Calculating gold from your fire assay ends with reading the equipment called the gold scale or “the balance”. This equipment should turn with 1/200 milligramme at most, and there is an advantage in using still more sensitive balances. A unit of 0.005 mg. corresponds to 24 grains per ton if 1 A.T. of ore has been taken, and 0.2 grain per ton if 12 A.T. have been taken. Great care must be exercised in placing the balance, to avoid vibration from machinery, traffic. In certain cases the foundation must be laid at a depth of many feet and a column built for the support of the balance. The column should then not be allowed to touch any part of the superstructure. Balances are made with more care than weights, which are almost always inaccurate when received from the manufacturer. Small weights often differ from their stated value by 5 per cent., or even more. For this reason riders of 5 mgs. and 0.5 mg. should be used, the smallest weight used in the pan being 5 mgs. The errors on riders are reduced according to their position on the beam. At a distance of one-tenth of the half-beam length from the centre, the error of the rider is reduced to one-tenth of its full amount. The rider must fit the beam so that it always lies in a plane perpendicular to it. The value of each division on the ivory scale is made, as nearly as possible,equal to 0.1 mg., 0.05 mg., 0.01 mg., or 0.005 mg., according to the sensitiveness of the balance, and the divisions are divided into 10 parts by the eye with the aid of a lens. The rider is always placed exactly on a subdividing mark on the beam, which should be nicked.
If riders are used, as suggested above, the difficulties due to errors in weights are reduced; but, if great accuracy is required, the riders must be examined, and, if necessary, adjusted by the assayer. It is necessary that the riders should be in concordance with the assay-ton weights. It may generally be assumed that the assay-ton weights and the 1 grm. or 0.5 grm. bullion assay weight are in agreement when received from the manufacturer. The errors on weights of 1 A.T. (29.166 grms.) or of 1 grm. rarely exceed 0.01 per cent. The assumption that these weights are correct involves a possible error of only 0.02 per cent., and this corresponds to only 0.1 grain of gold per ton on a 20-dwt. ore, and 0.01 grain of gold per ton on a 2-dwt. ore, amounts which are inappreciable. It is enough, therefore, to see that the rider bears the correct ratio to the gramme weight. This must be ascertained by building up a set of weights from 1 mg. to 1,000 mgs.
Examination of Assay Materials
The fluxes are usually free from the precious metals, but the litharge, red-lead, and granulated lead contain a small amount of silver and less gold. These are estimated by fusion with charcoal (or in the case of granulated lead by scorification) and cupellation. To prevent errors due to “ salting,” which may occur accidentally in an office where gold bullion or residues are handled, it is advisable to run a full blank charge at least once a week.
The weighings will be sufficiently accurate if they are made on a balance in which the divisions on the ivory scale are each equal to 0.005 mg. These divisions are divided into ten parts by the eye. Care must be taken to avoid unequal heating of the balance case. This can be watched if two thermometers reading to 0.01° C. are kept in the case—one near each end of the beam. In reading the indications of the balance, parallax is avoided by applying the eye to a pin-hole in a card fixed at about 12 inches from the ivory scale. A swinging ivory scale fixed to the beam and passing across a thread in a telescope gives still more exact readings. The position of rest of the balance is given by the formula, l1 + 2l2 + l3/4, or with greater exactness by l1 + 3l2 + 3l3 + l4/8, where l1,l2,l3,l4 are successive positions of arrest (end of swing) of the balance, l1 and l3, on one side of the middle of the scale, and l2 and on the other. If the middle of the scale is taken as zero, l2 and will be negative.
The riders must be of platinum or aluminium. Brass riders increase in weight, and gilded brass riders oxidise and increase in weight still more rapidly. Some gilded brass riders at the Royal Mint increased in weight from 5.0 mgs. to 5.025 mgs. in six months. Platinum riders, if too light, may be gilded in a cyanide bath, but light aluminium riders must be rejected. Any rider may be reduced in weight by light rubbing on a sheet of ground glass, or by other mechanical means. The error on a rider can in this way be made less than 0.005 mg. without difficulty, but, if greater accuracy is desired, it saves time to determine the error of the weight and to apply it as a correction to the result of each assay.
The gold can be weighed by the method of substitution by which the bias of the balance is eliminated. The principle of this method is to counterpoise the gold, then to remove it from the pan, and compensate for the loss of weight by a rider. If direct weighing is preferred, the position of rest of the balance when the pans are empty is determined, the gold is put in the pan, a rider put on the beam in such a position as to approximately equalise the effect of the weight of the gold, and then the position of rest again determined. The method of swings is always used to determine the position of rest, disregarding the first swings. An example is appended.
Best Position to Empty Balance
The swings, after the first two, are in succession + 3.4 divisions of the index, – 2.7 divisions, + 3.2 divs. (Plus divisions are always reckoned on the side where the weights are placed, minus divisions on the side where the gold is added.) The position of rest or true zero is ½ (3.4 + 3.2/2 – 2.7) = + 0.3 div. This determination is repeated after the parted gold assay pieces have all been weighed.
Value of the Division
A weight of, say, 0.1 mg. (by means of a rider) is placed on the weight side, and the previous work repeated. The swings are – 2.1, + 0.4, – 2.0, and the new position of rest is ½ (+ 0.4 – 2.1 + 2.0/2) = — 0.8, and 1.1 division = 0.1 milligramme. This value does not alter much from day to day.
Weight of Parted Gold
The gold is found to require about 1 milligramme to counterbalance it, and the swings are now – 4.6 + 0.2 — 4.4. The new position of rest is ½ ( 0.2 – 4.6 + 4.4/2 ) = – 2.15. The gold therefore weighs 1 — ( 2.45 x 0.1/1.1) milligramme—that is, 0.78 milligramme.
By weighing in this way results correct to 0.002 mg. are easily obtained by using the best assay balances in which the value of the scale division is 0.005 mg., and still greater accuracy is attainable by weighing the gold in each pan in succession and by taking means of a number of observations.how-to-calculate-gold-in-fire-assay