Manufacturer and builder 6, 1871
Two faults have been found with photography. One that the printing process is tedious, slow, and expensive; the other that the ordinary "silver prints" are not permanent. The latter defect has been over-come by the so-called carbon process,.which promisad, a few years ago, to supersede the silver printing; but the practical operation takes no much more time and trouble than the silver print, that photographers stilt adhere to the latter. Previously to this, from the earliest period in the history of the art, attempts have been made to produce, by the action of light, surfaces upon metal, stone, and other substances, which could be printed from with printer's ink by some of the process., the durability of the impressions obtained by which has been established by the specimens which have withstood the ravages of time for centuries. Some ten or twelve years ago, some progress was made in France in what was called "heliographic engraving," in which metal printing-plates were obtained by the action of light; and more recently, in this country, some attentions was bestowed upon the maturing of a process of this character. The experiments in this directions have, however, fallen far short of success; and, as we have heard nothing of them for two or three years, we suppose them to have been abandoned.
Messrs. Rockwood & Company, of this city, have recently introduced a system of obtaining the desired printing surfaces, invented in Copenhagen, which is of the simplest and most reliable nature. Under ordinary photographic negatives, prepared plates of glass, zinc, and stone are exposed to the action of light, and from these plates thousands of pictures are printed with all the facility of the ordinary lithograph upon a press with ink. The effect of the light upon the sensitized plate is to transform it into a veritable lithographic plate - the parts exposed to the action of light having an affinity for fatty or printer's ink, and the portion protected from light rejecting the ink and absorbing water. So, first, a wet roller is passed over a plate ready for the press, followed by an ink-roller, and the paper then placed on the press, and run through the rollers at the rate of about sixty or seventy an hour.
In order to understand the possibility of this process, it is necessary to know that when glue is dissolved in a solution ofbichromate of potash and dried in the dark, it may be dissolved again, or will absorb water readily; but when exposed to light, it becomes insoluble, and will even reject water; and the degree of insolubility will be in exact proportions to the degree of light to which it hiss been exposed. Hence when a surface covered with this bichromate of potash and gelatine has been exposed to light under an ordinary photographic negative, it will have obtained various degrees of insolubility, on different parts of its surface, according to the intensity of the light which has acted on these places, and will consequently ab-sorb or reject water in different, degrees. It is thus similar to a lithographic stone; the places whichlave rejected water will absorb the oily printing:ink, while those which have absorbed more or less water are more or less protected against the adhesion of this ink.
The specimens we have seen of this new process are of very great variety - portraits, landscapes, and copies of mechanical drawings, music, and ordinary letter-press. The portraits and landscapes are equal in appearance to the best specimens of photography obtained by the ordinary printing process; and the copies of mechanical and other drawings are far superior in clearness, fineness, and sharpness to any we haYe seen produced by the ordinary "photo-lithographic" process. The copies of letter-press are, some of them, so fine that they can not be read without a powerful magnifying-glass, yet perfectly clear and sharp.
There seems to be scarcely any limit to the application of this new art, which combines all the fidelity, delicacy, and vigor of photography with the absolute permanence of printer's ink; besides which, pictures can be multiplied by it at so reasonable a cost that it may be used for most of the ordinary illustrations now made by the various styles of engraving heretofore in use.