GOLD.— Construction of Cupels.

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and the greater part of the litharge is expelled by adifferent method.

The English method of cupellation or refining isas follows:—A piece of bar-iron, about half an inchthick and 4 inches deep, is bent into the form ofan oval hoop, and the ends of the bar are weldedtogether. The greater diameter of this hoop, whichis destined to form the containing circumference ofthe cupel or test, may be about 4 feet, and its lesser2 feet 6 inches; but the dimensions vary consider-ably according to circumstances. On the lowerpart of the hoop are fixed four or five flat bars ofiron, arranged parallel to each other in the directionof the short diameter. The first of these is placedabout 9 inches from one end of the oval, and theothers at about equal distances between this bar andthe other extremity. The hoop is then placed withthe cross bars downwards upon a solid floor, and aquantity of pounded and sifted bone-ash is beatenfirmly into it with a mallet or rammer until it isentirely filled, or the bone-ash is level with the upperedge of the hoop. The bone-ash employed forthis purpose is mixed with about one-tenth, bymeasure, of fern ashes, or one-fortieth by weightof American pearl-ashes, and moistened suffici-ently to become coherent by pressure. Thefern or pearl-ash has the property of givinggreater consistency to the bone-ash when heated.When the hoop or frame has been well filled withthis mixture and solidly beaten down, the surfaceis carefully scooped out with a trowel all roundthe centre, so as to form a shallow concavityof about 2J inches in depth, leaving a wall ofbone-ash all round, about 2 inches in thicknessat the top and 3 inches at the bottom, exceptat one end, where a thickness of 13 inchesis left; whilst the thickness of the sole itselfis reduced to 1 inch above the upper surfaceof the iron cross-pieces. At that end of thetest where the wall was left 13 inches thick,and which is termed the breast , a segment ofthe bone-ash contiguous to the hoop is removed.

This apparatus is represented in Figs. 25 and 26,of which the former is a plan, and the latter a longi-tudinal vertical section. In these drawings the lettersA a denote the iron hoop; a a, the tranverse bars;E indicates the cavity in the bone-ash ; B, the breast;and F, the space where the bone-ash is removedbetween the breast and the iron.

In cupels of more modern construction the bone-ash is not removed from the part F, as the litharge isfound to come into contact with the iron ring, onwhich it exerts a powerfully corroding action, whensuch an arrangement is adopted. In place of this,however, a round hole is cut in the centre of thebreast near F, to which channels are cut similar tothose shortly to be described.

When fully prepared, the test is allowed to dry,and is then placed in a furnace, constructed in allrespects like a common reverberatory, except that aspace is left open in the bed of the furnace to receivethe test, and that the width of the arch is much re-duced. The test, in fact, forms the bed of the furnace,

in which it is adjusted with the long diametertransversely. It is supported in its place by an ironring built into the masonry, or by iron bars placedunderneath, between the two walls of the furnace.The fire-place of a cupelling furnace is usually about2 feet in breadth, and 2 feet 6 inches in length. Thisis separated from the body or bed of the furnace bya fire-bridge 18 inches in breadth, so as to allow theflame and heated air to pass directly over the surfaceof the cupel, from whence it escapes through separateflues into a high chimney. At the side of the furnaceopposite to the breast of the cupel the nozzle of apair of large double bellows, or a fan-blast, is intro-duced.

When the cupel is inserted and the furnace lighted,the heat must be applied in the first place with muchcaution; for the test, if exposed suddenly to ahigh temperature before it has become sufficientlydry, is liable to split and fall to pieces. In the mean-time the lead containing the gold and silver, andwhich, from this circumstance, is termed rich lead, is

Fig. 25.

Fig. 26.

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brought to a state of fusion in a cast-iron pot, set inbrickwork at the side of the furnace; and when thetest has been cautiously raised to a cherry-red heat,the rich lead is laded into it till nearly full. Oxid-ation now proceeds rapidly. At first the lead becomescovered on the surface with a yellow or grayish dross;but on further raising the heat, the surface of thebath “uncovers,” and a film of melted litharge beginsto appear. Part of this litharge sinks into the bone-ash of the test, which it soon saturates. The blowingapparatus at the back of the test is now set in motion,and forces the litharge as it forms forward to thebreast, B, across the surface of which a channel iscut, through which the litharge begins to flow, andruns over at the end through the vacant space, F, intoa movable iron pot, placed on the floor for itsreception. The current of air introduced at the backof the cupel not only assists in clearing away thelitharge, but supplies the amount of oxygen neces-sary for its rapid formation. In proportion asthe litharge flows away, more of the melted lead is