Over-exposed negatives give flat and feeble positives, which are deficient in contrast and cannot be properly printed to a good tone.
There are limits therefore to the time of exposure which cannot be passed with impunity. When diffused light finds its way into the camera, fog is an evil which is greatly exaggerated by over-exposure ; and when the chemistry of the process is a little at fault, the effects of over-exposure are sometimes very remarkable and absurd.
Negatives may be developed with proto-sulphate of iron acidified with acetic or citric acid ; and some persons think this developer a much better one than either of those before described. The proportions are variable, and depend ou circumstances. In our hands the following formula succeeds very well :— Distilled water . 1 ounce Proto-sulphate of Iron . . 5 grains Acetic acid . . 5 minims The acid should be added to the proto-sulphate solution imme diately before use, and the proto-sulphate solution should be kept as much as possible IN the light. There should be no free nitric acid in the nitrate bath, and the collodion should be such as to assist the production of an organic compound of silver. Whdn this developer is used too strong, a white transparent stain is produced upon that part of the plate on which it is first poured ; but so long as this evil not occur the developer may be strengthened with advantage. The exposure need not much exceed one-half that which is required when pyrogallic acid is used. The chief merit of the iron developer is that it yields negatives abounding in half tone, and with exquisite detail. Any amount of density may be obtained with proper treatment. The negatives are brown, and not blue.
Some operators commence the development with iron, and finish with pyrogallic acid, the iron being, of course, carefully washed off before applying the pyrogallic. This is an excellent plan.
When the precautions for avoiding " fog" are strictly observed, negatives developed with iron are as perfectly transparent in the lights, as those developed with pyrogallic acid.
On the whole the iron developer appears to he superior to the other, in the hands of a skilful operator; and it is particularly serviceable in cold weather.
As a universal rule, a negative should be red and not grey in the first stage of the development.
To fix the picture—Pour over it either a saturated solution of Hyposulphite of soda, or a solution of cyanide of potassium from 5 to 10 grains to the ounce. As soon as all the yellow iodide of silver is dissolved, wash the plate well with water, and set it up to drY.
Hyposulphite of soda does not reduce the density of a red organic image so much as cyanide of potassium, but the plate requires to be washed much more thoroughly and carefully after it, for if this be not attended to, the hypo left in the film is apt to crystallize and destroy the picture.
Collodion negatives are not permanent unless they are varnished. The pyroxylin film is gradually decomposed by contact with air and moisture, and an oxide of nitrogen produced, which destroys the photograph. When properly varnished, however, they appear to be extremely permanent. Good spirit varnish is the best, and amber varnish one of the worst to employ. (See" Varnish.") When spirit varnish is use,d the plate should be warmed before the fire, both before and after the application, but not made too hot, or the negative would be dissolved Off the plate. The varnish is poured on in the same way as the collodion. It dries very quickly. Other varnishes are applied in the same way to a cold plate, which should, however, be thoroughly dry.
The negative is now fmished, It may be well to add a fen, words on the chemistry of this im portant process, although the general chemistry of photographic processes has been discussed at some length in a separate article on the Chemistry of Photography, g. v.
The collodion film is nearly inert, producing in a very slight degree the effects due to organic matter ; it must be considered as little more than the vehicle for supporting the sensitive iodide of silver, which is more sensitive to light when precipitated in a test tube, than when collodion is added to it. Collodion is not an accelerating substance ; on the contrary, it diminishes the sensitiveness of the mixed iodide and nitrate of silver. The reason why the collodion process is more sensitive than any other, is partly because the film is moist and transparent, and the sensitive chemicals are distributed through it in a finely divided state, and partly because the nitrate bath is nearly neutral, and the developer highly energetic.