The next question which arises is, “What can be reproduced or multiplied by means of photo-lithography?”
The answer is, “According to the existing state of the technique of photography and the perfection of the transfer process, everything.” Any line or wash drawing, any oil painting or other plastic object, any photograph of living creatures or lifeless objects—briefly, any photograph can, under certain conditions, be transferred to stone or a metal plate, and from this any number of impressions in greasy ink be obtained. Before the discovery of the method of breaking up half-tones into points or dots, only line drawings could be reproduced by photo-lithography. Since, however, it has been possible to break up half-tone into line or points or grain, nothing stands in the way of reproducing by this {20} method any kind of original. This breaking up of the half-tone into a regular or irregular grain can be effected by means of a crossed line screen in making the negative, as in autotypic, or half-tone, block-making, or independently of the making of the negative, as in the asphalt process of Orell, Füssli and Co., and in Bartös’ process and various other methods.
The undesirable half-tone must, however, always be broken up in some way into the requisite points or lines, as it is one of the characteristics of lithography that it is only possible to print from distinct figures on the stone. Lithography and letterpress printing produce impressions of sharply-defined lines or points, each of which must form a distinct whole. Any tone which is lighter or not absolutely black must be formed by lines or points standing in close proximity to one another. The tonality must be formed by stronger or weaker lines or points, by black and white spaces, or by more or less close line and point patterns. An absolutely closed tone, as in collotype or photogravure, is not attainable with lithography, and although numerous experiments have not been wanting to reach this end, the same have hitherto been without success.
The half-tones are broken up and separated into solid printable points:—
(a.) By the autotypic transfer, which is effected by placing in front of the sensitive plate when making the exposure the above-mentioned crossed-line screen.
(b.) By the so-called asphalt process, by means of preliminary graining of the stone before coating with asphalt and printing.
(c.) By means of the production of a grain on chromated gelatine.
(d.) By the use of a sand blast, which is the chief point of Bartös’ process.
(e.) By transfer of a close grating or network on to a polished stone, which forms the basis of the litho-heliogravure process of General-Director Chas. Eckstein, which is included in photo-lithography.
All these methods have for their purpose the breaking up of the light tones into regular or irregular but separate points. There are other methods besides those mentioned above which have the same purpose.