NITRATE BATH. Many of the failures in photographic opera tions, particularly in the negative collodion process, may be traced to the nitrate bath being out of order.
1st. It may not be of the right strength. Test it with the silver meter (q. v.), and add either water or nitrate of silver, as the case may require.
2nd. It may be alkaline to test paper. To a negative bath add acetic acid, a drop or two at a time, until acidity is established. To a positive bath add nitric acid. The bath becomes alkaline in consequence of the introduction of carbonate of potass, an impurity contained in iodide of potassium. An alkaline bath fogs the plate, and gives a very dense picture.
3rd. It may be too acid. This diminishes the sensitiveness of the film, and in the case of acidity from nitric acid produces grey metallic negatives devoid of density, and brings about by great over-exposure the reverse action of light. In the case of a great excess of acetic acid, the negatives are all black and white, and devoid of half-tone. Acidity keeps the lights clean and transparent. It is brought about either by the fermentation of the ether and alcohol which are continually added to the bath, or by the accu mulation of an acid salt, such as nitrate of cadmium. The remedy for excess of nitric or acetic acid is carbonate of soda. Add a solution of this, a few drops at a time, until the bath is rendered • alkaline, then acidify slightly with acetic acid, and filter. The addition of carbonate of soda produces a yellow turbidity, due to carbonate of silver; when acetic acid is added acetate of silver is formed, and carbonic acid liberated ; the acetate of silver is, for the most part, dissolved in the nitrate bath, the remainder is removed by filtering. The shaking of the bath in travelling frequently renders it acid by causing the fermentation of the organic matter.
4th. The picture may be 'fogged all over, and yet be thin and grey, and the bath acid. This state of things is not so easily
explained, nor is it easy to suggest a certain remedy. A plan which frequently answers is to add nitric' acid, and let the bath stand for a day or two, then to neutralize with carbonate of soda, and acidify slightly with acetic acid.
Sometimes a bath in this state is cured by exposing it for a day or two to sunshine, and thus throwing down a deposit of silver and organic matter.
Another plan is to add a solution of carbonate of soda to the nitrate bath until the whole of the silver is thrown down as yellow carbonate of silver. This should be allowed to settle and the clear liquid poured off. The carbonate should then be well washed in several waters, dried, and roasted in a crucible ; after which it may be dissolved in nitric acid, which liberates the carbonic acid and produces nitrate of silver ;—or if the presence of lead be suspected, acetic acid may be added, which forms insoluble acetate of silver and soluble acetate of lead ; the acetate of silver must then be well washed and lastly dissolved in nitric acid, and the nitrate of silver solution evaporated and crystallized.
Or, instead of throwing down the silver as a carbonate, it may be precipitated as an oxide, by adding solution of caustic potass. The oxide should then be treated in the same way as the carbonate.
The advantage of either of the above methods is that it separates the silver from the excess of nitmtes which may have accumulated in the bath.
Sometimes a bath may be cured by simply evaporating, crystal lizing, and redissolving the nitrate of silver in pure distilled water.
Nitric acid is an infallible remedy for nitrite of silver in a bath.
A nitrate bath is frequently put out of order by dirty plates being immersed in it.
Ammonia should never be added to the nitrate bath for negatives. Afized alkali is far better.