After the paper has been perfectly coated, or washed with the silvering solution, it should be placed in a perpendicular position to dry. I usually tack the paper on a board of the requisite size, and then stand it on one edge until it has drained and dried. As soon as dry, it is ready for use. This paper will not keep more than twelve hours, therefore the operator should silver in the morning the quantity required for the day. It is imperatively necessary that the silvered paper be kept in the dark. It is extremely sensitive to light, and a very brief exposure of the prepared sheet would render it unfit for use.
Printing the Positive.
The several kinds of apparatus used for holding the negative and the sensitive paper together, have already been given on [page 36], Figs. [31], [32], [33]. The paper having been salted and silvered, as just described, should be placed on the pad of the printing frame or glasses, with its sensitive surface up, and then the negative placed directly upon and in contact with it; then it is to be fastened together, when it will be ready for exposure to the direct rays of the sun. From 10 to 40 seconds will be found enough to give a sufficiently intense print.
The paper first changes to a slate color, and then to a brown or copper color t when of a dark slate color is about the proper time to take it out and immerse in the toning bath.
Fixing and Coloring Bath.
I have employed the proportions given by Mr. Hardwich in his Photographic Chemistry, page 209—Humphrey's American edition.
| Solution of chloride of gold, a quantity equivalent to 4 grains. | ||
| Nitrate of silver | 30 | " |
| Hyposulphite of soda | 2 | ounces. |
| Water | 8 | " |
"Dissolve the hyposulphite of soda in four ounces of the water, the chloride of gold in three ounces, the nitrate of silver in the remaining ounce; then pour the diluted chloride by degrees into the hyposulphite, stirring meanwhile with a glass rod; and afterwards the nitrate of silver in the same way. This order of mixing the solution is to be strictly observed; if it were reversed, the hyposulphite of soda being added to the chloride of gold, the result would be the reduction of metallic gold. The difference depends upon the fact that the hyposulphite of gold which is formed is an exceedingly unstable substance, and cannot exist in contact with unaltered chloride of gold. It is necessary that it should be dissolved by hyposulphite of soda immediately on its formation, and so rendered more permanent by conversion into a double salt of soda and gold.
"The time of coloration depends much upon the quantity of gold present, and may in some cases be extended to many hours. The results of a few experiments, performed roughly, appeared to indicate that the activity of this bath is less affected by depression of temperature than those prepared with tetrathionate. Certainly the injurious effects of prolonged immersion are not so evident as with the first two formulæ: the purity of the whites remains unaltered for many hours if the bath is new, but with an old bath there is a tendency to yellowness, which is probably caused by the presence of sulphuretted hydrogen. Fresh chloride of gold must be added from time to time, as it appears to be required."
After the impression has remained in the toning bath a sufficient length of time, it should be placed in a dish or sink of clean water, which should be changed several times—floating for at least 12 hours; then it may be taken out and hung up to dry.