ZINC 



1189 



In the arched chamber A are placed 48 cylindrical retorts, 3 feet 6 inches in 

 length from b to d, and 7 inches internal diameter. .These are made of refractory 

 fire clay, well baked and supported behind by ledges of masonry a, b. fig. 2152, 

 whilst in front, at c d, they rest on fire-clay saddles let into an iron framing. Short 

 conical fire-clay pipes, 10 inches in length from d to c, are fixed in the mouths of 

 these retorts by means of moistened clay, and project for a short distance beyond 

 the mouth of the furnace. To these are adapted thin wrought-iron cones 18 inches 

 in length from e tof, tapering off to the smaller extremity to an orifice of about three 

 quarters of an inch in diameter. The inclined position of the retorts, the method 

 of adjusting the pipes, and the general arrangement of the apparatus are shown in 

 fig. 2152, in which r, r, r, r, represent the nozzles of thin wrought iron. When a new 



2152 



furnace is first lighted the retorts are introduced without being previously baked, but 

 care must be taken that they be perfectly dry and seasoned, and for this reason it 

 is necessary to keep a large stock constantly on hand, in a storehouse artificially 

 heated by means of some of the flues of the establishment. The heat is gradually 

 increased during three or four days, at the end of which period charges of ore are 

 introduced, the clay cones are luted in their places, and the furnace is brought into full 

 working order. The charge of a furnace consists of 1,680 Ibs. of roasted blende, or 

 calcined calamine, and 840 Ibs. of coal-dust. The ore and coal-dust, after being finely 

 divided and intimately mixed, is slightly damped and subsequently introduced into the 

 retorts by means of a semi-cylindrical scoop, by the aid of which an experienced work- 

 man will effect the charging without spilling the smallest quantity of the mixture. 



In this country the retorts in the lower tier are usually not charged, as they are 

 extremely liable to be broken, and are therefore only employed to moderate the heat 

 of the furnace. On the Continent, however, the fireplace is frequently covered by a 

 hollow arch, and in that case every retort requires a charge of ore. 



The mixture introduced into the retorts varies, to a certain extent, with their 

 position in the furnace, for in spite of every precaution to prevent inequality of 

 temperature, it is found impossible to heat the whole of them alike, and those next 

 the fire, therefore, from being the most strongly heated, are liable to work off first. 

 As soon as the retorts have been charged the clay cones are luted into their places, 

 and carbonic oxide gas, which burns with a blue flame at the mouth of the cones, 

 quickly makes its appearance. The quantity of this gas gradually diminishes, and as 

 soon as the flame assumes a greenish-white hue, and white fumes are observed to be 

 evolved, the sheet-iron cones are put on, and the furnace at once enters into steady 

 action. From time to time, as the iron cones become choked with oxide, they are 

 taken off and gently tapped against some hard substance, so as to remove it, and 

 then replaced. The oxide thus collected is added to the mixture prepared for 

 the next charge. After the expiration of about six hours from the time of charging 

 the wrought-iron tubes are successively removed, and the metallic zinc scraped from 

 the clay-pipes into an iron ladle. This, when full, is skimmed, and the oxide added 



