and to Russia, where, having little artistic past to refer to,
designers and craftsmen display unequalled individuality and
force. It is from the Far East, however, that the most serious
rivalry may be anticipated. The metal-work of China and
Japan, so pleasantly naïve and inexpensive, though becoming
undesirably modified as to design through contact with European
buyers, is losing none of its matchless technique, which indeed
in Japan is still being developed. In any history of the art
revival the influence of such firms as Barbedienne and Christofle
in Paris and Tiffany in New York cannot be ignored.
(J. S. G.)
Industrial Metal-Work.
The malleability and ductility of metals lie at the basis of the work of the gold- and silver-smiths at one extreme, and of the boiler-maker at the other. Sheet metals can be made to assume almost any shape under the hammer, or by pressure, provided they are subjected to annealing to restore the property of malleability. The most awkward shapes, involving excessive extensions of metal, are produced by drawing processes between dies of iron and steel in power presses. All the common domestic utensils in tinned and enamelled ware, and all the ordinary patterns of the silversmiths, are similarly done. Frequent annealings are necessary to prevent fracture of the metal; but with these and the observance of certain other precautions of a practical character the degree of extension possible is enormous. Another illustration of the malleability of metal is afforded by metal spinning. A sheet of metal set revolving at a high speed, in a lathe is bent over into cup-shaped forms. with numerous mouldings, by a blunt hardened tool. A great deal of work is done in this way, though this sphere has also been invaded by the draw presses, whose output would seem incredible to those not familiar with the work. Objects that do not require annealing are produced by dozens per minute, and all the movements of feeding and stamping and removal are often automatic. The ductility of metals and alloys is utilized in wire and tube-drawing through dies on long benches. This work also requires frequent annealing, for otherwise the wires or tubes would rupture. Even hard steel is treated in this way to form tubes for the highest hydraulic and steam pressures.
Platers’ Work (see Boiler) is distinguished from work in sheet metals by the fact that plates have considerable thickness, which sheets have not. Plates range in thickness from 14 in. to 2 in., but for most purposes they do not go beyond 34 in. or 1 in. Over these thicknesses they are used chiefly for the largest marine boilers. Armour plates which are several inches in thickness do not come in this group, being a special article of manufacture. Sheets are of thicknesses of less than 14 in. This distinction of thickness is of importance in its bearing on workshop practice. A thin sheet requires a very different kind of treatment from a thick plate. Not only is more powerful machinery required for the latter, but in bending it allowance has to be made for the difference in radius of outer and inner layers, which increases with increase of thickness. Short, sharp bends which are readily made in thin sheets cannot be done in thick plates, as the metal would be stressed too much in the outer layers. The methods of union also differ, riveting being adopted for thick plates, and soldering or brazing generally for thin.
Coppersmiths’ Work is an important section of sheet-metal working. It is divided into two great departments; the domestic utensil side, on which the brazier’s craft is exercised; and the engineering side, which is concerned in some engine-work, locomotive and marine, and in the manufacture of brewers’ utensils. The methods of the first are allied to those of the tinman, those of the second to the methods of the plater. Tinsmiths’ work resembles the lighter part of the work of the coppersmith. There is no essential difference in dealing with tin (i.e. sheets of iron or steel coated with tin) and copper of the same thickness. Hence the craft of tinmen and braziers is carried on by the same individuals. There are, however, differences of treatment in detail, because copper is more malleable and softer than tin plate. The geometry of sheet-metal work and of platers’ and boiler-makers’ work is identical up to a certain stage. The divergence appears when plates are substituted for sheets. A thin sheet has for all practical purposes no thickness—that is, the geometrical pattern marked on it will develop the object required after it is bent. Nearly all patterns are the developments of the envelopes of geometrical solids of regular or irregular outlines, few of plane faces; when they are made up of combinations of plane faces, or of faces curved in one plane only, there is no difference in dealing with thin sheets or thick plates. But when curving occurs in different planes at right or other angles (hollowing), the metal has to be drawn or extended on the outside, and important differences arise. A typical form is the hemisphere, from which many modified forms are derived. The production of this is always a tedious task. It involves details of “wrinkling” and “razing,” if done by hand-work in copper. In thick plates it is not attempted by hand, but pressing is done between dies, or segments of the sphere are prepared separately and riveted together. In tin it is effected by stamping. In all work done in thick plates the dimensions marked out must have reference to the final shape of the article. Generally the dimensions are taken as in the middle of the plate, but they may be on the inside or outside according to circumstances. But in any case the thickness must enter into the calculations, whereas in thin sheets no account is taken of thickness.
Raised Work.—All the works in sheet metal that are bent in one plane only are easily made. The shapes of all polygonal and all cylindrical, and conical forms are obtained by simple development—that is, the envelopments of these bodies are marked out on a flat plane, and when cut, are bent or folded to give the required envelopes. Only common geometrical problems are involved in the case of sheets of sensible thickness, and allowances are made for thickness. But in those forms where curving must take place in different directions the layers or fibres of metal are made to glide over one another, extension taking place in some layers but not in others, and this goes on without producing much reduction in the thickness. This is only possible with malleable and ductile metals and alloys. As a general rule it is restricted to metals which are not cast, for, with some slight exceptions, it is impossible to produce relative movements of the layers in cast iron, steel or cast brass. But most rolled metals and alloys can be so treated, copper being the best for the purpose. The methods employed are “raising” by the hammer, and pressing in dies. But the severity of the treatment would tear the material asunder if rearrangement of the particles were not obtained by frequent annealing (q.v.).
If an object has to be beaten into concave form from a flat thin sheet, the outer portions must be hammered until they occupy smaller dimensions than on the flat sheet. If a circular disk is wrought into a hemisphere and the attempt is made to hammer the edges round, crumpling must occur. This in fact is the first operation, termed wrinkling, the edge showing a series of flutes. These flutes have to be obliterated by another series of hammerings termed razing. The result is that the object assumes a smooth concave and convex shape, without the thickness of the metal becoming reduced.
Cast Work.—The metals and alloys which are neither malleable nor ductile can only be worked into required shapes by melting and casting in moulds. Abundance of remains which date from the Neolithic period testify to the high antiquity of this class of work, and also to the great skill which the ancient founders had acquired. Statue-founding is a highly specialized department of metal-work, in which the artists of the middle ages excelled. Two methods have been employed, the cire-perdue, or wax process (see above), and the present, or all sand method. In the latter the artist provides a model in plaster from which the founder takes a mould within an encircling box. This mould must obviously be made in scores of little separate sections (false cores or drawbacks) to permit of their removal from the model without causing fracture of the sand. These are subsequently replaced piece by piece in the encircling frame, and a core made within it, leaving a space of 14 in. or thereabouts into which the metal is poured. The advantage of this process is that the artist’s model is not destroyed as in the cire-perdue, and if a “waster” results, a second mould can be taken. A large statue occupies from one to three months in the moulding.
The extreme tenuity of objects which are hammered, drawn or rolled cannot for obvious reasons be attained by casting. Casting also is complicated by the shrinkage which occurs in cooling down from the molten state, and in some alloys by the formation of eutectics, and the liquation of some constituents. The temperature of pouring is now known to be of more importance than was formerly suspected. The after-treatment of castings by annealing exercises great influence on results in malleable cast iron and steel.
There are many metals and alloys which are malleable and ductile, and also readily fused and cast. This is the case with gold, silver, copper, tin, lead and others, and especially with low carbon steel, which is first cast as an ingot, then annealed and rolled into plates as well as the thinnest sheets. The ancient wootz, and the products of the native furnaces of Africa are first cast, then hammered out thin. Many of the patent bronzes are by slight variations in the proportions of the constituents made suitable for casting, for forging, and for rolling into sheets. But in all the great modern manufacturing processes it is true that metals and alloys, though of the same name, have a different composition according as they are intended for casting on the one hand, or for forging, rolling and drawing on the other. Wrought or malleable iron has less of carbon and other elements in its composition than has cast iron. Steel intended for castings has slightly more carbon and other elements than the cast-steel ingot intended for rolling into plates. So also with the numerous bronzes, the phosphor, the delta, the aluminium and other alloys of copper; each is made in several grades to render it suitable for different kinds of treatment.
There are no materials used in manufacture of which the craftsman is able to vary the composition and physical qualities so extensively as the metals and their alloys. Much light has been thrown on facts which have long been known in a practical way, by the labours of the Alloys Research Committee of the Institution of Mechanical Engineers (England). These, together with independent researches into the heat treatment of steel and iron, have opened up many unsolved problems fraught with deepest interest and importance.
One of the most difficult problems with which the metal-worker