CALOTYPE PROCESS. (Gr. icaXoe, beautiful, rvroe, an impres sion.) Under this head are included certain processe,s for producing negatives on paper, which are identical in principle, and differ only in the manipulation.
The principle of the calotype process is as follows :-1st. To pre pare and expose in the camera a sheet of PAPER rendered sensitive by being impregnated with a mixture of iodide of silver, nitrate of silver, and org,anic matter. 2nd. To develop the picture by means of gallo-nitrate of silver or some other similar developer.
The chief difference between the various processes which are classed under the term calotype, consists in the mode of rendering 9 paper sensitive, that is in the order in which the ingredients are oduced, the mode of development being nearly the same in all, merely involving a difference in the strength of the developer the mode of applying it.
A sheet of sensitive calotype paper contains a large quantity (comparatively speaking) of iodide of silver, a small quantity of free nitrate of silver, a small quantity of organic matter, and a sufficient quantity of an organic acid to keep the lights of the picture clean during the development.
It will be seen that in one of the methods about to be described the paper itself contains sufficient organic matter ; but it will be observed that in this process the paper undergoes complete immer sion in a strong bath of nitrate of silver. On the other hand, in one
of the other methods it will be shown that when the organic matter in the paper has no opportunity of coming into contact with a strong bath of nitrate of silver, it becomes necessary to add organic matter of an energetic kind to the exciting solution. Bearing these facts in mind, the principle of the various calotype processes will be clearly understood.
The calotype processes may be classed under two heads. The first class comprises two processes in which the paper is first coated with insensitive iodide of silver, and then excited by a weak solution of acidified gallo -nitrate of silver; the second class, those processes in which the paper is first coated with an alkaline iodide, and then excited by immersion in a strong bath of nitrate of silver, the excess of free nitrate being subsequently removed by washing. Among the latter processes, one will be selected for minute description, which appears to be, of all the calotype processes, the simplest in manipula tion and most scientific in principle. In the processes of Class I., the paper is iodized with iodide of silver ; in those of Class II., with a soluble alkaline iodide. The first kind of iodized paper may, for convenience, be called " argento-iodized " paper, the second kind simply " iodized " paper.