This beautiful art was first proposed by Mr Perkins for the purpose of rendering the forgery of bank notes a matter of great difficulty; and there are two principles which peculiarly adapt it to that object: first, the perfect ident.i.ty of all the impressions, so that any variation in the minutest line would at once cause detection; secondly, that the original plates may be formed by the united labours of several artists most eminent in their respective departments; for as only one original of each design is necessary, the expense, even of the most elaborate engraving, will be trifling, compared with the mult.i.tude of copies produced from it.
125. It must, however, be admitted that the principle of copying itself furnishes an expedient for imitating any engraving or printed pattern, however complicated; and thus presents a difficulty which none of the schemes devised for the prevention of forgery appear to have yet effectually obviated. In attempting to imitate the most perfect banknote, the first process would be to place it with the printed side downwards upon a stone or other substance, on which, by pa.s.sing it through a rolling-press, it might be firmly fixed. The next object would be to discover some solvent which should dissolve the paper, but neither affect the printing-ink, nor injure the stone or substance to which it is attached. Water does not seem to do this effectually, and perhaps weak alkaline or acid solutions would be tried. If, however, this could be fully accomplished, and if the stone or other substance, used to retain the impression, had those properties which enable us to print from it, innumerable facsimiles of the note might obviously be made, and the imitation would be complete. Porcelain biscuit, which has recently been used with a black lead pencil for memorandum books, seems in some measure adapted for such trials, since its porosity may be diminished to any required extent by regulating the dilution of the glazing.
126. Gold and silver moulding. Many of the mouldings used by jewellers consist of thin slips of metal, which have received their form by pa.s.sing between steel rollers, on which the pattern is embossed or engraved; thus taking a succession of copies of the devices intended.
127. Ornamental papers. Sheets of paper coloured or covered with gold or silver leaf, and embossed with various patterns, are used for covering books, and for many ornamental purposes. The figures upon these are produced by the same process, that of pa.s.sing the sheets of paper between engraved rollers.
Of copying by stamping
128. This mode of copying is extensively employed in the arts. It is generally executed by means of large presses worked with a screw and heavy flywheel. The materials on which the copies are impressed are most frequently metals, and the process is sometimes executed when they are hot, and in one case when the metal is in a state between solidity and fluidity.
129. Coins and medals. The whole of the coins which circulate as money are produced by this mode of copying. The screw presses are either worked by manual labour, by water, or by steam power.
The mint which was sent a few years since to Calcutta was capable of coining 200,000 pieces a day. Medals, which usually have their figures in higher relief than coins, are produced by similar means; but a single blow is rarely sufficient to bring them to perfection, and the compression of the metal which arises from the first blow renders it too hard to receive many subsequent blows without injury to the die. It is therefore, after being struck, removed to a furnace, in which it is carefully heated red-hot and annealed, after which operation it is again placed between the dies, and receives additional blows. For medals, on which the figures are very prominent, these processes must be repeated many times. One of the largest medals. .h.i.therto struck underwent them nearly a hundred times before it was completed.
130. Ornaments for military accoutrements, and furniture.
These are usually of bra.s.s, and are stamped up out of solid or sheet bra.s.s by placing it between dies, and allowing a heavy weight to drop upon the upper die from a height of from five to fifteen feet.
131. b.u.t.tons and nail heads. b.u.t.tons embossed with crests or other devices are produced by the same means; and some of those which are plain receive their hemispherical form from the dies in which they are struck. The heads of several kinds of nails which are portions of spheres, or polyhedrons, are also formed by these means.
132. Of a process for copying, called in France clichee. This curious method of copying by stamping is applied to medals, and in some cases to forming stereotype plates. There exists a range of temperature previous to the melting point of several of the alloys of lead, tin, and antimony, in which the compound is neither solid, nor yet fluid. In this kind of pasty state it is placed in a box under a die, which descends upon it with considerable force. The blow drives the metal into the finest lines of the die, and the coldness of the latter immediately solidifies the whole ma.s.s. A quant.i.ty of the half-melted metal is scattered in all directions by the blow, and is retained by the sides of the box in which the process is carried on. The work thus produced is admirable for its sharpness, but has not the finished form of a piece just leaving the coining-press: the sides are ragged, and it must be trimmed, and its thickness equalized in the lathe.
Of copying by punching
133. This mode of copying consists in driving a steel punch through the substance to be cut, either by a blow or by pressure.
In some cases the object is to copy the aperture, and the substance separated from the plate is rejected; in other cases the small pieces cut out are the objects of the workman"s labour.
134. Punching iron plate for boilers. The steel punch used for this purpose is from three-eighths to three-quarters of an inch in diameter, and drives out a circular disk from a plate of iron from one-quarter to five eighths of an inch thick.
135. Punching tinned iron. The ornamental patterns of open work which decorate the tinned and j.a.panned wares in general use, are rarely punched by the workman who makes them. In London the art of punching out these patterns in screw-presses is carried on as a separate trade; and large quant.i.ties of sheet tin are thus perforated for cullenders, wine-strainers, borders of waiters, and other similar purposes. The perfection and the precision to which the art has been carried are remarkable. Sheets of copper, too, are punched with small holes about the hundredth of an inch in diameter, in such mult.i.tudes that more of the sheet metal is removed than remains behind; and plates of tin have been perforated with above three thousand holes in each square inch.
136. The inlaid plates of bra.s.s and rosewood, called buhl work, which ornament our furniture, are, in some instances, formed by punching; but in this case, both the parts cut out, and those which remain, are in many cases employed. In the remaining ill.u.s.trations of the art of copying by punching, the part made use of is that which is punched out.
137. Cards for guns. The subst.i.tution of a circular disk of thin card instead of paper, for retaining in its place the charge of a fowling-piece, is attended with considerable advantage. It would, however, be of little avail, unless an easy method was contrived of producing an unlimited number of cards, each exactly fitting the bore of the barrel. The small steel tool used for this purpose cuts out innumerable circles similar to its cutting end, each of which precisely fills the barrel for which it was designed.
138. Ornaments of gilt paper. The golden stars, leaves, and other devices, sold in shops for the purpose of ornamenting articles made of paper and pasteboard, and other fancy works, are cut by punches of various forms out of sheets of gilt paper.
139. Steel chains. The chain used in connecting the mainspring and fusee in watches and clocks, is composed of small pieces of sheet steel, and it is of great importance that each of these pieces should be of exactly the same size. The links are of two sorts; one of them consisting of a single oblong piece of steel with two holes in it, and the other formed by connecting two of the same pieces of steel, placed parallel to each other, and at a small distance apart, by two rivets. The two kinds of links occur alternately in the chain: each end of the single pieces being placed between the ends of two others, and connected with them by a rivet pa.s.sing through all three. If the rivet holes in the pieces for the double links are not precisely at equal distances, the chain will not be straight, and will, consequently, be unfit for its purpose.
Copying with elongation
140. In this species of copying there exists but little resemblance between the copy and the original. It is the cross-section only of the thing produced which is similar to the tool through which it pa.s.ses. When the substances to be operated upon are hard, they must frequently pa.s.s in succession through several holes, and it is in some cases necessary to anneal them at intervals.
141. Wire drawing. The metal to be converted into wire is made of a cylindrical form, and drawn forcibly through circular holes in plates of steel: at each pa.s.sage it becomes smaller.
and, when finished, its section at any point is a precise copy of the last hole through which it pa.s.sed. Upon the larger kinds of wire, fine lines may sometimes be traced, running longitudinally.
these arise from slight imperfections in the holes of the draw-plates. For many purposes of the arts, wire, the section of which is square or half round, is required: the same method of making it is pursued, except that the holes through which it is drawn are in such cases themselves square, or half-round, or of whatever other form the wire is required to be. A species of wire is made, the section of which resembles a star with from six to twelve rays; this is called pinion wire, and is used by the clockmakers. They file away all the rays from a short piece, except from about half an inch near one end: this becomes a pinion for a clock; and the leaves or teeth are already burnished and finished, from having pa.s.sed through the draw-plate.
142. Tube drawing. The art of forming tubes of uniform diameter is nearly similar in its mode of execution to wire drawing. The sheet bra.s.s is bent round and soldered so as to form a hollow cylinder; and if the diameter outside is that which is required to be uniform, it is drawn through a succession of holes, as in wire drawing: If the inside diameter is to be uniform, a succession of steel cylinders, called triblets, are drawn through the bra.s.s tube. In making tubes for telescopes, it is necessary that both the inside and outside should be uniform.
A steel triblet, therefore, is first pa.s.sed into the tube, which is then drawn through a succession of holes, until the outside diameter is reduced to the required size. The metal of which the tube is formed is condensed between these holes and the steel cylinder within; and when the latter is withdrawn the internal surface appears polished. The bra.s.s tube is considerably extended by this process, sometimes even to double its first length.
143. Leaden pipes. Leaden pipes for the conveyance of water were formerly made by casting; but it has been found that they can be made both cheaper and better by drawing them through holes in the manner last described. A cylinder of lead, of five or six inches in diameter and about two feet long, is cast with a small hole through its axis, and an iron triblet of about fifteen feet in length is forced into the hole. It is then drawn through a series of holes, until the lead is extended upon the triblet from one end to the other, and is of the proper thickness in proportion to the size of the pipe.
144. Iron rolling. When cylinders of iron of greater thickness than wire are required, they are formed by pa.s.sing wrought iron between rollers, each of which has sunk in it a semi-cylindrical groove; and as such rollers rarely touch accurately, a longitudinal line will usually be observed in the cylinders so manufactured. Bar iron is thus shaped into all the various forms of round, square, half-round, oval, etc. in which it occurs in commerce. A particular species of moulding is thus made, which resembles, in its section, that part of the frame of a window which separates two adjacent panes of gla.s.s. Being much stronger than wood, it can be considerably reduced in thickness, and consequently offers less obstruction to the light; it is much used for skylights.
145. It is sometimes required that the iron thus produced should not be of uniform thickness throughout. This is the case in bars for railroads, where greater depth is required towards the middle of the rail which is at the greatest distance from the supports. This form is produced by cutting the groove in the rollers deeper at those parts where additional strength is required, so that the hollow which surrounds the roller would, if it could be unwound, be a mould of the shape the iron is intended to fit.
146. Vermicelli. The various forms into which this paste is made are given by forcing it through holes in tin plate. It pa.s.ses through them, and appears on the other side in long strings. The cook makes use of the same method in preparing b.u.t.ter and ornamental pastry for the table, and the confectioner in forming cylindrical lozenges of various composition.
Of copying with altered dimensions
147. Of the pentagraph. This mode of copying is chiefly used for drawings or maps: the instrument is simple; and, although usually employed in reducing, is capable of enlarging the size of the copy. An automaton figure, exhibited in London a short time since, which drew profiles of its visitors, was regulated by a mechanism on this principle. A small aperture in the wall, opposite the seat in which the person is placed whose profile is taken, conceals a camera lucida, which is placed in an adjoining apartment: and an a.s.sistant, by moving a point, connected by a pentagraph with the hand of the automaton, over the outline of the head, causes the figure to trace a corresponding profile.
148. By turning. The art of turning might perhaps itself be cla.s.sed amongst the arts of copying. A steel axis, called a mandril, having a pulley attached to the middle of it, is supported at one end either by a conical point, or by a cylindrical collar, and at the other end by another collar, through which it pa.s.ses. The extremity which projects beyond this last collar is formed into a screw, by which various instruments, called chucks, can be attached to it. These chucks are intended to hold the various materials to be submitted to the operation of turning, and have a great variety of forms. The mandril with the chuck is made to revolve by a strap which pa.s.ses over the pulley that is attached to it, and likewise over a larger wheel moved either by the foot, or by its connection with steam or water power. All work which is executed on a mandril partakes in some measure of the irregularities in the form of that mandril; and the perfect circularity of section which ought to exist in every part of the work, can only be ensured by an equal accuracy in the mandril and its collar.
149. Rose engine turning. This elegant art depends in a great measure on copying. Circular plates of metal called rosettes, having various indentations on the surfaces and edges, are fixed on the mandril, which admits of a movement either end-wise or laterally: a fixed obstacle called the "touch", against which the rosettes are pressed by a spring, obliges the mandril to follow their indentations, and thus causes the cutting tool to trace out the same pattern on the work. The distance of the cutting tool from the centre being usually less than the radius of the rosette, causes the copy to be much diminished.
150. Copying dies. A lathe has been long known in France, and recently been used at the English mint for copying dies. A blunt point is carried by a very slow spiral movement successively over every part of the die to be copied, and is pressed by a weight into all the cavities; while a cutting point connected with it by the machine traverses the face of a piece of soft steel, in which it cuts the device of the original die on the same or on a diminished scale. The degree of excellence of the copy increases in proportion as it is smaller than the original. The die of a crown-piece will furnish by copy a very tolerable die for a sixpence. But the chief use to be expected from this lathe is to prepare all the coa.r.s.er parts, and leave only the finer and more expressive lines for the skill and genius of the artist.
151. Shoe-last making engine. An instrument not very unlike in principle was proposed for the purpose of making shoe lasts. A pattern last of a shoe for the right foot was placed in one part of the apparatus, and when the machine was moved, two pieces of wood, placed in another part which had been previously adjusted by screws, were cut into lasts greater or less than the original, as was desired; and although the pattern was for the right foot, one of the lasts was for the left, an effect which was produced by merely interposing a wheel which reversed the motion between the two pieces of wood to be cut into lasts.
152. Engine for copying busts. Many years since, the late Mr Watt amused himself with constructing an engine to produce copies of busts or statues, either of the same size as the original, or in a diminished proportion. The substances on which he operated were various, and some of the results were shewn to his friends, but the mechanism by which they were made has never been described. More recently, Mr Hawkins, who, nearly at the same time, had also contrived a similar machine, has placed it in the hands of an artist, who has made copies in ivory from a variety of busts. The art of multiplying in different sizes the figures of the sculptor, aided by that of rendering their acquisition cheap through the art of casting, promises to give additional value to his productions, and to diffuse more widely the pleasure arising from their possession.
153. Screw cutting. When this operation is performed in the lathe by means of a screw upon the mandril, it is essentially an art of copying, but it is only the number of threads in a given length which is copied; the form of the thread, and length as well as the diameter of the screw to be cut, are entirely independent of those from which the copy is made. There is another method of cutting screws in a lathe by means of one pattern screw, which, being connected by wheels with the mandril, guides the cutting point. In this process, unless the time of revolution of the mandril is the same as that of the screw which guides the cutting point, the number of threads in a given length will be different. If the mandril move quicker than the cutting point, the screw which is produced will be finer than the original; if it move slower, the copy will be more coa.r.s.e than the original. The screw thus generated may be finer or coa.r.s.er-- it may be larger or smaller in diameter--it may have the same or a greater number of threads than that from which it is copied; yet all the defects which exist in the original will be accurately transmitted, under the modified circ.u.mstances, to every individual generated from it.
154. Printing from copper plates with altered dimensions.
Some very singular specimens of an art of copying, not yet made public, were brought from Paris a few years since. A watchmaker in that city, of the name of Gonord, had contrived a method by which he could take from the same copperplate impressions of different sizes, either larger or smaller than the original design. Having procured four impressions of a parrot, surrounded by a circle, executed in this manner, I shewed them to the late Mr Lowry, an engraver equally distinguished for his skill, and for the many mechanical contrivances with which he enriched his art. The relative dimensions of the several impressions were 5.5, 6.3, 8.4, 15.0, so that the largest was nearly three times the linear size of the smallest; and Mr Lowry a.s.sured me, that he was unable to detect any lines in one which had not corresponding lines in the others. There appeared to be a difference in the quant.i.ty of ink, but none in the traces of the engraving; and, from the general appearance, it was conjectured that the largest but one was the original impression from the copperplate.
The means by which this singular operation was executed have not been published; but two conjectures were formed at the time which merit notice. It was supposed that the artist was in possession of some method of transferring the ink from the lines of a copperplate to the surface of some fluid, and of retransferring the impression from the fluid to paper. If this could be accomplished, the print would, in the first instance, be of exactly the same size as the copper from which it was derived; but if the fluid were contained in a vessel having the form of an inverted cone, with a small aperture at the bottom, the liquid might be lowered or raised in the vessel by gradual abstraction or addition through the apex of the cone; in this case, the surface to which the printing-ink adhered would diminish or enlarge, and in this altered state the impression might be retransferred to paper. It must be admitted, that this conjectural explanation is liable to very considerable difficulties; for, although the converse operation of taking an impression from a liquid surface has a parallel in the art of marbling paper, the possibility of transferring the ink from the copper to the fluid requires to be proved.
Another and more plausible explanation is founded on the elastic nature of the compound of glue and treacle, a substance already in use in transferring engravings to earthenware. It is conjectured, that an impression from the copperplate is taken upon a large sheet of this composition; that this sheet is then stretched in both directions, and that the ink thus expanded is transferred to paper. If the copy is required to be smaller than the original, the elastic substance must first be stretched, and then receive the impression from the copperplate: on removing the tension it will contract, and thus reduce the size of the design.
It is possible that one transfer may not in all cases suffice; as the extensibility of the composition of glue and treacle, although considerable, is still limited. Perhaps sheets of India rubber of uniform texture and thickness, may be found to answer better than this composition; or possibly the ink might be transferred from the copper plate to the surface of a bottle of this gum, which bottle might, after being expanded by forcing air into it, give up the enlarged impression to paper. As it would require considerable time to produce impressions in this manner, and there might arise some difficulty in making them all of precisely the same size, the process might be rendered more certain and expeditious by performing that part of the operation which depends on the enlargement or diminution of the design only once; and, instead of printing from the soft substance.
transferring the design from it to stone: thus a considerable portion of the work would be reduced to an art already well known, that of lithography. This idea receives some confirmation from the fact, that in another set of specimens, consisting of a map of St Petersburgh, of several sizes, a very short line, evidently an accidental defect, occurs in all the impressions of one particular size, but not in any of a different size.
155. Machine to produce engraving from medals. An instrument was contrived, a long time ago, and is described in the Manuel de Tourneur, by which copperplate engravings are produced from medals and other objects in relief. The medal and the copper are fixed on two sliding plates at right angles to each other, so connected that, when the plate on which the medal is fixed is raised vertically by a screw, the slide holding the copperplate is advanced by an equal quant.i.ty in the horizontal direction. The medal is fixed on the vertical slide with its face towards the copperplate, and a little above it.
A bar, terminating at one end in a tracing point, and at the other in a short arm, at right angles to the bar, and holding a diamond point, is placed horizontally above the copper; so that the tracing point shall touch the medal to which the bar is perpendicular, and the diamond point shall touch the copperplate to which the arm is perpendicular.
Under this arrangement, the bar being supposed to move parallel to itself, and consequently to the copper, if the tracing point pa.s.s over a flat part of the medal, the diamond point will draw a straight line of equal length upon the copper; but, if the tracing point pa.s.s over any projecting part of the medal, the deviation from the straight line by the diamond point, will be exactly equal to the elevation of the corresponding point of the medal above the rest of the surface. Thus, by the transit of this tracing point over any line upon the medal, the diamond will draw upon the copper a section of the medal through that line.
A screw is attached to the apparatus, so that if the medal be raised a very small quant.i.ty by the screw, the copperplate will be advanced by the same quant.i.ty, and thus a new line of section may be drawn: and, by continuing this process, the series of sectional lines on the copper produces the representation of the medal on a plane: the outline and the form of the figure arising from the sinuosities of the lines, and from their greater or less proximity. The effect of this kind of engraving is very striking; and in some specimens gives a high degree of apparent relief. It has been practised on plate gla.s.s, and is then additionally curious from the circ.u.mstance of the fine lines traced by the diamond being invisible, except in certain lights.
From this description, it will have been seen that the engraving on copper must be distorted; that is to say, that the projection on the copper cannot be the same as that which arises from a perpendicular projection of each point of the medal upon a plane parallel to itself. The position of the prominent parts will be more altered than that of the less elevated; and the greater the relief of the medal the more distorted will be its engraved representation. Mr John Bate, son of Mr Bate, of the Poultry, has contrived an improved machine, for which he has taken a patent, in which this source of distortion is remedied.
The head, in the t.i.tle page of the present volume, is copied from a medal of Roger Bacon, which forms one of a series of medals of eminent men, struck at the Royal Mint at Munich, and is the first of the published productions of this new art.(3*)
The inconvenience which arises from too high a relief in the medal, or in the bust, might be remedied by some mechanical contrivance, by which the deviation of the diamond point from the right line (which it would describe when the tracing point traverses a plane), would be made proportional not to the elevation of the corresponding point above the plane of the medal, but to its elevation above some other parallel plane removed to a fit distance behind it. Thus busts and statues might be reduced to any required degree of relief.