BBBB, fig. 2. is a perpendicular section of fig. 1.; aa end view of the rollers; b the ash-pit; c one of the ash-pit dampers; d the grate; e the plate upon which the muffle rests, and which is covered with loom nearly one inch thick; f the muffle in section representing the situation of the cupels; g the mouth plate, and upon it are laid pieces of charcoal, which during the process are ignited, and heat the air that is allowed to pass over the surface of the cupels, and which will be more fully explained in the sequel; h the interior of the furnace, exhibiting the fuel.
The total height of the furnace is 2 feet 6½ inches; from the bottom to the grate 6 inches, the grate, muffle, plate, and bed of loom with which it is covered, 3 inches; from the upper surface of the grate to the commencement of the funnel, e fig. 1., 21½ inches; the funnel e 6 inches. The square of the furnace which receives the muffle and fuel is 11½ inches, by 15 inches. The external sides of the furnace are made of plates of wrought-iron, and are lined with a 2 inch fire brick.
CCCC, fig. 3. is a horizontal section of the furnace over the grate, showing the width of the mouth-piece or plate of wrought iron, which is 6 inches, and the opening which receives the muffle-plate.
Fig. 4. represents the muffle or pot, which is 12 inches long, 6 inches broad inside; in the clear 6¾. In height 4½ inside measure, and nearly 5½ in the clear.
Fig. 5. the muffle-plate, and which is the same size as the bottom of the muffle.
Fig. 6. is a representation of the sliding door of the mouth-plate, as shown at d in fig. 1.
Fig. 7. a front view of the mouth-plate or piece, d fig. 1.
Fig. 8. representation of the mode of making, or shutting up with pieces of charcoal, the mouth of the furnace.
Fig. 9. a view of the cupel, which is generally one inch by ¾ of an inch deep.
Fig. 10. the teaser for cleaning the grate.
Fig. 11. a larger teaser, which is introduced at the top of the furnace, for keeping a complete supply of charcoal around the muffle.
Fig. 12. the tongs used for charging the assays into the cupels.
Fig. 13. represents a board of wood used as a register, and is divided into forty-five equal compartments, upon which the assays are placed previous 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 requires.
The furnace and implements which we have just
detailed, are such as are used in the Royal Mint and Goldsmiths' Hall in the city of London.
We 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 a Coupelle. Fig. 14. represents this furnace, and it is composed of a chimney or pipe of wrought-iron, a, and of the furnace B. It is 17½ inches high, and 7½ 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. 14. and 15. 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 2½ 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. 14. At the union of the conical and cylindrical part of the tube, there is placed a small gallery of iron e, fig. 14. 15. and 16. See also the plan of it, fig. 17. This gallery is both ingenious, useful, and necessary; upon it are placed the cupels, which are properly heated 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; and above that there is placed at g, a key or 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 high above the gallery for annealing or heating the cupels. The circular opening h, in the dome fig. 14., and as seen in the section fig. 15., 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 m, fig. 15.
The section of the furnace fig. 15. presents several openings; the principal, which is that of the muffle, is placed in i; it is shut with the semicircular door m, fig. 14., and as seen in the section m, fig. 15. 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. 15., 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 the rod of
Assaying. iron fig. 31., that the grate may be easily kept clean. This opening is shut at pleasure by the wedge represented at k, fig. 14. and 15.
Upon the back of the furnace is a horizontal slit p, fig. 15., which supports the fire brick S, fig. 15., and upon which the end of the muffle, if necessary, may rest: u, fig. 15., is the opening in the furnace, where the muffle is placed.
Fig. 19. is a plan of the grate of the furnace, and fig. 20. a horizontal view of it. These two figures show us the dimensions of the ellipse, and determines the general form of the furnace, and thickness of the grate. To give strength and solidity to the grate, it is encircled by a bar or hoop of iron. We see at z the groove in which the hoop of iron is fixed. The holes of the grate are truncated cones, having the greatest base below, that the ashes may more easily fall into the ash-pit. The letter V, fig. 15., shows the form of these holes. The grate is supported by a small bank or shelf, making part of the furnace, and as seen at a, fig. 15.
The ash-pit c has an opening y in front, fig. 15., and is shut, when necessary, by the mouth-piece r, fig. 14. and 15.
To give strength and solidity to the furnace, it is bound with hoops of iron at b, b, b, b, fig. 14.
Figs. 21. 22. and 23. are views of the muffle.
Fig. 24. is a view of a crucible for annealing gold.
Figs. 25. 26. and 27. are cupels of various sizes, to be used in the furnace. They are the same as those used by assayers in their ordinary furnaces.
Figs. 28. and 29. are views of the hand-shovels used for filling the furnace with charcoal; they should be made in size and form so as to fit the opening h in figs. 14. and 15.
Fig. 30. the smaller pincers or tongs, by which the assays are charged into the cupels, and by which the latter are withdrawn from the furnace.
Fig. 18. the teaser for cleaning the grate of the furnace.
Fig. 16. is a representation of the furnace first constructed by Messrs Anfrye and D'Arcet, and which was worked by means of a pair of bellows, which forced a current of air through the brass tube b, entering the ash-pit under the grate at the circular hole c, fig. 15. The strength of the blast or current of air can be regulated at pleasure by the stop-cock d, fig. 16.