283. Loading Plate Holders. Plate holders or dark slides should always be loaded in weak, non-actinic light, and preferably in absolute darkness. 2 It is desirable to practise filling the plate holders first in full daylight, using waste negatives for this purpose, and then, having become familiar with the operation, to get accustomed to working in complete darkness. 3 Sensitive materials should always be held by the edges. At the most the fingers should only come into contact with either the face or back of a plate or film to the extent of about three sixteenths of an inch from the edge in large sizes. Wherever the fingers touch the emulsion, even when they seem quite clean and dry, a minute quantity of greasy matter is deposited on the gelatine. This prevents the developer from penetrating the film, and produces a light finger mark on a dark ground in the developed Similar marks are sometimes found on a plate or film which has been packed with its sensitive surface in contact with the back of a plate on which there are finger-marks. For this reason, when plates are re-packed after exposure, they should always be placed face to face.
In order to use stiff cut-film in dark-slides for plates it is possible, in small and medium sizes, to slip the film into a film holder securing it on three sides. This arrangement does not ensure the necessary flatness with films of large and very large sizes, and these are held with their back in contact with a rigid support (glass or sheet metal) coated with an adhesive varnish containing vinyl resins (M. Hagedorn and A. Jung, 1931) or rubber ; if required, the film can be left on this support during development and further processing. The use of a blackened support can, in some measure, act as an anti falat ion protective.
Except in special cases, the sensitive surface of a plate or film should always be turned towards the outside when filling a dark slide. Autochrome and similar plates are, of course, exceptions, as are also plates intentionally reversed in the plate-holder for the purpose of obtaining reversed negatives. In such cases it is necessary to adjust the camera extension when focussing to allow for the thickness of the glass.
Although plates are always packed in a uniform manner, uncertainty may occasionally arise with regard to which is the sensitive side. In order to ascertain this, a corner may be touched cautiously, care being taken to touch the surface as near the edge as possible. The glass is smoother and feels colder than the emulsion. If still in doubt, moisten the thumb and first finger slightly and hold the corner of the plate between them. The sensitive emulsion
will stick to the finger the glass will not Since cut films are coated with gelatine on the rear side, these methods for recognizing the emulsion side cannot be adopted with them. In order to obtain the correct position, both during loading and dish development, a guiding mark is cut in one of the edges, so that when the film is held in such a position that the V-shaped cut is on the right at the top, the emulsion side of the film is then towards the operator.
Before loading is begun, the plate holders, which retain dust very easily, should be carefully cleaned.
The different types of plate holders and changing boxes vary considerably in the method of loading, so that in each case the instructions of the maker or the dealer should be consulted. 2 If the sheaths are fitted into the changing box in the wrong manner, they may jam or be damaged. The direction in which they are inserted, i.e. the position of the groove of the sheath relatively to the handle of the drawer, varies with different changing boxes. 3 Often, when fitting a plate with sharp edges into a plate holder or metal dark slide, small pieces of glass are broken off. Loading should therefore never be carried out over the open box, or over plate holders which have already been filled. As the plate holders are loaded, place them with their faces towards the wall to protect them from light and dust.
284. Dusting the Sensitive Surfaces. With very few exceptions, the emulsion surface of plates and films is perfectly clean when pur chased, for every precaution has been taken to this end in the factory, where the struggle against dust is often carried to greater extremes than in many surgical operating theatres, since only filtered air is allowed to enter the factory.
After development, however, a number of white spots (pinholes) are often found on the negative, each of which marks the shadow of a grain of dust which was present before exposure. In professional work these markings have to be carefully spotted out.
These dust particles are more frequently found on very fast plates, the surface of which is more matt than that of the slow emulsions and there fore much more liable to retain any dust which comes into contact with it.
The doubtful cleanliness of some dark-rooms, the use of fluffy paper for packing plates, chips of varnish, the fragments of glass produced in the loading of the dark slide or plate-holder, and the air eddies produced in a changing box by each operation of the drawer are sufficient to explain the presence of these dust particles, which are rarely found on roll films.