I have purposely reserved this subject for a distinct section, on account of its great general importance in the arts, and have placed it last, in order that the various applications of the hammer might have been rendered comparatively familiar; for although the plane surface, may appear to be of more easy attainnicnt than many of the complex forms which have been adverted to, such is by no means the case.
* Many ancient specimens of armour, gold and silver plate, vases and ornaments, are excellent examples of raised, chased, inlaid and engraved works, both as regards design and execution. In our own times, the Hungarian silversmith, Szentepeteri, has produced a very remarkable alto-relievo in copper, taken from Le Brun's picture of the battle of Arbela, in which some of the legs of the horses stand out and are entirely in relief from the background.
It would appear from a prior attempt, also exhibited, in which the artist had failed, as if the metal were cut through around the legs, and that the edges of the hole were drawn together to complete the background, whilst the edges of the removed piece were also stretched and curled backwards so as to unite at the hinder part of the limb. This singular chasing was exhibited in London in 1838 and 1851; and the author of "Hungary and Transylvania," (1839,) who visited the artist, during the progress of the work, speaks warmly of his unpretending skill.
The methods employed arc entirely different from that explained at page 247, in reference to flattening thick rigid plates, which are corrected by enlarging the concave side, with blows of the sharp rectangular edge of the hack hammer, applied within the concavity. A method which bears some analogy to that employed by the. joiner in straightening a board which is curved in its width, namely, the contraction of its convex side by exposure to heat, as adverted to at page 51. In thin metal plates neither of these modes is available, as the near proximity of the two sides causes both to be influenced in an almost equal degree by any mode of treatment.
Thin plates are flattened by means of solid and hollow blows, which hare been recently explained, but they require to be given with considerable judgment; and a successful result is only to be obtained by a nice discrimination and considerable practice. All therefore that can be here attempted is an examination of the principles concerned, and of the general practice pursued; as the process being confessedly one of a most difficult nature, success is only to be expected or attained by a strict and persevering regard to principle.
As respects thin works no figure is so easily distorted as the true plane, and this arises from the very minute difference which exists between the span or chord of a very flat arch, and its length measured around the curve. For example, imagining the span of an arch to be one inch, and the height of the same to be one-twentieth of an inch (a monstrous error as regards a flat plate), the curve would be only about one 200th of an inch longer than the span: and therefore, if any spot of one inch diameter, were stretched until, if unrestrained, it would become one inch and one 200th, in diameter, such spot would rise up as a bulge one-twentieth of an inch high. This trivial change of magnitude would be accomplished with very few blows of the hammer, and much less than this would probably distort the whole plate.
In general however, there would be not one error only, but several, the relationship of which would be more or less altered with nearly every blow of the hammer; thence arises the difficulty, as the plane surface cannot exist so long as any part of the plate is extended beyond its just and proportional size, and which it is a very critical point to arrive at.
There is another test of the unequal condition of flat works besides that of form, namely their equal or unequal states of elasticity, and which is an important point of observation to the workman. For instance, if we suppose a plate of metal to be exactly uniform in its condition, it will bend with equal facility at every point, so that bending a long spring or saw, will cause it to assume a true and easy curve; but supposing one part to be weaker than the remainder, the saw will bend more at the weak part, and the blade will become as it were two curves moving on a hinge. When such objects are held by the one extremity and vibrated, the perfect, will feel as a uniformly elastic cane; the imperfect, as a cane having a slight flaw, which renders it weak at one spot; and in this manner we partly judge of the truth of a hand-saw, as in shaking it violently by the handle, it will, if irregularly elastic, lean towards the character of the injured cane, a distinction easily appreciated.
A thin plate of metal can only be perfectly elastic, when it is either a true plane or a true curve, so that every point is under the same circumstances as to strength. Thus a hemisphere, as at a, fig. 287, possesses very great strength and rigidity owing to its convexity, but as the figure becomes less convex it decreases gradually in strength, and when it slides down to the plane surface, as at f, the metal assumes its weakest form.
A nearly plane surface will necessarily consist of a multitude of convexities or bulges varying in size and strength, connected by intermediate portions, which may be supposed to be plane surfaces; the whole may be considered as greatly exaggerated in the figure. The bulged parts are stronger than the plain flat parts, it follows that the bending will occur in preference at the plane or weak parts of the plate, precisely as in the injured cane.
When the bulges are large but shallow, they flap from side to side with a noise at every bending, as their very existence shows that they cannot rest upon the neutral or straight line; such parts are said to be buckled, their ready change of position renders them flaccid and yielding under the pressure of the fingers, and they are therefore called loose parts, but at the same time it is certain that they are too large.