A third patent has been obtained by Mr. Talbot, mainly involving the use of porcelain as a substitute for glass, and contains some useful facts noticed by Mr. Malone.
The first part of the patentee's invention consists in the use of plates of unglazed porcelain, to receive the photographic image. A plate intended for photographic purposes should be made of the finest materials employed by the manufacturers of porcelain; it should also be flat, very thin, and semi-transparent; if too thin, so that there would be a chance of breaking, it may be attached by means of cement to a piece of glass, to give it strength. The substance of the plate should be slightly porous, so as to enable it to imbibe and retain a sufficient quantity of the chemical solutions employed. To prepare the plate for use, it is first required to give it a coating of albumen, or white of eggs, laid on very evenly, and then gently dried at a fire. According as the plate is more or less porous, it requires more or less of the albuminous coating; it is best to employ a very close-grained porcelain, which requires but little white of egg. The prepared plate may be made sensitive to light in the same way in which a sheet of paper is rendered sensitive; .and we generally find the same methods applicable for photographic pictures on paper, applicable to those on porcelain plates, and one of the processes employed by the patentee is nearly the same as that patented by Mr. Talbot in 1841. The prepared plate is dipped into a solution of nitrate of silver, made by dissolving twenty-five grains of nitrate in one ounce of water: or the solution is spread over the plate uniformly with a brush; the plate is then dried, afterwards dipped into a solution of iodide of potassium, of the strength of about twenty-five grains of iodide to one ounce of water, again dried, and the surface rubbed clean and smooth with cotton. The plate is now of a pale-yellow colour, owing to the formation on its surface of iodide of silver. The plate, prepared as above directed, may be kept in this state until required, when it is to be rendered sensitive to light by washing it over with a solution of gallo-nitrate of silver, then placed in the camera; and the image obtained is to be rendered visible, and sufficiently strengthened, by another washing of the same liquid, aided by gentle warmth. The negative picture thus obtained is fixed by washing it with water, then with bromide of potassium, or, what is still better, hyposulphite of soda, and again several times in water. The plate of porcelain being semi-transparent, positive pictures can be obtained from, the above-mentioned negative ones by copying them in a copying-frame.
The picture obtained on porcelain can be altered or modified in appearance by the application of a strong heat, a process not applicable to pictures taken on paper. With respect to this part of their invention, the patentees claim :—" The obtaining by means of a camera, or copying-frame, photographic images or pictures upon slabs or plates of porcelain." The second part relates to the process which has been discovered and improved upon by Mr. Malone, who is associated with Mr. Fox Talbot in the patent. The patentees' improvement is a method of obtaining more complete fixation of photographic pictures on paper.
For this purpose, the print, after undergoing the usual fixing process, is dipped into a boiling solution of strong caustic potash, which changes the colour of the print, and usually, after a certain time, acquires something of a greenish tint, which indicates that the process is terminated.
The picture is then well washed and dried, and if the tint acquired by it is not pleasing to the eye, a slight exposure to the vapours of sulphuretted hydrogen will restore to it an agreeable brown or sepia tint. Under this treatment the picture diminishes in size, insomuch that if it were previously cut in two, and one part submitted to the potash process, and the other not, the two halves, when afterwards put together, would be found not to correspond. The advantages of this process for removing any iodine which, even after fixing with the hyposulphite, remains in the paper, is great, and it will tend much to preserve these beautiful transcripts of nature.
The patentee also claims as an improvement the use of varnished, or other transparent paper, impervious to water, as a substitute for glass, in certain circumstances, to support a film of albumen for photographic purposes. A sheet of writing-paper is brushed over with several coats of varnish on each side : it thus becomes extremely transparent. It is then brushed over on one side with albumen, or a mixture of albumen and gelatine, and dried. This film of albumen is capable of being rendered sensitive to light by exposing it to the vapour of iodine, and by following the rest of the process indicated in the preceding section of this specification. The advantages of using varnished or oil paper do not consist in any superiority of the images over those obtained upon glass, but in the greater convenience of using paper than glass in cases where a large number of pictures have to be made and carried about for considerable distances : besides this, there is a well-known kind of photographic pictures giving panoramic views of scenery, which are produced upon a curved surface by a movement of the object-glass of the camera. To the production of these images glass is hardly applicable, since it cannot be readily bent to the required curve and again straightened; but the case is met by employing talc, varnished paper, oiled paper, etc., instead of glass. It will be seen that the varnished paper acts as a support to the film of albumen or gelatine, which is the surface on which the light acts, and forms the picture. The next improvement consists in forming photographic pictures or images on the surfaces of polished steel plates. For this purpose, one part (by measure), of a saturated solution of iodide of potassium is mixed with 200 parts of albumen, and spread as evenly as possible upon the surface of a steel plate and dried by the heat of a gentle fire. The plate is then taken, and, whilst still warm, is washed over with an alcoholic solution of gallo-nitrate of silver, of moderate strength. It then becomes very sensitive, and easily receives a photographic image. If the plate be cold, the sensibility is considerably lower. The image obtained is fixed by washing with hyposulphite of soda, and finally with water.