A Text-book of Assaying: For the Use of Those Connected with Mines Part 20

The likelihood of two very faulty a.s.says being concordant is remote; but with very important work, as in selling parcels of ore, even this risk should be avoided, as concordance in these cases is demanded in the reports of two or more a.s.sayers. The following actual reports on a disputed a.s.say will ill.u.s.trate this: (a) 5 ozs. 1 dwt.; (b) 5 ozs.

10 dwts. 12 grains; (c) 5 ozs. 11 dwts.; (c) 5 ozs. 11 dwts. 12 grs.

The mean result of several a.s.says, unless there be some fault in the method, will be very fairly exact; and individual a.s.says, with an uncertainty of 1 in 20, may, by repet.i.tion, have this reduced to 1 in 100 or less.

~a.s.say Tons, etc.~--Having decided on taking a larger or smaller portion, the exact quant.i.ty to be used will be either some round number of grams, such as 50 or 100, easily calculable into percentage; or it will be that known as the "a.s.say Ton" (see page 13) or some simple multiple or fraction of it, which is easily calculable into ounces. The reports, too, are at least as often made as ounces in the short ton of 2000 lbs., as on the more orthodox ton of 2240 lbs. Now the short ton is equal to 29,166.6 troy ounces; and the corresponding "a.s.say ton" is got from it by replacing ounces by milligrams. The advantage of its use is that if one a.s.say ton of ore has been taken, the number of milligrams of gold obtained is also the number of ounces of gold in a ton of the ore, and there is absolutely no calculation. Even if half an a.s.say ton has been taken the only calculation needed is multiplying the milligrams by two. On the other hand with a charge of two a.s.say tons the milligrams need halving. Where weights of this kind (_i.e._, a.s.say tons) are not at hand they may be easily extemporised out of b.u.t.tons of tin or some suitable metal, and it is better to do this than to array out the grams and its fractions at each weighing. The sets of "a.s.say tons," however, are easily purchased. As stated on page 13, the a.s.say ton for 2240 lbs.

is 32.6667 grams; and for the short ton, 29.1667 grams. If, however, the round number of grams be used and the result brought by calculation to the produce on 100 grams, the conversion to ounces to the ton may be quickly effected by the help of the table on page 107. As this table only deals with the ton of 2240 lbs., it is supplemented here by a shortened one dealing only with _the produce of 100 grams_ and stating the result in _ounces troy to the short ton of 2000 lbs_.

~Estimation of Small Quant.i.ties of Gold.~--_By the Balance._ In estimating minute quant.i.ties of gold there are one or two points, of importance to an a.s.sayer only in this a.s.say, where they will often allow one to avoid the working of inconveniently large charges. One of these is known as "weighing by the method of vibrations."

TABLE FOR CALCULATING OUNCES TO THE SHORT TON FROM THE YIELD OF GOLD FROM 100 GRAMS OF ORE.

----------+---------+-----------+---------+-----------+--------- Milligram.|Ounces to|Milligrams.|Ounces to|Milligrams.|Ounces to | the Ton.| | the Ton.| | the Ton.

----------+---------+-----------+---------+-----------+--------- 0.01 | 0.003 | 0.4 | 0.117 | 7.0 | 2.042 0.02 | 0.006 | 0.5 | 0.145 | 8.0 | 2.333 0.03 | 0.009 | 0.6 | 0.175 | 9.0 | 2.625 0.04 | 0.012 | 0.7 | 0.204 | 10.0 | 2.916 0.05 | 0.014 | 0.8 | 0.233 | 20.0 | 5.833 0.06 | 0.017 | 0.9 | 0.262 | 30.0 | 8.750 0.07 | 0.020 | 1.0 | 0.292 | 40.0 | 11.666 0.08 | 0.023 | 2.0 | 0.583 | 50.0 | 14.583 0.09 | 0.026 | 3.0 | 0.875 | 60.0 | 17.500 0.10 | 0.029 | 4.0 | 1.167 | 70.0 | 20.416 0.20 | 0.058 | 5.0 | 1.458 | 80.0 | 23.333 0.30 | 0.087 | 6.0 | 1.750 | 90.0 | 26.250 ----------+---------+-----------+---------+-----------+---------

Suppose a balance at rest in perfect equilibrium, with the pointer exactly over the middle point of the scale. Let the scale be a series of points at equal distances along a horizontal line; then, if a small weight be placed on one pan, the pointer will deviate from its vertical position and come to rest opposite some definite part of the scale, which will depend upon the magnitude of the weight added. The law determining this position is a very simple one; the deviation as measured along the points of the scale varies directly as the weight added. For example, with an ordinarily sensitive balance, such as is used for general purposes, one milligram will move the pointer along, say, three divisions of the scale; then two milligrams will move it six divisions; half a milligram, one and a half divisions; and so on. Of course, with a more sensitive balance the deviations will be greater.

Now the point at which the needle comes to rest is also the middle point about which it vibrates when swinging. For example, if the needle swings from the third to the seventh division on the right then [(7+3)/2] it will come to rest on the fifth. In working by this method the following conventions are useful: Always place the b.u.t.ton to be weighed on the left pan of the balance, the weights on the right; count the divisions of the scale from the centre to right and left, marking the former + and the latter -; thus -5 is the fifth division to the left. Then the position of rest is half the algebraic sum of two readings. For example, let the readings be 7 to the right and 3 to the left, then (+7-3)/2 = +2. The mean division is the second division to the right. If the student will place himself in front of a balance and repeat the following observations and replace the figures here given by his own, he will have no difficulty in grasping the method. First determine the _bias_ of the balance; suppose the unloaded balance swings +1.25 and -1; the bias then is (1.25-1)/2 = +.125 or one-eighth of a division to the right. Now having put on the b.u.t.ton to be weighed let the readings be +7.5 and +9.25, and (7.5+9.25)/2 = +8.375. Then the effect of the b.u.t.ton has been to move the pointer from +.125 to +8.375, or 8.25 divisions to the right; we should, therefore, add the weight equivalent of 8.25 divisions to the weights, whatever they may be on the right hand pan of the balance; if the divisions were to the left (- divisions) we should subtract. The value of 1 division is easily determined. Suppose the b.u.t.ton in the example were a 1 milligram weight, then we should have found that 1 milligram = 8.25 divisions .'. 1 division = .121 milligram.

This method of working adds very considerably to the power of a balance in distinguis.h.i.+ng small quant.i.ties.

[Ill.u.s.tration: FIG. 44_a._]

_By the Microscope_.--The use of the microscope also is a real advantage in estimating the weights of minute b.u.t.tons of gold where there is no undue risk in sampling, and where an error of say 1 in 20 on the quant.i.ty of gold is tolerable. For ores with copper, lead, zinc, &c., as well as for tailings rather poor in gold, this leaves a wide field of usefulness. The method is described on page 440, but the description needs supplementing for those who are not accustomed to the use of a microscope. The eye-piece of a microscope (fig. 44_a_, A) unscrews at _a_, showing a diaphragm at _b_, which will serve as a support for an eye-piece micrometer. This last, B, is a scale engraved on gla.s.s, and may be purchased of any optical instrument maker, though it may be necessary to send the eye-piece to have it properly fitted. When resting on the diaphragm it is in focus for the upper lens, so that on looking through the microscope, the scale is clearly seen in whatever position the instrument may be as regards the object being looked at. Suppose this to be a small b.u.t.ton of gold on a shallow, flat watch-gla.s.s, on the stage of the microscope. Bring the b.u.t.ton under the "objective" (_i.e._, the nose of the microscope), which should be about a quarter of an inch above the watch-gla.s.s; then looking through the instrument, raise the tube until the b.u.t.ton of gold, or at least some dust on the gla.s.s, comes into focus. If the b.u.t.ton is not in the field, rest the thumbs and index fingers, using both hands, on the edge of the watch-gla.s.s, pressing lightly but steadily, and give the gla.s.s a slow, short, sweeping motion; the b.u.t.ton will perhaps appear as an ill-defined blackness, because not quite in focus. Bring this into the centre of the field. Raise or lower the microscope until the b.u.t.ton appears with sharp outlines. If the scale does not cover the b.u.t.ton, rotate the eye-piece; this will bring the scale into a new position. Since the divisions over the b.u.t.ton are less distinct than the others, it is best to read the latter. Thus, in fig. 44_b_, there are 36 divisions on one side of the b.u.t.ton, and 35 on the other, making altogether 71. The whole scale is 80, therefore the diameter of the b.u.t.ton is 9 divisions. The value of each division obviously varies with the magnifying power employed. With most microscopes there is a telescopic arrangement whereby the tube may be lengthened; if this be done and the b.u.t.ton again brought in focus, it will be seen that, as measured on the scale, the b.u.t.ton is much larger than before. It is evident, therefore, the micrometer must always be used in the same way. The method given in the appendix (page 440), for finding the value of the scale when gold b.u.t.tons are to be measured is easy and satisfactory. When the b.u.t.ton of gold is so small that there is considerable risk of losing it in transferring to a watch-gla.s.s, it may be measured on the cupel, but for this purpose it must be well illuminated; this is best done by concentrating light on it with a lens, or with what comes to the same thing, a clean flask filled with water.

[Ill.u.s.tration: FIG. 44_b._]

Most a.s.sayers, however, using a micrometer in this way, would like to know its absolute value. To do this, a stage micrometer must be purchased. This is like an ordinary microscope slide (fig. 44_a_, C), and when looked at through a microscope it shows (fig. 44_c_) lines ruled on the gla.s.s at distances of tenths and hundredths of a millimetre, ten of each, so that the full scale is 1.1 mm. In the case ill.u.s.trated, 60 divisions of the scale in the eye-piece are just equal to the 1.1 mm., therefore 1 division equals .0183 mm. A cube of this diameter would contain (.0183.0183.0183) .0000061285 cubic mm. The corresponding sphere is got by multiplying by .5236; this gives .000003209 cb. mm. The weight of 1 cb. mm. of water is 1 milligram; and, since gold is 19.2 times as heavy as water (sp. g. = 19.2), the contents in cb. mm. must be multiplied by 19.2. This gives .0000616 milligram as the weight of a sphere of gold measuring 1 division.

[Ill.u.s.tration: FIG. 44_c._]

If every result had to be calculated in this way the method would be very laborious; but, having the figures for the first division, those of the others may be calculated by multiplying by the cube of the corresponding number. Thus, for the third division (333 = 27), the content of the cube (.000006128527) is .0001655 cb. mm.; the content of the sphere (.00000320927) is .0000866 cb. mm.; and the corresponding sphere of gold (.000061627) is .00166 milligram. With the help of a table of cubes the whole calculation for 25 or 30 divisions may be made in half an hour, and the results preserved in the form of a table will simplify all future work.

~a.s.say Operations.~--The actual work of the a.s.say resolves itself into three operations:--(1) The fusion of the ore and concentration of the "fine metal" (_i.e._, gold and silver) in a b.u.t.ton of lead; (2) The cupellation of the lead, whereby a b.u.t.ton of fine metal is obtained; and (3) the "parting" of the gold which separates it from the accompanying silver. The following description takes the order as here given, but the student, in learning the method, should first practise cupellation if he has not already done so; next he should practise the separation of gold from silver, taking known weights of fine gold (p. 63), varying from .5 or .3 gram down to quite minute quant.i.ties, and not resting satisfied until a sensitive balance can barely distinguish between the weights of gold taken and found. It may be noted here that if he has not a flatting mill at his disposal, then for large b.u.t.tons it is better to make an alloy with eight or nine parts of silver to one of gold, and attack it with acid without previous flattening, rather than accept the risk and labour of beating out a less easily attacked alloy to the necessary thinness with a hammer. It is only after a sense of security in gold parting has been acquired, that the attack of an ore can be profitably accomplished, and even then simple and easy ores should be first taken, pa.s.sing on to others more difficult, either because of a more complex mineral composition or a difficulty in sampling.

~Concentration of the fine Metal in Lead.~--The best flux for quartz, which makes up the earthy matter of most gold ores, is soda, and this is best added as carbonate or bicarbonate. By theory,[20] 50 grams of quartz will require 88.5 grams of the carbonate, or 140 grams of the bicarbonate, to form sodium silicate, which is a gla.s.sy, easily-fusible substance, making a good slag. If the bicarbonate is used, and heat is applied gradually, steam and carbonic acid are given off at a comparatively low temperature, and the carbonate is left; at a higher temperature (about 800 C., or a cherry-red heat) the carbonate fuses attacking the quartz, and giving off more carbonic acid; as the heat increases, and the attack on the quartz (which of itself is infusible) becomes complete, the whole ma.s.s settles down to a liquid sodium silicate, which is sufficiently fluid to allow the gold and lead to settle to the bottom. The fluid slag does to a certain extent dissolve some of the crucible, but not seriously. In a perfect working of this experiment, the first evolution of gases (steam and carbonic acid) should be gentle, so as to run no risk of its blowing the fine powder out of the crucible; and the heat at which the second evolution of carbonic acid is produced should be maintained until the reaction is completed, so that there may be little or no formation of gas in the fused ma.s.s to cause an effervescence which may force some of the charge over the edges of the crucible. Of course, in practice the ideal fusion is not attained, but there is no difficulty in approaching it closely enough to prevent the charge at any time rising above the level it reached at first in the crucible, and this should be accomplished. It is usual with quartzose ores to rely mainly on the action of carbonate of soda, but not entirely. Litharge is also used; it forms, on fusion with quartz, a silicate of lead, which is a yellow gla.s.s, easily fusible, and more fluid in the furnace than silicate of soda is. By theory, 50 grams of quartz would require 186 grams of litharge.[21] The reaction takes place without evolution of gas, and in its working the only point is to so regulate the heat that the litharge shall not fuse and drain under the unattacked quartz, leaving it as a pasty ma.s.s on the surface. Now, if in making up a charge for 50 grams of ore, we took 100 grams of bicarbonate of soda (equivalent to about 63 grams of the carbonate), this being five-sevenths of 140 grams (which by itself would be sufficient), leaves two-sevenths of the quartz to be fluxed by other reagents: two-sevenths of 186 grams (say 52 grams) of litharge would serve for this purpose. But if we used 10 grams of borax, which has a fluxing action about equal to that of the litharge, then 40 grams of the latter, or (making an allowance for the quartz being not quite pure) say 35 grams, will suffice. The fluxes, then, for the 50 grams of ore would be: bicarbonate of soda 100 grams, litharge 35 grams, and borax 10 grams; we could decrease any of these, and proportionately increase either or both of the others, and still rely on getting a fusible slag, which is the whole of the function of a flux, considered simply as a flux. It should be remembered, however, that the slag is a bi-silicate or acid slag, and that its acid character is increased by increasing the proportion of borax.

But in addition to the fluxes there is required about 30 or 40 grams of lead to collect the silver and gold. This is best added as litharge (say 40 grams) and flour (4 grams), or charcoal powder (2 grams). See pages 93 and 94. The full charge, then, would be:

Ore 50 grams.

Bicarbonate of soda 100 "

Litharge 75 "

Borax 10 "

Flour 4 "

These should be mixed, placed in a suitable crucible (a G Battersea, round, will do), and heated, at first at a red heat, but finally much hotter, so as to get a fluid and clean slag. When the charge has been in tranquil fusion for some little time, take it out and pour it into an iron mould. When cold, detach the b.u.t.ton of lead. The slag should be gla.s.sy, all through alike, and easily separable from the metal. With ordinary ores, this slag may be considered as free from gold. In an experiment in which 90 milligrams of gold were added, the full amount was obtained from the lead produced by the first fusion. But in certain cases, more especially where large amounts of metallic oxides are present, the slag is not so clean, and with these the slag should be powdered, mixed with 40 grams of litharge and 4 of flour, and melted again; it is an advantage to add a small prill of say 2 or 3 milligrams of silver to the charge, as it insures a visible product in the cupellation. Indeed, this last precaution is a good one to be taken wherever there is reason to expect very small b.u.t.tons. It has the further advantage, that, if the quant.i.ty of silver necessary for inquartation is known, the right quant.i.ty may be added here, so as to save a subsequent operation.

~Ores containing Oxides of Iron.~--Of the metallic oxides likely to be present in a slag, oxide of iron is the most important. Gold is occasionally found in a matrix of this substance, and in the a.s.say of "concentrates" largely made up of pyrites, this oxide will be formed in the preliminary calcination. Now, the lower oxide of iron (ferrous oxide, FeO) is easy to deal with; fused borax will dissolve about its own weight of it, and a silicate of soda (such as makes up the bulk of a slag in a gold a.s.say) will take up at least half as much. But the higher oxide (ferric oxide, Fe_{2}O_{3}) is more refractory; even 6 parts of borax yields a poor product, and slags with any considerable percentage of it are not satisfactory. A student attempting to recover gold from some haemat.i.te (in which there was about half an ounce of the metal), found in the slag nearly a gram of gold, although in the first fusion the slag appeared perfectly fluid. There is, however, no difficulty in getting good slags, even with large quant.i.ties of iron. For example, with 50 grams of ferric oxide, 10 of quartz, 30 of borax, 30 of soda,[22] 50 of litharge, and 7 of flour, the result was quite satisfactory. So, too, was 25 of quartz, 50 of soda, 50 of litharge, and 7 of flour. It is well, however, in such cases to have an ample proportion of flux and to aim at a larger b.u.t.ton of lead than usual by increasing the proportion of flour or charcoal (see also page 91). A charge used on the Randt for roasted "concentrates" (which we may roughly speak of as quartz and ferric oxide), is one a.s.say ton (about 30 grams) each of ore, soda, and borax, and one and a half a.s.say ton of litharge and 2 grams of charcoal. Whilst, for the same material, from which most of the gold has been extracted by "chloridising," 2.5 tons each of ore, borax, and soda, 4 of litharge, and 4 grams of charcoal are needed. This quant.i.ty requires a large crucible (I Battersea, round). In this the proportion of silicate of soda and borax counted together is to the oxide of iron as 4 to 1, on the supposition that the quartz and oxide of iron of the ore are in about equal quant.i.ties; but, in the larger charge especially, much oxide of lead would also remain as a flux.

~Ores containing Sulphides.~--In a.s.saying ores containing a large proportion of pyrites or mispickel, or both, the best plan is to take a portion and calcine so as to convert it into a product of the kind just considered. The weighed portion of ore should be placed in a clean crucible and be heated to incipient redness: with pyrites the first effect is to drive off about half the sulphur as vapour which burns as flame over the ore. At this stage care should be taken that there is no great increase of temperature, otherwise there may be more or less fusion, which would spoil the operation. When the sulphur flame ceases the solid sulphide of iron burns with visible incandescence and the charge should now be stirred with a flattened iron rod so as to expose fresh portions to the air. The top of the furnace must be open, so that air may have free access to the crucible. When stirring is no longer followed by visible burning the heat may be raised to full redness. The crucible is then lifted out (the stirrer still resting in it) and if the charge gives off no odour of burning sulphur it is shaken out into an iron mortar and mixed with the fluxes, taking care to clean the stirrer in the mixture. The charge is then replaced in the crucible in which the roasting was done and fused in the furnace. The resulting b.u.t.ton of lead is cupelled for fine metal. Ores rich in sulphides requiring this treatment are frequently "concentrates." For their a.s.say take 1 a.s.say ton (30 grams), calcine and mix with an equal weight of soda and of borax (30 grams each), and half as much again of litharge (1.5 tons or 45 grams), and with 2 grams of charcoal or 5 grams of flour.

Where the sulphides are present in smaller proportion (10 per cent. or less), they may be taken as serving the purpose of flour or charcoal (see page 95); the sulphur and iron are oxidised at the expense of the litharge with a consequent separation of lead as metal. If the proportion of sulphides is not sufficient to give a large enough b.u.t.ton of lead, some charcoal or flour should be added. On the other hand, if they are in small excess and give a b.u.t.ton of lead somewhat sulphury, _i.e._, hard and brittle, it may be remedied by the judicious addition of nitre; this last reagent, however, should not be used in large quant.i.ty. A plan much used to prevent sulphury b.u.t.tons is to insert an iron rod or a nail in the charge in the crucible; the iron takes the sulphur forming sulphide of iron which in moderate quant.i.ty does not form a separate layer of matte but dissolves in the slag. A slag formed of 50 grams of quartz, 100 soda, and some borax, may take up in this way some 10 or 12 grams of sulphide of iron. If, however, an ore gives a layer of matte or speise, it is best to repeat the a.s.say by the method of calcining before fusion.

~Cyanide Charges, etc.~--In a.s.saying the "tailings" which are to be treated in a cyaniding plant the following charge is used:

Tailings 3 a.s.say tons or 100 grams.

Litharge 4.5 " 150 "

Soda 4.5 " 150 "

Borax .75 " 25 "

The sand is a.s.sayed without any further crus.h.i.+ng and the a.s.say is made in duplicate.

The residues after treatment with cyanide, differing from the tailings merely in being poorer in gold because of the extraction by the solution of cyanide, are run down with the same fluxes in the same relative proportions. But four charges of 2.5 a.s.say tons (say 75 grams) are worked, and two of the resulting b.u.t.tons are scorified together and then cupelled, etc., so as to give duplicate a.s.says on charges of 5 a.s.say tons. This is one of the cases in which it is desirable to add a small portion of silver before cupelling.

In a.s.saying the "cyanide liquors" for gold, 2 a.s.say tons of the liquor are measured out (58.3 c.c. for the ton of 2000 lbs., 65.3 c.c. for the other) and are evaporated to dryness in a lead dish weighing about 35 grams. Such a dish is easily extemporised out of a piece of lead foil, if the ordinary vessel is not at hand; but care must be taken that the lead is free from gold. The dish with the dried residue is then scorified and the resulting b.u.t.ton of lead is cupelled.

[Ill.u.s.tration: FIG. 44c.]

In some cases the fusion of the ore may be replaced by a treatment with solution of cyanide of pota.s.sium and the gold recovered from the solution in the way just described. For this purpose the ore should be in not too fine powder, otherwise there will be great difficulty in filtering; a sand which will pa.s.s a 30 sieve and having no large proportion of very fine stuff will do. Not less than 200 grams should be taken; and as an extraction apparatus a bell jar capable of holding half as much again may be used. Such a jar may be extemporised by cutting off the bottom of a bottle by leading a crack around it with a red hot poker; or a lamp chimney will serve the purpose. The smaller mouth of the jar is closed by a perforated cork provided with a clipped tube after the manner of a burette (see fig. 44c). In the jar, just over the cork, put a plug of loose asbestos or gla.s.s wool, or a piece of sponge to act as a filter; a layer of broken gla.s.s, coa.r.s.e at the bottom and fine at the top, will serve the same purpose. On this, place the charge of ore to be extracted. Prepare a solution of cyanide of pota.s.sium in water, with 5 or 10 grams of the salt to the litre. It may be that the whole point of the a.s.say depends on the solution being of a definite strength; as, for example, where the relative efficiency of solutions of different strengths is being determined, when it will be best to estimate the quant.i.ty of cyanide of pota.s.sium in the dilute solution by the method given at the end of this article (page 160). Pour the cyanide solution on to the ore, letting the first portions to come through run into the beaker, but as soon as the ore is thoroughly wetted close the clip and allow to stand for several hours. Then, opening the clip, run through more cyanide solution and then water, so as to wash the gold-carrying liquor thoroughly into the beaker. It is no matter if the liquor is a little bit turbid; transfer it to a lead dish, evaporate, scorify, and cupel in the usual fas.h.i.+on.

The a.s.say of gold-zinc slimes, which is the precipitate formed by zinc acting on cyanide solutions of gold, may be made by wrapping 2 or 3 grams in 40 grams of sheet lead and scorifying, cupelling, &c. The amount of impurity in the stuff varies greatly; it is usually calcined and mixed thoroughly with soda 40 per cent., borax 30 per cent., and sand 10 per cent., and melted in graphite pots. The b.u.t.tons of bullion obtained are afterwards remelted with borax and run into bars, the fineness of which varies from 600 to 830 thousandths. The bars are sampled by chipping off diagonally opposite corners: or better, by drilling, the drillings being freed from pieces of steel with the help of a magnet.

~Cupellation.~[23]--The cupellation of lead for gold differs very little from that of lead carrying silver. When the gold is accompanied by a larger proportion of silver, and both have to be determined, the cupellation must be conducted exactly as in a silver a.s.say, the usual precautions being taken to moderate the temperature so as to lessen the cupellation loss and to promote a slow and undisturbed solidification in order to avoid spirting. If, however, the gold predominates the finish should be effected at a higher heat, as the melting-point of gold is 100 higher than that of silver. The bad effect of a higher temperature in increasing the cupellation loss need hardly be considered in the case of such small b.u.t.tons of gold as are obtained in a.s.saying gold ores, as any loss there may be is hardly appreciable by the balance. With larger quant.i.ties of gold, however (as in a.s.saying gold bullion), this loss becomes important; and it is therefore necessary to very carefully regulate the temperature of the m.u.f.fle so as to minimise the loss.

~The cupels~ are made of well-burnt bone-ash, of the fineness of coa.r.s.e wheat flour, moistened with one-twelfth its weight of water and compressed into shape in suitable moulds. The moulds sold for this purpose are often of unsuitable shape. Since lead has a specific gravity of over 11, a cup to hold from 15 to 25 grams of molten lead need not have a capacity of more than about 2 c.c. A hollow about 1 inch across and 1/4 inch deep is sufficient; and the body of the cupel to absorb this weight of lead should itself weigh from 20 to 25 grams. The b.u.t.ton of lead in a gold a.s.say may be twice as heavy as this. For these larger b.u.t.tons a hollow 1-1/3 inch across and 1/3 inch deep will be sufficient.

If these larger cupels are not at hand the larger b.u.t.tons will have to be reduced in size by a scorification before cupelling. In some cases this preliminary scorification is advantageous or even necessary: this may be because the lead is hard and impure, or it may be that a very small b.u.t.ton of gold is expected. In the latter case it is best to scorify the lead down to something less than 1 gram, and to perform the cupellation on a specially prepared small fine cupel. These small cupels are best made by grinding the unsaturated portion of a used cupel to a fine powder, and compressing the dry powder into a small Berlin crucible or scorifier; the face should be made quite smooth by pressure from a pestle. On such cupels a small speck of gold (less than .01 milligram) will be left in a good shape and easily visible; but the cupel must be withdrawn from the m.u.f.fle as soon as the cupellation is finished to make sure of always getting the b.u.t.ton in good condition. In places, such as Mints, where large numbers of bullion a.s.says are regularly made a special form of cupel is used so that not less than six dozen a.s.says may all be cupelled at the same time in a m.u.f.fle of ordinary size. These cupels are square blocks, a little less than 2 inches across, and a little more than three quarters of an inch deep. Each block carries four hollows of about .7 inch across and .3 inch deep. A m.u.f.fle, on a floor s.p.a.ce of 6 inches by 12, would take 3 of these blocks abreast and 6 deep, and thus provide the means for 72 a.s.says.[24]

Cupels made with wet bone-ash should be slowly dried; and if in the m.u.f.fle they can be slowly brought to an orange-red heat it is all the better. Under no circ.u.mstances must the lead be placed on the cupel before the latter has been so thoroughly heated that it can no longer give off steam or gas of any kind. For this gas bubbling through the molten metal spatters it, thus spoiling one a.s.say and throwing doubt on all the rest. Again, the risk of freezing at the start is much greater with a cupel which has not been properly heated.

The best plan is to do all the cupellations in batches. After the m.u.f.fle has cooled down for the withdrawal of the last batch, and the old cupels have been taken out, the new cupels for the next batch should be put in their place. The furnace should then be stoked and made ready for the next cupellations; by the time the furnace is ready the cupels will be ready also. There should be no unnecessary handling of the cupels once they have been placed in the m.u.f.fle.

~The cupellation temperature for gold~ is an orange-red heat or perhaps a little hotter. Beginners, who are apt to overheat their furnace, should avoid a heat which can properly be called yellow. Dr. T.K.

Rose[25] has determined the temperature of a m.u.f.fle during the cupellation of gold-silver alloys at the Royal Mint. In one m.u.f.fle the temperature ranged from 1065 to 1095 C.; the lower temperature was of course in the front of the m.u.f.fle. In another it ranged from 1022 to 1062, and here the m.u.f.fle appeared to the eye "decidedly cooler than usual." The alloy left after cupelling was made up of 1 part of gold to 2-1/2 parts of silver, and was fused at 952; hence the usual temperature of cupellation was, say, 120 or 130 above the melting-point of the residual metal. To obtain some real knowledge as to the meaning of these figures, the student should prepare pointed pieces of the following metals: silver, which melts at 945; gold, which melts at 1035; and an alloy, half silver, half gold, which melts at 990.

These should be placed on clean cupels in a m.u.f.fle almost entirely closed; the temperature should be very slowly raised, and the appearance of the m.u.f.fle when each metal begins to melt should be carefully noted.

The cupelling temperature in Dr. Rose's experiment was as much above the melting-point of gold as this is above that of the silver-gold alloy.

The _finish of the cupellation_ of gold or gold-silver alloys is practically the same as with pure silver; there is the same thinning out of the litharge into a luminous film which becomes iridescent before the brightening. But the danger of spirting decreases as the proportion of gold becomes greater, and disappears when the gold is much over 30 per cent. Nevertheless it is well to let such b.u.t.tons become solid undisturbed and protected from draughts in the body of the m.u.f.fle. This means closing the m.u.f.fle and allowing the furnace to cool down somewhat before withdrawing the cupels. b.u.t.tons solidified in this way are more malleable than when they are withdrawn promptly on the finish of the cupellation. This is important with large b.u.t.tons, as in a bullion a.s.say. On the other hand, very small b.u.t.tons, especially such as have to be measured rather than weighed, should be withdrawn as soon as the luminous film has disappeared. For when this is done the b.u.t.ton can be loosened from the cupel by merely touching it with the point of a pin, and is then safely and easily transferred to a watch gla.s.s by touching it with the head of a pin which has been moistened. It adheres to this, and if the pin is not too wet comes off at once on touching the gla.s.s, or in any case will do so on gentle warming.

Molten gold, with little or no silver, has a peculiar colour which is easy to recognise; it is more globular than a b.u.t.ton of silver of the same size would be, and it shows less adhesion to the cupel. Just after becoming solid it glows beautifully, and this is so marked that it is a valuable help in finding the position of a b.u.t.ton when it is more than ordinarily minute.

If the b.u.t.ton left from cupellation is yellow it is at least half gold, and a rough guess as to the proportion of gold may be made from its yellowness; the rest of the metal is generally silver. The presence of platinum or one of the platinum group of metals makes the surface of the b.u.t.ton dull and crystalline. The native alloy of osmium and iridium does not alloy with gold, however, but falls to the bottom of the molten metal. It shows itself in the subsequent parting as a black spot or streak on the under surface.

The b.u.t.tons are removed from the cupel with a pair of pliers and then brushed to remove adherent litharge and bone-ash. Some a.s.sayers advise cleaning by dipping in warm dilute hydrochloric acid followed by was.h.i.+ng in water and drying. The b.u.t.ton is next weighed. When the quant.i.ty of silver obtained is not required to be known the weighing may sometimes be omitted. The next operation in either case is parting either with or without a previous inquartation.

_The loss of gold in cupellation_ is by no means always inconsiderable.

In three cupellations of 1 gram of gold with 20 grams of lead made purposely at a very high temperature the cupel absorbed 6.04, 6.20, and 6.45 milligrams of gold. Hence at a high temperature there may easily be a loss of more than half a per cent. of the gold. In ten cupellations with the same quant.i.ties of gold and lead, but at an ordinary temperature, the gold recovered from the cupels varied from 1.37 to 1.92 milligrams, and gave an average of 1.59 milligrams. In round numbers the cupellation loss of pure gold is .15 per cent.

But if the gold be alloyed with _silver_ the loss is diminished, as is shown by the following experiments. Gold, .3 gram, was cupelled with 10 grams of lead and varying amounts of silver, and the cupels were a.s.sayed for gold with the following results:

Silver in the alloy .3 gram .6 gram .9 gram Gold in the cupel .47 milligram .32 milligram .17 milligram

These, calculated on the .3 gram of gold, give the loss as .157, .107 and .057 per cent. respectively. The effect of _copper_, on the other hand, is to increase the cupellation loss, which, silver being absent, may from this cause rise to .3 per cent., even when the temperature is not excessive.

In the ordinary a.s.say of gold-copper alloys a constant weight of the alloy is always taken; hence as the weight of copper in a cupel charge increases, the weight of gold decreases. The silver, on the other hand, is always very nearly two and a half times as much as the gold, whatever its quant.i.ty may be. But the cupellation loss is smaller with less gold and greater with more copper, and it so happens in these a.s.says that these two opposites nearly neutralise one another. Mr. W.F. Lowe[26]