The fire, kindled in general at the lower part, but sometimes, however, at the middle chimney, spreads from spot to spot, 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. Rains, winds, variable seasons, and especially good primary arrangements of a calcination, have much influence on this process, which requires, besides, an almost incessant inspection at the beginning.
Nothing in general can be said as to the consumption of fuel, because it varies with its quality, as well as with the ores and the purpose in view. But 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. 681], [682], [683.] represent the roasting in mounds, as practised near Goslar in the Hartz, and at Chessy in the department of the Rhone. [Fig. 681.] is a vertical section in the line h c of [figs. 682.] and [683.] In [fig. 682.] there is shown in plan, only a little more than one half of the quadrangular truncated pyramid, which constitutes the heap. [Fig. 683.] 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. 681.], and c g h, [fig. 683.] C is a wooden chimney, formed within the heap of ore, at whose bottom c there is a little parcel of charcoal, d d are large lumps of ore distributed upon the wooden pile a a; e e are smaller fragments, to cover the larger; f f is rubbish and clay laid smoothly in a slope over the whole. g, [fig. 683.], a passage for air left under the bed of billets; of which there is a similar 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. 681.]
The kindling is thrown in by the chimney C. The charcoal c, and the wood a a, take fire; the sulphureous ores d e f are heated to such a high temperature as to vaporize the sulphur. In the Lower Hartz, a heap of this kind continues roasting during four months.
2. The second method. The difficulty of managing the fire in the roasting of substances containing little sulphur, with the greater difficulty of arranging and supporting in their place the schlichs to be roasted, and last of all, the necessity of giving successive fires to the same ores, or to inconsiderable quantities at a time, have led to the contrivance of surrounding the area on which the roasting takes place with three little walls, or with four, leaving a door in the one in front. This is what is called a walled area, and sometimes, improperly enough, a roasting furnace. Inside of these little walls, about 3 feet high, there are often vertical conduits or chimneys made to correspond with an opening on the ground level, in order to excite a draught of air in the adjacent parts. When the roasting is once set agoing, these chimneys can be opened or shut at their upper ends, according to the necessities of the process.
Several such furnaces are usually erected in connexion with each other by their lateral walls, and all terminated by a common wall, which forms their posterior part; sometimes they are covered with a shed supported partly by the back wall, built sufficiently high for this purpose. These dispositions are suitable for the roasting of schlichs, and in general of all matters which are to have several fires; a circumstance often indispensable to a due separation of the sulphur, arsenic, &c.
3. The furnaces employed for roasting the ores and the mattes differ much, according to the nature of the ores, and the size of the lumps. We shall content ourselves with referring to the principal forms.
When iron ores are to be roasted, which require but a simple calcination to disengage the combined water and carbonic acid, egg-shaped furnaces, similar to those in which limestone is burned in contact with fuel, may be conveniently employed; and they present the advantage of an operation which is continuous with a never-cooling apparatus. The analogy in the effects to be produced is so perfect, that the same furnace may be used for either object. Greater dimensions may, however, be given to those destined for the calcination of iron ores. But it must be remembered that this process is applicable only to ores broken into lumps, and not to ores in grains or powder.