It is impossible to indicate an average time of exposure without talking actinometer. We will take the simplest form, the Artigue Actinometer. Smear a strip of ordinary white paper with your ammonium bichromate solution, without alcohol ; dry it in the dark and slip it between the leaves of a book so that it protrudes by the length of an inch or so. Expose to light until the exposed portion does not darken any more. Then pull the strip out another inch, note the exact time and watch the darkening of the new portion. When the first and second have become equal in tone, note the time again. The number of minutes elapsed we will call an " Artigue Degree." It will vary from two or three minutes (in diffused light, of course) in summer, to half an hour or more in winter. The average good negative will take from one to one and a half degrees exposure ; a very thin negative will print in half a degree.
WASHING. I shall refer to what has been previously said about the keeping qualities of sensitized paper. For the same reasons it is wiser to wash the exposed print at once. This operation, for a batch of five prints, will take about twenty minutes, with half a dozen changes of water; Of course the first, second, and third changes must follow each other rapidly. Tepid water will ensure faster and more complete bleaching of the image. A still more rapid result may be obtained by adding a small quantity of bisulphite of soda to the third bath and rinsing thoroughly afterwards.
An hour in cold water, once the yellow stain has disappeared, or two or three minutes in lukewarm water will suffice, unless the print has been previously washed and dried. In that case it is better to let it soak for two hours, and to finish off with some lukewarm water.
The nature of the ink is most important. I have tried dozens of samples and I have also made some myself with the advice of expert engravers ; and I have come to the conclusion that all sorts of inks, except the non-drying lithographic transfer ink, can be of use - but that only two samples will meet the everyday requirements of the oil printer. These, for France, are represented by the Encre Machine and the Encre Taille Douce of Valette's, both of which are quite free from turpentine. I use several other kinds of home and commercial manufacture, but only for special and rare occurrences. With an ink of the thickness of Encre Machine for. fully exposed, and of Taille Douce for underexposed portions of a picture, one can work for months without feeling the want of any other ink. The day that want makes itself apparent, a simple addition of cooked or pure linseed oil will be sufficient to convince the oil printer that he had better print another picture with the right exposure.
Nevertheless, for extreme cases, I can recommend a prudent mixture of Roberson's Medium, but this medium contains a fair percentage of turpentine, dries quickly and consequently makes the coating tacky in a very short time. It is helpful when there is a necessity of darkening considerable areas of light tones, but I should advise nobody to use it as an habitual adjunct to engraving or machine ink - only when soft ink will not take under ordinary conditions.
When the gelatine print has been well soaked it must be placed in full light on a pad composed of at least half a dozen sheets of thick fluffless blotting paper, quite wet, and the gelatine surface thoroughly wiped with clean butter muslin. This must not be done timidly - there is no danger whatever of hurting the film. Examine the print from an angle and remove any drop or streak of water that may have remained. Then with an engraver's palette knife spread a small quantity of soft, and further on, of hard ink on a thick glass plate.
I have found that it is no easy undertaking to teach a beginner how to handle his brushes " in anima vili " ; I know that it is next to impossible to describe the inking action in print with sufficient accuracy to ensure adequate rendering of this action on the part of the reader. Perhaps it would be wiser to warn him against the things he is not to do ?
Let him first bear in mind that every application of the ink-charged brush is composed of two actions, each of which produces an opposite result - the downward and the upward action - separated by a period of contact, which we must admit is part of the first and downward movement. This first movement brings the ink into contact with the film. The second movement removes it, wholly or partly. These two actions may be so well balanced that no result will be perceptible, the ink applied by the downward action having been completely removed by the upward one. It follows that, according to the delicate pressure of the hand in applying the ink, and according to the varying elasticity of the wrist in removing the brush from its close contact with the film, varying quantities of ink may be applied or brought away.