For etching a solution of perchloride of iron of various strengths is used, 40, 37, 33, and 30° Beaumé. With subjects very rich in tones all four will be used; with simpler subjects one or the other may be omitted.
The etching will begin, as a rule, with the 40° solution. The ferric chloride will first dissolve the thin, hardened pigment film and then go through the open lines of the pattern, where it will attack the stone and etch the deepest parts.
Slowly the ferric chloride dissolves also the thicker pigment {82} film—begins also to etch the stone there, but less deeply. To tell when the four solutions should be used in turn, as well as the correct moment to interrupt the etching, requires considerable experience. The 30° solution, the weakest of the ferric chloride solutions, is used last, and in consequence of the greater proportion of water possesses a solvent power for the whole of the light-hardened pigment film. Accurate directions as to how long the etching with the different solutions is to be continued cannot be given, as this depends upon the character of the subject as well as upon the thickness of the pigment image. It may be always assumed, however, that any individual etching ought to last under no circumstances longer than three to four minutes. With well-covered stones the first etching should not be allowed to act so long as the last, and vice-versâ with less covered stones.
As soon as the image shows the correct character through the etching solution the stone should be brought under the tap in the trough and quickly washed with clean cold water. The surface of the image should then be flowed over with oil of turpentine in order to remove the asphalt and gelatine film, and finally well washed with the rose and allowed to dry. The picture will be found deeply etched into the stone. The stone is now smeared over with linseed oil, inked up, and further treated as in ordinary lithogravure.
In this process the negative pigment image is transferred to the stone and the printing image prepared by etching; the formation of the grain is produced by the pattern printed either on a polished stone, when it is converted into points, or the transfer may be effected on to an asphalted polished stone, when a network of lines will be obtained.
For dark subjects the latter process is better, for lighter subjects the grain.
Printing from such a plate has more similarities with etching printing than from stone; the effect of the ink is very good and expressive.
Various pictures executed by Eckstein and in our establishment in one or more colours have proved the value of this process in practice.
The foregoing are the different photo-lithographic methods for obtaining printing images in half-tone, and although in no way exhaustive, yet it appears to me that they are practically the most important.