A nitrate bath sometimes works badly in consequence of being too weak. Its strength should be tested occasionally with a silver metre. See " Nitrate Bath." The Bath may be kept for years in a glass, or gutta percha vessel, when the gutta percha is pure and unadulterated. Porcelain is objectionable, as nitrate of silver acts on the glaze. There is a substance used in America for baths and dishes, called " Photogra phic Ware," which resembles wedgewood ware, and has no glaze ; this is said to answer very well. The nitrate bath should be filtered occasionally.
To develope the image—Pour over the plate a solution containing :— Distilled water . 1 ounce Pyrogallic acid . . 1 grain Glacial acetic acid 20 minims, or Distilled water 2 ounces Pyrogallic acid . . 2 grains Citric acid . 3 grains.
Tt is convenient to make a small quantity of developer of three times the above strength, and to dilute it when wanted. You have, then, at hand a little strong solution to use, should occasion require. The developer will not keep longer than three or four weeks. A little alcohol added to it makes it flow more freely over the plate. It should be filtered if any floating particles appear in it. Citric acid is much cheaper to employ than acetic acid, and it answers very well, giving negatives of a more inky tint than acetic acid ; but it must be carefully weighed, as a grain or two more or less may make a considerable difference, citric being a very powerful acid in pho tography.
Although it is customary to call the acidified solution of pyrogallic acid the " developer," yet it must be borne in mind that this is not a developer unless it also contains nitrate of silver. The small quan tity of free nitrate of silver which remains on the wet plate is, in general, sufficient to complete the development, but sometimes it is necessary to add a few drops to the developer, towards the end of the process in order to render the negative sufficiently intense ; and this addition is always necessary when the bath contains a little free nitric acid. The picture should be developed as soon as possi ble after exposure, because, as the plate dries, the free nitrate becomes concentrated, and dissolves the iodide of silver, thereby destroying the impression produced by light.
It frequently happens, in hot weather, that the plate gets so dry during the exposure, that it is difficult to make the developer flow over it at once, and this produces stains, which spoil the picture. In
such a case the following is a good plan to adopt : After removing the plate from the slide, immerse it in a bath of distilled water. Then wash the measure which contains the developer thoroughly in water, afterwards with cyanide of potassium, and then with water again. This renders it chemically clean. Now pour into it the necessary quantity of developer, and add a few drops of fresh nitrate of silver solution. Remove the plate from the distilled water, and develop immediately with the above mixttire, which will now flow freely, and the picture will come out is quickly and as dense as before, and free from stains. The developing pyro gallo-nitrate of silver should not become discoloured in the measure. If it does, it is a proof that the measure is not thoroughly dean, and the result will probably be a bad proof, dirty, and stained.
When the details are fully out, and the blacks of the proper inten sity, wash the proof with water poured over it gently from a jug, and fix it in the manner to be described.
When a picture has not received sufficient exposure, the blacks become very intense while the lights remain clear and devoid of detail. Under these circumstances it is impossi,ble by any modifi cation of the treatment to produce a good negative. When the picture has been over exposed the effects are different according to the state of the bath and collodion. When the bath is acid with nitric acid, the effect of great over-exposure is to reverse the action of light in the black parts of the negative, and render them more or less transparent. When the bath is acid with acetic acid, and the picture exhibits the effects due to organic matthr, the over-exposed parts do not become dark so rapidly as those which have re,ceived the proper exposure, but remain red and transparent, having a burnt appearance, the edges of the red parts being, however, fringed with black, and the " solarization," as it is called, being more marked in the centre of the over-exposed part. When the bath is neutral, and the iodizer contains chloride of magnesium, and the developer citric acid, the over-exposed parts are sometimes those which acquire the greatest density and opacity, and what is called " solarization" is not then produced.