ALL the papers described hitherto have been of a kind needing daylight, or an electric light of high actinic value, to produce satisfactory results; the image also becomes fully visible and complete during exposure under the negative. We now have to consider a series of papers, sensitised with silver, in which the image does not print out, and which require only to be exposed to a very moderate light, even a wax candle or a lucifer match being generally sufficient.
These are papers coated with an emulsion in which bromide of silver is the principal haloid salt. The vehicle may be either collodion or gelatine, but the latter continues to be exclusively employed, especially since the introduction of the bromoil and ozobrome processes. The emulsion is of similar nature to that used for coating gelatine plates, but must not be ripened, and the proportion of gelatine is rather large. The following is a typical formula after Dr. Eder:
Ammonium Bromide.......30 „
A few grains of potassium iodide may be substituted for part of the bromide. The gelatine after prolonged soaking is dissolved with the bromide at a temperature of 130-135° Fahr.; and 30 gr. of silver nitrate dissolved in an ounce of water at the same temperature is added by degrees, the mixture being meanwhile well stirred or agitated. When thoroughly incorporated it is allowed to set, broken up, squeezed through thin canvas, and washed in three or four changes of water. Lastly, the emulsion is redissolved at a temperature of about 1300 Fahr., and coated on some pure paper (Saxe or Rive's), without much glaze upon it. The coating process will be greatly assisted if a few drops of glycerine are added after melting.
All operations with bromide paper, except exposure, must be conducted in the dark room, illuminated only by yellow or orange rays. There are many varieties of this paper sold, varying from very rough to smooth or glossy, the rough papers being most suitable for enlargements or large negatives. Most negatives may be adapted to this paper. Very hard ones should have short exposure very close to the source of light, soft negatives must have longer exposure at a distance of two or three feet. Even illumination of the whole printing surface is important. Practice soon enables the operator to gauge the time of printing and distance from the burner suitable for each negative; doubling the distance quadruples the length of exposure necessary. The light for printing may be either a gas-burner, an oil-lamp, or even a candle. Daylight is too rapid, 5 sec. often producing reversal. At 2 ft. from an ordinary incandescent burner the time of printing will probably be 15-30 sec, but it is advisable to test with narrow strips under an average negative before printing a batch. Some difficulty may be experienced at first in deciding which side of the paper is coated with emulsion, and must therefore be in contact with the negative, especially by the light of the yellow lamp. If the paper is laid on the palm of the hand the coated side will curve inwards; and if a corner is touched by the damp finger it will betray a slight stickiness.
Returning to the dark room, remove the print from the frame and place for 1 min. in clean water. Transfer to the developing dish and pour on the developer, taking care that the paper is well covered. Rock the dish for a minute or so, when, if the time of printing has been correctly gauged, the image will begin to appear. As soon as the details in the half-tones are faintly visible pour the developer back, and allow the print to continue gaining strength in the empty dish. When the required tint is nearly attained flood with three changes of water, and then transfer to the fixing bath of from 3 to 4 oz. of hypo to the pint of water.
Nearly all the developers given for plates are equally suitable for bromide papers when diluted to half their strength with water. The best for black tones are hydroquinone, eikonogen, metolquinol, and glycin. Rodinal and amidol are also good. A few drops of bromide solution should be added with most of these to avoid degradation of the high lights. Pyro is tabooed by most workers, but we have obtained good brown tints, and the acid alum clearing bath will remove all stains.
This developer is the traditional one for bromide papers, but we do not recommend it unless a very large amount of work of this kind is done. It involves a good deal of trouble, but gives bright, sparkling pictures and grey half-tones.
A. Potassium Oxalate (neutral)..... 5 oz.
Ammonium Bromide...... 6 gr.
Water......... 20 oz.
B. Ferrous Sulphate...... 1 oz.
Sulphuric Acid ....... 5 minims.
Water......... 3 oz.
At time of using pour 1 oz. of B into 6 oz. of A. The oxalate must not be poured into the iron solution, or a thick yellow precipitate of ferrous oxalate will form. After development wash the print in two changes of a solution of citric acid, 30 gr. to every pint of water, and then in three changes of pure water before fixing. Otherwise the iron salts will become insoluble and stain the print a pinkish yellow colour.
The bromide printing process has this convenience over any other, that, if a proof is wanted in a hurry, it may be taken from a negative within a minute after development. Wash the negative well, lay it face upwards in a dish of clean water, and bring in contact with it a piece of bromide paper of the same size. The two are squeegeed together, and then exposed in the printing frame; the exposure being about four times as long as would be the case with a dry, properly fixed negative. Bromide prints may be reduced and intensified in just the same way as ordinary negatives, except that the solutions are rendered more dilute for the former. Local reduction or intensification is easily accomplished by means of a brush dipped in a solution of copper bromide with glycerine. Blisters may be cured by the alum or formaline baths given under development of plates.