How to Measure & Detect Gold & Silver in Trace Amounts

How to Measure & Detect Gold & Silver in Trace Amounts

For a number of years I have, at odd times, tried to perfect a method of assay sufficiently delicate to find and estimate very small/trace quantities of gold and silver. The object in view was to examine rocks remote from veins or mineral areas, in order to test the probability of the lateral-secretion theory. Having succeeded in measuring approximately the amount of gold and silver contained in one cubic centimeter of sea-water, I present a detailed account of the method employed, with some of the results obtained.

Method of Trace Assay

The use of purified lead without other flux, blowpipe methods, extraction by cyanide, and measurement of the beads by the microscope.

Purified Lead

The method of testing is by cupellation. On a properly prepared cupel, it is easy to cupel and find a bead of 0.002 mm. diameter. Such a bead of silver weighs about 1/24,000,000 part of a milligramme. A bead 0.02 mm. in diameter weighs about 1/24,000 mg., and a close approximation can be made to its weight. It is desirable that the lead used in the assay should be low in silver so as to avoid large correction, but as the parted gold requires cupellation with lead it is essential to have the lead used of high purity. I have found the following method to give the best results of a number tried: Dissolve 500 grammes of sugar of lead in 1250 c.c. of water. Place in the center of the beaker a carbon rod for anode, and near the side a 4 by 9 in. lead plate for cathode. Attach one cell of a dry battery and leave it a week. The difference of potential in favor of the deposition of lead is about 0.1 volt; a small amount of lead is deposited which assays much higher in silver than the sample. The battery is disconnected and the solution is allowed to stand in contact with the electrodes for two months. It is then filtered and crystallized and preserved for use. Some of the crystals are ignited in a clean, iron spoon and the mixture of lead and oxides of lead is kept for use in the work. A portion of the calcined lead is reduced before the blowpipe on coal without flux; a part of the resulting metal is set aside as a source of pure lead, and the other part is assayed. The assays showed the lead to contain 1/21,300,000 of silver, as a mean of several trials. By fusing any of the ordinary re-agents with this lead and cupelling, a notable increase of silver is found. This is especially true for all sodium salts, cyanides and ferro-cyanides, and in a lesser degree for the mineral acids, lime, barium and borax—boracic acid being the only re-agent tested that did not show an increase of silver. A sample of Mercks C. P. carb. sod. (10 molecules H2O) showed over 3 grammes Ag per ton. A sample of KCN gave per metric ton 2605 milligrammes of silver and 147 of gold. This was a mean of several tests, and this sample was used in the assays reported below.

Trace Measurement

A microscope having powers of 40 and 60 diameters has in the eye-piece a micrometer ruled on glass. With the power of 40, one division is 0.02873 mm. With the power of 60, one division is 0.02001 mm. The latter is practically the same as a division on the ivory scale used in quantitative blowpipe- work. For careful work, both powers are used and a mean of the readings is taken. Thus, if a bead read 7.35 on the 60, it should read 5.12 on the 40. Using sunlight and reading the greatest diameter of the shadow of the bead, will usually permit readings to be made to .001 min. or one-twentieth of a scale-division.

The Form and Weight of Beads

The form of a bead is the resultant of three forces, namely, gravity and the surface-tensions of the metal and the litharge respectively. Experience has shown that if the cupellation be done in a uniform way, the beads will show a constant mutual relation of diameter, base and height. For small beads, under

watch-spring-clip-for-measuring-assay-beads

0.5 mm. in diameter, the action of gravity is much smaller than that of the other forces; hence, the beads tend to approach a spherical form, and for ordinary purposes it is sufficient to measure the diameter only, without removing from the cupel. Concerning this weight, there is much confusion: thus, in Cornwall’s Plattner the weight of a silver bead 1 mm. in diameter (No. 50 on the ivory scale) is said to be 3.48 mg. A few pages beyond is a table giving the weight of a similar bead (but

vertical-section-showing-bead-in-clip

measured in a jaw-micrometer) at 6.12 mg. A monograph by J. S. Curtis (6th report of the U. S. Geological Survey) gives a table for beads measured by cross-hairs and a micrometer-stage, and based upon perfect sphericity at zero, with a decrease of 20 per cent, for a bead 0.409 mm. in diameter, and an interpolated formula to satisfy these conditions. To test the matter, I devised a clip made of a smalldiagram-illustrating-calculation-of-weight-of-bead-from-measurements watch-spring, as shown in Fig. 1, which is about full size. The inside at a, b, is polished for 0.25 in. The bead is cleaned and inserted between the jaws of the clip, the base of the bead resting on b, slightly nearer the top than the bottom. The clip is placed on the stage and lighted from below. It should present an appearance like Figs. 2 and 3. The polished steel reflects the image and gives a sharp line, very helpful in measuring diameter, base and height.

A number of gold beads, varying in diameter between 0.1 and 3.3 mm., were separately weighed, and also measured in the clip above described for diameter (B) of base, diameter (D) of bead, and height (H) of bead. In order to deduce the volume from these measurements, the height (A) of the missing segment must be known. This cannot be directly measured; but an expression can be found, giving the volume of the bead as a fraction of the imaginary completed figure.

In Fig. 4, ab = B ; cd= D ; ko + oe =H, and amb is the segment cut off. Assuming the completed figure to be a sphere, ko = om = co = od = oa = R, or radius of the sphere; and from inspection of the diagram it is clear that ab/cd = sin. S (S being the angle aoe); and that ko + oe = H = R + R cos. S.

The volume of the segment of a sphere, divided by the volume of the sphere, is

volume-of-the-sphere-gold-and-silver

Calling this fraction P, the volume of the bead ackdb can be expressed as (1 — P) vol. of sphere, on the assumption that the completed figure is a sphere. But this would never be the case theoretically, and seldom practically (i.e., within the limits of practicable accuracy for the calculation here under consideration), for the obvious reason that the effect of gravity, and possibly of other forces, upon the liquid bead must distort it from a perfectly spherical shape. In other words, km in Fig. 4 could seldom be safely assumed as equal to cd. Another factor must therefore be introduced to correct the error due to this lack of sphericity. In Fig. 4,

km = cd/ke H = 2/1 + cos. S x H

Denoting by Q the coefficient of H in this expression, we have, as the final expression for the volume of the bead,

(1 – P) Q x π/6 D²H

The measurements being in millimeters, it is only necessary to multiply this volume by the specific gravity in order to obtain the weight of the bead.

A number of beads weighed on a good balance 73.27 mgs. The same beads calculated by the above formula measured 72.61 mgs. One bead of silver weighing 92 mgs. measured 91.463 mgs. Sin. S will not vary much if care be taken not to touch the bead with the tip of the flame after it sets. In gold buttons the usual effect is to increase H and diminish D, thus giving too small a value, if only D is read.

For heads too small to allow easy manipulation the measurements are best made upon the cupel. For such work, where the scale-division is 0.02001 mm., the value used is

Weight = D³ x 0.00007598 for gold (log. = 5.8807)
and D³ x 0.00004213 for silver (log. = 5.6246).

This agrees with the value given by Curtis for silver where the diameter = 0.08 mm.
(D in the above formula is the number of scale-divisions of 0.02001 mm. each.)
The above constants were used in the reduction of all the tests tabulated in this paper.

Fine Cupellation

Take elutriated bone-ash and grind it very fine in the agate mortar, heat to redness and preserve in a bottle. For the fine cupellation press in, lightly, ordinary bone-ash; heat it to expel moisture; then cover evenly with the fine ash and burnish with the smooth end of the agate-pestle; heat carefully, and, before placing the bead upon it, examine for cracks with a good lens. The cupel-surface should be smooth and polished. The bead of lead should not weigh more than 8 to 10 mg. and should be clean. As soon as fused, the bead should be kept moving. When it gets too small to be seen, the stain of litharge on the cupel shows its locus. Care must be taken not to overheat near the finish. By carefully observing the above directions 1 mg. of assayer’s lead will show a small head of silver. (Suppose the lead to contain 0.1 oz. of silver per ton; then 1 mg. will give a bead of 0.43 division or 0.0086 mm. diameter.)

Should the assay be for silver only, the cupel is placed upon the stage of the microscope and the diameter of the button is read; or, if it is large enough to handle, it can be detached, placed in the clip, and the three dimensions measured for volume of gold and silver.

Parting

The diameter of the alloy is read and estimated as silver, after which it is parted as follows: Procure a supply of clear white cups or saucers of a cheap grade, which should be free from dark specks, and which should stand the heat to be used, without cracking or losing the assay; break into pieces about 15 mm. square and cement a piece with sealing-wax to a strip of tin plate for a handle, placing the concave side uppermost. Clean the surface of the porcelain with nitric acid and wash it, leaving a drop of water on it. Place the cupel near it, and with the aid of a lens and chisel-pointed needle detach the bead. With a fine splinter of wood, previously wetted, touch the bead, and transfer it to the drop of water. Should the bead be too small for a hand-lens, take a fine needle; amalgamate the point with sodium amalgam; and, having the bead in focus on the stage, touch it with the point of the needle. Sufficient mercury will adhere to render the bead visible; and it can then be treated like a larger bead. With a capillary tube or dropper, add nitric acid very cautiously, warming the assay until the silver is seen to blacken and dissolve slowly. The gold will remain in a coherent mass. With a strip of filter-paper remove the acid; add strong acid, and warm again, after which the assay is washed sweet, and dried. With small amounts of gold great care must be taken to have a clean white surface to work upon, and dust must be kept away ; otherwise, it will be difficult to see and recognize the speck of gold.

Some of the calcined purified lead is reduced without flux on coal and kept in a clean box for use. Cut from it a small piece of 2 to 3 mg. weight; flatten it with the agate-pestle and turn up a corner for a handle. With the forceps cover the speck of gold and gently press down the lead. Next heat the corners of the porcelain before the blowpipe, carefully working inward to the test, and fuse the latter; after which, scorify the bead over the area covered by the lead when laid on, and cool. The bead detaches easily and with a clean bottom. Examine the bead with the lens to see that it is clean, and cupel as before directed. As gold will stand a high heat, it is advisable to measure, and, if there is any irregularity, reheat to melting point.

By observing the above directions, I have made beads which I estimated at of a scale-division. The bead was perfect in form and color, and with higher powers could easily have been measured with much precision. Such a bead is calculated as follows, according to the formula, weight = D³ x 0.0000759; D being 0.1 of a scale-division :

formula-weight-gold-and-silver

It is readily seen that we have here a method at once simple and delicate for detecting the presence and estimating the amount of the precious metals.

The Probable Error

As previously stated, the diameter of the bead can be read to 0.001 mm. Calling this the error, and taking the weight as KD³ (K being a coefficient) and its differential as 3KD² (.001), we have as the per cent, of error:

probable-error-gold-and-silver

The following table shows the weight of gold beads and the probable error:

weight-of-gold-beads

Cupellation Loss

A sample of about 331 mgs. of assayers’ sheet-lead, cut into small squares, mixed and cupelled, gave a bead covering 6 divisions, or 12 mm. in diameter. A sample of 41 mgs. gave a bead covering 3 divisions; and one of 5.2 mgs. ahead of 1.5 divisions; thus proving that there is no sensible departure in the proportion of loss for small beads from that of larger ones. Hence, in what follows, I have deducted the silver due to the lead cupelled and called the remainder the true amount.

Cyanide Assays

A sample of cyanide of potassium was tested by evaporating 2 grammes with 10 c.c. of water and 6 grammes of calcined lead. The dried mass was fused on a clean coal, and the resultant lead was weighed, cupelled, and the button parted. The value found as a mean of several trials was: silver, 2.605 grammes, and gold, 0.147 grammes, per metric ton. A 1-percent. solution was made and kept for use. The general method was to take 40 grammes of finely pulverized material, with 10 c.c. of KCN solution (= 100 mgs. KCN) and 50 c.c. of distilled water; place all in a flask and shake well, repeating the shaking at intervals for one or two days; then filter on a dry filter into a graduated vessel; note the c.c. used for assay; transfer to an evaporating dish; add 400 mgs. of calcined lead, and evaporate. The residue was then fused on a clean coal, the lead being weighed and cupelled, and the button parted, as above described.

This assay is made more certain by heating the coarsely pulverized rock to redness, quenching in water, evaporating to dryness, and then pulverizing, taking a sample for assay and grinding it in an agate mortar to a fine mud, and using a cyanide solution of one-third per cent., after which, the whole is transferred to a flask and made up to the desired weight of cyanide solution. The weighed sample of ore is triturated with only sufficient cyanide-solution for the purpose of proper grinding. When it is transferred, as above described, to a tared flask, the pestle, mortar and funnel are washed into the flask with either cyanide solution or water. It is desirable to have a known weight or volume of cyanide solution in the flask, because it will be filtered upon a dry filter, and usually some aliquot part of the original solution will be assayed. Ore can be pulverized finer in this way than by the dry method. The assay is usually made alkaline by this treatment,—a result which is beneficial for cyanide work.

Assaying Seawater for Gold

I have long thought that the sea-water was the chief source of gold and silver, and that some effort should be made to ascertain the facts and thus know what are the chances that the sedimentary rocks have been impregnated with small amounts of gold and silver. I have verified the statement of Sonnstadt that a small portion of BaCl2 will throw down a portion of the gold; also the statement of Dr. Don that heating the salts to redness and redissolving and assaying the residue would recover most of the gold. I have also evaporated to dryness, and find that after dissolving in water the residue carries a notable amount of gold and silver; also evaporating, or rather boiling with calcined lead acetate, fixes a larger amount of silver and some gold. The highest results were had by evaporating 1 to 10 c.c. of sea-water with 200 mgs. of lead per c.c., and fusing on coal, but the results were not concordant nor uniform, probably due to difference in heating and to volatilization of the silver. The gold was tolerably constant by this method; 500 c.c. were evaporated to dryness, redissolved in a minimum of water, and while boiling a 2-per-cent, solution of BaCl2 was added drop by drop to slight excess and again evaporated to dryness. The salts were heated to a bright red for ten minutes and redissolved and filtered ; the ppt. of barium sulphate and some lime were placed in a flask with some water and a drop of phenolphthalein. It required 4 drops of lime-water to produce a permanent color showing alkalinity. The solution was made up to 25 c.c. containing 40 mgs. of KCN, and shaken at intervals for a day—filtered and washed and the filtrate evaporated with 400 mgs. of lead, and the assay made as described. There were two assays made, which showed as follows:

Cyanide Method

Found per metric ton of water:

cyanide-method-gold-and-silver

A portion of the filtrate containing the chlorides was then assayed directly with lead, which gave for Ag 1010, 1241 and 1164 mgs. per ton. The three beads were then cupelled together and parted. There was a small bead of gold estimated at 1.5 mgs. per ton; from which I conclude that the amount present in this sample was of gold 12.6 mgs. and of silver not less than 1.500 grms. per ton. Dividing by .02756, the amount of salts in the sample, it becomes,

Per metric ton of salts:

per-metric-ton-of-salts

and for normal sea water, containing 3.5 per cent, of salts, gold, 16.0 mgs.; silver, 1.9 grms. (NOTE.—Several tests showed a much higher value for the silver.)

Sea weed and floating organic matter were tested by washing sweet, calcining and cyanide. In some cases large amounts of silver were found. Samples of the bay mud taken from the dredges gave from 3 to 18 cents in gold per ton. A sample of mud from Islais Creek channel, which is very foul from sewage, gave a very high assay in silver. Samples of organic matter taken near sewer outlets always assayed higher in silver than samples taken at more remote points.

As a result of the numerous tests made, I think it can be safely affirmed that organic matter reduces some silver from the sea water and probably some gold. The latter cannot be positively known, because the gold may have been carried in suspension and have become attached to the object assayed; but the method described might be used to examine organic matter taken at a point free from coastal influence. Personally my views, based upon my work done, are that the sediments as deposited are enriched by the reducing action of organic matter, and that the newly-formed stratum of mud contains not only the gold and silver due to the water present, but also some additional amount reduced by the organic matter.

Assays of Rocks

The statement of Dr. Don that country-rocks can be assayed by panning down a quantity and assaying the residue, has been tested, as well as the statement that pyrite must be present in order to find gold; and my experiments show that both statements are incorrect, or, at least, not in accord with my experience.

I have found that grinding poor quartz ore in an agate mortar to a fine slime, and removing the slime by water from time to time, is a very delicate test for gold. Most of the samples assayed and tabulated below were thus tested, and showed no gold. The method used for most of the samples was to crush to 60-mesh sieve and take 40 to 50 grms. of ore, and 60 c.c. of water containing 100 mgs. of KC. The stoppered bottle containing the assay was well shaken at intervals for a day or two, filtered on a dry filter, and the filtrate measured, and then evaporated with 400 mgs. of calcined lead acetate, and fused on coal b.b., as described. In these assays there are but two fluxes added, the lead and the cyanide; and as both have their tenor known, great confidence can be placed in the result. As an example, take No. (10), sample of Carrara, Italy, marble : Weight taken, 45 grms. + 60 c.c. water + 100 mgs. KC; time, 2 days; take 42 c.c. of solution for assay. Weight of Pb reduced 340 mgs.

Diameter of Ag Au bead, 5.45 divisions = 0.109 mm.
Diameter of Ag Au bead, 1.55 divisions = 0.031 mm.

Calculation of Gold & Silver Amounts

gold-silver-calculation

As the cupellation loss is greater, it is disregarded.

cupellation-loss-gold-and-silver

Extracted from, Carrara Marble

extracted-gold-silver

The following table comprises results of assays of rocks taken remote from veins or known regions of mineral values. The results are reported in milligrams per 1000 kilograms of ore assayed:

  1. Granite. Porcupine Flat near Lake Tenaya, Cal., Au, 104; Ag, 7660.
  2. Granite from Lake Tenaya, Cal., Au, 137; Ag, 1220.
  3. Granite, headwaters American river, Cal., Au, 115; Ag, 940.
  4. Syenite, Candelaria, Nevada, Au, 720; Ag, 15,430.
  5. Granite, Candelaria, Nevada, Au, 1130 ; Ag, 5590.
  6. Sandstone, Colusa county, Cal., sample from Hayward Building, Au, 39; Ag, 540.
  7. Sandstone from quarry, Angel Island, Cal., Au, 24; Ag, 450.
  8. Sandstone, Russian Hill, San Francisco, Cal., Au, 21; Ag, 320.
  9. Marble, Columbia quarry, Tuolumne county, Cal., Au, 5; Ag, 212.
  10. Marble, Carrara, Italy, Au, 8.63; Ag, 201.
  11. Basalt, Paving block, from Petaluma, Cal., Au, 26; Ag, 547.
  12. Diabase, Mariposa county, Cal., Au, 76; Ag, 7440.

From the above assays it is seen that the average silver is about 20 times that of the gold, or, excluding Nos. 4 and 5, the ratio is about 30 to 1, and this high ratio tends to confirm the belief that the gold and silver were deposited with the rock and have since remained with it. Assuming that the water has an underground circulation, then the source of the gold and silver can be found in the country-rocks, and we need not go to the unknown regions below for a source of supply. It should be remarked that the above assays by cyanide do not purport to be the value of the rocks but only of the amount extracted. Some check work on one of the samples showed 20 per cent, more by calcining and grinding to a fine mud. I have also found the above methods useful in cyanide tests, notably in experimental work for testing the rate of solution; from a definite amount of solution 1 to 5 c.c. is taken, and either evaporated with lead or the gold is precipitated by AgNO3, and the precipitate reduced and parted, and estimated as above. The method of course requires some calculation for the amounts subtracted from time to time; but, on the other hand, it is all from the same sample. Much valuable information can be rapidly gained from a few well-chosen tests of this kind.

The method is of great value in tests in regions remote from assay offices. One gram of properly prepared sample treated either by direct fusion or a larger amount amalgamated, and direct fusion of 0.5 gram will closely check the ordinary fire assay on ores as low as .2 oz. Au per ton.

Acknowledgments are due to Mr. H. W. Turner, U. S. Geological Survey, for specimens furnished for assay, and to Mr. Newton M. Bell, of San Francisco, for the use of his laboratory and assistance rendered during the investigation.