SILVER. —Assay. Cupellation'.
759
Ratio of Weight of Lead toWeight of Assay.
Ratio of Weight of Lead toWeight of Copper Present.
1000
3-10th to 1
__
950
3 to 1
60 to 1
900
7 to 1
70 to 1
800
10 to 1
50 to 1
700
12 to 1
40 to 1
6U0
14 to 1
35 to 1
500
16 or 17 to 1
32 or 34 to 1
400
16 or 17 to 1
27 or 28 to 1
300
16 or 17 to 1
23 or 24 to 1
200
16 or 17 to 1
20 or 21 to 1
100
16 or 17 to 1
18 or 19 to 1
Pure copper.
16 or 17 to 1
• 16 or 17 to 1
Makins, however, points out that there are greatdifficulties in working fine silver with less than threetimes its weight of lead, and he considers thatresults obtained on alloys very rich in silver are notsatisfactory when a smaller quantity is used. Heemploys six times its weight for the assay of metalof the English standard (925 parts to 1000), andincreases the proportion for coarser varieties.
In the above table the numbers in the secondcolumn, which express the quantity of lead neces-sary to be added in each case, are multiples of theweight of alloy on which the operation is performed.Hence, in the third line, for example, in which thealloy consists of 900 of silver and 100 of copper foreach grain of alloy taken, 7 grains of lead will benecessary ; and as each grain of alloy contains onlyone-tenth of its weight in copper, it follows that theratio of the lead to the copper is as 70 to 1.
It will be remarked also that here, as in the tablegiven for an alloy of gold and copper, the proportionof lead to be employed for a silver alloy below thestandard of 500 remains constantly the same.
Although silver is not appreciably volatilized whenmelted alone, yet it is so to a certain extent in thepresence of lead, and therefore the cupellation shouldbe made at the lowest temperature at which theabsorption of the litharge can be effected; and evenwhen every precaution is used, there is always a lossof silver in cupellation, for which allowance mustbe made.
It must be carefully noted, however, that the heatmust be raised as much as possible, in consequenceof the necessity of removing all foreign metals onthe cupel—the furnace process not being followed,as in the case of gold, by treatment of the button innitric acid. The cooling' also of the furnace mustbe accomplished with considerable care, in order toavoid loss of silver by the expulsion of oxygen,which attends the rapid solidification of silver buttons.The heat should therefore be so adjusted that onlyslight fumes are visible over the cupels, and thefused metal should be very luminous and easilydistinguishable from the cupel. With too low atemperature, crystals of litharge are liable to collectround the button; and with an excessive heat, thefumes rise rapidly to the crown of the muffle. Incooling down the furnace prior to withdrawing theassays, special care must be taken to avoid theadmission of cold air into the muffle, as this wouldbe followed by the “spitting” mentioned above. A
convenient method of avoiding such an accidentconsists in surrounding the closed door of the mufflewith glowing coals, and in not opening it until theupper coals in the furnace are black.
When other metals besides lead and silver arepresent in an alloy, the cupel usually affords indica-tions, from which it is generally possible to judge oftheir nature, and roughly of the amount in whichthey exist.
The following are the results of experiments madein order to ascertain the influence of the severalmetals on the nature of the resulting button, andthe action on the cupel; but the appearance due toeach metal is so often masked by the presence ofanother, that the indications given cannot be con-sidered of much practical value:—
Aluminium. —Yellowish stain, with scoria whichfloat on the surface of the button.
Antimony. —Light yellow slag, which sometimescracks the cupel.
Arsenic. —Sage green stain.
Cadmium. —Black ring near the top of the cupel.
Chromium. —Dark brick-red stain.
Cohalt. —Dark green scoria and greenish stain.
Copper. —Dark brown or green colour.
Iron .—Dark red-brown stain at commencement ofoperation, leaving a dark ring on the cupel.
Lead. —Straw or orange-yellow colour.
Manganese. —Dark bluish-black stain and corrosionof the cupel.
Nickel. —Dark green scoria and greenish stains.
Palladium and Platinum . — Green stain and verycrystalline button.
Tin. —Forms a grey slag.
Zinc. —Yellow ring on cupel; the metal burnswith a brilliant flame, gives out copious vapours,and corrodes the cupel.
On withdrawing from the muffle, the buttons areusually struck on their sides, on an anvil, prior tobrushing them with a stiff brush, in order to remove alladhering bone-asli, when they are ready for weighing.The loss by volatilization and absorption in the cupelbeing much greater than in the assay of gold, it isusual in works on assaying to give a table exhibitingthe results of experiments made in the French Mint,with a view to ascertain the amount of such loss inalloys of all degrees of fineness. Such a table is,however, of but little use to the assayer, as the lossdepends on so many varying circumstances that itis far from being constant, even when every pre-caution is taken. The best method of eliminatingit is the use of a number of “ check ” assays madeon fine silver, side by side with the metal underexamination, in the manner recommended in thearticle on the assay of gold. Since, however, theloss is much greater, and varies considerably through-out the muffle, it is necessary that a greater numberof check assays be made. In the Royal Mint ninechecks are worked in a fire of 45 assays, or one ineach row, and a correction is applied to each assaydeduced from the checks in its immediate neigh-bourhood. Considerable judgment is required inapplying such corrections to the weights obtained.