PART II.PHOTO-ENGRAVING IN HALF-TONE.CHAPTER I.RETROSPECTIVE.
The former chapters have treated entirely upon the production of blocks in line—i. e., where the picture has been made by a draughtsman, the half-tones and gradations being communicated by a greater or less thickness of line, or by dots, or stipple, or hatching.
The picture for such blocks may have been specially drawn for the process, the same size or larger, or it may be a copy of some woodcut or engraving already in existence, but if it is desired to reproduce blocks from drawings, paintings, or photographs, then an entirely different method must be adopted, and the smooth gradations of half-tone levelled, so to speak, so as to bring the high lights and the shadows upon one plane.
In photographs from nature (or from washed drawings or paintings) the scale of gradations runs, as it were, in a series of short steps from the deepest shadow to the highest light, and a block made, say in bichromated gelatine, from such a negative can give no half-tones, as the inking roller could only touch the deep shadows properly.
Now the subject of making photographs applicable for the illustration of letter-press, instead of woodcuts, has occupied the attention of experimentalists from the early days of the art-science, as the records of the Patent Office show.
The first patent, dated 1852, bears the honored name of Mr. Fox Talbot, and although it is for intaglio printing, and therefore a little out of place under the above heading, still it claims our attention as giving a method for breaking up the half-tones of the photograph, by placing muslin, crape, etc., between the photographic cliche and the sensitive surface; or a glass plate may be covered with fine lines, or glass may be coated with powder, which is caused to adhere.{64}
In 1854, Paul Pretsch broke up the half-tone by the reticulation of gelatine, caused by the admixture with iodide of silver and bichromate of potash; this was spread upon a silvered copper plate, dried, and exposed to light under the half-tone negative, then washed in cold water and borax, or carbonate of soda, then in alcohol, coated with copal varnish, and immersed in a weak solution of tannin, after which an electrotype could be made, or a transfer made to zinc or stone.
In 1855, A. J. Berchtold produced a grain by printing upon a photograph in black or in any color, from a plate or block or other surface, or by perforating or making strokes, lines, or dots upon it, by roller or other instrument. Repatented in 1883, by Brown, Barnes, and Bell.
In 1860, E. J. Asser used starch, and in 1865, J. W. Swan used a tissue of gelatine mixed with charcoal or other chemically inert grit; in the same year Messrs. E. & J. Bullock published, perhaps, the most important specification, describing all, or nearly all, practical methods of obtaining grain, the most important of which are the placing of any fine fabric between the lens and the sensitive surface, or between the camera and the object, or copies of granulated or recticulated structures or fabrics could be used, or such copy could be placed in contact with negatives, and both copied together.
In 1879, J. W. Swan made negatives by moving, during exposure, the Bullock’s screen, placed in front of a sensitive plate. Meisenbach’s method, patented a little later, is somewhat similar.
If a Woodbury relief is thinly coated with transfer ink, and then laid upon a piece of ordinary litho transfer paper which has been embossed with lines, or dots, or stipple, by being pressed in contact with wire gauze or an engraved plate, and the inked relief and the embossed litho transfer are then subjected to heavy pressure, a grained image is impressed upon the transfer paper, which can be transferred to zinc, and then etched in relief.
The method of making a grained negative to be now described, shortly stated, is done by interposing a screen, either before the sensitive plate in the dark slide of the camera (when copying a photograph) or placing the screen behind a transparency on glass when transmitted light is used. In the first instance the image projected upon the sensitive plate, having first to pass through the screen, is broken up by the dots upon the screen, the result being a definite grained negative. In the second instance, the screen being placed in contact with the transparency, a similar result follows. The first method is most generally used, as the print does not require the careful focussing necessitated by the second.