706 



ROASTING ORES. 



the proportion of 40 to 480, or as 1 to 12, exclusive of the saving of one man's labour. 

 The cylinder of an ordinary locomotive-engine boiler 8 feet 6 inches long and 3 feet 

 diameter can be riveted and the plates fitted completely by the machine in 4 hours ; 

 whilst to execute the same work by hand would require with an extra man 20 hours. 

 The work produced by the machine is likewise of a superior kind to that made in 

 the ordinary manner ; the rivets being found stronger, and the boilers more free from 

 leakage, and more perfect in every respect. The riveting is done without noise, and 

 thus is almost entirely removed the constant deafening clamour of the boiler-maker's 

 hammer. 



ROAN. The name of a common leather used for book-binding, and for slippers. 

 It is prepared from sheep-skin by tanning with sumach. See LEATHER. 



ROASTING ORES. The operation of roasting is executed by various pro- 

 cesses, relatively to the nature of the ores, the quality of the fuel, and to the olTject 

 in view. 



Three principal methods may be distinguished : 1, the roasting in heaps in the open 

 air, the most simple of the whole ; 2, the roasting executed between little walls, and 

 which may be called case-roasting (Eost-stadel in German); and 3, roasting in 

 furnaces. 



1. The roasting in the open air, and in heaps more or less considerable, is practised 

 upon iron ores, and such as are pyritous or bituminous. The operation consists in 

 general in spreading over the plane area, often bottomed with beaten clay, billets of 

 wood arranged like the bars of a gridiron, and sometimes laid crosswise over one 

 another, so as to form a uniform flat bed. Sometimes wood-charcoal is added, so as to 

 fill up the interstices, and to prevent the ore from falling between the other pieces 

 of fuel. Coal is also employed in moderately small lumps ; and even occasionally 

 turf. The ore, either simply broken into pieces, or sometimes under the form of 

 schlich (fine pyritous sand), is piled up over the fuel ; usually alternate beds of fuel and 

 ore are formed. 



The fire, kindled in general at the lower part, but sometimes, however, at the 

 middle, gradually spreads, putting the operation in train. The combustion must 

 be so conducted as to be slow and suffocated, to prolong the ustulation, and let the 

 whole mass be equably penetrated with heat. The means employed to direct the fire, 

 are to cover outwardly with earth the portions where too much activity is displayed, 

 and to pierce with holes or to give air to those where it is imperfectly developed. Kains, 

 winds, variable seasons, and especially good primary arrangements of a calcination, 

 have much influence on this process, which requires besides an almost incessant in- 

 spection at the beginning. 



It may be laid down as a good rule, to employ no more fuel than is strictly necessary 

 for the kind of calcination in hand, and for supporting the combustion ; for an excess 

 of fuel would produce, besides an expense uselessly incurred, the inconvenience, at 

 times very serious, of such a heat as may melt or vitrify the ores : a result entirely 

 the reverse of a well-conducted ustulation. 



Figs. 1712, 1713, 1714, represent the roasting in mounds, as practised near 

 Goslar in the Hartz, and at Chessy in the Department of the Khone. Fig. 1712 is a 



1713 



vertical section, in the line he, of fig. 1714. In jig. 1713 there is shown in plan, 

 only a little more than one-half of the quadrangular truncated pyramid, which con- 

 stitutes the heap. Fig. 1714 shows a little more than one-fourth of a bed of wood, 

 arranged at the bottom of the pyramid, as shown by a a, fig. 1712, and cgh, fig. 

 1714. c is a wooden chimney, formed within the heap of ore, at whose bottom, c, 

 there is a little parcel of charcoal; dd are large lumps of ore distributed upon 

 the wooden pile a a; ee are smaller fragments to cover the larger; //is rubbish 

 and clay laid smoothly in a slope over the whole. g,fig. 1714, a passage for air left 

 under the bed of billets, of which there is a smaller one in each of the four sides of 

 the base a a, so that two principal currents of air cross under the upright axis c c, of 

 the truncated pyramid indicated in fig. 1712. 



Burning wood is thrown in by the chimney c. The charcoal and tho wood take 

 fire ; the sulphurous ores, d ef, are heated to such a high temperature as to vaporise 



