In order to effectually remove the yellow surface (which is the unacted-upon sensitive salts and hence upon their removal the permanence of the print depends) three successive applications of the acid bath should be resorted to, the prints remaining for 5 to 10 minutes in each, and then finally washed in running water for a quarter-of-an-hour, dried between blotting paper or in any other manner preferred, and the platinotype print is finished and ready for mounting.
It should be seen from the foregoing general outline of the process that for directness, simplicity, and for the short time in which a finished print may be produced that platinotype stands alone amongst printing methods.
There are, however, some points needing careful consideration at each stage of the print's production, and to these we may now pay attention.
PRACTICAL CONSIDERATIONS AS REGARDS EXPOSURE.
As has already been stated right exposure constitutes the crux of the whole process; this once mastered the rest of the performance—development, clearing in acid and washing—is so simple that the chance of failure is remote.
Hence the greater need of paying especial attention to the question of exposure or printing.
Obviously, the duration of time of exposure cannot be fixed, not even to the extent it can be in bromide printing or any other method of printing with artificial light which may be a definite and permanent quantity.
The variable quality of the daylight and the density of the negative are both fluctuating factors in the calculation and hence some means may advisedly be resorted to for acquiring a sort of exposure index suitable for each individual negative and every variety of light.
First let it be noted that even with very great over-exposure the image will not become wholly visible, whilst to the inexperienced eye but little change takes place in the appearance of the printed image after the correct exposure has been reached.
If then the print has been over-exposed, the fact is not made evident until the print is subjected to the influence of the oxalate developing bath.