The undermost, called the hearth, or crucible, in which the cast-iron collects, is a right rectangular prism, elongated in a line perpendicular to the axes of the tuyères. The sides of the hearth consist in general of refractory sandstone (fire-stone), obtained mostly from the bed of the coal basin, called millstone grit; and the bottom of the hearth is formed of a large block of the same nature, laid on a cast-iron plate.
The second portion is also made of the same refractory grit stone. It has the form of a quadrangular pyramidal, approaching considerably to a prism, from the smallness of the angle included between the sides and the axis.
The third portion or lower body of the furnace is conical, but here the interior space suddenly expands; the slope outwards at this part seems to have a great influence on the quality of the cast-iron obtained from the furnace. When No. 2. of the blackest kind is wanted for castings, the inclination of this cavity of the furnace is in general less considerable than when No. 2. cast iron for conversion into bar iron is required. The inclination of this conical chamber, called the boshes, varies from 55 to 60 degrees with the horizon. The diameter of this part is equal to that of the belly, and is from 11 to 13 feet. The boshes are built of masonry, as shown in [figs. 583], [584.]
The fourth part, which constitutes about two-thirds of the height of the furnace from the base of the hearth up to the throat, presents the figure of a surface of revolution, generated by a curve whose concavity is turned towards the axis of the furnace, and whose last tangent towards the bottom is almost vertical. This surface is sloped off with that of the boshes (étalages in French), so that no sharp angle may exist at the belly. In some furnaces of considerable dimensions, as in that with three tuyères, this portion of the furnace is cylindrical for a certain height.
The following measurements represent the interior structure of two well-going furnaces.
| No. 1. | No. 2. | |||
|---|---|---|---|---|
| Feet. | Feet. | |||
| Height from the hearth to the throat or mouth | 45 | 49 | ||
| Height of the crucible or hearth | 6 | 1⁄2 | 6 | |
| H—ht of the boshes | 8 | 7 | ||
| H—ht of the cone | 30 | 1⁄2 | 36 | |
| H—ht of the chimney or mouth | 8 | 12 | 3⁄4 | |
| Width of the bottom of the hearth | 2 | 1⁄2 | 2 | |
| Ditto at its upper end | 3 | 2 | 2⁄3 | |
| Ditto of the boshes | 12 | 2⁄3 | 13 | 1⁄2 |
| Ditto at one-third of the belly | 12 | 11 | 1⁄2 | |
| Ditto at two-thirds of ditto | 8 | 2⁄3 | 9 | 1⁄2 |
| Ditto at the mouth | 4 | 1⁄2 | 3 | 2⁄3 |
| Inclination of the boshes | 59 | ° | 52 | ° |
The conical orifice called the tuyère, in which the tapered pipes are placed, for imparting the blast, is seen near the bottom of the furnace, [fig. 583.] at A. Nose tubes of various sizes, from 2 to 4 inches in diameter, are applied to the extremity of the main blast-pipe. Under A is the bottom of the hearth, which, in large furnaces, may be two feet square. B is the top of the hearth, about two feet six inches square. A, B, is the height of the hearth, about six feet six inches. B shows the round bottom of the conical or funnel part, called in this country, the boshes, standing upon the square area of the hearth. C is the top of the boshes, which may be about 12 feet in diameter, and 8 feet in perpendicular height. D is the furnace top or mouth (gueulard in French), at which the materials are charged. It may be 41⁄2 feet in diameter. The line between C, D, is the height of the internal cavity of the furnace, from the top of the boshes upwards, supposed to be 30 feet. A, D, is the total height of the interior of the furnace, reckoned at 441⁄2 feet. E E is the lining, which is built in the nicest manner with the best fire-bricks, from 12 to 14 inches long, 3 inches thick, and curved to suit the circle of the cone. A vacancy of 3 inches wide is left all round the outside of the first lining by the builder; which is sometimes filled with coak dust, but more generally with sand firmly rammed. This void space in the brick-work is for the purpose of allowing for any expansion which might occur, either by an increase in the bulk of the building, or by the pressure and weight of the materials when descending to the bottom of the furnace. Exterior to E E is a second lining of fire-bricks similar to the first. At F, on either side, is a cast-iron lintel, 81⁄2 feet long, by 10 inches square, upon which the bottom of the arches is supported. F, G, is the rise of the tuyère arch, which may be 14 feet high upon the outside, and 18 feet wide. The extreme size of the bottom or sole of the hearth, upon each side of A, may be 10 feet square. This part and the boshing stones, are preferably made from a coarse sandstone grit, containing large rounded grains of quartz, united by a siliceo-argillaceous cement.