IRON. 



"ting to about half the weight of the mineral to be 



1 iio eartliy iron ore muft be treated accoiding to the re- 

 u.k of the humid aiialyfis ; fucli earth being added as a flux, 

 as will make the mod fufible compound with that found by 

 analyfis to be prefent. Tlie carbonaceous matter may be 

 from y to i the weiglit of oxyd of iron. 



The furnace bell calculated for thefe experiments is called 

 an aflay furnace, and is capable of producing a great lieat. 

 See Furnace. 



The crucibles fhouid be very fmall, not capable of holding 

 more than three fluid ounces ; and they fliould be provided 

 with covers turned to them in a lathe before they are 

 burned. The aflay may be deemed finifhed when the whole 

 is in a ftate of fulion, and the metallic button feparated, 

 which, being weighed, will give t!ic/>iT ««/j_j.- of iron in 

 th; ore. 



§ 3. ReJuRlon of Ores, and ManufaFiurt of P'lg.'tr.^n. 



Although iron in its pure ftate is almoil an infufible fub- 

 flance, it is capable of alTuming the hquid form, by being 

 combined with other matter. With fulphur it forms a fufible 

 mafs, of which we ftiall treat hereafter. It is alfo rendered 

 fufible at a temperature fomething higher than that required 

 to melt copper, by being combined with about -J-th of its 

 weight of carbon. It is to this compound in different pro- 

 portions that the name of pig. iron is given, and it is fo de- 

 nominated, becaufe it is call into mafies of a femi-cylindrical 

 fhape, called //f J-. 



The fufibility of this compound of iron and carbon, 

 enables us to extract the metal from the ore to the greateft 

 advantage. It is now comsion, particularly in our own 

 country, to obtain the ircn in this form, previous to making 

 it into bar, or malleable iron. Formerly call metal was not 

 much in ufe, except for the manufadlure of bar-iron and 

 fteel ; w'hile in the prefent day, a much greater proportion 

 of it is confumed in that ftate than in any other ; and the 

 majority of our furnaces, too, are folely employed for this 

 branch of manufafture. 



The procefs by which pig-iron is obtained from the dif- 

 ferent iron ores is called ymfAinj; and the furnaces employed 

 for the purpofe are caUed fmelt'tng, or hlajl furtiaca. See 

 BL.\ST_/ar«arir, and Blowing. 



The ores of iron require different treatment in the fmelting 

 procefs, according to the quantity of heterogeneous matter 

 with which the metal is combined. 



In all the ores the iron is in the ftate of oxyd, and would 

 at leaft require a ftrong heat in contaft with combuftible 

 matter for their reduction. In moll, the oxyd of iron 

 is combined with a confiderable proportion of earthy matter, 

 and they are then denominated iron-Jloms. Thefe may be 

 generally divided into two claftes ; the one cal ed argilla- 

 ceous, from abounding with excefs of alumine, or clay ; and 

 the other calcareous, from lime being their principal earthy 

 conftituent. Tlie forner of thefe iron-ftones is by far the 

 mod abundant in this country. 



But, befides the earthy matter and oxygen in this clafs of 

 metallic minerals, many of them contam fulphur, which is 

 doubtlefs combined with the iron in the ftate of pyrites. 

 Arfenic and manganefe are alfo fometimes united with them. 

 Of the above, the arfenic and fulphur are extricated, pre- 

 vioufly to fmelting, by the procefs called roajling. For 

 this purpofe, the ftone is ftratified with refufe pit coal, and 

 burnt in large heaps in the open air. The heat is fufficient to 

 diffipate the greateft part of the above volatile materials, 

 leaving behind the earth and oxyd of iron ; and alfo the 

 manganefe, when the ore abounds with that metal. 



In the procefs of fmelting, two things are abfolutcly 

 efliential to the fcparation of the iron. Firft, the metal itielf 

 muft be rendered fluid, whicii will then, by its great fpecific 

 gravity, defcend to the loweft parts of the furnace, and 

 iome other compound muft, at the fame time, be eliminated 

 in a liquid form, fo as to float upon its furfacc, and defend 

 it from the influence of the blaft. If the ore confifted of 

 iron and oxygen alone, the carbon of the coke would com- 

 bine with the oxygen ; and an excefs of carbon would alfo 

 unite with the iron to render it liquid at that temperature ; 

 but here would be a deficiency of the fluid vitreous matter 

 neceffary to the defence of the iron from the oxygen of the 

 blaft. Hence it will be necefl'iiry to employ fon-.e fi:brtance 

 with fuch iron-ore, which fliall be capable of forming a hquid 

 fcoria, or cinder, for the prefervation of the carburetted iron 

 when once obtained. So far as obfervation has dictated, it 

 would feem that the cinder cannot be too perfeftly fluid. 

 The principles on which the fufibility of the cinder depends, 

 are not fimply confined to the materials ufed in the fmelting 

 oi iron, but refer to all compound fufible matter with which 

 we are acquainted. It may be obferved, in general, and, 

 indeed, almoft without exception, that the fufibility of an 

 alloy of two metals is fufible at a temperature much lefs 

 than the arithmetical mean between the fiifiiig points of the 

 metals themfelves. For inftance, an alloy of lead and tin 

 is more fufible than either of the metals compofmg it, and a 

 fimilar mixture of copper and lilver may be ufed as a folder 

 for either filver or copper feparately. This property is not 

 lefs confpicuous in the earths. None cf them in their pure 

 ftate can be fufed in our hotteft furnaces ; nor fcarcely 

 with a ftream of oxygen gas ; aithougli certain proportions 

 of ihcm are, together, fufible at the heat of a moderate 

 air-furnace. Lime p.nd clay, when feparately taken, may be 

 conf.dered as incapable of fufion at any degree of heat,' yet 

 produced in furnaces ; and lliU, in certain proportions, they 

 are too fufible to be made into even bricks or crucibles. It 

 will appear, from thefe facls, that the iron-mafter cannot 

 pay too much attention to the fubjeft of the relative fufibi- 

 lity of the earths in jifterent proportions. Moll of the iron- 

 ores of this country are argillaceous ; that is, confift, befides 

 oxyd of iron, of a fmall quantity of filex or flint, and a large 

 proportion of clay. LiT.eftone has always been employed 

 for fuch ores, and, by combining with the clay and flint, as well 

 as with a fmall portion of the oxyd of iron, forms a fcoria or 

 cinder, cafily capable of fufit^n. Since, however, the pro- 

 portions of thefe earths in the ore cannot be uniform, the 

 quantity of limeftone to be added ought to vary accordi:ig 

 to circumftances. We may hence infer, that if the fufibility 

 of the cinder depends upon the particular proportions of 

 the earths prefent, the iron-maker ought to poflefs a very 

 perfect knowledge of the relative fufibility of difl^erent com- 

 binations of thefe bodies, and being at the fame time aware 

 of the component parts of the ore to be reduced, he will not 

 be at a lofs what ftiould be added in the furnace, for the 

 purpofe of producing the moft fufible cinder. But it will 

 be proper here to obferve, that the earths prefent, however 

 accurate may be their proportions, will not of themfelves 

 form a cinder of fufficient fufibility, without the united aid 

 of the oxyd of iron. This fuft will be very familiar to thofe 

 who have had experience in the ufe of fire-bricks. Clays 

 which are free from that ingredient do not burn of a red 

 colour ; and hence the white appearance of fire-bricks is a 

 tolerable teft of their geodncfs. But thofe, on the contrary, 

 which exhibit a rednefs on being fired, are eafily fufed, and 

 unfit, confequently, to be ufed in thofe fituations which are 

 expofed to great heat. The proportions i:f lime, clay, and 

 oxyd of irou, necefiary to conftitute the moft fufible com- 

 pound) 



