There are three ways of preventing the presence of free nitric acid in the nitrate bath, The first is, to fuse the nitrate of silver ; the second to neutralize the nitric acid with carbonate of soda ; the third to add acetate or citrate of soda to the nitrate bath.
By fusing nitrate of silver the whole of the free nitric acid is driven off. The salt is then in general rendered slightly alkaline, a fault which may easily be remedied by the addition of acetic acid to the solution ; and sometimes, which is muoh worse, nitrite of silver is formed, in consequence of the presence of nitrate of potass in the silver salt. Heat drives off oxygen from nitrate of potass, and forms nitrite of potass, which in its turn is oxidized at the ex pense of the nitrate of silver, and nitrite of silver produced. The heat required for fusing nitrate of silver is not sufficient to decom pose it and form nitrite. Nitrite of silver is a most objectionable substance to introduce into the nitrate bath. A very few crystals of it added to a large bath will completely put it out of order, and cause it to produce foggy negatives, looking as if they had been ex posed to diffused light. Acetic acid will not remedy this evil. The only remedy is to add nitric acid, and so convert the nitrite into nitrate. It appears therefore, that fused nitrate of silver is liable to contain nitrite, and is consequently a dangerous substance to intro duce into a bath, unless the purity of the nitrate of silver has been fully established, before fusing it.
The second plan of getting rid of free nitric acid from the bath, is to neutralize it with pure carbonate of soda, a solution of which should be added drop by drop, until the bath becomes decidedly al kaline, after which it should be acidified with acetic acid. This is an excellent mode of dealing with an acid bath.
The third plan is, to add acetate or citrate of soda to the bath. The soda having a stronger affinity for the nitric than for the organic acid, leaves the latter and combines with the former, thus produc ing nitrate of soda, which is inert, and acetate or citrate of silver, both of which latter salts are slightly soluble in the nitrate bath, and have a marked effect in increasing the redness and density of the ne gative, and at the same time rendering the film less sensitive.
A bath made with fused nitrate of silver acts very well when the nitrate is perfectly pure, and the alkalinity of the solution neutralized by acetic acid.
The effect of free acetic acid in the bath is to keep the lights clean, and slightly to impair the sensitiveness of the plate.
When a nitrate bath is accurately neutral it gives the most sen sitive film, but in this state it is very liable to get out of order, and either become allcaline from the introduction of carbonate of potass with the collodion, or acid from the fermentation of organic matter which it may contain. Acidity is frequently produced in this way when travelling, in consequence of the bath being kept for a long time in a state of agitation.
It is impossible to obtain a good picture with an allcaline bath. The developer is immediately decomposed and a precipitate thrown down all over the plate. The film is not in this case more sensitive than when the bath is neutral.
The acidity of a bath should never be neutralized with ammonia, because ammonia has a strong tendency to escape from its compounds. Carbonate of soda is the proper alkali to employ. When added to the bath yellow insoluble carbonate of silver is formed which is immediately decomposed, and carbonic acid liberated with efferver scence. This goes on until the whole of the nitric acid is neutra lized, after which the insoluble c,arbonate of silver is precipitated. This is again decomposed by the acetic acid afterwards added, and acetate of silver formed.
On the whole it will be seen that the best formula for the nitrate bath is that in which pure recrystallized nitrate of silver is used, con taining only a very small excess of nitric acid. The negatives are not then so excessively dense as to produce chalky positives, and the process is generally manageable as regards exposure for out-of door subjects, where the range of light and shade is the greatest that occurs. When there are moving figures in a view which cannot be taken instantaneously, and when violent contrasts of light and shade occur, an exposure of two or three minutes is far preferable to one of twenty or thirty seconds,—and then this ldnd of bath will be found the best to employ, for the plate may then be exposed until the picture is covered with detail without any part being overloaded with a dense impenetrable precipitate. Excessive density is the great evil to avoid in negative photography, and beautiful modula tion of tone with perfect rendering of all the details the great desid eratum ; provided always that a proper amount of contrasts between the highest lights and deepest shades is at the same tirrie introduced.