By J. McINTOSH, F.R.P.S.,
Secretary of the Royal Photographic Society.
In applying methods of intensification and reduction, it is necessary for complete success that the photographer should have not only a knowledge of what these processes will accomplish but also a thorough understanding of the faults that are to be remedied and the process of discriminating between the remedial processes. The experienced photographer knows at least the defects of his negatives, even if he has not practised the arts of correcting them, but the photographer who has only recently taken to the camera has naturally everything to learn. It will be assumed in what follows that the reader is in the position of one asking for information on all points.
The faults in negatives which are curable by intensification or reduction are due to incorrect exposure Or incorrect development and very likely to both. Those due to the second cause are much more easily dealt with than those resulting from the first, and the inexperienced photographer is advised to spare no pains in acquiring the power of judging the correct exposure under all conditions. The actinometers now sold for this purpose simplify the matter to such an extent that comparatively little is left to the judgment of the worker. The cost of an actinometer, the time spent in understanding it and the cost of a box or two of plates expended in experimental work will be speedily recouped in the improved quality of the negatives and in the saving of labour that would otherwise have to be expended in correcting faults. Intensification and reduction should be regarded as processes to be applied as seldom as possible, as the need for them is, as a rule, due to errors on the part of the worker.
J. McIntosh, F.R.P.S. By W. Watmough,
A plate may be correctly exposed, over-exposed or underexposed. A correctly exposed plate may be correctly developed, or it may be over-developed or under-developed. An incorrectly exposed plate will never make a perfect negative under any conditions, but it may be lightly developed or fully developed. As the inexperienced worker may have some difficulty in classifying his faulty negatives it is desirable to indicate the appearance of the prints which can be obtained from them before remedial measures are adopted, and to specify the simplest remedies to which resort should be had. It should be understood that no one process of intensification or of reduction is the best under all circumstances and at the risk of imparting uncalled-for information it may be pointed out that intensification increases the density of a negative, that is, it makes it stronger, while reduction lessens the density.
We may take first the case of a plate which has been correctly exposed. If the developer is one which has been correctly compounded the image will appear in from 20 to 45 seconds. The highlights will first show as black patches, the half-tones will gradually appear, then the details in the shadow portions and finally the whole surface, with the exception of the margins, will darken over. If at this stage the negative is rinsed and washed it will be perfect of its kind, but if development is carried too far the negative will become so dense that even in bright light printing may occupy several hours, and it will be found that before the details of the highlights are visible in the print, the shadows will be heavy masses of ungraduated black.
Obviously it is necessary to get rid of the excess of density, and in doing so the high-lights and shadows should be reduced proportionately. This does not mean that the density is to be reduced equally over the surface of the plate, since in that case the shadows would lose more in proportion than the high-lights. The only agent which will do what is required is ammonium persulphate. Only a sufficient quantity of the salt for the work in hand should be taken, distilled water should, if possible, be used and a fresh portion of the solution should be employed for each negative. The formula is : -
Ammonium persulphate ........
Sulphuric acid (10% solution)
Distilled water ............
Another dish containing a five per cent, solution of sodium sulphite should be at hand and an unused fixing bath of ordinary strength is also required. The negative is immersed in the persulphate solution, care being taken to flood it all over, and it must be closely watched. The persulphate solution is somewhat capricious in its action, but as a rule it works very slowly at first, increasing in speed as the action progresses, so that once reduction of density is seen to be taking place a close watch must be kept lest the action should proceed too far and the image be entirely obliterated. When the density is judged to be sufficiently reduced the negative is to be at once, and without washing, transferred to the sodium sulphite solution for five minutes. It must then be well washed, placed in the hypo bath for a few minutes, thoroughly washed and dried. The sodium sulphite checks the action of the persulphate at once and the hypo bath removes any salts which may have formed in the gelatine, but a thorough washing is required before the negative is placed in the fixing bath, since, if any acid remained in the film, the hypo would be decomposed and the negative be stained a deep yellow with probably considerable danger to its permanence.