Winter daylight is of very weak activity, and printing on P.O.P. becomes a tedious operation. An excellent substitute as a printing medium is provided by Bromide Paper - under which heading fall Gaslight Papers, too. The printing of these papers can be conveniently done by exposure to artificial light, either gas or a paraffin lamp.
Unlike P.O.P., the image on bromide paper - as previously mentioned under "Sensitive Materials" - remains invisible, and requires to be brought out by development. The negative which gives good results with P.O.P. generally does so with bromide papers. The character of the print may be slightly varied according to the amount of exposure given. Thus, by slight under-exposure a well-contrasted and vigorous one may be obtained, while by slight over-exposure one tending to flatness will result.
The paper is sold in two speeds (fast and slow) and two surfaces (rough and smooth), and of different colours - white and cream. The emulsion in the latter variety is spread upon tinted paper, the result is a black image on a cream ground instead of the usual black on white, which is by far in the greatest demand.
There is at times some difficulty in determining which is the sensitive side of the paper. A quick method of finding out which is the sensitive surface is to moisten the thumb and finger and with them hold just a corner of the paper for a second or two; the gelatine will soften and stick, indicating the coated side. A second method is to allow the paper to be exposed to the air in the dark-room, it will curl inwards to the sensitive side. A third is to pass the finger over the surface of the paper; the sensitive side is indicated by the edges being a little rough, caused by the gelatine lifting a little when cut through at the time the paper is cut up into pieces.
The negative and paper are placed in the frame, film to film, in exactly the same manner as for the other printing processes.
For the exposure it is advisable to always use the same source of illumination and at the same intensity. Then, by grouping the negatives according to their density, a fairly certain result will always be obtained. To get an idea of the printing values of different negatives, a little experimental work is recommended. Select three negatives, one thin, one medium and one dense. Working with the thin one first, it is placed in a printing-frame and upon it a piece of slow bromide paper; the back is then put in and fastened down. Cover the front of the frame with a piece of thick brown paper or cardboard. It is now taken to a lighted gas jet and held at a distance of about twelve inches from it. About one-third of the frame is uncovered and an exposure of five seconds given; another third part is uncovered and a further five seconds given; lastly the whole of the frame is uncovered and another five seconds counted. The frame must then be quickly covered up and taken to the dark-room. The exposures will represent five, ten and fifteen seconds respectively, and will be indicated on the print after development.
The other two negatives should be treated in the same way, but for the negative of medium density begin with ten seconds, and for the dense one fifteen seconds. With the information gathered from the results of these experimental exposures, it should be fairly easy to refer any negative to its respective class and expose accordingly. This experimental treatment may be carried out to ascertain the correct exposure for any negative, and the result should be noted in a book for future reference. Such notes greatly facilitate work if prints are required from the same negative at different times.
If a lamp is used as the source of illumination, the exposure must be longer than for gas; with incandescent gas the exposure will be about half that of the ordinary gas.
Distance from the illuminant materially affects the length of exposure. If a certain exposure were required at a foot from the light, at two feet it would be four times as long, and at three feet nine times. A useful piece of apparatus is an exposure-board, marked off in distances of six-inch intervals; this allows the easel to be quickly adjusted at any distance from the gas jet. Fig. 45 shows how an exposure-board may be arranged - it can be made any length. A is the baseboard along which the easel travels, and which may be divided into spaces as above suggested. B is the gas jet arranged with the tap close at hand to be under immediate control; the tube is connected by india-rubber tubing with the gas-fitting of the room. C is the easel against which the printing-frame is placed.
The paper must not be exposed to active light until after development and fixation. The process of development is the same as for films. The paper must be soaked in water for a short time before the developing solution is poured on. It will then lie flat in the dish and the solution will flow more evenly over it, with less risk of air bubbles-forming. The Hydroquinone Developer given on page 70 will be found useful for bromide paper, but it must be diluted with an equal quantity of water. The fixing bath is the same as for plates.
After the developer is poured on, the dish must be gently rocked and the print carefully watched. The image makes its appearance gradually, and continues to grow. As soon as it has acquired nearly its full strength, it should be immediately transferred to the fixer, in which it will acquire slightly more strength. If it were carried to its full strength in the developer, the increase in the fixer would render it slightly overdone.
The prints are allowed to remain in the fixer for at least ten minutes - fifteen minutes will do no harm. It is best to err on the side of safety, and it is not possible to judge the extent of fixation from the appearance of the surface, as is the case with plates. After fixing, the print must be thoroughly washed in the same way as P.O.P. prints.