c. Addition of certain organic matters.—It has long been remarked that the use of bodies like Albumen, Gelatine, Caseine, etc., which combine with Oxides of Silver, retard the action of light upon Iodide of Silver; and the recent observations of the Author enable him to confirm this statement. It is probable that one cause, amongst others, of the great sensibility of the Collodion film is due to the fact that Pyroxyline is a substance peculiarly indifferent to the Salts of Silver, exhibiting no tendency to reduce them to the metallic state; and it is proved by experiment that the addition of Grape Sugar, or of the resinous body, Glycyrrhizine, which resembles Albumen in causing a white precipitate in strong solution of Nitrate of Silver, renders necessary a longer exposure in the Camera. Alkaline Citrates have a still more marked effect, as also have Tartrates, Oxalates, etc.
d. Impurities in the soluble Iodides.—Commercial Iodide of Potassium often contains Iodate of Potash, which is found to have a retarding effect upon the action of light; also Carbonate of Potash, which, in Collodion, produces Iodoform,[14] and in the paper processes, where "Aceto-Nitrate" is used for sensitizing, forms Acetate of Silver. Iodoform has a marked influence in diminishing the sensitiveness of Iodide of Silver; Acetate of Silver may perhaps increase it a little by securing the absence of free Nitric Acid ([p. 117]). Iodide of Potassium prepared by the process in which Sulphuretted Hydrogen and Alcohol are used, and having a smell of Garlic, contains probably Xanthate of Potash, and is nearly useless for Photography.
[14] See the Vocabulary, Part III., Art. Iodoform.
Commercial Iodide of Cadmium is a purer salt than the Iodide of Potassium, and may be advantageously substituted for it; but it possesses the property of coagulating Albumen, and hence cannot be employed in conjunction with that substance.
e. Presence of free Iodine.—Both in the waxed paper and the Collodion processes, the solutions often contain a small quantity of free Iodine. This Iodine, in contact with the Nitrate of Silver of the Bath, produces a mixed Iodide and Iodate of Silver, and liberates Nitric Acid. It thus retards the sensitiveness of the film in proportion to the quantity of Iodine present. Collodion of a full yellow colour is perceptibly less sensitive than the same rendered colourless; and when enough Iodine has been liberated to give a red or brown tint, double the original exposure will probably be required.
If brown Collodion be much used, the Nitrate Bath may by degrees become sufficiently contaminated with free Nitric Acid to interfere with the sensitiveness of the film; but if colourless or lemon-yellow tinted Collodion be employed, this evil need not be anticipated.
Certain substances may be added to coloured Collodion which possess the property of counteracting the retarding influence of the free Iodine, such, for instance, as the Oils of Cloves, Cinnamon, etc.; they probably act in virtue of their affinity for Oxygen, by preventing the formation of Iodate of Silver. In colourless Collodion they produce little or no effect, neither do they remove the insensitiveness of the film when dependent upon a too acid condition of the Nitrate Bath.
f. Addition of Bromide or Chloride to Collodion.—In the Daguerreotype a very exalted state of sensibility is obtained by exposing the silvered plate first to the vapour of Iodine, and afterwards to that of Bromine or Chlorine; but this rule does not apply to the Collodion process, which differs essentially in principle. Soluble Bromides added to Collodion lessen its sensibility to an appreciable extent, as also do Chlorides. This rule however may perhaps be liable to an exception when artificial light is used, which contains a greater proportion of the rays of small refrangibility, known to act more powerfully upon the Bromide than upon the Iodide of Silver ([p. 66]).
g. Density of the sensitive film.—When the proportion of soluble Iodide in the iodizing solution is too great, the film is very dense, and the Iodide of Silver is apt to burst out upon the surface, and fall away in loose flakes into the Bath. This condition, which is highly unfavourable to sensitiveness, is very common in Collodion, and constitutes what is termed "over-iodizing." The Iodide, in fact, is formed in such a case too much upon the surface, and consequently, when the fixing agent is applied, the image not being retained by the film, is washed off and lost.
On the other hand, the sensibility of the film is not lessened by reducing the amount of Iodide in Collodion to a minimum, if all the solutions are neutral; but the pale blue films formed by a dilute Collodion, and which almost rival the Daguerreotype itself in delicacy, are nearly useless in practice; for if free Iodine or other bodies of a retarding nature are present in any quantity, either in the Collodion or in the Bath, they almost destroy the action of a weak light, producing a far more injurious effect than if the film were more yellow and opaque.