilar. 



The charge confifts of twelve tons, each pot containing 



])x tons of iron ; and it is neceflary that all the bars converted 



4P at 



