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Copper ore is also produced and smelted with the lead ores if it does not contain more than 6 per cent of copper and a sufficient percentage of silver. Still another kind of ore occurring abundantly in the district may be designated as fluxing ore, its chief constituents being iron pyrites, zinc blende, quartz, calcspar, about 0.03 per cent of silver, and a little galena and copper.

Besides these local ores, foreign ores are bought and smelted in considerable quantities at Freiberg, occasioning a great variety of operations.

Nearly all ore is received from the dressing works in form of slimes, which have to be roasted. The furnaces used for this operation have two beds, as represented in Figs. 30 and 31.

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The flames pass from the fireplace A over the lower bed B up to the upper bed C, pass over this and carry the sulphurous acid, resulting in this operation as a secondary product, along with them through the flue D to an extensive condensing apparatus.

The charge, consisting of 50 per cent of first class lead ore, 30 per cent of second class lead ore, and 20 per cent of silver or copper ore, is introduced into the upper bed and gradually pushed towards a hole communicating with the lower one, through which it falls, and is raked forward.

The charge is thus exposed to a constantly increasing temperature, the final degree of heat being sufficient to cause the ore to fuse. Fresh ore is charged whenever the preceding charge is moved on a step, thus making the process a continuous one.

One ton of ore requires from eight to sixteen hours for complete roasting, the time depending upon the size of the furnace.