The annoying small transparent spots and microscopic holes, often found in the negative, are generally to be ascribed to dust on the plate at the time of exposure. Plates should never be kept for more than a day or two in the dark slide; somehow or other they will always accumulate dust there, and may possible absorb moisture from the air. The practice of wetting the plate with water before developing is another cause of these holes. Sometimes, but very rarely, they are due to defects in the gelatine. If the pinholes are very minute we must make up our minds philosophically to ignore them. Larger holes may be filled up with a mixture of India ink and gum water, with two drops of formaline. A very finely pointed brush must be used and the colour, almost dry, toned down to match the surroundings. The operation requires skill and exactness, and the colour tone must not be too dark or it will appear as white spots on the print.
Irregular rounded patches of lighter tint, frequently bounded by a fine dark line, are caused by the fact that the developer did not flow over the whole plate at once, and action commenced much later on these portions. Strangely enough, however long development may be protracted, these portions never "catch up" to the rest. Patches of varying density are due, as a rule, to parts of the negative drying before the rest, and at a different temperature. For instance, when half the film was dry, the plate may have been held near a fire to finish it off quickly.
If the whole plate is masked by a grey fog and the margin covered by the rebate of a dark slide remains clear, we know that the fault is due to a leaky camera. A round halo in the very middle shows that the automatic shutter is slightly out of gear and lets in light while the camera closes. If the fog covers the plate, rebate edges included, either the developer contained an excess of alkali, or the dark-room light is defective. Streaks of light from the angles towards the middle indicate that the joints of the dark slide are becoming loose, or that the slide does not fit close enough in the camera. Green fog is nowadays a rarity except with very stale plates, and will generally disappear in the clearing bath. Black spots of various sizes, some of them with curved tails, are caused by particles of developer insufficiently dissolved.
Powdery substance on the face of the film is due to impure chemicals giving a precipitate insoluble in water. Often this consists of a salt of lime from hard water used in washing the plate.
Very frequently our negative, although otherwise satisfactory, is lacking in density or contrast, and is so thin and weak that we cannot get good prints from it. It is possible in that case to deposit on the existing image a new one of chromium, copper, or silver. Before treatment the negative must be quite free from all traces of hypo (unless the chromium method is adopted) and if there is any surface fog it must first be removed by reduction (q.v.).
The old-fashioned and still widely practised mode of intensification by mercury is not one which is desirable. Apart from the deadly poisonous nature of mercury bichloride, the action of this substance on gelatine, the tendency of the negative to fade, and numerous other incidental uncertainties, all cry out against its adoption, now that safe and more efficient substitutes are available. In the mercury process the negative is first bleached white in a solution consisting of:
Mercury Bichloride......1/4 oz.
Hydrochloric Acid......10 minims.
The bleaching generally being fully accomplished within ten minutes. The plate is then washed in three or four changes of water and either redeveloped in any clean-working developer, such as hydroquinone or rodinal; or simply blackened by immersion in a bath of water 4 oz., ammonia 1 dram.
A. Potassium Bromide..... 20 gr.
Bichloride of Mercury..... 20 „
Water........ 2 oz.
B. Cyanide of Potassium (sticks) ... 20 gr.
Nitrate of Silver...... 20 „
Water......... 2 oz.
The silver and cyanide must be dissolved separately, each in an ounce of water, and then the cyanide poured in until the precipitate is not quite redissolved. Bleach the plate in solution A, and then, after washing, transfer to B. The solutions may be used over and over again, and, if intensification has been carried too far, on immersion in the hypo bath the plate will revert to its original state.
This intensifier is nearly as effective as chloride of mercury, and is attended by fewer inconveniences.
Dissolve separately, each in 2 ounces of hot water,
Potassium Bromide . . . . . . 1/4 oz.
Copper Sulphate....... 1/2 „ and then mix and if necessary filter, to get rid of the white precipitate. The resulting solution may be used to bleach the negative, which may then either be blackened with nitrate of silver or redeveloped. Only very careful rinsing is correct with a negative bleached with copper bromide, on account of the white deposit being not quite impregnable.
This is the simplest, and at the same time the most reliable of intensifiers, and we have used it exclusively since the introduction of Mr. C. Welborne Piper's formula, five or six years ago. The presence of hypo in the film need cause no anxiety; just a rinse after fixing is enough; and the operation may be repeated several times if the first application does not give enough density. Three grades may be used according to circumstances: A gives moderate density, B considerable, and C great intensification.