1902 Encyclopedia > Assaying

Assaying




ASSAYING. This term is used in metallurgy to denote a chemical operation in which the quantity of one ingredient of a mineral or alloy is determined ; it is chiefly used in reference to the precious metals, gold and silver, and it is in this connection that the subject will here be treated of. In the wider acceptation of the term in which it is used amongst practical metallurgists, assaying means almost the same thing as the quantitative estimation of one constituent of a compound, when the process adopted is one which has to be frequently repeated in a laboratory, and the results are required for commercial purposes. In this sense we speak not only of the assay of gold and silver, but of other metals, such as lead and copper, of non-metallic elements, such as sulphur and iodine, and even of compounds such as nitre. The operations of assaying were, until recently, chiefly performed by what is called the dry method, but of late years the processes of volumetric analysis have been so largely introduced into the metallurgical laboratory, that the wet method is almost as much used as the dry method. In the processes of assaying the precious metals described in the following pages, the reader will have both these terms explained, for gold is assayed in the dry, while silver is assayed in the wet way.
The precious metals, gold and silver, being almost uni-versally used as convenient representatives of value, and as such passing frequently between one country and another, it is of the utmost importance to ascertain, quickly and accurately, the marketable value of any sample of gold or silver bullion. Were these metals invariably used in their pure state, their commercial value would be in direct pro-portion to their weight, and all that would have to be known would be the actual value of a pound of gold or silver ; but the metals exist in commerce in the form of alloys or mixtures containing an indefinite amount of base metaL Gold is generally alloyed with copper and silver, whilst silver ia generally alloyed with copper. The problem is, therefore, to ascertain by some ready process, which admits of extreme accuracy as well as moderate rapidity, the exact proportion of pure gold present in an alloy, ore, or mixture containing this metal; and inasmuch as silver is also a precious metal, the assay of gold almost always involves the assay of the silver which accompanies it, for in many cases the amount of silver present may be sufficient to increase the commercial value of the substance under assay. It is very seldom, however, that the copper or other metal present is in sufficient quantity to be of value, unless, in-deed, the substance under assay be a copper ore or pyrites containing only traces of gold. In the case of silver assay, when the base metal is copper, it is generally neglected. It is, however, frequently necessary to examine silver for gold, for, formerly, the methods of parting these two precious metals were by no means so exact as they are now, and on this account old silver frequently contains an amount of gold which it will pay well to extract by modern methods. The principle of assaying gold and silver is very simple theoretically, but in practice great experience is necessary to ensure accuracy, and there is no branch of business which more demands personal and undivided attention.
All substances containing gold may be divided into two classes. The first class comprises ores containing gold in a mineralised form. These include graphic tellurium and foliated tellurium, and are of no commercial importance. For the present purpose, we need simply mention the substances in the second class, which consist of alloys of gold, and include native gold, containing from 65 to 99 per cent, of gold; palladium gold, containing about 86 per cent, of gold; rhodium gold, containing from 59 to 66 per cent, of gold; gold amalgam, containing 38 per cent, of gold; and artificial alloys, as gold coin, jewellery, <fcc. Of the foregoing list, the only alloys which are of commercial importance are, native gold and artificial alloy*. Native


gold is commonly found in a quartzose gangue, and nearly always associated with iron and copper pyrites, mispickel or arsenical pyrites, blende, galena, many antimonial minerals, and nearly all the primitive rocks.
The only artificial alloys of gold which will be specially noticed here are the standard gold of this realm, and alloys of gold with silver or copper used for jewellery. The stan-dard of alloys of gold is expressed in fractions of unity. It is assumed that there are 24 carats in unity, and ffnds in the carat. Standard gold, in the 24 carats, contains 22 carats of pure or fine gold, as it is called, and two carats of alloying metal, either silver or copper, or a mixture of the two. Standard gold is, therefore, called 22-carat gold. In a similar way articles of jewellery are denominated 12-carat, 16-carat, 18-carat, &c, according to the propor-tion of fine gold alloyed with the inferior metal An ordi-nary assay report of gold expresses the variation from the standard, and not the fine metal contained in it, and it is, therefore, marked as either better or worse than standard. The standard of gold being 22 carats fine and 2 alloy, an ingot of gold found to contain only 21 carats pure go'd would be reported worse 1 carat. If it contained 23¿ carats, it would then be reported better 1J carat.
The processes by which gold is generally assayed are cupellation, when the alloy consists of copper, and part-ing, when the alloy consists of silver. Generally speaking, both operations are necessary. We will describe them as they would be performed in practice. When the stan-dard of the alloy to be examined is not approximatively known, a preliminary assay must be made to ascertain the quantity of lead necessary to fuse with the gold alloy. But in most cases this is unnecessary, as, from the circumstances of the case, the standard of the alloy is generally known within sufficiently close limits.
The process of cupellation is briefly as follows:—The gold alloy is fused with a quantity of lead, and a little silver if silver is already present. The resulting alloy, which is called the lead button, is then submitted to fusion on a very porous support, made of bone-ash, and called a cupel. The fusion being effected in a current of air, the lead oxidises. The heat is sufficient to keep the resulting oxide of lead fused, and the porous cupel has the property of absorbing melted oxide of lead without taking up any of the metallic globule, exactly in the same way that blotting-paper will absorb water whilst it will not touch a globule of mercury. The heat being continued, and the current of air always passing over the surface of the melted lead button, and the oxide of lead or litharg'e being sucked up by the cupel as fast as it is formed, the metallic globule rapidly diminishes in size until at last all the lead has been got rid of. Now, if this were the only action, little good would have been gained, for we should simply have put lead into the gold alloy, and then taken it out again; but another action goes on whilst the lead is oxidising in the current of air. Other metals, except the silver and gold, also oxidise, and are carried by the melted litharge into the cupel. If the lead is therefore rightly proportioned to the standard of alloy, the resulting button will consist of only gold and silver, and these are separated by the operation of parting, which consists in boiling the alloy (after rolling it to a thin plate) in strong nitric acid, which dissolves the silver and leaves the gold as a coherent sponge. To effect this parting properly, the proportion of silver to gold should be as 3 to 1. The operation by which the alloy is brought to this standard is termed quartation or inquartation, and consists in fusing the alloys in a cupel with lead and the quantity of fine silver or fine gold neces-sary to bring it to the desired composition.
What is called the Trial of the Pyx is an ancient cere-mony which takes place about once in every three years, at which the standard coin of the realm is carefully assayed. For a description of this see the article COINAGE.
It is unnecessary in the present work to describe the various delicate operations which we have briefly alluded to above, but we will describe the implements and furnaces which have been introduced and adopted in the Royal Mint by Mr H. W. Field, the late resident assay-master of that establishment.
Fig. 1 is the front elevation of the furnace; a, a view of the front iron roller on which it rests ; b, the ash-pit; ce are the dampers moving horizontally from side to side towards each other, meeting exactly in the centre; d, the muffle door by which the as-says are introduced; ee, the door slides. So far, the furnace is similar to that formerly in use, except that the bars on which the muffle stands run from front to back, and are movable, ren-dering the removal of the brick-work unneces-sary. By this means the muffle stand is easily introduced, and, having steady pins on the under side, it is raised about an inch above the bars. The furnace measures 2 feet 10 inches in height, 1 foot 7 inches in width, and 1 foot 11 inches in depth. Instead of the furnace, as formerly, be-ing fed at the top, the fuel is charged by the door h, which also affords the means of regulating the draught, and of throwing a current of air through the muffle by the door d. This door has a bar k, traversing about two-thirds of it, running easily from the top towards the bottom within ii, with a ketch m, on each side to keep it close. These are made on an incline, and about 3 J inches long, so as to allow the travers-ing bar to slide freely when the door is not required to be closed. In this manner the door may be opened from a quarter of an inch to the extent of three inches. This feeding and regulating door is fixed by hinges 11 to the front part of the iron frame covering the brick lining on the top of the furnace. On this frame rests the square dome r, the front of which, w, is removable by two handles nn; and by taking out the two thumb-screws oo, the door and part of the frame come away, leaving a large opening, so as to enable the furnace to be cleaned, the muffle repaired, <fcc. The furnace should be placed in a recess, under a chimney, with a movable iron ceiling t, about 1 foot above the dome, fitting close in every part, so that the draught of air may pass through the furnace. A door, or flap, x, is attached to the iron ceiling by a hinge opening on the side of the recess, with means to fix it at any point required, so that the current of air may be regulated by the operator; s, a swivel door affords another mode for damping the furnace.
Fig. 2 is a section of the furnace fig. 1; aa, the two rollers on which the furnace is placed; bb, the slides on which

ash-pit doors run; c, the door and ash-pit; d, the iron casing to the furnace; e, the brick lining; /, the ash-pit; gg, the two bars inserted in the brick lining,—one in front, one at the Dack, supporting the furnace bars, which can easily be remov-ed at pleasure; A, one of the bars on which the muffle plate rests; i, a movable tray on which the mouth coal is placed; h, a section of a muffle charged with its full complement of 50 cupels, showing also the rows of holes over each row of cu-pels, through which a current of air passes; similar holes are placed at the back in three rows; they are not pierced through horizontally, but slope towards the ceiling of the muffle at such an angle so as to exclude the ashes; I represents the extra covering of fire-clay; m, the an-thracite coal, showing the level; n, the feed-ing and regulating door; o, the ketch or inclined plane on which the sliding-bar travels; p, the door, with running staples in which the bar slides; q, the mode in which the movable front is brought round and fixed by the thumb-screws r; sv, the hood; t, handle for removing the front; wx, the damper and handle.
Fig. 3 represents the upper interior view of the furnace bars with the muffle stand or plate, showing also the space in-tended for the fuel.
Fig. 4 is the mouth of the muffle door, representing the mode of regulating the current of air by cylinders of charcoal.
Fig. 5 is the movable muffle door.
Fig. 5.
Fig. 6.
Fig. 8 is an annealing-iron for softening the assays after they are flattened and rolled. It resembles a square bar of iron about |th of an inch thick, having strengthening
Fig. 6 is a representation of a muffle, 14^ inches long, 7^ in-ches wide, until it begins to taper at about 1J inches from the front (see fig. 7), when it does not ex-ceed 5^ inches. The height is 6£ inches, in the clear 5f inches. Its sides are perforated with holes about a quarter of an inch in diameter.
pieces rivetted at each end and two in the middle bbbb,

between which are receiving places for the assays aaa.
The apertures are made
diagonally, as shown by ccc, MI^IIIMlllt^iirfii'oill^
that the assays may not fall (\ ''^Pffia
completely to the bottom of IppHHHlBHBR
the box, so that they may lah'-.
be conveniently removed. nJHfflJH
The under part of the box e
has a kind of double keel _
7 . .. i .. . _ FIG. 8.—Annealing Iron,
o rivetted on it, so that in
taking it from the furnace there be no danger of upsetting it on the annealing trident.
FIG. 9.—Trident.
Fig. 10 represents the cupel mould; fig. 11 being the
section of the same, showing the four pieces of which it
consists. The mould is circu-
lar, made of forged steel nicely
turned. There are several
substances of which cupels
may be made, but the one in
general use is the ash of burnt
bones. This consists princi-
pally of phosphate of lime,
with a little carbonate and
some fluoride of calcium. The
bones of sheep and horses are
best for cupels. They should
be burnt until they are quite
white, care being taken not to
heat the bone-earth too strong-
ly. It must then be finely
ground, sifted, and washed FIG.
several times with boiling dis-
tilled water till all soluble salts FIG. 11. — Seo-
are removed. The finest par- tion of Cupel
tides of the powdered bone- Mouli
earth will remain longest suspended in the washing waters.
This must be allowed to settle separately, and should be
reserved for giving a final coating to the surface of the cupels.
For the body of the cupels the bone-ash should be about as
fine as wheat flour. The bone-ash being moistened with
a quantity of water, just sufficient to make the particles
adhere, is put into the mould a, fig. 11, and pressed down
level with the surface. The mould is then put together,
as in fig. 10, and the pestle struck with a hammer so as to
compress the bone earth into a solid cake. The surface of
the cupel may then have sifted over it a little of the very
fine levigated bone-ash, and the pestle again hammered on
it. The pestle is to be turned lightly round so as to smooth
the inner surface of the cupel, and then withdrawn. The
cupel is removed from the mould by gentle pressure on
the narrowest end. It must be dried gently by a stove,
and lastly ignited in a muffle to expel all moisture. It is
then ready for use.
Fig 12 shows the appearance of the finished cupel, which is generally 1 inch by £ths of an inch.
The lead used in cupeilation should be of the Flg' 12' greatest purity, because, as most lead contains a small portion of silver, this silver would necessarily combine with the assay and vitiate the accuracy of the result. Another important consideration i3 the quantity of lead to be used with each assay. This information is generally obtained by an ex-perimental assay, unless, as most frequently happens, the circumstances of the case enable the assayer to judge the
Fig. 9 represents the trident for removing the annealing-iron from tie furnace.


FIG. 9.—Trident.
Fig. 10 represents the cupel mould; fig. 11 being the
section of the same, showing the four pieces of which it
consists. The mould is circu-
lar, made of forged steel nicely
turned. There are several
substances of which cupels
may be made, but the one in
general use is the ash of burnt
bones. This consists princi-
pally of phosphate of lime,
with a little carbonate and
some fluoride of calcium. The
bones of sheep and horses are
best for cupels. They should
be burnt until they are quite
white, care being taken not to
heat the bone-earth too strong-
ly. It must then be finely
ground, sifted, and washed FIG.
several times with boiling dis-
tilled water till all soluble salts FIG. 11. — Seo-
are removed. The finest par- tion of Cupel
tides of the powdered bone- Mouli
earth will remain longest suspended in the washing waters.
This must be allowed to settle separately, and should be
reserved for giving a final coating to the surface of the cupels.
For the body of the cupels the bone-ash should be about as
fine as wheat flour. The bone-ash being moistened with
a quantity of water, just sufficient to make the particles
adhere, is put into the mould a, fig. 11, and pressed down
level with the surface. The mould is then put together,
as in fig. 10, and the pestle struck with a hammer so as to
compress the bone earth into a solid cake. The surface of
the cupel may then have sifted over it a little of the very
fine levigated bone-ash, and the pestle again hammered on
it. The pestle is to be turned lightly round so as to smooth
the inner surface of the cupel, and then withdrawn. The
cupel is removed from the mould by gentle pressure on
the narrowest end. It must be dried gently by a stove,
and lastly ignited in a muffle to expel all moisture. It is
then ready for use.
Fig 12 shows the appearance of the finished cupel, which is generally 1 inch by £ths of an inch.
The lead used in cupeilation should be of the Flg' 12' greatest purity, because, as most lead contains a small portion of silver, this silver would necessarily combine with the assay and vitiate the accuracy of the result. Another important consideration i3 the quantity of lead to be used with each assay. This information is generally obtained by an ex-perimental assay, unless, as most frequently happens, the circumstances of the case enable the assayer to judge the


upproximate standard of the alloy with sufficient accuracy. The sample of gold alloy is either cut from a corner, or drilled out of the centre of an ingot, care being taken to secure uniformity of composition with the whole mass. The amount of lead to be added to the alloy varies with the proportion of base metal present; as a rule, from 10 to 20 parts are required to 1 of alloy.

The amount of lead having been determined, the alloy is wrapped up in a known quantity (say one-half of that required for its purification), formed into a case somewhat resembling a thimble, great care being taken to make the joints firm and close so that no gold shall escape. The re-quisite quantity of silver is added at the same time. When a number of assays are made at the same time, they are arranged, enveloped in their lead cases, on a board divided into compartments corresponding in number and position with the cupels into which they are intended to be charged. As the assayer generally makes two or more trials of the same piece, so that great accuracy may be secured, it is his practice to give one assay a side place in the muffle, and the second a middle one, in order to check any irregularity in the result. When a sufficient number of assays are weighed and arranged upon the board in the manner referred to, and the furnace as well as the cupels raised to the neces-sary point of heat, the charging tongs are then taken, and the rest of the lead and silver apportioned to each assay placed individually upon the cupels, beginning at the back of the muffle. The lead added in this case is not flattened, but is a piece of known weight—various sizes of which, as well as cases, are kept in stock by the assayer. The lead so placed in the furnace rapidly melts, and becomes covered with a gray oxide, but soon after appears fluid and bright. At this point the assays are added by means of a pair of tongs (fig. 13), great care being taken that no part overhangs or touches the edge of the cupel. The assays are thus drawn into the mass of molten lead, and any particles of gold are in this manner prevented from adhering to the sides of the cupels in charging, sufficient despatch being used to obviate the fusion of the assay in its transition. The assays being charged in order on their respective cupels, and the furnace previously filled with fuel, the door of the muffle is partly closed, and the progress of the cupellation watched. Too much air must not enter, or the muffle will be chilled and the progress retarded, whilst if too little enters the operation will be too slow.
At first dense fumes will be observed to rise from the melted metal, indicating the oxidation and subsequent volatilisation of the lead. These after continuing some time are followed by the appearance of small luminous points on the surface, which increase in size and brilliancy as the operation progresses. Then a minute stream of red fused oxide of lead is seen to flow from the top of the metal globule and circulate round, when it is carried down and absorbed by the cupel. This is caused by the oxida-tion of the lead by the air, which at the same time oxidises the other metals, except silver, which accompany the gold. As the cupellation advances the fumes gradually lessen in density till they disappear altogether. The melted button at this stage is observed to become more convex and round, and as the last vestiges of the lead and alloy are being carried off, it assumes a cloudy appearance on the surface, changing to large bright points of the fused oxide, till at length it is nearly freed from all impurity. At this point the gold-silver alloy displays some singular and beautiful characteristics. Deprived of all the base alloy 3ave the last minute portion that tarnished its lustre, it has become bright and pure, and finally it gives forth from its surface iridescent circulating bands of light, which indicate the suc-cessful completion of the operation. The globules after being cooled, are removed from the cupels with a pair of pincers and carefully cleaned. They are then placed in their com-partments and weighed with the greatest nicety. Finally, they are submitted to the operation of parting. This is effected by boiling with strong nitric acid, which dissolves the silver and leaves the gold as a sponge. The gold-silver globule is first passed through a flattening mill, and reduced to long thin strips, which are annealed and then rolled up into a corkscrew spiral, so that the acid may penetrate between each fold. The spirals are now transferred in order to small platinum cups arranged on a frame, so that they can be simultaneously lowered into and removed from the nitric acid. They are kept in the hot acid for the requisite time, then washed, and the residual.gold sponge, which possesses considerable coherence, and retains the shape of the original spiral, is carefully dried and finally weighed with the greatest possible accuracy.
We shall now proceed to describe briefly the process of silver assay. The cupellation process does not differ except in details from that of gold,—the outline already given will, therefore, be a sufficient description. Like gold, silver occurs in two classes of combinations,—mineralised, as in red silver ore, chloride of silver, argentiferous galena, <fcc., or as metallic silver and its alloys. In metallurgi-cal establishments, where silver occurs in small propor-tion with metallic sulphides, such as those of lead or copper, the process of cupellation is generally adopted not only for quantitative estimation of the amount of silver present in the ore, but also for its extraction on the large scale. In the assay of silver bullion, however, the process of cupellation is now almost entirely superseded by a volumetric process, devised by the distinguished French chemist, Gay-Lussac, by whose influence it was introduced into the Paris Mint. The process consists in determining the fineness of silver bullion by the quantity of a standard solution of common salt necessary to precipitate fully and exactly the silver contained in a known weight of alloy. This process is based on the following principles:—
The alloy previously dissolved in nitric acid is mixed with a standard solution of common salt, which precipitates the silver as chloride, a compound perfectly insoluble in water and even in acids. The quantity of chloride of silver precipitated is determined not by its weight, which would be less exact, and occupy too much time, but by the weight or volume of the standard solution of common salt neces-sary to precipitate exactly the silver previously dissolved in nitric acid. The term of complete precipitation of the silver can be readily recognised by the cessation of all cloudiness when the salt solution is gradually poured into that of the nitrate of silver. One milligramme of that metal is readily detected in 150 grammes of liquid, and even a half or a quarter of a milligramme may be detected if the liquid be perfectly bright before the addition of the salt solution. By violent agitation during a minute or two, the liquid, rendered milky by the precipitation of chloride of silver, becomes sufficiently bright after a few moments' repose to allow of the effect of the addition of half a milligramme of silver to be perceptible. Filtration of the liquid is more efficacious than agitation; but the latter, which is much more rapid, generally suffices. The presence of copper, lead, or any other metal, with the exception of mercury in the silver solution, has no sensible influence on the quantity of salt required for precipitation ; in other words, the same quantity of silver, pure or alloyed, requires for its precipitation a constant quantity of the standard salt solution. Supposing that 1 gramme of pure silver be the quantity operated on, the solution of salt required to precipitate exactly the whole of the silver ought to be of

such strength that, if it be measured by weight, it shall weigh exactly 100 grammes, or if by volume, 100 cubic centimetres. This quantity of salt solution is divided into 1000 parts, called thousandths. The standard of an alloy of silver is generally the number of thousandths of solution of salt necessary to precipitate the silver contained in a gramme of the alloy.
The operations of assaying depend for their accuracy on the perfection of the balance used to ascertain the weights of the metals taken, and the resulting globules. In a good assay balance three essentials are indispensable:—(1.) It should be quick in its action ; (2.) It should be constant and uniform; (3.) It should be extremely sensitive and delicate, indicating the minutest shades of difference. Assay balances, as now constructed, are capable of indicating a difference of the ten-thousandth of a grain. For a description of the modern chemical balance, see article BALANCE. (w. C.)








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