The manipulations just described are much facilitated by using a copying-frame or press, which consists of a square wooden frame with a thick plate-glass window; upon this are placed the negative picture and the copying paper, and the two are brought in close contact by means of a board at the back pressed by a hand-screw. (Fig. 138.) After the photogenic drawing or positive copy is taken, it is fixed by being placed in a solution of hyposulphite of soda, consisting of one fluid ounce of saturated solution to eight of water. The saturated solution of hyposulphite of soda is conveniently kept in a large bottle for use, and in order to improve the colour a very little chloride of gold is added to the fixing solution, the picture must now be thoroughly washed, dried, and pressed.
Fig. 138.
The back of the copying-frame, showing the hand-screw and pressure-board. The plate glass inside is set in the base of the frame, and is of course the part exposed to the light.
Second Experiment.
Another mode of preparing the copying paper, called albumen paper, is to take the whites of four eggs, and four ounces of distilled water containing one hundred and sixty grains of chloride of ammonium; these are beaten up with a fork or a bundle of feathers, and as the froth is produced it is skimmed off by a silver spoon into another basin, or a beaker glass, and being allowed to settle for twelve hours it is strained through fine muslin, and is ready for use. The best paper is floated on the surface of this liquid for three minutes, taken out, and dried at once on a hot plate.
In floating paper one corner is first laid down, and care taken not to enclose any air bubbles, which would prevent the fluid wetting the paper, whilst the remainder of the paper is slowly laid upon the surface of the fluid.
The albumen paper is excited by laying it for five minutes on a solution of nitrate of silver, seventy-two grains to the ounce of water, and when dry it will keep for three days. This copying paper is used in the same manner as the last, and fresh eggs only must be used in its preparation, because stale ones soon cause the copy to change and blacken all over from the liberation of sulphur, which unites with the silver. The colour of the copy is sometimes improved by a solution of hot potash, and by dipping the well-washed picture, after the use of the hyposulphite of soda, in a very dilute solution of hydrosulphuret of ammonia.
Third Experiment.
In the Daguerreotype process, a silver plate, after being thoroughly cleaned and polished, is exposed to the vapour of iodine, and is thus rendered so sensitive that it may be at once exposed in the camera. In the Talbotype process, the same principle is apparent, and paper is prepared by first covering its surface with iodide of silver, which is afterwards rendered sensitive to the action of light by means of an excess of nitrate of silver, as follows:—