By ROBERT DEMACHY.
Honorary Fellow of the Royal Photographic Society.
If a sheet of paper covered with a thin layer of gelatine is sensitized with bichromate, dried in the dark and exposed to daylight under a negative, it will be found that the surface of the print, after soaking some time in water, will retain unequally any greasy ink applied with appropriate tools. The positive image of the negative plate will be the result of this application. This unequal receptivity is due to the fact that wet and swollen gelatine in its normal condition will not retain greasy ink, while the addition of bichromate combined with the action of light on the same will tan or grain it sufficiently to retain the aforesaid ink. This explanation is so simple that oil printers are apt to forget it, without realizing that it holds the secret of success and the remedy for failure.
My materials for working the oil process are all comprised in the following list: a 6% solution of ammonium bichromate, - double the quantity of pure alcohol - a drawing board - a flat hog's-hair brush for sensitizing - a dozen special (stag's-foot) brushes of different sizes, and two or three straight cut - a dozen sheets of thick fluffless blotting paper - two thick glass plates - a tin of Valette's Encre machine - another of Encre Tattle Douce - a tube of Roberson's medium - a roll of Illingworth's double transfer paper, No. 125 - one of 119 - one of No. 151 Gravure White. (The Autotype double transfer papers and Mr. Manly's specially prepared paper work well. I have not had the occasion to try Mr. Rawlins' last papers).
Self Portrait, R. Demachy.
Any description of negative may give a passable or even a good print with oils in the hands of an expert, but I prefer to work with the kind of negative that will allow me the greatest scope for freedom in treatment : one that is absolutely free from fog. It may be fairly dense or it may be very thin, but in all cases it must have clean shadows and translucent blacks. Over-exposed and over-developed negatives, of the kind that a sickly fashion has made popular for so-called pictorial effects, are eminently unsuitable for the oil process.
SENSITIZING. To cover six whole-plate sheets of double transfer paper, take five cubic centimetres (1 1/2 drams) of ammonium bichromate 6% solution, and add ten cubic centimetres (3 drams) of alcohol* (90 degrees). Pin a sheet of paper on your drawing board by its four corners, dip the extremity of your flat hog's-hair brush in the alcoholic mixture, draw it once horizontally along the upper part of the sheet, and with rapid downward strokes gather the solution from the top streak and cover the whole sheet. I have found that the quantity of liquid applied with the first horizontal stroke is just sufficient to cover the rest of the paper without superfluous flooding, which is sometimes difficult to remove equally, but only the tip of the brush must be immersed in the bichromate solution. The gelatine coating of double transfer papers is made so thin that it will only absorb a very small bulk of liquid. If the quantity applied is superior to the power of absorption, the solution will settle in small pools and it will cause streaks under the action of the brush. In such a case, the streaks may be removed by passing a dry and perfectly clean camel's-hair or badger softener over the still wet surface of the sheet.
* Rectified spirits of wine may be used, but not methylated spirit.
This operation must be performed in subdued light and the wet sensitized paper removed immediately to a dry and perfectly dark place. Desiccation will be completed in ten to fifteen minutes according to hygrometric conditions.
Sensitized paper may keep its qualities for several days - but also it may not. I have had fair results with sensitized paper a week old, and I have met with streaks and spots on a two days' old sheet. The moral is that we had best expose as soon as possible after the paper is quite dry. Thus one cause of possible failure will be eliminated.
Accurate exposure is always an important factor in photographic work. Here it becomes all important. It is all very well to talk about control and local inking, but control and local inking can only be used with an obedient film and the film is obedient only when its surface offers the maximum quality of love and distaste for greasy ink, if you will excuse such unphotographic terms. This special degree can only be produced by a special degree of exposure. Above this point, these qualities of love and distaste will merge into one of general acceptance of the greasy ink - below it, the quality of distaste will become predominant, and the ink will be repelled.