† Mr. Perkins's admirable process of transfer engraving may be thus explained. A soft steel plate was first engraved with the required subject in the most finished style of art, either by hand or mechanically, or the two combined, and the plate was then hardened. A decarbonized steel cylinder was next rolled over the hardened plate by powerful machinery, until the engraved impression appeared in relief, the hollow lines of the original becoming ridges upon the cylinder. The roller was reconverted to the condition of ordinary steel and hardened, after which it served for returning the impression to any number of decarbonized plates, every one of which became absolutely a counterpart of the original; and every plate when hardened would yield the enormous number of 150,000 impressions without any perceptible difference between the first and the last
In the event of any accident occurring to the transfer roller, the original plate still existed, from which another or any required number of rollers could be made, and from these rollers any number of new plates, all capable of producing as many impressions as above cited.
Specimens of Mr. Perkins's process may be seen in the 38th volume of the Transactions of the Society of Arts; and it is there stated by the inventor, that to decarbonise the plates they were placed in the vertical position in cast-iron boxes not less than 3/4 of an inch thick, and surrounded on all sides by a stratum of iron filings not less than 1/2 an inch thick; the boxes were then placed in a furnace, and after having been heated, were Buffered to cool in the most gradual manner by stopping off but without any very advantageous results; for instance, it has been recommended to harden small cylindrical wins, by rolling all the air-passage and covering the boxes with a layer of cinders 6 or 7 inches deep. The re-conversion was similarly accomplished, but with the charcoal from leather sifted fine, and on removal from the boxes at the end of from three to five hours the plates were immediately plunged vertically into cold water.
The late Mr. Warren was instrumental in bringing into common use the thin steel plates, similar to those previously used for copper; these ware annealed at a higher temperature, in earthenware boxes filled with pounded oyster-shells. - See Trans. Soc. of Arts, vol. xll p. 88.
The practice at the Bank of England, introduced by the late Mr. John Oldham, and followed under the superintendence of his son Mr. Thomas Oldham, is to anneal at one time four cast-iron boxes, each containing from three to six steel plates, surrounded on all sides with fine charcoal mixed with an equal quantity of chalk and driven in hard.
The reverberetory furnace employed, has a circular cast-iron plate or bed upon which the four boxes are fastened by wedges, and as the plate revolves very slowly and continually by the steam engine employed in working the printing presses and other machinery, the plates are exposed in the most equal manner to the heat, and when the proper temperature is attained, all the apertures are carefully closed and luted to extend the cooling over a space of at least forty-eight hours.
The surfaces of the cylinders and plates are thus rendered exceedingly soft, to the depth of about the 82nd of an inch, "so as to become more like lead than anything else," and thus much of their surfaces must be turned or planed off: the device is raised in the transfer-press upon the natural soft steel of the rollers, under a pressure of some tons, and these are hardened without any intentional application of the case-hardening process, as the simple steel is undoubtedly very superior in all respects to that which has been decarbonised and reconverted.
The plates themselves are used in the soft state, as they then admit of reparation by the transfer rollers; and the process is found to be more economical, as the risk of warping is avoided, and they may be easily repaired. The dates and numbers are at present printed as a second process by letterpress printing, with the machines invented by the late Mr. Bramah, and which have been engraved and described in different books.
In hardening engraved plates, rollers, dies, and similar works, it is of the greatest importance to preserve the surface unimpaired; and as steel is very liable to oxidation at the red heat if exposed to the air for even a few seconds, and which oxidised scale will in some cases nearly remove, or at any rate injure, the subject produced upon its surface, it is of great importance to conduct the heating and cooling with the most complete exclusion of the air.
Mr. Thomas Oldham introduced a mode of hardening the transfer rollers which appears as near to perfection as possible, and by it, instead of the works acquiring the ordinary black and grey tints, and a minute roughness, like the surface of the finest emery paper, the steel comes out of the water as smooth to the touch as at first, and mottled with all the beautiful tints seen on case-hardened gun-locks. The method is simply as follows:
The work to be hardened is inclosed in a wrought iron box with a loose cover, a false bottom, and with three oars projecting from its surface about midway; the steel is surrounded on all aides with carbon from leather, driven in hard, and the them when heated between cold metallic surfaces to retain them perfectly straight. This might probably answer, but unfortunately cylindrical steel wires supply but a very insignificant portion of our wants.
Another mode tried by Dr.Wollaston was to inclose the piece of steel in a tube filled with Newton's fusible alloy, the whole to be heated to redness and plunged in cold water; the object was released by immersion in boiling water, which melted the alloy, and the piece came out perfectly unaltered in form, and quite hard. This mode is too circuitous for common practice, and the reason why it is to be always successful is not very apparent.
Mr. Perkins resorted to a very simple practice with the view of lessening the distortion of his engraved steel plates, by boiling the water in which they were to be hardened to drive off the air, and plunging them vertically; and as the plates were required to be tempered to a straw colour, instead of allowing them to remain in the water until entirely cold, he removed them whilst the inside was still hot, and placed them on the top of a clear fire until the tallow with which they were rubbed, smoked; the plate was then returned to the water for a few moments, and so on alternately until they were quite cold, the surface never being allowed to exceed the tempering heat.
cover and bottom are carefully luted with moist clay. Thus prepared, the case is placed in the vertical position, in a bridle fixed across a great tub, which is then filled with water almost to touch the false bottom of the case. The latter is now heated in the furnace as quickly as will allow the uniform penetration of the heat.
When sufficiently hot, it is removed to its place in the hardening tub, the cover of the iron box is removed, and the neck or gudgeon of the cylinder is grasped beneath the surface of the carbon, with a long pair of tongs, upon which a coupler is dropped to secure the grasp. It only remains for the individual to hold the tongs with a glove whilst a smart tap of a hammer is given on their extremity; this knocks out the false bottom of the case, and the cylinder and tongs are instantly immersed in the water; the tongs prevent the cylinder from falling on its side, and thus injuring its delicate but still hot surface. For square plates, a suitable frame is attached by four slight claws, and it is the frame which is seized by the tongs; the latter are sometimes held by a chain, which removes the risk of accident to the individual. In some cases, the work assumes a striated and mackled appearance, evident to the touch as well as the sight, and which is to be attributed to an imperfect manufacture of the steel.
Mr. Oldham informed me that in the Paris Mint, the dies are inclosed in the soot of burnt wood; and that in our own Mint, the dies are hardened by a powerful jet of water. He also added, that his workpeople have the impression that steel is reduced to its softest state by inclosure with lime and ox-gall. The reader is referred to Sir J. Robinson's note U, in the appendix of vol. ii. page 970.
From various observations, it appears on the whole to be the best in thick works thus to combine the hardening and temper-tag processes, instead of allowing the objects to become entirely cold, and then to re-heat them for tempering. To ascertain the time when the plate should be first removed from the water, Mr. Perkins heated a piece of steel to the straw colour, and dipped it into water to learn the sound it made; and when the hardened plate caused the same sound, it was considered to be cooled to the right degree, and was immediately withdrawn.*
I will conclude these numerous examples and remarks by one of a very curious, massive, and perfect kind, in which the harden-ing is sure to occur without loss of figure, unless the work break under the process. I refer to the locomotive wheels with hardened steel tires, patented by Mr. Daniel Gooch, and which may be viewed as the most ponderous example of hardening as the tires of the eight-foot wheels weigh about 10 cwt., and consist of about one-third steel, and there seems no reason why this diameter might not be greatly exceeded. The materials for the tires are first swaged separately, and then welded together under the heavy hammer at the steelworks, after which they are bent to the circle, welded, and turned to certain gages. The tire is now heated to redness in a circular furnace; during the time it is getting hot, the iron wheel, previously turned to the right diameter, is bolted down upon a face-plate; the tire expands with the heat, and when at a cherry-red, it is dropped over the wheel, for which it was previously too small, and is also hastily bolted down to the surface plate, the whole load is quickly immersed by a swing crane into a tank of water about five feet deep, and hauled up and down until nearly cold; the steel tires are not afterwards tempered, †
* Trans. Soc. of Arte, vol. xxxviii. p. 55. † The patentees consider the steel tires to be eight times harder than those of wrought-iron, and to be proportionately durable, and from the very ingenious method employed in their construction they are little exposed to accident The spokes are forged out of flat bars with T formed heads; these are arranged radially in the founder's mould, whilst the cast-iron center is poured around them; the ends of the T heads are then welded together to constitute the periphery of the wheel or inner tire, and little wedge-form pieces are inserted where there is any deficiency of iron.
The wheel is then chucked on a lathe, bored, and turned on the edge, not cylindrically, but like the meeting of two cones, and about one quarter of an inch higher in the middle than on the two edges. The compound tire is turned to the