[19] Read the observations on the Explosive Properties of Fulminating Gold in the Vocabulary, Part III.
The Author explains the action of these Salts of Gold upon the Positive print as follows:—they are unstable, and contain an excess of Sulphur loosely combined; hence, when placed in contact with the image, which has an affinity for Sulphur, the existing compound is broken up, and Sulphuret of Silver, Sulphuric Acid, and metallic Gold are the results. That a minute proportion of Sulphuret of Silver is formed seems certain; but the change must be superficial, as the stability of the print is very little lessened when the process is properly performed.
Sel Or employed as a toning agent.—This process, which was communicated to the 'Photographic Journal' by Mr. Sutton of Jersey, has been found serviceable.
The prints are first washed in water, to which is added a little Chloride of Sodium, to decompose the free Nitrate of Silver. They are then immersed in a dilute solution of "Sel d'or," or double Hyposulphite of Gold and Soda, which quickly changes the tint from red to purple without destroying any of the details or lighter shades. Lastly, the Hyposulphite of Soda is employed to fix the print in the usual way.
This process differs theoretically from the last in some important particulars. The toning solution is applied to the print before fixing, which experience proves to have an important influence upon the result, it having been found that when the print is previously acted upon by Hyposulphite of Soda, the rapidity of deposition of the Gold is interfered with;—thus, a dilute solution of Sel d'or colours a print rapidly, but if to this same liquid a few crystals of Hyposulphite of Soda be added, the picture becomes red and may be kept in the Bath for comparatively a long time without acquiring the purple tones.
As Hyposulphite of Soda in excess lessens the action of the Sel d'or, so on the other hand the addition of an acid increases it. The acid does not precipitate Sulphur, as might be expected from a knowledge of the reaction of Hyposulphite with acid bodies ([p. 137]), but it favours the reduction of metallic Gold. Hence it is usual to add a little Hydrochloric Acid to the toning solution of Sel d'or, to increase the rapidity and perfection of the colouring process.
THE CONDITIONS WHICH AFFECT THE ACTION OF THE FIXING AND TONING BATH OF GOLD AND HYPOSULPHITE OF SODA.
Although the process of toning Positives by Sel d'or is very certain in its results and gives good tints, yet, as involving a somewhat greater expenditure of time and trouble, it is not at present universally adopted. The ordinary plan of fixing and toning in one bath has been proved to yield permanent prints if the proper precautions are observed, but it is quite necessary, in order to ensure success, that the conditions by which its action is modified should be understood. The more important of these are as follows:—
a. The AGE of the Bath.—When Chloride of Gold is added to Hyposulphite of Soda, several unstable salts are produced, which decompose by keeping. Hence the solution is very active during the first few days after mixing; but at the expiration of some weeks or months, if not used, it becomes almost inert, a reddish deposit of Gold first forming, and eventually a mixture of black Sulphuret of Silver and Sulphur, the former of which often adheres to the sides of the bottle in dense shining laminæ.