For drawings by application, less care is required than for the camera obscura. With a very soft flat brush apply the solution on both sides of the prepared paper, until it appears equally absorbed; place it in close contact with the object to be copied, and expose it to sunshine. The exposure should continue until the parts of the paper exposed to uninterrupted light, which first change to a pale yellow, are seen to brown a little. The observance of this simple rule will be found of very great advantage in practice. Immersion for a short time in soft water removes the brown hue, and renders the bright parts of the picture clearer than they would be otherwise.
Engravings to be copied by this process—which they are most beautifully—should be soaked in water and superimposed on the photographic papers, quite wet. If the paper is intended to be used in the camera, it is best to soak it in the bleaching solution until a slight change is apparent, from chemical action on the silver : it is then to be stretched on a slight frame of wood, which is made to fit the camera, and not allowed to touch in any part but at the edges; placed in the dark chamber of the camera at the proper focus, and pointed to the object of which a copy is required, which with good sunshine is effected in about twenty minutes, varying of course with the degree of sensibility manifested by the paper. If the wetted paper is placed upon any porous body, it will be found, owing to the capillary communication established between different points, that the solution is removed from some parts to others, and different states of sensitiveness induced. Another advantage of the frame is, the paper being by the moisture rendered semi-transparent, the light penetrates and acts to a greater depth; thus cutting out fine lines which would otherwise be lost. However, if the camera is large, there is an objection to the frame; the solution is apt to gather into drops, and act intensely on small spots, to the injury of the general effect. When using a large sheet, the safest course is to spread it out when wetted upon a piece of very clean wet glass, great care being taken that the paper and glass are in close contact. The picture is not formed so quickly when the glass is used, as when the paper is extended on a frame, owing to the evaporation being slightly retarded. The additional time required—about one-sixth longer—is, however, in most cases, of little consequence.
The picture being formed by the influence of sunshine, it is required, to render it unchangeable by any further action of the luminous fluid, not only that the salt of iodine be entirely removed from the paper, but that the iodide of silver which is formed be also dissolved out of the drawing.
By well washing the drawing in warm water, the iodide of potassium is removed, and the pictures thus prepared have been stated to be permanent; and if they are kept in a portfolio, and only occasionally exposed, they are really so : for I shall show presently, that they have the property of being restored in the dark to the state in which they were prior to the destructive action of light. A drawing which I executed in June, 1839, which has often been exposed for days successively to the action of sunshine, and has altogether been very little cared for, continues to this date (December, 1853) as perfect as at first. These photographs will not, however, bear long-continued exposure without injury —about three months in summer, or six weeks in winter, being sufficient to destroy them. As this gradual decay involves some very curious and interesting chemical phenomena, I shall make no excuse for dwelling on the subject a little.
The drawing fades first in the dark parts, and as they are perceived to lose their definedness, the lights are seen to darken, until at last the contrast between light and shadow is very weak.
If a dark paper is washed with an iodide in solution, and exposed to sunshine, it is first bleached, becoming yellow; then the light again darkens it. If, when quite dry, it is carefully kept from the light, it will be found in a few days to be again restored to its original yellow colour, which may be again darkened by exposure, and the yellow colour be again restored in the dark. The sensitiveness to the influence of light diminishes after each exposure, but I have not been enabled to arrive at the point at which this entirely ceases. If a dark paper, bleached by an iodide and light, be again darkened, and then placed in a bottle of water, the yellow is much more quickly restored, and bubbles of gas will escape freely, which will be found to be oxygen. By enclosing pieces of iodized paper in a tube to darken, we discover, as might have been expected, some hydrogen is set free. If the paper is then well dried, and carefully shut up in a warm dry tube, it remains dark ; moisten the tube or the paper, and the yellowness is speedily restored.
Take a photograph thus formed, and place it in a vessel of water: in a few clays it will fade out, and bubbles of oxygen will gather around the sides. If the water is examined there will be found no trace of either silver or iodine. Thus it is evident the action has been confined to the paper.
We see that the iodide of silver has the power of separating hydrogen from its combinations. I cannot regard this singular salt of silver as a definite compound : it appears to me to combine with iodine in uncertain proportions. In the process of darkening, the liberation of hydrogen is certain; but I have not in any one instance been enabled to detect free iodine: of course it must exist, either in the darkened surface, or in combination with the unaffected under layer : possibly this may be the iodide of silver, with iodine in simple mixture, which, when light acts no longer on the preparation, is liberated, combines with the hydrogen of that portion of moisture which the hygrometric nature of the paper is sure to furnish, and as an hydriodate again attacks the darkened surface, restoring thus the iodide of silver. This is strikingly illustrative of the fading of the photograph.