Four baths are used, each of different strengths, the strongest is used first, the weakest last. I quote from the catalogue of the Boston Art Museum, of the exhibition illustrating the technical methods of the Reproductive Arts and Photo-Mechanical Processes, held January 8, 1892: "Photo-aquatint (photogravure) for the production of half-tone intaglio plates from photographs from nature, paintings, etc. A dry aquatint ground is laid on a metal plate, and over this is mounted a gelatine negative film, made by the pigment printing process. To obtain this negative film a reversed positive on glass has first to be made. The reason why this positive must be reversed will become clear when the nature of the manipulations in the pigment printing process, which involves the turning of the film, are considered. The film mounted on the plate is a washout relief, thickest in those parts which are to show white in the impressions from the plate, and gradually growing thinner toward the darkest parts, where it is thinnest. The film acts as a 'resist' to the mordant, allowing it to pass freely in the thinnest parts, and less freely as it increases in thickness. If, however, the film were mounted on the bare plate, and the biting then proceeded with, the result would be of no practical use, as the plate would present merely shallow hollows, incapable of holding the ink, and which would therefore be wiped out in the attempt to clean the surface of the plate. This is, however, prevented by the aquatint ground, which allows the mordant to circulate only in the channels around the resinous particles of which it consists, and thus produces a grain precisely as in ordinary aquatinting. The mordant used in perchloride of iron, which is a 'still mordant,' i.e., one which does not evolve bubbles of gas. An effervescent mordant cannot be used as the bubbles rising under the film would tear it up. In biting, successive baths of varying strength are made.
"A strong solution of perchloride of iron penetrates only the thinner parts of the film, whereas a weaker acts also through the thicker parts. The biting, therefore, begins with a strong solution, which acts only in the darkest parts, and followed up with weaker and weaker solutions, which continue the biting in the darks and at the same time carry it on gradually toward the lights. If necessary, the plate is worked over with the burnisher to brighten the lights, and with roulettes, etc., to strengthen the darks."
Purchase nine (9) pounds of perchloride of iron in crystals (45 cents per pound), take a wide-mouthed gallon jar, place within half a gallon of distilled water, add the iron until it tests 30 deg. by a Beaumé hydrometer, pour off enough to fill a one-litre glass stoppered bottle, after filtering through absorbent cotton. Keep adding the iron to the jar until the strength of each bath is reached. To the strongest solution add half a drachm of C. P. muriatic acid, and to the weakest half a drachm C. P. nitric acid; the nitric acid is added so that in the last biting a good final nip is given to the copper.
I here give my own formula, with those recommended by others.
The four (4) baths should be well aired for a day (in broad pans) in the open air before filtering.
Formula for Acid Baths.
(H. R. Blaney.)
| No. 1 | should | register | to | Beaumé's | scale | 42 deg. |
| No. 2 | " | " | " | " | " | 37 deg. |
| No. 3 | " | " | " | " | " | 33 deg. |
| No. 4 | " | " | " | " | " | 30 deg. |
The temperature of the bath to be at 63 deg. Fahr. when tested.