INTRODUCTORY.
Assaying has for its object the determination of the quantities of those constituents of a material which add to or detract from its value in the arts and manufactures. The methods of assaying are mainly those of analytical chemistry, and are limited by various practical considerations to the determination of the constituents of a small parcel, which is frequently only a few grains, and rarely more than a few ounces, in weight. From these determinations calculations are made, which have reference to a mass of material of, perhaps, hundreds of tons. But in all cases, whether the mass under consideration be large or small, whether the material be obtained by mining, grown, or manufactured, the assayer is supposed to receive a small quantity, called "the sample," which is, or ought to be, the exact counterpart of the mass of material that is being dealt with. The taking and making of this sample is termed "sampling"; and the men whose special work it is to select such samples are "the samplers."
But although "sampling" is thus distinct from "assaying," the assayer should be familiar with the principles of sampling, and rigorous in the application of these principles in the selecting, from the sample sent him, that smaller portion upon which he performs his operations.
Sampling.—In the case of gases, there is absolutely no trouble in mixing. The only difficulty is in drawing off a fair sample where, as in flues, the body of the gas is in motion, and varies a little in composition from time to time. In this case, care must be taken to draw off uniformly a sufficient volume of the gas during a prolonged period; any portion of this larger volume may then be taken for the analytical operation.
In the case of liquids, which mix more or less easily—and this class includes metals, &c., in the state of fusion—more or less severe agitation, followed by the immediate withdrawal of a portion, will yield a fairly representative sample.
In the case of solids, the whole mass must be crushed, and, if not already of fairly uniform quality, mixed, before sampling can take place. Most of the material which a sampler is called upon to deal with, is, however, in a more or less divided state and fairly uniform. In practice it is assumed that 5 per cent. of the whole (= 1/20th), if taken in portions of equal weight and at frequent and regular intervals, will represent the mass from which it was taken. Taking a heap of ore, A, and selecting one out of every twenty spade-, bag-, barrow-, or wagon-fuls, according to the quantity of stuff in the heap, there is obtained a second heap, B, containing one-twentieth of the stuff of the heap A. If we crush the stuff in B until this heap contains approximately the same number of stones as A did—which means, crushing every stone in B into about twenty pieces—B will become the counterpart of A. Selecting in the same manner 5 per cent. of B, there is got a third heap, C. This alternate reduction and pulverising must be carried on until a sample of suitable size is obtained. This may be expressed very clearly thus:—
A = 1000 tons of rocks and lumpy ore.
B = 50 " " rough stones, 1/20th of A.
C = 2.5 " " small stones, 1/20th of B.
D = 0.125 " " coarse powder, 1/20th of C.
If the material to be sampled is already a dry powder, 5 per cent. of it should be heaped in a cone; each lot being added on the apex of the cone already formed, so that it may distribute itself by falling evenly in all directions. When the cone is completed, convert it into a low frustrum of a cone by drawing stuff uniformly and in a direct line from the centre to the circumference. Draw two diameters at right angles to each other, and reserving any two alternate quarters, reject the others. Mix; and form another cone, and proceed until a sample is got of the bulk required.
This is the usual plan, and all samples should be treated in this way when the stuff is fine enough to fall evenly down the sides of a cone.
Samples as they reach the assay office are seldom in a fit state for the work of the assayer; they are generally too coarse, and ought always to be more than he wants for any particular determination. The portion he requires should never be taken at hap-hazard; the sample must be reduced systematically to the quantity required.
1. If the sample is a liquid: it is sufficient to shake the bottle, and take out a measured or weighed quantity for the assay.
2. If a liquid with a solid in suspension: measure the whole of it. Filter. Make up the filtrate with the wash-water or water to the original bulk. Assay it. Dry and weigh the residue, and make a separate assay of it.
3. If of a creamy consistency, free from heavy particles: mix well; spread out evenly on a glazed tile. Take up equal portions at equal distances. Mix and assay.
4. If a mud of coarse and fine particles, or of particles of unequal density: weigh and transfer to a porcelain dish, or weigh in the dish. Dry at 100° C., weigh. Treat the residue as a solid capable of being powdered.
5. If a solid capable of being powdered, or already powdered: heap up into a cone; flatten with a spatula; divide along two diameters at right angles, and carefully reject the whole of two alternate quarters, brushing away any fine powder. Mix the other quarters, and repeat (if necessary). For small quantities a fine state of division is essential.
6. If a solid with metallic particles: powder and pass through a sieve; the metallic particles will not pass through. Weigh both portions and assay separately. Sifting should be followed by a very thorough mixing.
7. If a metal or alloy in bar or ingot: clean the upper surface of the bar, and bore through the bar. Use the borings. If the ingot or bar is small, cut it through and file the section. Filings must be freed from fragments of the file by means of a magnet; and from oil, if any be present, by washing with a suitable solvent.[1] Where practicable, metals and alloys are best sampled by melting and granulating. The student must carefully avoid any chance of mixing dirt or particles of other samples with the particular sample which he is preparing. One ore should be done at a time, and when finished, it should be labelled and wrapped up, or bottled, before starting on a fresh sample.
When an ore requires to be very finely ground in an agate mortar, it is often advisable to mix with a little pure alcohol and rub until free from grit; dry at 100° C. and mix well before weighing.
When an assay is required of a quantity of ore made up of parcels of different weight and quality, each parcel should be separately sampled and parts of each sample, bearing to each other the same proportion by weight as the original parcels, should be taken and mixed. For example, a lot of ore is made up of one parcel of A, 570 tons, one of B, 180 tons, and another of C, 50 tons; a sample representing the whole may be got by mixing 57 parts of a sample of A with 18 parts of a sample of B, and 5 parts of a sample of C.
A bruising plate, like that in fig. 2, is convenient for general office work. The slab is of cast iron, about an inch thick. It is firmly supported on a solid block of wood, and pivoted for convenience in emptying. The bruising-hammer is steel-faced, about 4 inches square, and 1-1/2 inch thick. The block is firmly fixed to a small table or tressel, so that the slab is about 2 feet 6 inches from the ground. The slab is cleaned, and the sample collected with the help of a stiff-haired brush.
Drying: Determination of Moisture.—In practice, the moisture is generally determined by the samplers, and the proportion is specified in grains per pound on the label attached to the sample when it reaches the assay office. The method adopted is usually to dry 1 lb. = 7000 grs. of the ore in a frying-pan heated over a gas flame, or in an ordinary oven, until a cold bright piece of metal or glass is no longer damped when held over it. The loss of weight in grains = moisture.
Properly, however, this work should be done by the assayer, if only for the following reason. It is assumed that the dry ore of the sampler and of the assayer are the same thing; according to the nature of the ore, this may or may not be the case. The assayer, however, uses the sample which he has dried for his moisture-determination, as the dry ore on which he makes his other assays, and no variation in moisture would influence the other and more important determinations. Some ores are sent to the smelter with from 5 to 15 per cent. of adherent water. In these cases it is best to spread out the sample, and taking equal portions fairly at regular intervals, weigh into a Berlin dish 20 grams. This should then be dried over a sand-bath, or if the ore is likely to be injured by excess of heat, over a water-bath until the weight is constant. The loss of weight multiplied by 5 gives the percentage of water present.
Example:—
| Weight of dish + wolfram | 32.67 | grms. |
| " " dish | 12.67 | " |
| ——— | ||
| " " wolfram | 20.00 | " |
| " " dish + wolfram | 32.67 | " |
| " " " dried | 30.15 | " |
| ——— | ||
| " " water | 2.52 | " |
| 2.52 × 5 = 12.6 | 12.6%. |
There are other ores which are not apparently wet, but in the state called "air-dried." It is easier to take fair samples of these, and, consequently, it is not necessary to use so large a quantity as 20 grams. But with a smaller quantity, extra precautions must be taken. All dry solids at ordinary temperatures absorb moisture from the air. The amount varies with the nature of the material and with the quantity of surface exposed. Light bulky powders absorb more than heavy ones, because of the greater condensing surface. It is on this account that it is well to weigh substances, which have been dried, between close-fitting watch-glasses. The method of determining moisture is to weigh out into the glasses 5 grams of ore, and dry in the water-oven until there is no further loss of weight. On taking the glasses out of the oven, they should be at once closed, the clip put on, and after cooling in a desiccator weighed. If after a second trial the loss is the same, or only increased by a milligram, the determination is finished.
Example:—
| Weight of glasses + pyrites | 31.0470 | grms. |
| " " glasses | 26.0470 | " |
| ——— | ||
| " " pyrites | 5.0000 | " |
| " " glasses + pyrites, dried 1 hour | 30.8965 | " |
| " " " " dried 1-1/2 " | 30.8957 | " |
| " " " " | 31.0470 | " |
| " " " " dried | 30.8957 | " |
| ——— | ||
| " " moisture | 0.1513 | " |
| 0.1513 × 20 = 3.026 | 3.02%. |
Sometimes it may be advisable to dry 10 grams, in which case multiplying the loss by 10 will give the percentage. The dried ore should be transferred to a weighing-tube (fig. 3), and reserved for the subsequent determinations. The weighing-tube with the ore must be marked, and kept in a desiccator.
Most ores and inorganic substances can be dried, and their moisture determined by the loss in this way. When, however, the substance contains another somewhat volatile ingredient, it is exposed over sulphuric acid in a desiccator for two days (if in vacuo, all the better), and the loss determined. Moisture in dynamite should be determined in this way.
When water is simply mechanically mixed with a substance it presents but little difficulty. The combined water is a different matter. Slaked lime, even when perfectly dry, contains much water; and if the water of soda crystals were separated and frozen, it would occupy a volume equal to that of the original crystals. Perfectly dry substances may contain much water, and this combined water is retained by different materials with very unequal vigour. Sodium sulphate and sodium phosphate crystals lose water even when exposed under ordinary conditions to dry air. Soda crystals when heated melt, and at a moderate temperature give off their water with ebullition. The temperature at which all the water is given up varies with each particular salt; the actual determination of the water in each case will require somewhat different treatment. Such determinations, however, are seldom required; and from a practical point of view this combined water causes no trouble.
In assaying ores, we term "moisture" all water which is lost by exposure in a water-oven at 100° C., and the "dry ore" is the ore which has been dried at this temperature. No advantage, but rather endless confusion, would be caused by varying the temperature with the object of estimating the whole of the water which a hydrated salt may contain. The results of the assay of the other components should be calculated on the "dry ore." One advantage of this is obvious:—The dry ore has a constant composition, and the results of all assays of it will be the same, no matter when made; the moisture, however, may vary from day to day, and would be influenced by a passing shower of rain. It is well to limit this variability to the moisture by considering it apart, and thus avoid having the percentage, say, of copper rising and falling under the influence of the weather.
In the case of certain salts, however, such as soda crystals and hydrated sulphate of copper (when these constitute the bulk of the substance to be assayed), it is as well to perform the assay on the moist, or at any rate air-dried, substance.[2] It would be equally convenient to calculate on the substance dried at 100° C.; but in this case it would be well, in order to avoid a somewhat shallow criticism, to replace the term "moisture" by the longer but equivalent phrase "water lost at 100° C."
Calculation and Statement of Results.—By far the most generally convenient method of stating the results of an assay is that of the percentage or parts in a hundred, and to avoid a needlessly troublesome calculation it is well to take such a quantity of ore for each assay as by a simple multiplication will yield the percentage. In these calculations decimals are freely employed, and students should make themselves familiar with the methods of using them.
Other methods of statement are in use, and have advantages in certain special cases. With bullion the parts in a thousand are given, and in those cases in which the percentage is very small, as in water analysis, it is convenient to report on parts in 100,000, or even on parts per 1,000,000. These are easily got from the corresponding percentages by shifting the decimal point one, three, or four places to the right. Thus 92.5 per cent. is 925 per thousand; and 0.0036 per cent. is 3.6 per 100,000, or 36 per million.
With ores of tin, silver, and gold, the result is stated as so many cwts., lbs., or ozs., in the ton. With dressed tin ores as they are sent to the smelter, the produce is given in cwts. and quarters to the ton. The corresponding percentage may be obtained by multiplying by five; or, inversely, if the percentage is given, the produce may be got by dividing by five. A produce of 13-1/2 equals a percentage of 13.5 × 5 = 67.5; and a percentage of 70.0 equals a produce of 70 / 5 = 14. With tin ores as raised (in which the percentage is small) the reduction must be carried to pounds per ton. One per cent. equals 22.4 lbs. to the ton; consequently, if we multiply the percentage by 22.4, the produce will be given. Thus, if an ore contains 6.7 per cent. of oxide of tin, the produce is 6.7 × 22.4 = 150 lbs. (or 1 cwt., 1 quarter, and 10 lbs.) to the ton. With gold and silver ores, the proportion of precious metal is small, and it is necessary to carry the reduction to ozs. and dwts. to the ton; and since gold and silver are sold by troy weight, whilst the ton is avoirdupois, it is of importance to remember that the ounces in the two systems are not the same. A ton contains 15,680,000 grains, which equal 653,333.3 dwts. or 32,666.6 ozs. (troy). The following rules are useful:—
To get ozs. (troy) per ton, multiply parts per 100,000 by 0.327;
To get dwts. per ton, multiply parts per 100,000 by 6.53;
To get grains per ton, multiply parts per 100,000 by 156.8.
Where liquids are being assayed, cubic centimetres are held to be equivalent to grams, and the usual method of statement is, "so many parts by weight in so many by measure." Where the statement is made as grams per litre or grains per gallon, there can be no doubt as to what is meant; and even if it be expressed in parts per 100,000, parts by weight in a measured volume must be understood unless the contrary is expressly stated.
In some cases, where the density of the solution differs greatly from that of water, the percentage by weight may be given; and in others, mixtures of two or more liquids, the percentages may be given by volume or by weight; as so many c.c. in 100 c.c., or as so many grams in 100 grams, or even as so many grams in 100 c.c. In such cases it must be distinctly shown which method of statement is adopted.
One grain per gallon means 1 grain in 70,000 grain-measures, or one part in 70,000. Dividing by 7 and multiplying by 10 will convert grains per gallon into parts per 100,000. Inversely, dividing by 10 and multiplying by 7, will convert parts per 100,000 into grains per gallon.
Grams per litre are parts per 1000; multiplying by 100 will give parts per 100,000, and multiplying by 70 will give grains per gallon.
Among foreign systems of weights, the French is by far the best. Kilograms (2.205 lbs.) per quintal (220.5 lbs.) are parts per cent.; and grams (15.43 grs.) per quintal are parts per 100,000. From the rule already given, grams per quintal may be converted into ounces to the ton by multiplying by 0.327.
The German loths per centner (1/2 oz. (avoirdupois) to 100 lbs.) equal parts per 3200; they are converted into parts per cent. by dividing by 32, or into ounces (troy) per ton by multiplying by 10.208.
In the United States, as a sort of compromise between the avoirdupois and metric systems, a ton is taken as 2000 lbs. There, too, the custom is adopted of reporting the gold and silver contents of an ore as so many dollars and cents to the ton. In the case of gold, an ounce is considered to be worth 20.6718 dollars. With silver, the nominal value is 1.2929 dollars per ounce, but frequently in assay reports it is taken as one dollar. The practice is objectionable. The prices of metals vary with the fluctuations of the market, and if the assayer fixed the price, the date of his report would be all important; if, on the other hand, he takes a fixed price which does not at all times agree with the market one, it leaves a path open for the deception of those unacquainted with the custom. American "dollars on the ton of 2000 lbs." may be converted into "ounces in the ton of 2240 lbs." by dividing by 1.1544 in the case of silver, and by 18.457 in the case of gold.
Laboratory Books and Report Forms.—The record which the assayer makes of his work must be clear and neat, so that reference, even after an interval of years, should be certain and easy. One method should be adopted and adhered to. Where there are a large number of samples, three books are required.
Sample Book.—This contains particulars of the samples (marks, &c.), which are entered by the office-clerk as they arrive. He at the same time puts on each sample the distinguishing number.
Example of Page of Sample Book.
| Date. | Number. | Sample. | Remarks. |
| Feb. 1 | 482 | Tough Copper | For Arsenic. |
| " 2 | X | Piece of Metal | For Ni and Cu. |
| " | 483 | Tough Copper. | |
| " | 73 | Silver Precipitate, 4 casks, 24 cwt. 1 qr. | With Letter. |
| " | 494 | Purple Ore, 200 tons. | |
| " | 1 J.T. | Lead Ore, 1 J.T. | From Corsica. |
| " | 2 J.T. | " 2 J.T. |
Laboratory Book. This is the Assayer's note-book, in which he enters clearly the particulars of his work—the results obtained, as well as how these results were arrived at. The calculations should be done on scrap-paper, and should not be entered, although, of course, detail enough must be shown to enable the results to be recalculated.
Example of Page of Laboratory Book.
____________________________________________________________
Purple Ore 5 grams
19/10/89 0.0042 grm.
0.0021 "
———
Colorimetric 0.0063 × 20 = 0.13% Copper
______________________________________________________________
482
Tough Copper 10 grams
Feb. 1/89 10.5 c.c. Uranium.
= 0.52% Arsenic
______________________________________________________________
2082
Tough Copper 10 grams
12.7 c.c. Uranium.
= 0.63% Arsenic
______________________________________________________________
491 10 grams
Tough Copper 13.7 c.c. Uranium
Feb. 1/89
= 0.68% Arsenic
______________________________________________________________
Standard of Uranium acetate.
0.150 gram As2O3 = 23.3 c.c. Uranium.
∴ 100 cc. Uranium = 0.5 gram As.
______________________________________________________________
10071 5 grams
Tin Ore Cruc. and SnO2 9.6065 grms.
Feb. 3/89 Cruc. and Ash 9.4235 "
———
SnO2 = 0.1830 = 2.88% Tin
______________________________________________________________
The Assay Book.—This is the Official book, and is a combination of the Sample and Laboratory books. It corresponds with the report-forms. Without being loaded with detail, it should contain sufficient to characterise each sample.
Example of Page of Assay Book.
Description of Sample.
| Date. | Material. | Weight. | No. | Water Lost at 100° C. | Assay on the Dry Material. | Date Reported. | ||||
| 1889 | ton | cwt | qrs | lbs | ||||||
| Feb. 1 | Tough cake copper | ... | ... | ... | ... | 482 | ... | Arsenic, 0.52% | 7 | |
| " | Tough cake copper | ... | ... | ... | ... | 2082 | ... | Arsenic, 0.63% | 7 | |
| " | Tough cake copper | ... | ... | ... | ... | 491 | ... | Arsenic, 0.68% | 7 | |
| Feb. 2 | Nickel disc for C.R. | ... | ... | ... | ... | X | ... | Copper, 73.75 | 7 | |
| Nickel, 24.34 | ||||||||||
| Iron, 2.18 | ||||||||||
| ——— | ||||||||||
| 100.27 | ||||||||||
| ——— | ||||||||||
| " | Silver precipitate, 4 casks | ... | 24 | 1 | 0 | 73 | Not det. | Silver, 4.851 | 10 | |
| Gold, 0.0215 | ... | |||||||||
| Lead, 19.37 | ... | |||||||||
| Zinc, 2.00 | ... | |||||||||
| Silver, 1584.7 ozs. per ton | ... | |||||||||
| Gold, 7.0 ozs. per ton | ... | |||||||||
| " | Purple ore | ... | 200 | ... | ... | ... | 494 | Not det. | Copper, 0.13% | 11 |
| Sulphur 0.15% | ... |
When the number of samples is small, the Sample Book may be omitted, and the entries made in the Assay Book as the samples arrive.
Report-forms. These should entail as little writing as possible in making out the report. For general purposes the form given on p. 12 is useful.
The quantity of substance to be taken for any particular assay depends largely upon the method of assay adopted. There are, however, some general considerations which should be remembered, and some devices for simplifying the calculations which should be discussed.
The smaller the percentage of the substance to be determined, the larger should be the amount of the ore taken. The following table will give a general idea as to this:—
| Percentage of the substance to be determined. | Amount of ore, &c. to be weighed. |
| 100-10 | 1 gram. |
| 10-5 | 2 grams. |
| 5-1 | 5 " |
| 1-0.1 | 10 " |
| 0.1-0.01 | 20 " |
ASSAY NOTE
The rougher the method of assay adopted, the larger should be the quantity of ore taken. If the degree of accuracy attainable with the methods and instruments at the assayer's service is known, it is easy to calculate what quantity should be taken for any particular case. If the results are good within 0.001 gram, then, taking 1 gram of ore we can report within 0.1 per cent., or if they are good within 0.0002 gram, taking 20 grams of ore, we can report within 1 part per 100,000, or very closely within 6-1/2 dwt. to the ton. If it is wished to be yet more particular in reporting, larger quantities must be taken. The difficulty of manipulating very small or very large precipitates, &c., must be borne in mind. So, too, must the fact that the greater the weight of the final product of an assay, the less, as a rule, is the percentage error. The distinction between absolute and percentage error, often overlooked, is important. If 0.5 gram of silver be cupelled with 20 grams of lead, there may be obtained a button of 0.495 gram; the absolute loss is 0.005 gram, and this equals 1 per cent. of the silver present. Similarly, cupelling 0.1 gram, the resulting button may be 0.098; the absolute loss is only 0.002 gram, but this equals 2 per cent. of the silver present. In the same way the student should see that the two results, 91.5 per cent. and 92.0 per cent., are really more concordant than the results 9.1 per cent. and 9.2 per cent.
A device often adopted in practice where a large number of assays of one kind are made, and the report is given as so many ounces or pounds to the ton, is that known as the assay ton. The assay ton may be any arbitrary and convenient weight, but its subdivisions must bear to it the same relations as pounds and ounces bear to the actual ton. On the other hand, in a laboratory where many kinds of work are performed, different sets of weights of this kind would only tend to confusion, even if they were not unnecessary. With a set of gram weights and its subdivisions anything may be done. If it is desired to report as pounds to the ton, then, since there are 2240 lbs. to the ton, a weight of 2.240 grams may be taken as the assay ton, and each 0.001 gram yielded will equal 1 lb., or 22.4 grams may represent the ton, and each 0.01 gram a pound. Similarly, since there are 32,666.6 ozs. troy to the ton; if we take 32.6667 grams as the assay ton, each 0.001 gram will equal 1 oz. to the ton. In some cases it may be convenient to have, in addition to the usual gram weights, one or other of the "assay tons" mentioned above, but generally it is better to work on a purely decimal system, and convert when required into ounces per ton, &c., either by actual calculation or by reference to a set of tables.
Practical Exercises.
The student should practise such calculations as the following:—
1. Calculate the percentages in the following cases:—
(a) Ore taken, 2 grams; copper found, 0.2155.
(b) " 1.5 gram; iron found, 0.8340.
(c) " 30 grams; lead found, 23.2.
2. Calculate the parts per thousand in the following:—
(a) Bullion taken, 1.1 gram; silver found, 1.017.
(b) " 1.14 gram; silver found, 1.026.
(c) " 0.6 gram; gold found, 0.5500.
3. Calculate parts per 100,000 in the following:—
(a) Ore taken, 20 grams; silver found, 0.0075.
(b) " 50 grams; gold found, 0.0026.
(c) Water taken, 500 c.c.; solids found, 0.1205.
4. Calculate cwts. to the ton in the following:—
(a) Ore taken, 5 grams; tin found, 2.816.
(b) " 5 grams; tin found, 3.128.
(c) An ore with 68.2 per cent. of tin.
5. Calculate lbs. to the ton in the following:—
(a) An ore with 3.28 per cent. oxide of tin.
(b) Ore taken, 20 grams; oxide of tin found, 1.67.
6. Calculate ozs. (troy) to the ton in the following:—
(a) Ore taken, 50 grams; gold found, 0.0035.
(b) " 20 grams; silver found, 0.0287.
(c) " 25 grains; silver found, 0.0164.
7. Calculate in grains per gallon:—
(a) 0.51 gram per litre.
(b) 24.6 parts per 100,000.
(c) Solution taken, 100 c.c.; copper found, 0.0045 gram.
(d) " 50 c.c.; iron found, 0.165 gram.
8. Convert into ozs. (troy) per ton:—
(a) 7 loths per centner.
(b) 30 grams per quintal.
(c) 15 parts per 100,000.