CHEOME IRON 



799 



CHROME IRON, or CRROXVXZTZi. The only ore of chromium which occurs in 

 sufficient abundance for the purposes of art is the octahedral chrome ore, commonly 

 called chromate of iron, though it is rather a compound of the oxides of chromium and of 

 iron. It is formed on the type of magnetic iron-ore, being a combination of a protoxide 

 with a sesquioxide the protoxide metal in chrome iron-ore being chiefly iron, and the 

 sesqxiioxido metal chiefly chromium ; hence, the formula is FeO.Cr 2 0. 3 (2*eCr 2 O 4 ). The 

 fracture of this mineral is imperfect conchoidal, or uneven. Hardness = 5'5 ; specific 

 gravity 4'4 to 4-5 ; but the usual chrome ore found in the market varies from 3 to 4. 

 It lustre is semi-metallic or resinous ; colour, iron or brownish black ; streak,yellowish 

 to reddish brown. It is sometimes magnetic. Before the blowpipe it is infusible 

 alone, but in borax it is slowly soluble, forming a beautiful emerald-green bead ; 

 fused with nitre it forms a yellow solution in water. 



Chrome ore was first discovered in the Var department in France ; it is also found, 

 chiefly in serpentine, in Saxony, Silesia, Bohemia, Transylvania, the Banat, and Styria ; 

 in Norway at Roraas, in the Ural near Katherinenberg, in the United States at the 

 Barehills near Baltimore, Chester in Massachusetts, and Hoboken in New Jersey. In 

 Scotland it is found in the parishes of Kildrum and Towie in Aberdeenshire, in the lime- 

 stone near Portsoy in Banffshire, near Ben Lawes in Perthshire, and at Buchanan in 

 Stirlingshire. It occurs massive and in considerable quantity in serpentine-rocks at 

 Swinaness, and Haroldswick in Unst, one of the Shetlands ; in Fetlar and in other 

 of the smaller Shetland Islands. 



Composition of Chrome Iron Ores. 



(1) from St. Domingo, analysed by Berthier ; (2) from Roraas, in Norway, analysed by Von Kobell ; 

 (3) from Baltimore, analysed by Seybert; (4) crystallised, from Baltimore, analysed by Abich ; (5) 

 analysed by Klaproth. 



The chief application of this ore is to the production of Chromate of Potash, from 

 which salt the various other preparations of this metal used in the arts are obtained. 



Treatment of the Ore. According to the old method it is reduced to a fine powder, 

 by being ground in a mill under ponderous edge-wheels, and sifted. It is then mixed 

 with one third or one half its weight of coarsely-bruised nitre, and exposed to a 

 powerful heat for several hours, on a reverberatory hearth, where it is stirred about 

 occasionally. In the large manufactories of this country, the ignition of the above 

 mixture in pots is laid aside as too operose and expensive. The calcined matter is 

 raked out and lixiviated with water. The bright yellow solution is then evaporated 

 briskly and the chromate of potash falls down in the form of a granular salt, which is 

 lifted out from time to time from the bottom with a largo ladle, perforated with small 

 holes, and thrown into a draining box. The saline powder may be formed into regular 

 crystals of neutral Chromate of Potash, by solution in water and slow evaporation ; 

 or it may be converted into a more beautiful crystalline body, the Bichromate of 

 Potash, by treating its concentrated solution with nitric, muriatic, sulphuric, or acetic 

 acid, or indeed any acid exercising a stronger affinity for the second atom of the 

 potash than the chromic acid does. 



The first great improvement in this manfacture was the dispensing with nitre, 

 and oxidising entirely by means of air admitted into the reverberatory furnace, in 

 which the ore mixed with carbonate of potash is calcined. Stromeyer afterwards 

 suggested the addition of lime, by which the oxidation was much quickened, and Mr. 

 Charles Watt substituted the sulphates of potash and soda for the nitrates of those 

 alkalis. The sulphate was first intimately mixed with the ground ore, and then the 

 lime well incorporated with the mixture, which was heated to bright redness, for four 

 hours with frequent stirring. 



In 1847 Mr. Tilgmann obtained a patent for the use of felspar in the manufacture 

 of certain alkaline salts, and amongst them of chromate of potash : ho directs 4 parts 

 by weight of felspar, 4 parts of lime, or an equivalent quantity of carbonate of lime, 

 and one part of chromic ore, all in fine powder, to be intimately mixed together, and 



