CHAPTER VI.

THE ACID BATHS.--HOW TO MAKE THEM AND METHOD OF BITING THROUGH THE GELATINE.

Perchloride of iron C.P. is the acid generally used for this purpose; it is a still acid, and if the room is well ventilated no harm to health results, but care must be taken to air the baths after making to get rid of the surplus chlorine.

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 Beaume 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 Beaume'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.

(DENISON'S.)

No. 1 should be made to register Beaume's scale, 45 deg., the percentage of perchloride in this solution is 47, and the specific gravity 1.444.

No. 2, 40 deg.; percentage, 41; spec. grav. 1.375 No. 3, 38 " " 38; " 1.339 No. 4, 35 " " 35; " 1.313 No. 5, 27 " " 27; " 1.225

From an article in the _Photographic News_ (English), Nov. 1, 1889, as practiced in India.

BITING BATH.

(WATERHOUSE.)

No. 1, sp. grav., 1.444; ap. per ct. of Fe_{2}, Cl_{6} = 47 No. 2, " 1.375; " " 41 No. 3, " 1.339; " " 38 No. 4, " 1.313; " " 35 No. 5, " 1.225; " " 27

A stronger solution of 48 deg. has been tried (by the above) but has no penetrating power through even the thinnest film.

ANOTHER FORMULA.

For large plates, 20 lbs. perchloride of iron and distilled water, until weight amounts to 1.500 grammes per 1000 c.c. From this four (4) solutions are made, at

No. 1, 42 deg. Beaume; spec. grav. 1.420 No. 2, 38 " " " 1.375 No. 3, 35 " " " 1.330 No. 4, 31 " " " 1.285

The plate is now ready for biting. Keep a record of the bitings, and length of time for each one, for after-study; also note the time of exposure of the tissue, age of same, etc., etc.

Pour the acid from a glass graduate with one sweep over the plate, removing all bubbles with a feather, noting the time of immersion so as to guide you. Start with 42 deg., having ready the 37 deg. in another graduate, watch carefully the action of the acid, and if the resist has been properly printed, the action of the acid will show after a minute; if longer it means a generally longer biting for each bath.

AVERAGE BITINGS.

42 deg., No. 1 5 minutes 37 deg., No. 2 5 minutes 33 deg., No. 3 2 minutes 30 deg., No. 4 2 minutes

Temperature of bath at 70 deg. Fahr., with No. 103 tissue.

Total of different bitings, from 10 to 25 minutes, according to depth of printing. It always varies. There is no hard and fast rule; you must in time learn to judge by your eye alone. The acid will first attack the thinnest part of the film, wherever that may be, and when the darkening of the copper ceases to spread to the next thickest parts, instantly pour off the acid, and pour on the 37 deg. Do not allow the atmosphere to act on the gelatine while biting any longer than is necessary to pour off one bath and quickly pour on a new one. The 37 and 33 deg. baths are for the middle tones, the 30 deg. for the most delicate ones. The action of each bath is cumulative, the 37 deg. biting a little where the 42 deg. had bitten, the 33 deg. doing the same for those before it, besides taking care of itself, and the 30 deg. attacking all more or less. During the biting with the 30 deg. solution, it should be continued until the whites just turn color, and a minute beyond; that is, the copper should begin to show a very little under the thickest and darkest film.

(Note that in the carbon resist the shadows are transparent and the high lights are opaque.)

The length of the last biting very seldom is over two minutes. It is better to overbite your darks, and underbite your lights, if you vary any.

The amount of moisture in the air and the heat of the day influences the length of biting. In hot weather in summer it is very difficult to work the process, the walling wax being discarded and the copper (back and edges protected by varnish) placed in a porcelain tray, surrounded by ice-water and kept at 65 to 70 deg., and the acid pured over the plate to the depth of one inch.