Withcharcoal,18gramsof lead."flour,18"""tartar,22""
Comparing these with the results with litharge, in the previous table it will be seen that the same quantity of reducing agent has in each case brought down 16 grams less of lead, so that a larger amount of the reducing agent must be added to get a button ofthe same weight as that obtained with litharge. To get a button of a desired weight, say 22 grams, we must add reducing agent sufficient to throw down 22 + 16 or 38 grams of lead, which would require 3.4 grams of flour. If this amount of flour is fused with 80 grams of red lead, a button of lead weighing 22 grams will be formed, the other 16 grams being kept up by the oxygen of the red lead.
If the quantity of red lead differs from 80 grams, this rule must be modified. With 40 grams of red lead, for example, we should add an excess of reducing agent sufficient to throw down 8 grams of lead instead of 16. Similarly, with 160 grams of red lead, we should add enough to throw down 32 grams.
The following rule will enable one to calculate the weight of flour required to produce a button of lead of any desired weight from any given quantity of red lead. Each 5 grams of red lead present diminishes the weight of the lead by 1 gram. If then wedivide the weight of red lead in a charge by 5, and add this to the weight of lead required, the sum divided by 11 will give the weight of flour which must be added. Using 80 grams of red lead and wanting a button of 20 grams, we should add 3.3 grams of flour.
80/5 = 16; 16+20 = 36; 36/11 = 3.3 nearly.
The following are some results obtained which will illustrate the rule:—
Red Lead used.Flour used.Lead got.40grams3grams25.0grams100"3"13.5"80"4"30.0"80"5"40.0"
On the Reducing Effect of Metallic Sulphides, and the Counteracting Effect of Nitre.—The sulphides found in ores will reduce a button of lead from oxide of lead just as flour does; and, as charcoal, flour and tartar differ in their reducing power, so equal weights of the different mineral sulphides throw down different weights of lead.
One gram of iron pyrites yields about 11 grams of lead. One gram of copper pyrites, blende, fahlerz, or mispickel, yields 7 or 8 grams of lead, whilst 1 gram of antimonite will give 6, and 1 gram of galena only a little over 3 grams. It is evident that if an ore carries much of these sulphides, the quantity of lead reduced will be very much larger than that required for an assay. To counteract this effect nitre is added;1 gram is added for each 4 grams of lead in excess of that required. For example: with a 20-gram charge of an ore containing 50 per cent. of pyrites, if no nitre were added, 110 grams of lead would be got; or, if therewas not sufficient oxide of lead to yield this quantity of metal, the button would be sulphury. To reduce the weight of the button by 80 grammes, we should add 20 grams of nitre, if litharge were used; or if red lead were used, we should add 16 grams of nitre,since the oxidizing effect of 20 grams of red lead is equivalent to that of 1 of nitre, and since 80 grams of red lead are generally used in a charge. Two assays of an ore of this kind with these quantities of nitre gave 26.0 grams of lead with litharge, and 22.5 grams with red lead.
It is best to use in these assays 80 grams of red lead, 20 of soda, and 5 of borax, with 20 grams of the ore. If the lead got by the preliminary fusion in a small crucible with litharge (described under "ores containing much sulphides") is known, the following table will indicate the quantity of nitre, or flour, to be added with this charge:—
Lead got in Preliminary Fusion with 3 grams of Ore.Flour to be added to the Assay.Nitre to be added to the Assay.0.0 gram3.3 gramsnone3.0 grams1.3 gram—6.0 "none4.0 grams9.0 "—9.0 "12.0 "—14.0 "15.0 "—19.0 "18.0 "—24.0 "21.0 "—29.0 "
If litharge is used in the assay instead of red lead 4 grams more nitre, or 1.5 gram less flour must be used. When more than a few grams of nitre are added to a charge the proportion of "soda" and borax should be increased, because one of the products of the reaction of nitre upon sulphides in the presence of soda is sulphate of soda, and because the "soda" thus used up no longer serves as a flux; more borax should be added, as it is the best flux for the metallic oxides which are formed in the process. If in an assay too large a button of lead is got, even after this calculation has been made, and the assay is repeated, add 1 gram more nitre for each 4 grams of lead in excess. Sometimes the assay appears tranquil before the nitre has produced its full effect; in such cases it is well to seize the crucible with the tongs and mix its fused contents by rotating them; if this causes an effervescence, the crucible should be replaced in the fire and the fusion continued. The following experiments will illustrate the extent to which the above rules may be relied on. In all of them the standard flux was used, viz.:—80 grams of red lead, 20 of soda, and 5 of borax.
Pyrites55552.55101520Quartz—20—2017.515105Nitre——55—41628.541Lead got42.536.016.019.011.522.522.526.527.5
Copper Pyrites8888Quartz—12—12Nitre——44Lead got47.534.033.026.0
Antimonite8888Quartz—12—12Nitre——44Lead got29.026.013.013.0
Galena101010101520Quartz.—15—155—Nitre——333.57Lead got17.019.08.08.018.518.5
A similar set of experiments, with 80 grams of litharge instead of 80 grams of red lead, gave:—
Pyrites4444710Quartz—15—151310Nitre——5512.520Lead got46.540.525.524.527.026.5Copper Pyrites5555Quartz—15—15Nitre——55Lead got44.532.523.025.0Blende555510Quartz—15—1510Nitre——5515Lead got41.538.521.522.521.6Antimonite555510Quartz—15—1510Nitre——5510Lead got31.032.511.512.518.7Galena101010101520Quartz—15—155—Nitre——557.511Lead got33.533.513.014.019.522.7
The variation in some of these experiments, in which we might have expected similar results, is due to the fact that the sulphur, and in some cases the metals, are capable of two degrees of oxidation. For example: theoretically 1 gram of iron pyrites (FeS2) would yield 8.6 grams of lead if the sulphur were oxidised to sulphurous oxide (SO2), and the iron to ferrous oxide (FeO); whilst if the sulphur were oxidised to sulphate (SO3), and the iron to ferric oxide, 12.9 grams of lead will be thrown down. Similarly the yield with copper pyrites would be 7.5 or 11.6; with blende, 6.4 or 8.5; with antimonite, 5.5 or 8; and with galena, 2.6 or 3.4. As regards the metals, the lower oxide will always be formed if the assay is carried out properly (fused under a cover, and with a sufficiency of reducing agent). But the proportion of sulphur oxidised completely will vary with the conditions of the assay. With a slag containing much soda the tendency will be to form sulphate, and, in consequence, a big reduction of lead; whilst with an acid slag containing much quartz the tendency will be for the sulphur to go off as sulphurous oxide (SO2). In a fusion with litharge alone all the sulphur will be liberated as the lower oxide, whilst with much soda it will be wholly converted into sulphate. For example: 3 grams of an ore containing a good deal of pyrites and a little galena, gave, when fused with litharge, 16.5 grams of lead. A similar charge, containing in addition 20.0 grams of soda, gave 22.5 grams of lead.
It will be noted from the experiments that 1 gram of nitre kept up on the average 4 grams of lead; the range being from 3.2 with acid slags to 5.3 with very basic ones. These facts serve to explain some apparently irregular results got in practice.
The process is as follows:—The cupels, which should have been made some time before and stored in a dry place, are first cleaned by gentle rubbing with the finger and blowing off the loose dust; and then placed in a hot muffle and heated to redness for from 5 to 10 minutes before the alloy to be cupelled is placed on them. The reasons for this are sufficiently obvious: the sudden evolution of much steam will blow a cupel to pieces; and, if the whole of the water has not been removed before the cupel is filled with molten lead, the escaping steam will bubble through, and scatter about particles of the metal. If some particles of unburnt carbon remain in the bone ash, a similar result will be produced by the escape of bubbles of carbonic acid as soon as the fused litharge comes in contact with them. The cupels having been prepared are arranged in a definite order in the muffle, and the assay buttonsare arranged in a corresponding order on some suitable tray (cupel tray, fig. 41); the heat of the muffle being at bright redness. Then with the help of the tongs (fig. 42) the assay buttons should be placed each in its proper cupel; a note having been previously made of the position it is to occupy, and the door of the muffle closed.
Fig. 41.
This part of the work should be done promptly, so as not to unduly cool the muffle: the start requires a fairly high temperature, and is a critical part of the process. A black crust forms at once on the surface of the lead; but this ought soon to fuse and flow in greasy drops from off the face of the metal, so as to leave the latter fluid with a well-defined outline, and much brighter than the cupel. If this clearing does not take place, the buttons are said to be frozen; in which case the temperature must be raised, some pieces of charcoal put in the muffle, and the door closed. If they still do not clear, the heat must have been much too low, and it is best to reject them and repeat the assays.
Fig. 42.
When the buttons have cleared it is well to check the draught of the furnace, and to partly open the door of the muffle, so as to work at as low a temperature as is compatible with the continuation of the process.[11]Too low a temperature is indicated by the freezing of the buttons and the consequent spoiling of the assays. Experience soon enables one to judge when the heat is getting too low. A commoner error is to have the heat too high: it should be remembered that that which was high enough to clear the buttons at starting is more than sufficient to keep the process going. At the finish a higher temperature is again required: therefore the door of the muffle should be closed and the furnace urged. The finish is easily recognised. The drops of litharge which in the earlier stages flow steadily from the surface of the alloy, thin off later to a luminous film. At the end this film appears in commotion, then presents a brilliant play of colours, and, with a sudden extinction, the operation is finished. The metal again glows for an instant whilst becoming solid.
If the button is a small one the cupel is withdrawn at once and placed on that square of the cupel tray which corresponds tothe position it occupied in the muffle. If, however, it is fairly large precautions must be taken to prevent spirting.
Molten silver dissolves oxygen from the air and gives it off on solidifying; the escape of the gas on sudden cooling is violent and, by throwing off particles of the metal, may cause loss. This is called "vegetation" or "spirting." The silver is apparently solid when spirting takes place; the crust breaks suddenly and some of the metal is forced out. The evil is best guarded against by slow cooling and avoiding draughts. With large buttons of silver precautions should never be omitted. One plan is to allow the cupels to cool in the muffle itself, the mouth being closed with hot charcoal. Another is to cover the cupel with another cupel previously heated to redness; in this case the silver cools between two hot cupels, and, of course, cools slowly. A third plan is to withdraw the cupel to the door of the muffle, holding it until it begins to get solid and then immediately to put it back into the hotter part of the muffle.
Silver remains after cupellation in flattened elliptical buttons, adhering but only slightly to the cupel. Its upper surface should show faint markings as if it were crystalline. The presence of platinum renders it still more crystalline, but removes the characteristic lustre and renders the metal dull and grey. Copper, if not completely removed, has a very marked effect on the appearance of the button: the metal is spread out, damping, as it were, and firmly adhering to the cupel, which latter in the neighbourhood of the metal is almost black with oxide of copper. Sometimes the silver button is globular, or even more sharply rounded on its under than on its upper surface; it is said that this is due to the presence of lead. Gold may be present even to the extent of 50 per cent. without showing any yellow colour.
The appearance of the cupel affords some useful information. The presence of cracks evidently due to shrinkage indicates a badly made cupel. If, however, they are accompanied by a peculiar unfolding of the cupel, the margin losing its distinctness, it is because of the presence of antimony. When lead is the only easily oxidisable metal present, the stained portion of cupel is yellow when cold. A greenish tint may be due to small quantities of copper or, perhaps, nickel, cobalt, or platinum. Larger quantities of copper give a greenish grey or almost black colour. A dark green and corroded cupel may be due to iron. Rings of pale-coloured scoria may be due to tin, zinc, antimony, or arsenic. When the cupel shows signs of the presence of these metals in objectionable quantity, it is well to repeat the assay and scorify so as to remove them before cupellation.
The button should be detached from the cold cupel by seizingwith a pair of pliers: the under surface should be distorted by squeezing or hammering the button so as to loosen the adhering bone ash. The cleaning is easily completed by rubbing with a clean hard brush. After cleaning the buttons are best put on a tray of marked watch-glasses, and then taken to the balance and weighed. The weight of silver got needs a small correction; (1) by deducting for the amount of silver introduced by the lead or oxide of lead used in the assay;[12]and (2) by adding for the cupellation loss.
Loss in Cupellation.—During the whole process of cupelling a silver lead alloy a more or less abundant fume may be observed rising from the cupel. This furnishes an evident loss of lead and a possible loss of silver; for although silver at the temperature of cupellation gives off no appreciable vapour, it is known that such fume formed on a large scale contains silver. It is, however, difficult to believe that the small amount of lead vapourised carries with it a weighable amount of silver. That it does not do so in the ordinary way of working is shown by the fact that a button of silver equal in weight to the silver lost in cupelling may be got by smelting the cupel and cupelling the resulting button of lead. The loss of silver by volatilisation is altogether inconsiderable, unless the temperature at which the operation is performed is much too high.
Another possible source of loss is the infiltration of small particles of alloy into the cupel. The cupel is necessarily porous, and particles of metal may perhaps drain into it, more especially if the bone ash is not in fine powder; but if this is the main source of loss it is hard to see why, in cupelling equal weights of silver and gold, the loss is not equal in each case. It is not easy to believe that the mere filtration of the fused alloy will effect such a change in the proportion of the metals as that which actually occurs. For example: a cupel on which an alloy consisting of 0.80 gram of silver, 0.47 gram of gold, and 25 grams of lead had been cupelled, was found to contain 7-1/2 milligrams of silver, and rather less than half a milligram of gold. Assuming, for the sake of argument, that the gold present had filtered into the cupel in the form of small drops of alloy, it would have been accompanied by less than a milligram of silver, and the presence of the extra 6 or 7 milligrams of silver must have been due to a different cause. There can, thus, be little doubt that the cause of the greater part of the "cupellation loss" is a chemical one and cannot be counteracted by a mechanical contrivance.[13]In cupellation,then, there is a loss, apart from imperfect working, inherent in the process itself; and as the amount of this loss varies under different conditions, it is necessary to study it somewhat in detail.
The following experiments are taken without selection from the work of one student. Three experiments were made for each determination, and the mean result is given. By "range" is meant the difference between the highest and lowest result and the percentage loss is calculated on the silver present. The silver added in the lead used has been deducted.
Effect of Varying Lead.—In each experiment 0.4 gram of silver was taken and cupelled with the lead. The silver loss and "range" are expressed in milligrams.
Lead Used.Silver Lost.Range.Percentage Loss.Grams.106.51.01.62207.01.01.754012.01.53.006012.70.53.17
The loss increases with the lead used.
Effect of Varying Temperature.—0.4 gram of silver was cupelled with 20 grams of lead.
Temperature.Silver Lost.Range.Percentage Loss.Bright red7.01.01.75Clear yellow17.31.74.32
The difference in temperature in these experiments was much greater than would occur even with careless work.
Effect of Varying Silver.—20 grams of lead were used in each cupellation.
Silver Taken.Silver Lost.Range.Percentage Loss.Milligrams.12.50.70.25.625.01.40.15.650.01.60.43.2100.02.90.32.9200.05.60.52.8400.07.01.01.7800.09.71.01.2
It will be seen that, although the quantity of silver lost increaseswith the silver present, the percentage loss is greater on the smaller buttons.
The following results are often quoted:—Cupelling 1 grain of silver with 10 grains of lead, the loss was 1.22 per cent.; 10 grains of silver with 100 grains of lead, loss 1.13 per cent.; 25 grains of silver cupelled with 250 grains of lead, lost 1.07 per cent. The proportion of silver to lead was the same in the three experiments, and the largest button gave the best result. Evidently, if the quantities of lead had been the same in the three experiments (say, 250 grains in each case), the loss on the smaller quantities of silver would appear worse in the comparison.
In judging these results, it must be borne in mind that it is difficult to regulate the temperature, &c., in consecutive experiments so as to get exactly similar results, so that the range in consecutive cupellations is greater than that in a batch cupelled side by side.
Effect of Copper and Antimony.—0.1 gram of silver was cupelled with 20 grams of lead, and to one batch 0.5 gram of antimony, and to another 0.5 gram of copper was added.
Silver Lost.Range.Loss in Percentage.Without addition2.90.32.9With antimony3.20.23.2With copper4.91.74.9
Perhaps the antimony has so small an effect because it is eliminated in the earlier part of the process, while the silver is still alloyed with, and protected by, a large proportion of lead; whilst the copper on the other hand makes its fiercest attack towards the close, when the silver is least capable of resisting it. The ill effects of copper are most strongly felt when the quantity of lead present is not sufficient to remove it: the coppery button of silver got under these conditions is very considerably less than the weight of silver originally taken.
Although the above is a fair statement of the loss attending average work, it will not do in very important and exact work to place too much reliance on the figures given, or, indeed, on any other set of figures, with the object of correcting the result of an assay. Each man must rely on his own work.
It is easy to determine what allowance must be made for the loss in cupellation by cupelling side by side with the assay piece an alloy of similar andknowncomposition. For, if the two pieces are very nearly alike, we may justly conclude that the loss on each will be the same; and if, further, we take the average of three or four such determinations we shall get results accurate within 0.1 per cent. The method of getting such results may be best explainedby one or two illustrations. This method of working is termed "assaying by checks."
Suppose we have an alloy of silver and lead in unknown proportions and that by cupelling two lots of 10 grams each there is got from I. 0.1226 gram of silver, and from II. 0.1229 gram. We should know from general experience that the actual quantity of silver present was from 2 to 4 milligrams more than this. To determine more exactly what the loss is, the following plan is recommended:—The two silver buttons are wrapped up each in 10 grams of lead, and cupelled side by side with two other lots of 10 grams of the original alloy. If now the two buttons I. and II. weigh 0.1202 and 0.1203, they will have suffered in this second cupellation an average loss of 2.5 milligrams. Suppose the two fresh lots of alloy gave 0.1233 and 0.1235 of silver, the average loss on these would also be 2.5 milligrams. Add this loss to each result, and take the mean; which is in this case 0.1259.
If copper is present in the alloy as well as silver, it is necessary to add about the same quantity of copper to the checks as is supposed, or known, to be present in the assays. If the substance to be assayed is an alloy of silver and copper, first cupel 0.5 gram of it, with, say, 10 grams of lead, and weigh the resulting button of silver, in order to get an approximate knowledge of its composition. Suppose the button weighs 0.3935 gram. We know that this is below the truth: for the sake of round numbers take it as 0.4, and assume that the rest of the alloy (0.1 gram) was copper. Two check pieces are then weighed out, each containing 0.4 gram silver and 0.1 gram of copper wrapped in 5 grams of lead. Of course the silver must be pure. And there is also weighed out two (or better, four) assay pieces each containing half a gram of the alloy wrapped in 5 grams of lead. The whole lot are then cupelled as nearly as possible under the same conditions. With four assay pieces, the cupels should be placed close together in two rows of three across the muffle; the two check pieces are put in the middle cupels. Suppose the buttons of silver got weighed as follows:—
Check piecesI.0.3940II.0.3945Assay piecesI.0.3905II.0.3912III.0.3910IV.0.3909
The average loss on the two check pieces is 5.7 milligrams, and the average result of the four assay pieces is 0.3909. Add the average loss to the average result, and there is got the corrected result, 0.3966. And if 0.5 gram of alloy contain 0.3966 of silver, 1000 will contain 793.2 of silver, and this is the degree of fineness.
A correction for the loss in cupellation is always made in thisway when rich alloys are being assayed; and in the case of rich ores it may be done after the manner of the first of the above illustrations. There is another method of working which relies more on experiment. This is to smelt the cupel as described further on (p. 114), and to again cupel the resulting button of lead. The button of silver got in this second cupellation is added to that first obtained. It will sometimes, but not often, happen that the two buttons together will slightly exceed in weight the silver which was actually present. This is because of the retention in the buttons of a small quantity of lead. It has been stated that the proportion of lead thus retained may be as much as 1% of the silver present; this, however, can only be under exceptional conditions. A determination of the actual silver in the buttons got in the series of cupellations quoted on pages 102, 103, gave an average percentage of 99.85, so that even with the larger buttons the effect of the retained lead would be only to increase the weight by about 1 milligram. In the method of working with checks, the retained lead has no disturbing influence.
The proportion of lead requiredfor the cupellation of any particular alloy requires consideration. With too much lead the time occupied in the process is increased, and so is the loss of silver; on the other hand, too little lead is of greater disadvantage than too much. From 8 to 16 parts of lead are required for each part of silver alloy, or, if gold is present, about twice as much as this must be used. For the cupellation of 1 gram of a silver copper alloy containing different percentages of copper, the following quantities of lead should be used:—
Percentage of Copper in Alloy.Lead Required.56grams108"2010"3012"4014"50-10016-18"
The alloy, in not too large pieces, is wrapped in the required weight of lead foil and charged into the cupel at once; or the lead may be put in first, and, when the cupellation has fairly started, the alloy may be added wrapped in tissue paper; or a portion of the lead may be first started and the alloy wrapped in the remaining lead and subsequently added. The cupellation of large quantities of alloy or of alloys which contain tin, antimony, iron, or any substance which produces a scoria, or corrodes the cupel, must be preceded by a scorification. The advantages of this are that the slag is poorer in precious metalthan that found on a cupel and is more easily collected and cleaned; that larger quantities of metal can be treated, and that, even if the substance is in part infusible, or produces at the start a clinkery mass or scoria, the oxide of lead gradually accumulates, fluxes the solid matters, and produces a good final result; but if the oxide of lead by itself is not sufficient for the purpose, borax or some other flux can be easily added.
If the button of silver got is very small its weight may be estimated from its size; but it must be remembered that the weight varies as the cube of the diameter. If one button has twice the diameter of another it is eight times as heavy and so on. Scales specially constructed for measuring silver and gold buttons may be purchased; but it is much better to make the measurement with the help of a microscope provided with an eyepiece micrometer.
If the length of the long diameter of a silver button be taken the following table will give the corresponding weight in milligrams:—
Diameter.Weight.Diameter.Weight.0.04 inch3.60.015 inch0.190.035 "2.40.014 "0.150.03 "1.50.013 "0.120.025 "0.90.012 "0.0970.02 "0.450.011 "0.0750.019 "0.40.010 "0.0560.018 "0.330.008 "0.0280.017 "0.270.006 "0.0120.016 "0.230.004 "0.004
The weight of a corresponding button of gold is got by multiplying by 2.25. These figures are based on those given by Plattner, and apply only to buttons of such shape as those left after cupellation. A sphere of silver 0.01 inch in diameter would weigh 0.09 milligram, and a similar sphere of gold weighs 0.167 milligram.
It is safer, however, to compare with a micrometer the diameter of the button whose weight has to be determined with that of a standard button of nearly equal size whose weight is known. The weights of the two buttons are proportional to the cubes of their diameters. This plan of working is described more fully in Appendix B., page 440.
Calculation of the Results.—After deducting for the silver added, and correcting for the cupellation loss, the calculation is made in the usual way; reporting as so many parts per thousand in the case of rich alloys and as so many ounces andpennyweights, or better as ounces and decimals of an ounce, in the case of poor alloys and ores.
In this last case, however, it is less fatiguing to refer to a set of tables which give, either directly or by means of simple addition, the produce corresponding to any weight obtained from certain given weights of the substance. The following table gives the produce in ounces and decimals of an ounce per ton of 2240 pounds:—
Weight of Metal got.Weight of Ore taken.3 grams.5 grams.20 grams.50 grams.100 grams.0.00011.090.650.160.060.030.00022.181.310.330.130.060.00033.271.960.490.200.100.00044.362.610.650.260.130.00055.443.270.820.330.160.00066.533.920.980.390.190.00077.624.571.140.460.230.00088.715.231.310.520.260.00099.805.881.470.590.290.00110.896.531.630.650.330.00221.7813.073.271.310.650.00332.6719.604.901.960.980.00443.5626.136.532.611.310.00554.4432.678.173.271.630.00665.3339.209.803.921.960.00776.2245.7311.434.572.290.00887.1152.2713.075.232.610.00998.0058.8014.705.882.940.01108.8965.3316.336.533.270.02217.78130.6732.6713.076.530.03326.67196.0049.0019.609.800.04435.56261.3365.3326.1313.070.05544.44326.6781.6732.6716.330.06653.33392.0098.0039.2019.600.07762.22457.33114.3345.7322.870.08871.11522.67130.6752.2726.130.09980.00588.00147.0058.8029.400.11088.89653.33163.3365.3332.670.22177.781306.67326.67130.6765.330.33266.671960.00490.00196.0098.000.44355.562613.33653.33261.33130.670.55444.443266.67816.67326.67163.330.66533.333920.00980.00392.00196.000.77622.224573.331143.33457.33228.670.88711.115226.671306.67522.67261.330.99800.005880.001470.00588.00294.001.010888.896533.331633.33653.33326.67
When, as in this table, the fraction of an ounce is expressed by two places of decimals, it may be reduced to pennyweights (dwts.)by dividing by 5. For example, 0.40 of an ounce is 8 dwts. The fraction of a dwt. similarly expressed may be converted into grains with sufficient exactness by dividing by 4. Thus, 1.63 ozs. equal 1 oz. 12.60 dwts., or 1 oz. 12 dwts. 15 grains. In England it is usual to report in ounces and decimals of an ounce.
The way to use the table is best shown by an example. Suppose a button of silver weighing 0.0435 gram was obtained from 20 grams of ore. Look down the 20-gram column of the table, and select the values corresponding to each figure of the weight, thus:—
0.04 = 65.33 ozs. to the ton0.003 = 4.90 "0.0005 = 0.82 "——————0.0435 = 71.05 "
Add these together. The produce is 71.05 ozs., or 71 ozs. 1 dwt. to the ton.
Or, suppose an ore is known to contain 1.24 per cent. of silver. Look down the 100-gram column, select the values, and add them together as before.
1.0 = 326.67 ozs. per ton0.2 = 65.33 "0.04 = 13.07 "——————1.24 = 405.07 "
This gives 405 ozs. 1 dwt. 10 grains to the ton.
The calculation becomes more complicated when, as is frequently the case, the ore contains metallic particles. These show themselves by refusing to pass through the sieve when the ore is powdered. When they are present, a large portion, or if feasible the whole, of the sample is powdered and sifted. The weights of the sifted portion and of the "metallics," or prills, are taken; the sum of these weights gives that of the whole of the sample taken. It is very important that nothing be lost during the operation of powdering.
Each portion has to be assayed separately. It is usual to assay a portion of the sifted sample, say, 20 or 50 grams, and to add to the produce of this its share of the "metallics." This way of calculating, which is more convenient than correct, is illustrated by the following example:—