METALLURGY: 133 
of these furnaces a blast of air is used to urge the fire, and increase the 
heat. 
1. Oren Furnaces. Open furnaces are the simplest used in metallurgic 
operations. Pl. 27, jig. 1, is an example, in which the walls are but 2 or 
3 feet high; strictly, this should be considered as several furnaces, with 
division walls between them. The roasting of minerals is performed in 
these furnaces in the following manner: The floor is covered with a layer 
of fuel, upon which the ore is placed and the fuel is lighted. Those ores 
containing sulphur and bitumen require but little fuel, as when once heated 
to a certain point they take fire and burn of themselves. Others, as iron 
ores, which contain no combustible matter, require considerable fuel to 
effect the roasting. Ores are often roasted in pits in the earth, in which 
ease a high and dry locality must be chosen; frequently the operation is 
performed in heaps in the open air, which is often considered the most 
available method. Pl. 27, jig. 2, shows the liquation furnace used for 
separating silver from lead ores; the walls, a, are inclined towards each 
other, and on the top are covered with plates of iron, which leave narrow 
openings between them, their whole length. The material to be operated 
upon is placed upon the inclined plates, and the fuel beneath and all around 
it; the lead, as it melts, drops through the openings between the plates, and 
collects in the receptacle 6. /%gs. 3,4, and 5, represent a blomary or forge- 
fire for the reviving of iron; it consists of low masonry work, with an exca- 
vation in the hearth, to contain the metal to be operated upon. Fig.3,abed 
are four cast-iron plates, the bottom of the hearth forming a fifth; e is the 
opening through which the tuyere passes; three or four inches above the 
bottom of the hearth is a row of holes or a slit to let off the cinders. The 
hearth and tuyere are hollow, and water is kept circulating through them. 
An open copper furnace, seen in jig. 6, differs from the blomary principally 
in having a spherical hearth; @ is the crucible. The masonry, which par- 
tially surrounds the hearth, is for the purpose of better concentrating the 
fire; 4 is the tuyere; c, the back wall through which the tuyere passes; d, 
a partition wall dividing the space above the hearth. 
Pl. 27, jig. 7, is a view of an open silver refining furnace; a, the open- 
ing for the tuyere; 6, the cupel crucibles. The cupel consists of a crucible 
of iron, in which the wood and bone ashes are rammed, on the surface of 
which is a depression for receiving the silver which is to be refined. 
2. Srack Furnaces. The signification of the word stack sufficiently 
explains the general character of this class of furnace, the interior space 
being open at the top, and entirely closed with masonry, forming a shaft or 
stack which receives the material to be heated, either alone or mixed with 
the fuel, the atmosphere necessary to combustion being supplied at the 
bottom, either by the draught or by a blowing apparatus ; those operated by 
draught alone are used for roasting only. P2. 27, figs. 11 @ and 1143, are ver- 
tical sections, at right angles to each other, of a Swedish furnace of this 
description ; @ is the stack which is to be filled with the material to be 
operated upon ; d is the fire space, at the bottom of which is a grate; e the 
ash-pit. The fire space is covered with massive iron bars, laid close to each 
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