(1) According to W. Brooks, the greatest drawback to gelatine plates has been the want of a proper intensifier after fixing. Many negatives are perfect in every respect except for the want of a little more density to bring them up to proper printing quality. Several methods of mercurial intensification have been put forward; but to no purpose, as the negative changes by light more or less. The objection to mercury in any form is that when you attempt to alter a negative you never know how much or how little intensity you will get after its application; again, there is always more or less clogging or blocking up of the fine detail, with the result, in 9 cases out of 10, of absolute ruin to the negative. Many thousands of negatives have been sacrificed in this way.
Photographers are more certain than formerly, in the early days of the gelatine process, in getting their negatives somewhere near the mark as to density. But at times one is apt to be a little out in judgment, and fall a little short; and for the want of a ready and reliable system of intensification many stick fast. Brooks made innumerable experiments in this direction, and felt certain that the result must be gained by redevelopment with silver after fixing, as with a wet plate.
It is a well-known fact that to develop a gelatine plate with alkaline pyro, the developer must be much more powerful than we dare use to develop a collodion emulsion plate. In his own work Brooks uses a much stronger developer than that usually recommended - generally as much as 10 minims of liquor ammonia in his developer, well restrained with ammonium bromide; and, by being able to use such a powerful developer, it occurred to him that, to gain the desired end, we must work in the same direction as regards an intensifier, and he found his surmises correct. Some years since, he tried other fixing or clearing agents than soda hyposulphite, as when this salt was used it was a risk to use silver and pyro afterwards. He employed potassium cyanide with success at times, but found the films were not always in the proper condition. At one time also he used ammonium sulphocyanide; this had certain drawbacks, and it required a great deal of washing to get rid of it, but there was a slight gain over soda hyposulphite, as the film did not stain nearly so much on the application of pyro and silver. But with all this it was very slow work, and something was always wanting.
He never found the preliminary wash of iodine of much advantage to a gelatine plate; it worked well for wet plates, but not so well for gelatine.
There is another defect that is very vexatious in gelatine plates; that is, some of the plates, especially the larger sizes, have a thin end to them, sometimes owing to the glass not being perfectly flat. With the intensifier about to be proposed, Brooks finds it very easy to get local intensity on such parts in just the same way as we used to do with wet plates, namely, by pouring the developer on and off the place that may require it. He is also able to intensify a mere ghost of an image to good printing density with but very little trouble, providing the shadows are free from deposit and quite clear. The slightest veil in these parts comes up with the image, and a favourable result is not obtained. If the shadows are only slightly veiled, allowing them to remain in the hyposulphite solution for 1/2 hour may clear them; if not, recourse must be had to other reagents, such as iron perchloride followed by hypo. Care, however, must be taken that its action is even.
Before attempting to intensify the negative, all traces of soda hyposulphite must be eliminated, and there is nothing better for the purpose than the alum and citric acid solution. Make a stock ' solution, consisting of a saturated solution of common alum, with 1 oz. citric acid dissolved in every 10 oz. After the negative has been well washed, place it in a dilute solution of the above (1 part to 4 of water), and allow it to remain for about 1/4 hour; then place in a developing measure (supposing a half-plate is being operated upon) about 2 dr. of the stock alum and citric-acid solution, and add to it about 2-3 gr. of dry pyrogallic acid. Give it a shake round to dissolve; then drop in 3 or 4 drops of a 20-gr. solution of silver nitrate, and apply to the plate, holding the latter on a pneumatic holder and pouring off from alternate corners. If the film repel the solution, just run the finger or a brush kept (clean) for the purpose over the repellent portion of the film. This is very energetic, and the alum in the solution keeps back any trace of hypo that may be lurking about.
It is not satisfactory to work in a dish when intensifying, as the back of the plate gets very cloudy; and sometimes, as the solution gets brown, it apparently discolours the film, but that all comes right afterwards. Silver is added according to the density required. When sufficient density is obtained, well wash and place for about 5-10 minutes in the hypo fixing bath; well wash again, and place in the dilute alum and citric solution. This will remove all colour, and if there were any greenish-yellow look about the negative before intensification it will be found to have all disappeared, and the result is a negative in all respects equal to the finest wet plate. Care must be taken not to work the alum and citric bath too much, so as to foul it with the hypo. (Brit. Jour. Plwtog.)
(2) Notwithstanding the fact that a good deal of attention has been given to the subject of intensifiers, but few photographers are satisfied with the results obtainable with the mercury methods. Arnold Spiller thinks that a perfect intensifier for gelatine plates, as compared with the silver redeveloper for collodion films, does not exist; yet there are several processes, if used with care, that answer well for most purposes, and perfectly in a few cases. It will, no doubt, be of interest to some readers to explain here the difference between the silver solution acting with collodion film and the mercury with gelatine.