(9) That not a few of the votaries of the art would forsake mercurial intensification for silver and iron or other permanent redevelopment, were they assured of the absence of abnormal or other stains, may be taken for granted. A. Donald brings forward a method of intensifying gelatine negatives with silver and iron which has rendered good service since the summer of 1880.
The chemicals necessary are identical with those used with wet collodion, with the exception of the chloro-iodo bath, which apparently decomposes the active principle or constituent in the gelatine that causes the well-known pink fog. Neither iodine nor sodium chloride by itself will prevent the pink stain, but a combination of both as prescribed. The image is, probably, converted into silver iodide, which in turn gives place, to some extent, to the excess of chloride, forming both iodide and chloride of silver in the film, and during the decomposition the fog-producer is destroyed or rendered harmless. At all events, the negative is very readily and successfully intensified after its immersion in the bath.
The following are the requisite solutions : (a) saturated solution of common salt; (6) ruby solution of iodine in potassium iodide; (c) 10-gr. solution of iron protosulphate; (d) 15-gr. solution of silver nitrate; (e) a very weak solution of potassium cyanide.
When the crystals in solution (d) are dissolved, a few drops of a saturated solution of soda carbonate should be added, and the bottle placed in the light until clear. The silver should then be filtered and acidified with acetic acid (eschew nitric) enough to turn litmus paper red. To solution (c), 1/2 oz. glacial acetic acid must be added to every 15 oz. water. Test a mixture of (c) and (d) thus: Pour into a developing cup 1/2 oz. of (c), and add (say) 1/2 dr. of (d). If the combination turn muddy in less than 5 minutes, add more acetic acid to the stock bottle (c), until it remains clear when mixed with the silver solution for the specified time.
To intensify the negative: It is premised that the negative has been "alumed" and washed thoroughly to free it from the fixing agent. If the film be tender it is best, in the first place, to dry it. Now pour into a clean measure sufficient of (a) to cover the plate; then add a few drops of (b), until the colour of amber be attained. Immediately place the negative in a clean tray, and pour over it the chloro-iodo solution. Leave the plate in this bath for 4-5 minutes - the thicker the film the longer it should remain - then well wash it under the tap. As vegetable and other extraneous substances from the washing-water ofter adhere tenaciously to the surface of the film, thereby causing irregular markings, at this stage any such should be gently removed by a tuft of cotton wool dipped in water.
The next operation must be done in the dark room - one in which wet collodion can be worked - or by candle- or gas-light. To proceed: into a developing cup shake a few drops of (d), and add (say) for a half-plate almost1/2 oz. of (c). Flow this on the negative, and let it remain for 3-4 minutes - not longer - gently rocking the plate the while; then quickly place it under the tap until all greasy lines have disappeared. It will now be seen that a considerable increase of density is the result. One operation generally suffices; but if the image be still too thin, clean out the cup and repeat the dose. The plate will be thin, indeed, if it requires a third application. After a good wash, place the negative in solution (e) for 1-2 minutes, when it may be examined in daylight. Well wash again, then dry, and varnish.
If ordinary precautions be taken with the above method, there will be little fear of red fog; but if, from inadvertence or other cause, it should appear, immediately immerse the negative in a fresh solution of (a) and (6). It has the valuable property to a great degree of clearing off the stain. Should the fog be deep and obstinate, place out of doors the plate in a solution of potassium sulphide, when it (the stain) will gradually vanish. This treatment intensifies the negative. It simply requires to be well washed in running water, dried, and varnished. (Brit. Jour. Photog.)
(10) Abney's method for increasing the sensitiveness of the ferrous-oxalate developer. Add 10-40 drops, according to requirement, of a 25-gr. solution of hyposulphite to 3 oz. of developer.
(11) Cowell's Clearing Solution.
Alalia......2 parts Citric acid.. .. 1 part Water .. ..10 parts.
Edwards makes this sherry-coloured with iron perchloride.