[Fig. 67.], a larger teaser, which is introduced at the top of the furnace, for keeping a complete supply of charcoal around the muffle.
[Fig. 68.], the tongs used for charging the assays into the cups.
[Fig. 69.] represents a board of wood used as a register, and is divided into 45 equal compartments, upon which the assays are placed previously to their being introduced into the furnace. When the operation is performed, the cupels are placed in the furnace in situations corresponding to these assays on the board. By these means all confusion is avoided, and without this regularity it would be impossible to preserve the accuracy which the delicate operations of the assayer require.
I shall now proceed to a description of a small assay furnace, invented by Messrs. Anfrye and d’Arcet, of Paris. They term it, Le Petit Fourneau à Coupelle. [Fig. 70.] represents this furnace, and it is composed of a chimney or pipe of wrought iron a, and of the furnace B. It is 171⁄2 inches high, and 71⁄4 inches wide. The furnace is formed of three pieces; of a dome A; the body of the furnace B; and the ash-pit C, which is used as the base of the furnace, [fig. 70.] and [71.] The principal piece, or body of the furnace, B, has the form of a hollow tower, or of a hollow cylinder, flattened equally at the two opposite sides parallel to the axis, in such a manner that the horizontal section is elliptical. The foot which supports it is a hollow truncated cone, flattened in like manner upon the two opposite sides, and having consequently for its basis two ellipses of different diameters; the smallest ought to be equal to that of the furnace, so that the bottom of the latter may exactly fit it. The dome, which forms an arch above the furnace, has also its base elliptical, whilst that of the superior orifice by which the smoke goes out preserves the cylindrical form. The tube of wrought iron is 18 inches long and 21⁄2 inches diameter, having one of its ends a little enlarged, and slightly conical, that it may be exactly fitted or jointed upon the upper part of the furnace dome d, [fig. 70.] At the union of the conical and cylindrical parts of the tube, there is placed a small gallery of iron, e, [fig. 70], [71.] See also a plan of it, [fig. 72.] This gallery is both ingenious and useful. Upon it are placed the cupels, which are thus annealed during the ordinary work of the furnace, that they may be introduced into the muffle, when it is brought into its proper degree of heat. A little above this gallery is a door f, by which, if thought proper, the charcoal could be introduced into the furnace; above that there is placed at g a throttle valve, which is used for regulating the draught of the furnace at pleasure. Messrs. Anfrye and d’Arcet say, that, to give the furnace the necessary degree of heat so as to work the assays of gold, the tube must be about 18 inches above the gallery, for annealing or heating the cupels. The circular opening h, in the dome, [fig. 70.], and as seen in the section, [fig. 71.], is used to introduce the charcoal into the furnace: it is also used to inspect the interior of the furnace, and to arrange the charcoal round the muffle. This opening is kept shut during the working of the furnace, with the mouth-piece, of which the face is seen at n, [fig. 71.]
The section of the furnace, [fig. 71.], presents several openings, the principal of which is that of the muffle; it is placed at i; it is shut with the semicircular door m, [fig. 70.], and seen in the section m, [fig. 71.] In front of this opening, is the table or shelf, upon which the door of the muffle is made to advance or recede; the letter q, [fig. 71.], shows the face, side, and cross section of the shelf, which makes part of the furnace. Immediately under the shelf, is a horizontal slit, l, which is pierced at the level of the upper part of the grate, and used for the introduction of a slender rod of iron, that the grate may be easily kept clean. This opening is shut at pleasure, by the wedge represented at k, [fig. 70.] and [71.]