Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter IX.

THE salts employed for sensitizing plain collodion for the reception of the actinic impression, are the iodides and bromides of different metal, as of potassium, sodium, ammonium, lithium, zinc, iron, calcium, cadmium, etc.

Iodides and bromides, which are soluble, in ether and alcohol, can alone be employed in the preparation of sensitized collodion, in order to produce, by decomposition in and on the film, an iodide and a bromide of silver, which are insoluble. In so extensive a choice of materials it is a difficult matter to collect all the advantages of a given iodide or bromide over its neighbors; so that it has not yet been decided which is the most appropriate iodide or bromide.

If each soluble iodide or bromide were equally applicable in a photographic sense, then the choice would be influenced by pecuniary considerations of cost and the quantities required; and if by weight the iodides and bromides were equal in price, the selection would fall upon that iodide and bromide whose chemical equivalent is the least; for the less the combining proportion of a given chemical substance, the less the quantity required to produce a given effect. Guided by this consideration. of the subject, the iodide and bromide of lithium would claim our first attention; after lithium come magnesium, ammonium, calcium, sodium, iron, zinc, potassium, cadmium, etc. The solubility of the respective iodides and bromides in a mixture of ether and alcohol. will naturally form a second consideration; and, thirdly, a very important property must have its due weight in the scales, and that is the stability of the given salt in the ethereal solution. The alkaline iodides and bromides are all soluble, so that lithium stands, perhaps, quite as high as the rest in this respect. In absolute alcohol. the iodide of potassium is not soluble to the same extent as iodide of ammonium. The latter iodide is the most easily decomposed. On this account it is regarded as a more sensitive iodizer; it is also quicker; but on the same account it is unstable and undergoes spontaneous decomposition. The iodide of ammonium, as well as that of potassium, is very capricious.

The bromide of silver is sensitive to light as well as the iodide and the chloride; but the spectral rays have not the same influence on either of these three salts. The actinic impression on the iodide and bromide of silver is invisible or latent, and requires the aid of some developing agent to snake manifest the effect of light; whilst the impression made on the chloride of silver becomes manifest in proportion to the intensity and duration of light.

The photographed image of the solar spectrum is much broader on the bromide of silver film, than on the iodide film. In the former case, the violet, the indigo, the blue, and partially the green produce actinic action; whilst in the latter the blue part is but partially represented. Equal portions on the violet side and external to the violet color produce an equal impression on either of the films. The greater capacity of the bromized film has induced photographers to attribute to bromine qualities specially adapted to landscape-photography, where the greens occupy so large a space. By the introduction of the bromides into collodion, together with the iodides, much discussion has arisen to determine the precise action of the former. Certain collodions with certain baths are acknowledged to undergo an improvement when a bromide is a part of the sensitizer; the picture is softened, that is, the middle tints are more pronounced, or the lights and shades more agreeably graded with the bromo-iodizer, than with the simple iodizer. On this account, probably, bromides have been regarded by many as accelerators, or substances which render collodion more sensitive to light. On this ground alone the deduction would be false. The capacity for comprehending a greater range of colors is possessed by the bromo-iodized collodions. This, perhaps, is the only true and legitimate deduction that can 1>e drawn in the case; they are considered by very high authority, on the contrary, as deduced from experiments carefully conducted, to be retarders of the actinic action. In consequence of the greater comprehensiveness, as regards colors, of the bromides over the iodides, it may be concluded, that there are very few cases in which the bromo-iodized collodion can not be appropriately preferred to the simply iodized. collodion; the exceptions being the copying of engravings, plain or uncolored photographs, maps, letter-press printing, etc., where the iodized collodion alone possesses all the capacity required.

A peculiarity has been discovered in reference to iodized collodion. Some sorts of collodion are suited for one iodizer, and some for another. As a general rule, a cadmium iodide glutinizes collodion; whereas an alkaline iodide liquefies it. The natural deduction from these circumstances is this: a glutinous or tenacious collodion is suited for sensitizing with iodide of ammonium, or iodide of potassium; for it becomes thereby less tenacious, and flows better. Such collodion soon attains its maximum amount of sensitiveness, and almost with the same facility begins to deteriorate; it is very unstable, and not permanent in any degree of sensitiveness. On the other hand an alcohol collodion, which is in a condition to flow easily, is, in fact, thin and liquid, can be rendered more glutinous by a cadmium iodide. Collodion thus iodized is much more stable than when iodized with the alkaline iodides, but it attains its maximum degree of sensitiveness very slowly, that is, it takes a longer time to ripen than the first-mentioned collodion; but when ripe, it retains its sensitiveness much longer, is in fact a stable collodion. Coupling these two facts together, attempts have been made to combine the iodide of cadmium with an alkaline iodide in such proportions as to comprehend the peculiar advantages of either, that is, the stability and permanency of the one with the quick sensitiveness of the other, and the mutual tempering of either toward a medium glutinosity or liquefaction. The result of such experiments indicates that the cadmium salt must exceed the alkaline salt in quantity. As soon as the highest degree of sensitiveness and stability can be established by means of the iodides alone, it remains then to combine with these a certain proportion of a bromide to communicate to the collodion a greater capacity for colors. Notwithstanding that this is, in my opinion, the view we have to take of the matter, it must be confessed that the best working quantities of the iodides, or of the bromo-iodides have not yet been satisfactorily determined. The difficulty that stands in the way of this determination is increased by the peculiar condition of the nitrate of silver bath, whether it be acid, neutral or alkaline; and furthermore whether it be rendered acid by nitric acid or acetic acid; or whether it contain carbonate of soda or acetate of soda. A cadmium iodized, or bromo-iodized collodion sensitized in a bath of nitrate of silver rendered slightly acid with nitric acid, produces irreproachable pictures, but not more rapidly than a bath containing acetic acid, acetate of soda, or carbonate of soda, when these happen to be in a happy mood; but the latter are very unstable, whilst the former remains for a long time constant, and is regarded accordingly the proper bath for the cadmium collodion. It must not be forgotten that acids are retarders of sensitiveness, and that consequently a bath that yields a picture without spots, stains, or fogginess is preferable in the ratio as it approaches neutrality. A bath containing either acetate of soda or carbonate of soda is, when in its best condition, an accelerator; but it is very unstable, deteriorates very quickly, and at present no means are known to rectify the evil and preserve or restore the sensitiveness.

The iodides and bromides most generally employed by the photographer are those of lithium, potassium, sodium, ammonium, cadmium, and silver.