PART V.PHOTO-LITHOGRAPHY IN HALF-TONE.CHAPTER I.
This process, like photo-engraving, has been the subject of many applications to the Patent Office, but the first notable progress was made by Messrs. Bullock, in 1865. They seem to have gone into the subject thoroughly, as the elaborate and practical specifications prove, but although the patentees issued some fine specimens of their work, and advertised the sale of prepared paper, nothing came of it.
Messrs. Bullock’s method comprises the printing from grained stone tile, or a stone upon which has been laid a transfer, from a stipple plate, or a plate engraved in lines or dots—upon sensitive transfer paper in stiff ink.
The sensitive paper, with such imprint upon it, is exposed to light under a negative, the specks of ink forming a medium for breaking up the half-tones.
Suitable grain may be obtained from machine stippled plates, as well as from grained stone.
Half-tone photo-lithography may also be made by making, from a proof pulled from a machine stipple, or a ruled plate, or a grained stone, a wet, collodion negative, which, being stripped from its glass support (by any of the means mentioned in the chapter on stripped films) is placed between the ordinary half-tone negative and the sensitive surface—zinc, coated with bichromated albumen, or with bitumen, or either of the transfer papers treated of in Part IV.—the result will be a grained transfer.
Grained zinc, coated with bichromated albumen, or with bitumen, and exposed under a very thin half-tone negative, will also yield transfers suitable for a good many subjects, but the most successful methods are modifications of the collographic printing processes.
Transfers made by any of the methods mentioned in the following chapters may be transferred to polished zinc, and then etched in relief, as directed in Part I.