In this state of things, it became a very interesting object of inquiry to ascertain how many impressions may be taken from a plate of soft or decarbonized steel; and it was found that such a plate, prepared according to Mr. Warren's process, is capable of affording several thousand copies without undergoing any visible wear. In pi.00f of this, impressions were laid before the committee by Mr. Warren from the plates of decarbonized steel, executed by' him, the one for an edition of Mackenzie's works, pub lished by Cadull, the other for an edition of Beattie and Collins, published by 1ZiNington. These exhibit, both in the landscape and in the figures, the most elaborate and delicate work; five thousand impres sions have been taken from one, and four thousand from the other, and yet between one of the first and one of the last impressions, it was impossible to detect any perceptible difference.
If Mr. Warren had carried on his experiments alone, working by himself till he had brought his plan to perfection, it is probable that, at the period of his death, the evidence of the great importance of his discovery would by no means have been so com plete as it actually w as, and the result of his exertions might have been lost, to the great detriment of the profession, and of the f.ir fame of this eminent artist; but selfishness and secrecy in anything, which related to the improvement of the art to which he was attached, formed no part of his character; and all his discoveries, both those relating to the prepa rations of the plates as well as those w hick have reference to the engraving upon them, were unre servedly and grituitously communicated. The con sequence of this liberality was, that besides the plates of Mr. Warren's own engraving produced before the committee, impressions were shown of portraits engraved on decarbonized steel, for the Evangelical Magazine, demonstrating that after 25,000 copies have been taken, the plates still remain in a good state, and are not yet in want of repair. Mr. Mar stated, that having made an engraving on one of Mr. Warren's plates, he did not take his own proofs till after the 8000th impression; and, in another in stance, the engraving being a portrait, Mr. La Hie, the printer, certified that the artist's own proofs were not taken off till after the 20,000th impression.
Mr. Warren's original process for decarbonizing the steel plates, consisted in procuring a box; or case of iron, and covering the bottom of it with a mixture of iron turnings and pounded oyster shells; on this a steel plate is laid, another•bed of the mixture is then added, and so on alternately, till the box is full, taking care that the test of.the compositions shall
form the upper as well as the lower layer. The box so charged is thus to be placed in a furnace, and to be kept for several hours at the highest heat which it will bear without melting, after which, being al lowed to cool gradually, the plates are found to be reduced, for the most part, to the state of soft decar bonized steel.
Mr. Hughes, a copper-plate maker, having been instructed by Mr. Warren in his process, and finding that. the steel did not always turn out sufficiently and uniformly soft (particularly for the purpose of en gravers in mezzotinto), imagined that these occasional defects were owing to a deficiency of heat in the cementing process; accordingly, he substituted a case or oven of refractory clay, for the cast iron one, and then applying a considerably higher heat than the cast iron box would have endured without melting, was enabled to obtain plates so soft that they may be bent over the knee. Each plate requires two or more cementations; and, as the first cementation warps them more or less, Mr. IVarren was in the habit of rectifying them by means of a hammer. Mr. Hughes finds that the places struck by the hammer are apt to be less softened by the second cementation than the other parts, and therefore, that plates so treated will . often turn out unequal in hardness. llis own practice is to use a mallet, and as little force as possible, in de taching the cement from the surface, and in rectifying the plate.
The plate. being cleaned and polished (but not too highly), is ready for the engraver. 'When it comes into his hands, the first operation is to lay the etching ground, in doing which the plate must be rather less heated than is usual with copper, otherwise the ground, as it cools, contracts, presenting a honey combed surface, and leaving parts of the plate un covered. The same defect is apt to occur ir the plate is too highly polished. The ground should be laid rather thicker than on copper.
Various menstrunms were made trial of by Mr. Warren, for biting in with. Nitric acid, considerably more diluted than Ter copper, "as mad(' use of, with, upon the whole, good success. Nitrate or mercury was found to blunt or round the e L., of the lines; acetic acid, with a small portion of nitrate or copper, produced the same effects; sulphate of copper bit light tints very beautifully, but its farther action rendered the lines rough. The best menstruum, however, is half an ounce of crystallized nitrate of copper, dis solved in a pint and a half of distilled water, and a few drops of nitric acid added to the solution. This will be found to bite both deeper and clearer than dilute nitric acid.