17. Some paper is to be met with, containing traces of bleaching chlorides, which does not require any previous preparation ; but in general it will be found necessary to prepare the paper by slightly impregnating it with a minute quantity of common salt. This may be done by dipping it in a solution in which the salt can barely be tasted, or of the strength of from thirty to forty grains to a pint of water. The paper, after being pressed in clean blotting-paper, has merely to be dried and smoothed, when it will be fit for use.
18. The ammonia-nitrate of silver is applied to the paper in the manner described in § 3 ; and, when perfectly dry, the negative picture to be copied is to be applied to it, with its face in contact with the sensitive side. The back of the negative picture being uppermost, they are to be pressed into close contact by means of a plate of glass; and, thus secured, they are to be exposed to the light of the sun and sky. The exposed parts of the sensitive paper will speedily change to lilac, slate-blue, deepening towards black ; and the light, gradually penetrating through the semi-transparent negative picture, will imprint upon the sensitive paper beneath a positive impression. The negative picture, or matrix, being, slightly tacked to the sensitive paper by two mere particles of wafer, the progress of the operation may from time to time be observed, and stopped at the moment when the picture is finished.
19. It ought then, as soon as possible, to be soaked in warm water, and fixed in the manner described in § 14.
20. In theses pictures there is a curious and beautiful variety in the tints of colour they will occasionally assume, varying from a rich golden orange to purple and black. This effect depends in a great degree upon the paper itself; but it is modified considerably by the strength of the hyposulphite, the length of the time exposed to it, by the capacity of the paper to imbibe it, and partly, perhaps, by the nature of the light. Warm sepia-coloured pictures may generally be obtained by drying the paper, by pressure, and making it imbibe the hyposulphite supplied in liberal quantity.
The paper of " I. Whatman, Turkey Mill," seems to give pictures of the finest colour, and, upon the whole, to answer best for the purpose.
If the chemical agents employed be pure, the operator, who keeps in view the intention of each separate process, and either adopting the manipulation recommended, or improving upon it from his own resources, may rely with confidence upon a satisfactory result.
This calotype paper is so exceedingly sensitive to the influence of light, that very beautiful photographic copies of lace, feathers, leaves, and such like articles, may be made by the light of a common coal-gas flame, or an Argand lamp. The mode of proceeding is precisely that described for obtaining the ordinary photographic drawings by daylight, only substituting the calotype paper, which should be damp, for the common photographic.
When exposing the prepared paper to the light, it should be held about four or five inches from the flame, and the time required will be about three minutes.
But little remains to be added to this very clear and satisfactory description of the calotype process,—to which, indeed, is mainly due the perfection to which it has arrived both at home and abroad.
There are, however, a few modifications which must be noticed, as tending to simplify the details in some cases, and to improve the general effects in others. In the main, however, it will be found that Mr. Cundell's process of manipulation is almost as good as any that can be adopted : and that gentleman certainly merits the thanks of the patentee, and of all photographic artists.
Many modifications of Mr. Talbot's mode of manipulating have been introduced with very variable advantages. I have, however, found that nearly every variety of paper requires some peculiar method to excite it to its maximum degree of sensibility. A few of the published methods may be noticed, as, under different circumstances, they may prove useful.
Mr. Robert Bingham, who has operated with such success, adopts the following process:—
Apply to the paper a solution of nitrate of silver, containing 100 grains of that salt to 1 ounce of distilled water. When nearly, but not quite dry, dip it into a solution of iodide of potassium, of the strength of 25 grains of the salt to 1 ounce of distilled water, drain it, wash it, and then allow it to dry. Now brush it over with aceto-nitrate of silver, made by dissolving 50 grains of nitrate of silver in 1 ounce of distilled water, to which is added one-sixth its volume of strong acetic acid. Dry it with bibulous paper, and it is now ready for receiving the image. When the impression has been received, it must be washed with a saturated solution of gallic acid, and exposed to a steam heat, a jet of steam from the spout of a tea-kettle, or any convenient vessel. The image will be gradually brought out, and may be fixed with hyposulphite of soda. It will be observed that in this process the solutions of nitrate of silver and of gallic acid are not mixed before application to the paper, as in Mr. Talbot's process.
Mr. Channing, of Boston, very much simplified the calotype process. He directs that the paper should be first washed over with 60 grains of crystallized nitrate of silver, dissolved in 1 ounce of distilled water, and when dry, with a solution of 10 grains of the iodide of potassium in 1 ounce of water : it is then to be washed with water, and dried between folds of blotting paper: the sensibility of the paper is correctly said to be much improved by combining a little chloride of sodium with the iodide of potassium : 5 grains of the latter salt, and rather less than this of the former, in an ounce of water, may be employed advantageously.
To use this paper of Mr. Channing's, where time is an object, it is necessary to wash it immediately before it is placed in the camera obscura, with a weak solution of nitrate of silver, to which a drop or two only of gallic acid has been added. The picture is subsequently developed by the gallo-nitrate of silver, as already described.