IV. Assay Vessels.

14. GENERAL REMARKS.

The form of the assay vessels, as well as the materials from which they are made, varies according to the object for which they are to be used. The principal distinction is, whether they are to be employed in the dry or wet method.

15. ASSAY VESSELS FOR THE DRY METHOD.

A. Clay vessels.-These are required to be more or less refractory according to the heat to which they are to be exposed (their refractory quality depends on the proportion between silicic acid and alumina and the quantity of fluxing agents-ferric oxide, lime, alkalies, magnesia—which may be present). They must allow of being suddenly heated and cooled without cracking (fat, contracting clay requires to be mixed with quartz, chamotte," graphite). The vessels should be corroded as little as possible by the substances heated in them, but, as a general rule, this can never be entirely prevented. (It may be done to some extent by making the sides of the crucibles thicker, or by giving a finer grain to the stuff of which they are made. This should be made as compact as possible, by mixing the clay with chamotte instead of quartz. The interior of the crucible should be made very smooth, and it should be fired in the kiln as strongly as

Kerl, Thonwaarenindustrie, 1879, p. 491. Percy, Metallurgy, vol. I. 1875, p. 111.

[Chamotte is a mixture of unburnt fire-clay and dust of fire-bricks, glass pots, or seggars.-TRANSLATOR.]

possible.) The vessels should further be very compact. This can be accomplished by giving a suitable grain to the mass, exercising great care in moulding and firing them strongly. The compactness of the vessels is tested. by fusing metallic sulphides, such as galena, several times in them. They are made either by a plug and mould (roasting dishes and scorifiers, Upper Harz crucibles for lead smelting) or they are turned upon the potter's wheel (crucibles and larger melting pots).

The principal vessels, etc., are— 1. Vessels without feet.

a. Roasting dishes (Fig. 7).-They are flat, smooth inside, not very refractory, 8 to 10 millimeters (0.31 to 0.39 inch) deep and 50 to 80 millimeters (1.96 to 3.14 inches) wide. They are used in the manner indicated on p. 34.

Fig. 37.

b. Scorification or calcining vessels (Fig. 37).—They have a thick bottom and sides, very smooth interior, and are very compact. To avoid being corroded by lead oxide, they should be made of clay mixed with chamotte. They are 40 to 50 millimeters (1.57 to 1.96 inches) wide in the clear, 15 to 20 millimeters (0.58 to 0.78 inch) deep, with a bottom 10 millimeters (0.39 inch) or more thick.

c. Refining dishes.-They have either the same form as the flat roasting dishes, but are fire resistant and one edge is somewhat ground down, or they are made from fragments (Fig. 38) of crucibles (Fig. 42), and are then 70 to 80 millimeters (2.75 to 3.14 inches) long; or they are shaped like a flat saucer with feet. These are 30 millimeters (1.18 inches) wide, with a total height of 25 millimeters (0.98 inch) (Hungary).

d. Crucibles.-These are of various forms and sizes, large and small (Figs. 39 to 41). They are respectively 32 and 45 millimeters (1.25 and 1.76 inches) high in the clear, and have a total height of 39 and 52 millimeters

(1.53 and 2.04 inches), a clear width of 33 and 43 millimeters (1.29 and 1.68 inches), and are not very refractory. Fig. 40 shows a large and a small Cornish crucible for the assay of copper. They are very refractory. Their respective dimensions are: diameter on the top 80 and 68 millimeters (3.14 and 2.67 inches), total height 84 and 60 millimeters (3.3 and 2.36 inches). Fig. 41 represents a crucible for iron. This is lined by means of a wooden plug with charcoal powder b (this is first moistened with starch paste, molasses, or clay), or it is lined with a mixture of 90 to 95 per cent. retort graphite, 5 per cent. rosin, and some petroleum, and burned with exclusion of air. They are covered with the perforated lid c. They are 37 millimeters (1.45 inches) high and 25 millimeters (0.98 inch) wide, and, after they have been lined, respectively 22 millimeters (0.86 inch) and 10 millimeters (0.39 inch). They are very refractory (Hessian pots). The French pots are especially refractory and smooth inside.

Graphite crucibles are made of graphite mixed with clay. They are very smooth inside, and very refractory. Those used in Cornwall for assays of tin are 80 milli

meters (3.14 inches) wide on the top, and 50 millimeters (1.96 inches) on the bottom, have a clear height of 74 millimeters (2.91 inches), and a total height of 90 millimeters (3.54 inches).

Soapstone crucibles, if gradually heated, are adapted for all smelting purposes. They are infusible, not affected by alkalies, and become harder by burning.

Fig. 42.

2. Vessels with feet. Crucibles for lead and copper smelting (a, Fig. 42). -The latter are more refractory than the first. They are 25 to 32 millimeters (0.98 to 1.25 inches) wide on the top, 40 to 50 millimeters (1.57 to 1.96 inches) in the centre, 83 to 85 millimeters (3.26 to 3.34 inches) high in the clear, with a total height of 110 to 120 millimeters (4.32 to 4.71 inches). Sometimes there is a depression in the bottom for the reception of the regulus, and the foot, when broken off, may serve as a cover.

a

Fig. 43 shows a crucible for smelting iron, lined with powdered charcoal (see above). These crucibles are 45 millimeters (1.76 inches) wide, and 55 millime

ters (2.16 inches) high in the clear, with a total Fig. 43. height of 90 millimeters (3.54 inches).

3. Other clay vessels.-In this category belong muffles (p. 49), retorts, and tubes.

U

B. Wrought-iron vessels. Crucibles for assay of lead, with or without lip.-These are from 8 to 12 centimeters (3.14, to 4.72 inches) high, and 5 to 8 centimeters (1.96 to 3.14 inches) wide. The sides are from 10 to 12 millimeters (0.39 to 0.47 inch) thick, and the bottom from 2 to 3 centimeters (0.78 to 1.18 inches). Other iron vessels used are, tubes and retorts, and cast-iron muffles.

C. Vessels of bone-ash: Cupels (Fig. 44).-They are

Fig. 44.

Fig. 45.

B

made either of bone-ash alone, or with an addition of a little wood-ashes, or pearl-ash, to the water used for moistening the bone-ash, which addition decreases their power of conducting heat. The bones are burnt white throughout, are then powdered and washed. The dried powder, which should be about as fine as coarse wheat flour, is used for the principal mass, while a finer flour is reserved for a final coating. The cupels are formed by filling and driving the prepared bone-ash into a mould made for the purpose (Fig. 45), B, the pestle; A, the mould; or they are pressed.1 Ordinary Freiberg cupels for ores consist of 3 volumes soap-boilers' ash, and 1 volume bone-ash. Their outer diameter is 35 millimeters (1.37 inches), diameter in the clear 24 to 25 millimeters (0.94 to 0.98 inch). They are 10 to 12 millimeters (0.39 to 0.47 inch) high in the clear, with a total height of 18 millimeters (0.7 inch). Fine or mint cupels consist of 2 volumes soap boilers' ash, and 3 volumes bone-ash. Their total diameter is 26 millimeters (1.02 inches), with a clear diameter of 18 millimeters (0.7 inch); their total height is 14 millimeters (0.55 inch). Cupels in order to be perfect should dry very slowly, and be thoroughly ignited before they are used. They should be white, and, besides a certain degree of solidity, should possess the requisite porosity to absorb litharge (when taken up with the tongs they must not crumble, but it should be possible to crush

A

1 B. u. h. Ztg. 1868, p. 154.