The object of re-floating the print is thus explained. When a print is first raised from the developer, the liquid adhering to its surface contains only a small quantity of platinum salt (the developer being weak in this ingredient), and the amount of salt so taken up is usually insufficient to supply the necessary quantity of platinum pigment to the shadows and darker parts of the print; by re-floating, a fresh supply of this pigment-forming liquid is supplied, and the number of floatings required is determined by the strength of the light impression.
After the print has been twice floated it should be held in the hand, face upwards, and the process of development carefully watched. When the half-tones have sufficiently appeared, and have become free from the granulation usually visible in the first stages of development, and the shadows also are sufficiently strong, the print should be at once immersed in the acid clearing bath. In some cases it takes a full minute to complete a development, but the moment selected for arresting it is to be decided by the taste and judgment of the operator. During the progress of the development it may sometimes be noticed that the shadows are slightly rusty in color, and appear to hang back. This indicates the necessity of another flotation on the developer. Or, instead of re-floating, some of the solution may be applied to the shadows by means of a large camel-hair pencil.
A useful development, and one securing economy to small workers, is by means of a broad camel-hair brush, but it requires a little practice. The brush must be well wetted and the strokes given with fair rapidity. It is usually better to begin at the edge of a print and to let each suc ceeding stroke overlap the previous one, then, as soon as the print is covered, repeat the operation with strokes at right angles to the first series so as to render the coating as even as possible. The brush should be dipped in the developer before each stroke, or, at any rate, before every second one. And before beginning another print the brush should be washed in the developer in order to detach any salt which it may have derived from the previous print.
In using the floating method, air bells are sometimes formed on the surface of the print, but this only rarely happens when the surface is developed in its moist state. If any should appear on the print, after its first floating, they are best removed by again laying the print on the solution and then smartly sliding the print over the surface of the liquid.
A good method of floating is to lay one end or edge of the print upon the solution near the right-hand end of the dish; then, with a sliding motion towards the left, lower the print with an even movement, until it is entirely in contact with the liquid.
In order to avoid scum-marks on the prints it is very important to rock the developing dish between the development of each print. The rocking should be so managed that the devel oper is driven, in a wave, against the side of the dish, so that the surface scum may be broken up and sunk in the liquid.
Watzeck's Platinotype Process.—Paper is coated with Arrowroot i gramme Cold saturated solution of sodium oxalate 5o c.c.
The solution is boiled until all sediment is dissolved. It is then applied to the paper two or more times, according to the porous nature of the paper.
For black tones the paper is sensitized in Sat. sol. potassium chloroplatinite 5 c.c.
Sat. sol. double oxalate of soda and iron 8 c.c.
Sat. sol. potassium chlorate 3 drops For sepia tones the following proportions are taken: Sat. sol. potassium chloroplatinite 5 c.c.
Sat. sol. double oxalate of soda and iron 4 c.c.
Sat. sol. neutral oxalate of soda ............. 3 c.c.
Sat. sol. mercuric chloride i c.c.
Sat. sol. potash chlorate 3 drops More chlorate increases the contrasts and a smaller quantity of mercury gives darker tones. The solution of double oxalate of iron and soda is sensitive to light. The best results are ob tained by drying the paper at a temperature of 35 deg. C.
Pizzighelli's Printing-out Platinotype Process.—A remarkable advance in platinum printing processes, introduced by Captain Pizzighelli, by which means the print is directly produced upon the platinum paper in the printing frame. The principles upon which the process is based are the following: (i.) By adding thickening materials to the sensitizing solution, the latter is pre vented from penetrating the substance of the paper. (2.) If one of the substances used as a " developer " is added directly to the sensitizing solution, a reduction of the platinum salt takes place in the printing frame itself under the influence of the moisture of the air. The advantages of this process will be at once apparent. The previous preparation of the paper is dispensed with, and the progress of the printing can be watched, and, further, the developing process is also done away with.