This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Theory Of Printing. The paper is placed in a printing frame, under a negative, and exposed to daylight. At once the color begins to change, the exposed parts assuming a pink tint, then a reddish brown or purple color, and finally, if allowed to print for a sufficient length of time, the most exposed parts become extremely deep, causing a metallic lustre, or bronze appearance. The discoloration is due to the decomposition, by light, of the sensitive silver salt in the emulsion on the paper. Silver chloride is one of the chemicals forming a large portion of the constituents of the emulsion, and this substance loses its chlorine upon exposure to light and approaches nearer to the metallic silver. The bronzing of a much exposed print is due to the substance having become almost pure silver. To assist in the formation of the visible image free silver nitrate is present in the emulsion (to absorb the chlorine set free on the reducion of the silver chloride), besides citric acid and many other chemicals. Silver nitrate and citric acid are soluble, i. c, they dissolve in water, while the silver chloride and its colored products of decomposition are insoluble.
Theory Of Washing. The paper when sufficiently printed is removed from the frame and given a preliminary washing in water. This washing is necessary in order to remove, or wash out, from the emulsion the soluble compounds, so that only insoluble ones are present during toning. There are two reasons for this; first, a gold toning bath is ordinarily alkaline and, therefore, all acids (such as citric acid) must be washed out of the emulsion; second, silver nitrate will seriously interfere with the gold chloride in the toning bath, and must be removed. So important is the removal of the free silver nitrate that a salt bath is sometimes used before toning, the bath being a weak solution of ordinary kitchen salt. Salt or sodium chloride, chemically speaking, reacts readily on the silver nitrate and produces an insoluble silver chloride. The sodium nitrate which has been formed by this reaction readily washes out of the emulsion. A bath containing a pinch of salt to sixteen ounces of water will insure the destruction of all free nitrate; consequently, washing in water will remove all remaining soluble chemicals from the emulsion. Thus, all that is left in the emulsion prior to toning is a colored image, consisting of reduced silver compounds in the gelatin or collodion film.
Theory Of Toning. The print is next toned, in order to impart to it a pleasing color and to render the image permanent. The light-affected salts of silver possess the power of precipitating many metals when the salts of these metals are in solution. In more simple language this means that an image formed on a printing-out paper, if placed in a solution of say gold chloride, will cause the gold chloride to decompose and the gold to be precipitated. Theoretically speaking, it would be possible to employ any metallic salt, but in practice gold and platinum have been found to give the most lasting service.
12. The toning bath may be prepared by making up a very weak solution of gold chloride and rendering it slightly alkaline with bicarbonate of soda, or borax.
13. As a print lies in this solution, the silver forming the image in the emulsion causes the gold bath to be decomposed, and exceedingly fine particles of metallic gold are deposited on this silver image, thereby coloring it first brown, then chocolate, afterward purple, and finally blue black. Each particle of gold chloride that is thus split up gives off its chlorine, which unites with the silver of the image and forms the silver chloride. It will thus be seen that for each atom or particle of gold deposited we get at least one atom of silver converted into a soluble substance, and thus the image is robbed of that silver. This is one
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A TOKYO WATERWAY.
Study No. 3
By Wm. H. Phillips of the reasons why an overtoned print becomes weak. When the necessary amount of gold has been deposited, or when the print is sufficiently toned, it is again washed in water in order to remove the residual solution from the film.
14. As it is essential for the gold toning bath to be alkaline - that is, not acid - advantage may be taken of a neutral salt, such as common salt, to check toning abruptly, but it is far better to omit this if possible, as there is a danger of its interfering with the image.