SILVER.—Dry Assay.
757
the boiling point previous to the addition of theprecipitant. When the whole of the chloride hassettled at the bottom of the vessel, the supernatantliquor is carefully drawn off by means of a glasssyphon ; and the chloride, after being well washedby repeated decantations with water slightly acidu-lated with nitric acid, is transferred to a thinporcelain capsule, the weight of which has beenaccurately ascertained. It is carefully dried in a waterbath, and then heated over the flame of a gas-burneror spirit lamp until it fuses, and the crucible withits contents are again weighed while warm. Thedifference between the first and second weighingsgives the amount of the chloride, from which that ofthe silver can be deduced by the simple proportion:—
As 100 : 75-27 : : K, or the weight of chlorideobtained, to x, the weight of silver in the oreoperated on ; since every 100 parts of the chloridecorrespond to 75-27 parts of metallic silver.
This principle is applied to the assay of bullion inthe Indian mints, and will be found referred to underthe head, “ Humid Methods of Assay,” at page 760.
Valuation of Silver Alloys .—In the article on Gold(vol. ii. page 96) it has already been explained thatsilver assays may be reported either on the French ordecimal system, or on what is called the trade system.On this latter the assay pound (usually 12 grainstroy) is divided into 12 parts called “ ounces,” each“ounce” into 20 “pennyweights,” and each “penny-weight” into 24 “excess grains.” Thus the wholeassay pound contains 5760 excess grains, and thedivision is exactly analogous to that of the troypound; any report, therefore, on this system givesat once, without calculation, the amount of finesilver per troy pound of the alloy. The followingtables, similar in form to those given in the Valua-tion of Gold Alloys , will be found useful:—
Ounces.
Decimal
Equivalent.
Penny-
weights.
Decimal
Equivalent.
Excess
Grains.
Decimal
Equivalent.
i
83-3
1
4-16
1
0-17361
2
8-3
2
0-3472
2
166*6
3
12*49
3
0-52083
4
0-694
3
250-0
4
16-6
5
0-86805
5
20-83
6
1-0416
4
333-3
6
24-9
7
1-2L527
8
1-38
7
29-16
9
1-56249
5
416-6
8
33-3
10
1-7361
9
37-49
11
1-90972
6
500'0
2-083
10
41-6
12
11
45-82
13
2-25691
7
583"3
14
2-4305
12
49-9
15
2-60416
8
666-6
13
54-16
16
2-7
14
58-3
17
2-95138
9
750-0
15
62-49
18
3-1249
16
66-6
19
3-29861
10
833-3
17
70-83
20
3*472
21
3-64583
11
916-6
18
74-9
22
3-8194
19
79-16
23
3-99305
12
1000-0
20
83*3
24
4*16
Decimal
Penny-
Grains.
Equivalent.
weights.
Ounces.
1
0-1736
Assay Pound.
1
24
4-13
1
20
480
83 6
1
12
240
5760
1000-0
The terms betterness and worseness (indicatedby B and W) are used to show that the alloycontains a greater or less proportion of the preci-ous metal than the standard alloy, which containsTtnnn or D- ozs - 2 dwts. per lb. troy. Excessgrains, however, whether used in conjunction withB or W, always express betterness. Thus a report,B, 6J dwts. 5 excess grains, would indicate an alloycontaining 5 grains per lb. of silver more than thatcontained in an alloy of the standard, B, 6J dwts.;and further, a report, W, 6j dwts. 5 excess grains,would not indicate an alloy containing 5 grainsper lb. less than the report, W, 6J dwts., but thisamount in excess of that report. This differencewill be more clear if the decimal equivalents begiven:—■
Trade Report. Decimal Equivalent
B, 6J 5. 952-95
B, 6} 0. 952-08
W, 6$ 5. 898-82
W, 6£ 0, 897-95
Assay of the Alloys and Ores of Silver inthe Dry Way. —In this process the object sought isto obtain the silver and other metals that may bepresent in the form of an alloy with lead, which isafterwards introduced into the muffle, and cupelled inthe ordinary way. All the oxidisable metals are thusremoved, the silver remaining behind in a pure state—that is, free from all metals except gold, platinum,and any other non-oxidisable metals that may bepresent. Should appreciable quantities of suchmetals be known to be present, the dry method ofassay is not applicable in its simple form, but mustbe supplemented by the use of nitric acid, as de-scribed under the heading, “ Parting Assays,” inthe assay of gold (vol. ii. page 122); or one of thewet methods of assay of silver, subsequently de-scribed, may be resorted to.
The preliminary operations to which mineralscontaining silver are subjected, and the form of thecupels and other apparatus employed, have beenfully described in connection with the analogousprocesses followed in the assaying of gold ores, andtherefore it will be quite unnecessary to repeat themin this place.
Ores, according to their nature, are either (1)fused with a reducing flux, such as charcoal or blackflux in conjunction with litharge, in order to pro-duce metallic lead in which to collect the silver ; or(2) fused with an oxidising agent, such as lithargeor nitre ; or (8) scorified.
Ores of silver, in which the metals that are presentexist in the form of reducible oxides, are commonlytreated according to the first of these methods. Theproportion of litharge employed for this purpose,says Phillips, must be varied according to circum-