ASSAYING. 



ASSAYINi;. 



i . . 



~Juti*> ot oommoo Mb is prepared of uoh a strength, that 1000 

 drain* of it are exactly sufficient to precipitate tan grain* of pun silver. 

 Ten grain* of the alloy far examination are placed in stoppered bottle, 

 oapabl* of holding about *iz ounce* of water, and by the aid of a genUe 

 bait ro dissolved in two dnchm* of nitric add, of specific gravity, 

 1-J5 ; the solution of nit U then placed in 

 burette (.*;. a) capable of holding rather 

 more than 1000 grama. The burette, when 

 filled with the dilution, ia weighed before 

 being used, and the liquid n added to the 

 nitrate of rilrer in the bottle; when it in 

 supposed that the silver ia nearly all pre- 

 cipitated, the liquor ia briskly agitated in 

 the bottle, and the precipitate is allowed 

 to subside ; a drop or two more of the solu- 

 tion of salt ia then add. .1 ; if a precipitate 

 be produced, the liquid is again agitated, and when clear more of the 

 solution u added, as before, as long u any turbidity is produced by the 

 addition. When a cloud cease* to be formed, the proportion of solution 

 of salt which ha* been added i* ascertained, by weighing the burette a 

 second time. The number of grain* of the solution employed indicate* 

 the degree of fineness of the alloy." 



(hid. The assaying of gold is performed, to a certain extent, 

 exactly in the same way as that of silver by the dry way ; and if the 

 gold were alloyed only with copper, the process would be as simple 

 as that of silver assaying. Usually, however, gold contains silver. and 

 this cannot be got rid of by cupellation ; the parting process is there- 

 fore had recourse to. This consists in dissolving the silver by dilute 

 nitric acid, which leaves the gold perfectly pure, unless the silver is BO 

 small in quantity as to be protected by the gold from the action i.f 

 the acid, which is very commonly the case. To obviate this difficulty. 

 the gold alloy, supposing it to weigh twelve grains, is to have thirty- 

 six grains of pure silver added to it (hence the name quartatitm given 

 to this process), and to be cupelled with one hundred and eight grains 

 of lead. The button obtained is to be flattened into a plate of about 

 one inch and a half long, and four or five lines broad, returned to the 

 furnace, kept for some time at a red heat, taken out and Buffered to 

 cool, and rolled up about the size of a quill. This is to be put in a 

 matrass with about three times its weight of nitric acid, of sp. gr. 1 '25, 

 and heated on a sand-bath. By the action of the acid the silver is 

 dissolved, and the cvrarli, as they "are termed, of gold, are left of a dull- 

 brown colour, and without any metallic appearance ; these are repeat- 

 edly washed with distilled water, and heated in small clay crucibles to 

 bright redness. The pieces of gold having thus acquired their usual 

 appearance and properties, are to be weighed, the absolute loss in 

 weight indicating the purity of the alloy subjected to trial. 



Iron are* are chiefly of three kinds : the impure protocarbonate, 

 commonly called the argillaceous iron ore ; the peroxide, including the 

 specular and hematite iron ores ; the black or magnetic ore, which is a 

 compound of the protoxide and peroxide. The argillaceous iron ore is 

 that which supplies by far the greatest proportion obtained in Britain ; 

 the htcmatite occurs in North Lancashire and many other places ; the 

 mines of Elba yield the specular ore ; whilst the Swedish iron is ob- 

 tained principally from the magnetic ore. 



Various methods have been proposed for assaying these ores.'-but the 

 principle is in all of them the same; it is that of separating the 

 oxygen from the iron, by the greater affinity of charcoal for that 

 element at high temperatures. The operation of the charcoal is fre- 

 quently assisted by the use of a flux to combine with the earthy 

 matter, and to convert it into such a glass as will let the melted metal 

 easily fall through, and form, on cooling, a clean button. A flux com- 

 posed of lime and bottle glass has been used, or the clay which accom- 

 panies the argillaceous iron ore is to be burnt and mixed with an equal 

 weight of lime ; 200 grains of the powdered ore may be mixed with an 

 equal weight of this lux and forty grains of powdered charcoal ; the 

 mixture, put into a Cornish or Hessian crucible, is to be heated in a 

 wind-furnace or a forge. It is not always easy to apportion the char- 

 coal exactly to the oxide of iron in the ore ; when it is either too large 

 or too small, the product of iron is deficient, and this will be denoted 

 by the iiii]irrfection of the glass. 



In the Supplement to the ' Encyclopedia Britannica,' Mr. Muxliet 

 has given the results of using various fluxes with an iron ore that 

 yielded forty-six per cent, of the metal; and it appears that the 

 following mixture of the ore and substances, all of course reduced to 

 powder, gave the largest proportion of iron : ore, 200 grains, lime 100, 

 borax 100, charcoal 40, gave 91 of metal ; it is therefore evident that 

 only one-half per cent, of iron remained in the glass. 



According to M. Descotils (' Ann. de Chimie,' t. 84, p. 188), the 

 earthy portion of the argillaceous iron ore is frequently such as to 

 form a glaa* without adding any flux whatever to the charcoal. Me 

 Used crucibles lined with a mixture of clay and charcoal ; and thus, 

 Mimng many other assays, with nearly itniUr results, an ore which 

 was found by analysis to contain about 37} per cent, of iron yielded 

 M per cent, of the metal, and the glass was of excellent quality. 



Cofjprr aro, with reference to the mode of assaying them, may be 

 divided into two rleeere those that contain sulphur, and those that do 

 not The former class may be subdivided into such as also contain 

 iron pyrites, arsenic, tin, lead, sine, Ac., with a considerable quantity of 



earthy matter; and such as are composed principally of a mixt 

 the sulpburets of copper and iron, with but small portions, if . 

 other metallic or earthy minerals. 



To treat the first subdivision of the sulphureous ores (which 

 tute a large proportion of all copper ore- -at Britain), a flux 



should be prepared by mixing the following [ingredients in the under- 

 mentioned weights : 



1 ditto 

 1 ditto 

 1 ditto 

 t ditto 



Fluor ipar, 

 Slaked urn*, 

 Borax, 



Red argot (Impure tartar), 

 Nitre, 



all finely powdered and well mixed. 



The sample of ore being reduced to a coarse powder, take 400* 

 grains of it, and calcine it in a (' rni-h r Hessian crucible, at n 

 rate red heat, for fifteen or twenty minutes, stirring it repeated! 

 an iron rod flattened at the end. During this operation the <>< will 

 increase considerably in bulk, and it .-h..uld n< ver I 

 this begins to diminish. Having taken out the crucible 

 allow it to cool, fill the furnace t with fuel, and put on the e 

 increase the heat. When cool, mix the ore. without taking it 

 the crucible, with about 400 grains of the prepared flux, and c< 

 surface of the mixture with common salt ; introduce it into the furnace, 

 Hi. I continue it therein, at a white heat, until the whole is well n. 

 which will be known by the surface of the mass assuming a smooth 

 and quiet aspect. If the furnace is in good condition, this will gene- 

 rally be effected in about twenty minutes. Should the operator have 

 reason to think that the mixture in the crucible has not melted thin, 

 so as to allow the metallic regulus to subside through the lag, he may 

 project upon it a mixture of a scruple each of nitre, borax, and argol ; 

 and this may be again repeated if necessary, adding, howev. 

 grains of flowers of sulphur. When thoroughly melted. ]".ur il. 

 tents of the crucible into a hemispherical iron mould, prc\ 

 warmed and greased ; allow it to become solid, and then quench it in 

 water. Separate the button of regulus from the slag with a small 

 hammer ; it ought to be round and well defined, of a reddish-brown 

 colour with shades of blue, or else bluish-white. When of the i' 

 colour, it contains a little more sulphur than the latter. Should the 

 button of regulus exhibit a brilliant bluish-white surface, the slag 

 should be rcmelted with two drachms of red argol, and a scruple each 

 of slaked lime and sulphur, which will give a small button of re^u!'"- 

 to be added to the former. It may here be remarked, that a but 

 regulus with a nucleus of metallic copper should always be i. , 

 and a fresh assay commenced, calcining the ore less. Aid if. when tint 

 slag and button of regulus are quenched in water, it render* th. 

 immediately turbid and of a dirty orange-yellow colour, it should also 

 be rejected, the ore in this case also having been too much calcii 

 too large a quantity of nitre used. On the contrary, if the i 

 does not collect in a compact well-defined button, but spreads under 

 the slag a considerable way up the sides of the mould, and of a dull- 

 brown aspect, the ore has not been sufficiently calcined. 



The regulus must now be calcined : for which purpose reduc. 

 powder, and expose it in a clean porcelain crucible to a very dull red 

 heat, constantly stirring it with a flattened iron wire. As the o]~ 

 proceeds, the heat must be increased and the stirring continued, until 

 the whole of the sulphur is dissipated. Especial care must be taken, 

 particularly -at the commencement of the operation, to prevent the 

 regulus from clotting or sticking together, which in canned by excess 

 of heat or want of stirring, and much retards the op.iati. n The 

 same remark applies also to the calcination of ores. It 

 importance that the whole of the sulphur should be expelled in thin 

 operation, and this will be greatly facilitated by projecting gradually 

 about twenty grains of carbonate of ammonia into the crucible towards 

 the end of the operation. 



The crucible having been removed from the furnace, and allo , 

 cool, add to the calcined regulus about a drachm each of bur 

 red argol, with a scruple of nitre, covering the whole with common 

 salt. Melt the mixture well, and ]K>ur it into a mould as 1 

 (|iiench it in water, and knock off the slag (which reserve) from the, 

 metallic button. The latter is now termed coarse copper, and n 

 to be refined ; for which purpose return the crucible to ' 

 putting into it the button of copper, upon which, when mel'.d, 

 project about half a drachm of flux (prepared as below), and the like 

 quantity of common salt. Shut up the furnace for about two minutes, 

 or until the flux is well melted, and then pour out into the mould as 

 before. Separate the flux (which reserve) from the button; and if the 



The weight used by mayors of copper ore* ii 400 Troy grain", marked 100 

 (technically called ctntt), which i subdivided down to one, and that again to 

 one-eighth. Ore giving a button that weigh" ten three-eighths Is mid to produce 

 ten three-eighths per cent., and so on. The average of all the copper ores 

 smelted in Great Britain U about eight and a half per i-mt. 



t The furnace used for assaying copper ores is a simple air furnace, about 

 seven Inches square and fourteen Inches deep, communicating with a chimnry by 

 a lateral flu* five Inches wide by two deep. The fuel U coke, broken to piece* 

 about the aim of walnuts, the small lifted out. Cornish crucible* are used, and 

 require neither stand nor cover, being kept with the mouth just above the 

 surface of the fuel.' 



