Home >> Photography Theory And Practice >> Non Actinic Lighting Dark Room Lamps to Shutters 127 >> Physical Development Before or

Physical Development Before or After Fixing 395

image, cc, silver, gr, water, solution, developer and oz

PHYSICAL DEVELOPMENT BEFORE OR AFTER FIXING 395. Physical Development before Fixing. Physical development, which consists in the deposition of nascent silver formed in the developer on the nuclei of the latent image, and which is the normal method employed for Exposure test the effect of bromide on a developer. This is done by measuring approximately the ratio of the times of exposure which, with and without bromide, give two tones which differ in density from the respective fog values by the same amount.

The resistance of a developer to oxidation may be determined by comparing two samples of the same solution maintained under the following conditions : One is kept in a full and well-stoppered bottle ; the other is kept in an open dish with free access to the air, precautions being taken to keep up the volume of the latter sample in case of evaporation of water.

the development of the image in the wet collodion process, is also applicable to gelatino bromide emulsions. As a rule, however, the method has no advantages over ordinary chemical development. The precipitation of silver may be brought about in solutions which are alkaline, neutral or acid. One of the most commonly employed methods is that of Liippo Cramer, 1903, 1923).

The stock solution is prepared as follows Nktol . . 130 to 175 gr. to 20 gnu.) Citric acid . 2 OZ. (Ioo Soda citrate . . 130 gr. (i5 grin.) Water, to make . 20 Oz. (1,000 c.c.) The formation of mildew in this solution may be prevented by the addition of a small quantity of phenol. When required for use, take 3.1 oz. (roo c.c.) of this solution and add from 50 to 170 minims (3 to pp c.c.) of a 10 per cent solution of silver nitrate. Development is very slow. As soon as the developer becomes turbid it must be replaced by a fresh lot.

Plates of recent manufacture must be used and the exposures given must be large.' One condition of success is that all dishes employed must be quite clean it is best to use a glass dish which has been very carefully cleaned.

It has been proposed (A. Schmidt, 180) that when a plate has been found to be over-exposed chemical development should be stopped at once by washing in water, the plate being then treated by a physical developer.

Since physical development affects only the surface layers of an emulsion, C. E. K. Alves (1909) has proposed development by this method for fine-grained emulsions, coated in specially thin layers, for the purpose of increasing their resolving power. At present physical develop mmt of this kind is merely of theoretical interest.

396. Physical Development after Fixing. The fact that the latent image may be developed by deposition of silver or mercury after the sensitive silver salt has been dissolved by a fixing, bath was pointed out in 1858 by Young in the case of collodion plates and in 18q4 by Kogelmann, in the case of gelatino-bromide plates. This method has scarcely any practical

value, but is of some experimental interest, for which reason the recently improved methods of A. and L. Lumiere and A. Seyewetz (1924) may be mentioned.

Ample exposures must be given and these must be proportionally greater if the most sensitive emulsions are used. The plate must be fixed in an alkaline bath, e.g. one in which roo minims (ro c.c.) of ammonia (22° Be.) are added to 20 oz. (I. ,000 c.c.) of 30 per cent hypo. It must not be kept in the fixing bath more than 5 minutes, and must then be washed in several changes of water made alkaline by the addition of ammonia. As soon as fixing is complete all further operations may be carried out in full light.

The developers which appear to be most con venient are prepared from two stock solutions— Silver Mercury (A) Soda sulphite, anhydrous _ x,58o gr. 1,580 gr.

(itio gnu.) (x8o grm.) Silver Ili trate (eocY„ solution) . x( oz. — (75 c.c.) Mercuric bromide gr.

(9 gem.) Water, to make - . 20 02. 20 02.

(1,000 C.c.) (0,000 C.C.) (1_3) Soda sulphite, anhydrous . 175 gr. 075 gr.

(20 gnu.) (20 grew) Parapheivlencdiamioe (base) 175 gr.

(20 gnu.) Metol . . 275 gr.

(20 grin.) Water, to make . 20 02. 2002.

41,000 cc.) (1,000 For use, mix 5 volumes of (A) with t volume of (13).

Development must be carried out in a glass dish which has been very thoroughly cleaned, otherwise the greater part of the metal will be deposited on the sides of the dish instead of on the image.

The image appears very slowly and its density only attains a value sufficient for printing after several hours. The bath itself, however, is almost completely used up in about an hour, and must be renewed hourly. The image, clear grey by reflection, is slightly violet by trans mitted light, even after very prolonged develop ment. After intensification, an image is obtained of density suitable for printing, even if develop ment has been allowed to continue only for about half an hour. Under the microscope the image resulting from prolonged development by silver (48 hours) is seen to consist of sharply defined hexagonal grains, indicating that a crystallLation of silver round the nuclei of the latent image has occurred.

The characteristic curves of negatives treated by these methods have the same form as those corresponding with chemical development, but by sufficiently prolonging development it is possible to obtain much higher gamma values (M. Hanot and W. Guillemet, 1928).