The outer curve a a, fig. 274, p. 402, which represents the copper dish when the puckers have been removed, will not be sent into the hemispherical form, or the inner line d d, at one process, but will progressively assume the curvatures b b, c c, and sometimes many others: neither will the work be changed from the curve a a, to that of b b, at one sweep, nor as with the burnisher in spinning, even by one consecutive ring or wave. The hammer must necessarily operate by successive blows arranged in circles, the proximity of which circles, will at length include within their range the entire sweep a a, or b b, each of which is called a course: and before proceeding from one course or sweep to the next, the metal requires to be annealed.
Figs. 276 and 277 explain the transition or conversion from the first sweep a, to the second sweep b; the black lines represent the metal after a few circles of blows have been given. Fig. 277 shows the narrow edge of the raising-hammer, in the act of descending upon the center of the head or stake, and as a tangent to the circle; it first throws in a little rim at 1, which connects the new and old sweeps by a curve or ogee; then another little circle 2, will be similarly gathered in, then 3, 4, 5, and so on, up to the edge. Now the artifice consists in making the intervals both of the great sweeps, a, b, c, fig. 274, and of the little waves 1, 2, 3, of fig. 277, as large as practicable, provided they do not cause the exterior metal to pucker or become in plaits, as this would endanger its ultimately cracking at those places, whan the metal might bars become plaited.
In thus raising the metal, it necessarily becomes thickened from its contraction in diameter , but as in fig. 270 the hammer at h, gives a hollow blow and bends, whilst the part s, gives a solid blow and thine, the two effects are thus combined; and when they are duly proportioned, by a hammer more or less round, and blows more or less oblique, the true thickness as well as the desired change of figure arc both obtained.
It is easier to get the hemisphere by a little excess of thinning, or by a superfluity of blows: so that the less skilful workman will use a piece of copper of seven inches diameter, with additional blows, for a six inch hemisphere: but the more skilful will take a piece of seven and a half inches diameter, and obtain the work with less labour. Occasionally, when the work is common and thin, from three to six hemispheres or other pieces are hollowed together, the outer piece is cut as a hexagon or octagon, and its angles are bent over to embrace the inner pieces, before the process of hollowing is begun, and which scarcely consumes more time than for one only. This is a general practice in hollowing tin-works, such as the covers of saucepans, as the number of thicknesses divide the strength of the blows; the several pieces are then twisted round at intervals, so as to arrange them in a different order, which mixes the little imperfections, and tends to their mutual correction: the raising process represented in fig. 277 is also performed upon two or three pieces at a time, when they are sufficiently thin to permit it.*
* One of the most conspicuous and remarkable examples of raised works, is the ball and cross of St Paul's Cathedral, London. The old ball consisted of sixteen pieces riveted together; the present, also 6 feet diameter and \ inch thick, was raised in two pieces only, and may therefore bo considered to mark the improvement in the coppersmith's art in making large works, such as sugar-pans, stills, &c
The metal was first thinned and partly formed under the tilt-hammer at the copper-mills, or sunk in a concave bed; the raising was effected precisely as explained in fig. 277, and with hammers but little larger than usual; the two parts ware riveted together in their place, and the joint is concealed by the ornamental band.
All the work is modern, and is mostly hammered up, except the cast gun-metal consoles beneath the ball, which formed part of the original metallic edifice; a name to which it is justly entitled, the height being 89 feet, and the weight of copper 3 1/4 tons. The new bell and cross were erected in 1821, by Messrs. Kepp, of
Having conveyed the full particulars for raising a hemispherical shape, the modifications of treatment required for various other forms will be sufficiently apparent. Thus, below the dotted lines a d, in fig. 278, the sweeps are exactly the same as in fig. 274, but the metal rises higher from having been originally larger: in the courses g h, it is first kept rather thicker on the edge, and towards the conclusion, it is thinned on the edge to the common substance, and curled over by hollow blows from within, although the whole figure might be produced by external blows, but which would be a more tedious method.
On the other hand, by the continuance of the raising-in, explained by diagram 277, the metal would be gathered into a smaller diameter through the steps ijk l, in the latter of which the metal would become thickened, unless the solid or thinning blows were allowed to predominate. If enough metal had been given in the first instance, when the mouth had been contracted as to the form of a teapot, it might be extended upwards as a cylindrical neck, in the manner explained in fig. 271, p. 400, and curled over at the top, as on the opposite side of fig. 278, at h.
To lessen the labour of raising works from a single flat plate, soldering is sometimes resorted to; thus the teapots, figs. 207 and 209, p. 383, might be made in two dished pieces, and soldered at the largest diameter; the lofty vase or coffeepot, fig. 211, could be made from a cylinder of midway diameter soldered up the side, the bulge being set-out by thinning the metal, and the contraction above being drawn-in by hollow blows.