The advance of photography : its history and modern applications (1911)

30 THE ADVANCE OF PHOTOGRAPHY A piece of paper coated with white of egg and moistened with a solution of common salt is laid in a dish on a solution of nitrate of silver, and allowed to remain for about a minute. The solution is absorbed by the floating paper, and chloride of silver is formed by double decomposition of the nitrate of silver with the common salt. The wet paper is but slightly sensitive ; it becomes fully sensitive only after being dried. The dry paper, saturated with chloride and nitrate of silver, is then pressed upon the negative in the printing frame (fig. 8). Then the whole is exposed to the light. The same process ensues which we have described in the part dealing with lichtpaus paper ; the light shines through the clear places of the negative and colours the paper tying under them dark, the paper under the dark places of the negative remains white, while it assumes a slight colour under the half-tones. In this manner a faithful positive copy of the negative is produced, presenting a beautiful violet-brown tint. We know from the description of the lichtpaus process that this print would not remain unaltered in the light for long, because the paper is still sensitive to light. The salts of silver contained in it must be removed if the impression is to be made lasting. To this end a solution of hypo-sulphite of soda is employed. If the prints be immersed in this solution, they become durable in the light ; but, unfortunately, they suffer a peculiar change of colour, assuming an ugly brown tint. This tint is of no consequence in technical and scientific pictures, but detracts greatly from portraits and landscapes ; and in order to give these a more agreeable tint, before fixing