DRYING MARKS These in negatives are most frequently patches or portions which are stronger or weaker than the remainder of the image, such portions pro ducing corresponding defects in the print. These markings may be due to defective working, or to causes beyond the worker's control. With regard to the first, a plate may be drying very slowly, and when partly dry the conditions of drying may be changed to accelerate the drying of those parts that still remain wet. The part accelerated will almost always show greater density than the remainder, and at times a well-defined mark may separate the two parts.
The second cause is uneven drying of the plate in the course of manufacture. When plates are racked for drying, no matter how perfect the ventilation of the drying-room may be, the edges, where the emulsion always tends to thin ness of coating, will always dry more quickly than the centre, the result being that a line appears round the edges. The exact cause of
this is a little obscure, but the most satisfactory explanation is that given by Homolka, who ascribes the foggy line—for this is what edge drying marks actually are—to a diffusion of the faint traces of soluble haloid left in the emulsion from the thin dry edge to the thicker and moister centre. Actual drying marks from uneven dry ing of plates during the manufacture are now rarely met with, though in the early days of dry plate photography they were of frequent occur rence, and manifested themselves in precisely the same manner as in the unequal drying of nega tives—that is, by central patches of greater or less density due to this portion being more or less sensitive than the margins.