264 ASSAY 



arc more expensive than other varieties of clay crucibles. The so-called white 

 fluxing pots made in London of French fire-clay hare the same shape, but are rather 

 thicker than the true French crucibles. They ulso answer very well for assay 

 purposes. 



Black-lead, or Plumbago Crucibles. These pots are made of a mixture of plum- 

 bago and fire-clay. The best varieties withstand sudden alternations of tempera- 

 tures; resist the highest temperatures without softening or cracking, but they 

 slowly burn away externally by repeated use. They are employed in iron and tin 

 assaying. 



Iron Crucibks. Wrought-iron pots of various sizes and shapes are used in lead 

 assaying. Those made out of one piece of iron, without a weld, are preferable ; but 

 tJiey cost more than those made from boiler-plate, or iron-tubing by hammering and 

 welding. 



Porcelain Crucibles. These crucibles are made of various sizes and shapes. They 

 are chiefly used in assays by analysis. 



Boasting Dishes. Flat, shallow, thin circular vessels of fire-clay of various sizes, 

 used for the calcination of ores, &c., in muffles. The most useful sizes vary from 2 to 

 3 inches in diameter, and from to f of an inch in depth, inside measure. 



Scorififrs. Cup-shaped vessels of fire-clay used in assaying silver ores ; they 

 should withstand sudden alternations of temperature, and resist the corrosive action 

 of litharge and metallic oxides. Those commonly used are from 1^ to 2 inches 

 diameter at top, and from ~ to % of an inch in depth, inside measure. 



Cupels. Small circular vessels, having a shallow hemispherical cavity. They are 

 generally made of bone-ash, or other material which is porous, and resists the corro- 

 sive action of litharge at the required temperature. They vary from ^ inch to 2 

 inches in diameter, according to circumstances. 



Tongs, Scoops, Stirring Bods, Ingot Moulds, $c. For a description of these imple- 

 ments, see the assays of. the various metals. 



Balances. For general assay purposes, three kinds of balances will suffice. (1.) 

 To carry 500 grains, and turn with t^th of a grain : this may be used for gold and 

 silver assays, and for all purposes of exact weighing. (2.) To carry 1000 grains, and 

 turn with from ith to ~^th of a grain. This will bo found suitable for the dry assay 

 of copper, tin, lead, &c. (3.) To carry 10,000 grains, and turn with ^th to ^th of 

 a grain. Used for weighing out fluxes and other substances. In special branches of 

 assaying, where a largo number of assays of the same kind have to be made, it is de- 

 sirable to have balances constructed suitable for that specific work. 



Weights. For general assay purposes the grain weights divided on the decimal 

 system are most convenient. Special weights are used by tin assayers, copper as- 

 sayers, bullion assayers, and others, to facilitate calculation. These will be noticed 

 under the respective metals when necessary. 



Burettes, pipettes, and any special apparatus employed in wet assays, or otherwise, 

 will be described under the assays of the several metals, when necessary. 



FLUXES, KEAGENTS, AND OTHER SUBSTANCES USED IN ASSAYING. 



They should be kept in covered earthenware jars, or in a covered long rectangular 

 wooden box divided into compartments ; all fluxes, when practicable, should be used 

 in the dry state. Those employed in the dry way may be classified as follows : 



FLUXES. Carbonate of Soda, dried. Crystallised carbonate of soda contains about 

 63 per cent, of water, which it loses on heating. Bicarbonate of soda is a convenient 

 substitute. Carbonate of potash may also bo used instead. The alkaline carbonates 

 form fusible compounds with silica, &c. 



Borax or Biborate of Soda, dried or calcined. Crystallised borax contains about 

 47 per cent, of water; when heated it loses this water, and swells up to a 

 very light bulky mass, which is dried or calcined borax. If the temperature is in- 

 creased, it melts into a clear, colourless liquid, which on cooling constitutes glass of 

 borax. It forms fusible compounds with earthy and metallic oxides, as lime, oxido 

 of iron, &c. 



Glass. White glass, free from oxide of lead, such as plate-glass or window-glass, 

 is suitable for some purposes. Green bottle-glass may also be used where the presence 

 of oxido of iron is not objectionable. It serves to increase the fusibility of earthy 

 silicates, &c., and in some cases as a substitute for borax. 



Silica. White sand, or powdered quartz ; it serves as a flux for oxides of iron, lime, 

 &c. 



. Fluor Spar, Fluor, or Fluoride of Calcium. It should be selected free from galena, 

 copper pyrites, and other minerals. It forms very fusible compounds with sulphate 

 of baryta, sulphate of lime, phosphate of lime) silica, &c. 



