A very short time after the publication of Mr. Talbot's processes, which I anxiously repeated with various modifications, I discovered a singular property in the iodide of potassium (hydrio-date of potash) of again whitening the paper darkened by exposure, and also, that the bleaching process was very much accelerated by the influence of light. Early in the year 1839, Lassaigne, Mr. Talbot, Sir John Herschel, and Dr. Fyfe appear to have fallen on the same discovery.
As this process, giving by one operation pictures with their lights correct is of much interest, I gave it for a very considerable time my undivided attention. The most extraordinary character of these salts is, that a very slight difference in the strength of the solutions, in the composition of the photographic paper, or in the character of the incident light, produces totally opposite effects; in one case the paper is rapidly whitened, in the other a deep blackness is produced almost as rapidly. Sometimes these opposing actions are in equilibrium, and then the paper continues for a long time perfectly insensible.
I am inclined to hope these researches have reduced to certainty their somewhat inconstant effects, and rendered this method of producing photographs one of the most easy, as it is the most beautiful. That the various positions I wish to establish may be completely understood, and to insure the same results in other hands, it will be necessary to enter into a somewhat detailed account of the various kinds of paper used, and to give tolerably full directions for successfully using them, either in the camera, or for drawings by application,—to examine attentively the effects of different organic and inorganic preparations on the paper, and to analyze the influence of the different rays upon it. See also Part I., Chapter VI., Section VII., page 82.
These particulars will be copied chiefly from my paper, "On the Use of the Hydriodic Salts as Photographic Agents," published in the London and Edinburgh Philosophical Magazine for September and October 1840, to which will be added the results of my experiments since that time.
The variable texture of the finest kinds of paper occasioning irregularities of imbibition, is a constant source of annoyance, deforming the drawings with dark patches, which are very difficult to remove: consequently my first endeavours were directed to the formation of a surface on which the photographic preparations might be spread with perfect uniformity.
A variety of sizes were used with very uncertain results. Nearly all the animal glutens appear to possess a colorific property, which may render them available in many of the negative processes ; but they all seem to protect the darkened silver from the action of the solutions of the iodides. The gums are acted on by the nitrate of silver, and browned, independent of light, which browning considerably mars the effect of the finished picture. It is a singular fact, that the tragacanth and acacia gums render the pictures produced much less permanent. I therefore found it necessary for general practice to abandon the use of all sizes, except such as enter into the composition of the paper in the manufacture. It occurred to me that it might be possible to saturate the paper with a metallic solution, which should be of itself entirely uninfluenced by light, on which the silver coating might be spread without suffering any material chemical change. The results being curious, and illustrative of some of the peculiarities of the process, it will be interesting to study a few of them.
These salts, when used in small proportions, appeared to overcome many of the first difficulties, but all the drawings on papers thus prepared faded out in the dark. If, after these photographs have faded entirely out, they are soaked for a short time in a solution of the ferrocyanate of potash, and then are exposed to the light, the picture is revived, but with reversed lights and shadows.
These salts have been much used by Sir John Herschel, both in the negative and positive processes, and, it appears, with considerable success. I found a tolerably good result when I used a saturated solution; but papers thus prepared required a stronger light than other kinds. When I used weaker solutions, the drawings were covered with black patches. On these a little further explanation is required. When the strong solution has been used, the iodine which has not been expended in forming the iodide of silver—which form the lights of the picture—goes to form the iodide of lead. This iodide is soluble in boiling water, and is easily removed from the paper. When the weaker solution of lead has been used, instead of the formation of an iodide, the hydriodate exerts one of its peculiar functions in producing an oxide of the metal.
These salts, in any quantities, render the action of the iodides very quick ; and, when used in moderate proportions, they appeared to promise at first much assistance in quickening the process. I have obtained, with papers into the preparation of which nitrate of copper has entered, perfect camera views in ten minutes; but experience has proved their inapplicability, the edges of the parts in shadow being destroyed by chemical action.
Chlorides of Gold and Platinum act similarly to each other.
They remain inactive until the picture is formed; then a rapid oxidation of these metals takes place, and all the bright parts of the picture are darkened.
An extensive variety of preparations, metallic and non-metallic, was used with like effects, and I am convinced that the only plan of obtaining a perfectly equal surface, without impairing the sensitiveness of the paper, is careful manipulation with the ordinary muriates and silver solutions.
By attention to the following directions, which are simple in their character, but arrived at by a long series of inquiries, any one may prepare photographic papers on which the bleaching solutions shall act with perfect uniformity.