The CU′PEL (kū′-pĕl) is a small, porous, shallow crucible, usually made of bone ashes or burnt horn. The powder (slightly moistened with water) is placed in a circular steel mould, and after being pressed down tight, is finished off with a rammer having a convex face of polished steel, which is forcibly struck with a mallet, until the mass becomes sufficiently hard and adherent. The newly formed cupel is then carefully removed and exposed in the air for a fortnight or three weeks to dry. Fig. 1 represents a cupel in section, and fig. 2 the tongs used for charging it. The best weight for cupels ranges between 180 and 200 gr. Those used at the Royal Mint are made of the calcined cores of ox-horns.
Proc. of Ass. The muffle, with the cupels properly arranged on the ‘muffle-plate,’ is placed in the furnace, and the charcoal added and lighted at the top by means of a few ignited pieces thrown on last. After the cupels have been exposed for about half an hour, and have become white-hot, the lead (see below) is put into them by means of the tongs. As soon as this becomes bright red and ‘circulating,’ as it is called, the specimen for assay, wrapped in a small piece of paper or lead-foil, is added. The fire is now kept up strongly until the metal enters the lead and circulates well, when the heat, slightly diminished, is so regulated that the assay appears convex and more glowing than the cupel itself, whilst the ‘undulations’ circulate in all directions, and the middle of the metal appears smooth, with a margin of litharge which is freely absorbed by the cupel. When the metal becomes bright and shining, or, in technical language, begins to ‘lighten,’ and prismatic hues suddenly flash across the globules, and undulate and cross each other, followed by the metal becoming very brilliant and clear, and at length fixed and solid (called the ‘brightening’), the separation is ended and the process complete. The cupels are then drawn to the mouth of the muffle, and allowed to cool slowly. When quite cold, the resulting ‘button,’ if of silver, is removed by the pliers or tongs from the cupel, and after being flattened on a small anvil of polished steel, with a polished steel hammer, to detach adhering oxide of lead, and cleaned with a small hard brush, is very accurately weighed. The weight is that of the pure silver; and the difference between the weight of the alloy before cupellation, and that of the button of pure metal, represents the proportion of alloy in the sample examined. (See below.) In the case of gold, the ‘button’ has to undergo the subsequent operations of quartation, parting, and annealing, before it is weighed, as described under that metal.
Assayer’s weights, &c. The materials used in assaying are accurately weighed in a balance of the most susceptible description; and the weights are given in terms of the ‘notation’ employed by assayers. The ‘fineness,’ ‘richness,’ or ‘degree of purity’ of gold is expressed in carats. Pure gold is spoken of as 24 carats fine; and any other sample containing in 24 parts only 12, 18, 22, &c., parts of pure gold, is said to be of as many carats fine. Every carat is nominally divided into 4 ‘assay-grains,’ each assay-grain into ‘quarters,’ and each quarter into ‘eighths’ (= 1⁄32 carat), giving 768 “reports” for gold. On this system fractional alloys are commonly spoken of as of so many ‘carats and thirty-seconds fine.’ The real quantity taken for assay, technically termed the ‘assay-pound,’ is, however, very small, generally either 12 gr. or 6 gr., which makes each assayer’s eighth-grain, or “report,” equal to either the 1⁄64 or 1⁄128 gr. Troy, as the case may be. The nominal assayer’s gold carat is 12 gr. The “journey-weight of gold” is 15 lbs. Troy (= 701 sovereigns = 1402 half-sovereigns).
The ‘fineness,’ ‘richness,’ or ‘purity’ of silver was formerly expressed in pennyweights; but is now generally reckoned in 1000ths, which admits of greater accuracy. Pure silver was said to be silver of 12 pennyweights.” If it contained 1, 2, or 3 parts of alloy, it was termed “silver of 11, 10, or 9 pennyweights,” as the case might be. Every assayer’s pennyweight was nominally divided into 24 gr., and hence gave 288 fine grains, or ‘reports,’ for silver. The fineness of specimens containing odd grains was given in pennyweights and fine grains. The ‘assay-pound’ for silver, on this system, may be 24 Troy gr., when 2 real grains are equal to 1 ‘fine pennyweight,’ and 1⁄12 real gr. equal to 1 ‘fine-grain.’ In the decimal method pure silver is = 1000. The usual weight of silver taken for the ‘assay-pound,’ when the fineness
is reckoned in 1000ths, is 20 Troy gr., every real grain of which represents 50⁄1000th of fineness; and so on of smaller divisions. The mint “journeyweight of silver” is 60 lbs. Troy (=3920 shillings, or a like value in other denominations).
Ratio. Cupellation, which is the distinctive and most important operation in assaying gold and silver, is founded upon the feeble affinity which these metals have for oxygen, in comparison with copper, tin, and other cheaper metals; and on the tendency which these latter metals have to oxidise rapidly in contact with lead at a high temperature, and to sink with it into any porous earthen vessel, in a thin, glassy or vitriform state. The conditions essential to the success of the process, and which are found in the precious metals, are—that “the metal from which we wish to part the oxides must not be volatile;” and that “it should also melt and form a button at the heat of cupellation; for otherwise it would continue disseminated, attached to the portion of oxide spread over the cupel, and incapable of being collected.”[95]
[95] Ure’s ‘Dict. of Arts, M., & M.,’ 5th ed., i, 214.
Concluding Remarks. The art of assaying requires very great care, skill, and experience, for its due exercise; and from the costliness of the precious metals, and their general employment for coin, jewelry, plate, &c., is of the utmost importance both to individuals and governments. Such is the extreme delicacy of the operation of cupellation that, without the requisites alluded to, it is more likely to fail than to give reliable results. An assay is thought to be good when the ‘button’ or ‘bead’ separates readily from the cupel, has a round form, with a brilliant upper surface, and the lower one granular and of a dead metallic lustre. When the upper surface is ‘dead’ and ‘flat,’ too much heat has been employed; and in the case of silver, some of the metal may have been lost by fuming or absorption. When the bead adheres to the cupel, or is spongy, variegated, or has scales of litharge still adhering to it, either too little heat has been used, or the process has been stopped before the assay was complete. The remedy is re-exposure to heat in the cupel, adding a little powdered charcoal or a few small pieces of paper, and continuing the heat until the metal ‘brightens’ and ‘circulates’ freely. The lead employed must be absolutely pure, or that technically called ‘poor lead,’ and, for this purpose, is commonly prepared by the reduction of refined litharge mixed with some carbonaceous matter, by heat; but, according to the late T. H. Henry, “lead reduced from the litharge of commerce usually contains from 10 to 15 dwt. of silver per ton.” These remarks apply equally to gold and silver.
The process of assaying by the cupel, however skilfully conducted, gives much less accurate results, especially with silver, than the method of chemical analysis, often termed ‘humid’ or ‘volumetrical assay,’ whilst it is, in all cases, much more troublesome and expensive, and with compounds containing only small quantities of the precious metals, is not to be depended on. See Gold, Silver; also Carat, Cupellation, Parting, Liquation, Quartation, Refining, &c. (and below).[96]