THE CHIEF FAILURES IN NEGATIVE-MAKING 430. Preliminary Note. The enumeration of the faults which may occur during the various phases of the process of negative-making may be sufficient to discourage the novice in photo graphy ; it has, however, no other purpose than to allow the cause of a failure to be discovered. It must be admitted that any list of possible defects is never complete, unexpected failures sometimes occurring which cannot be traced to their causes.
When a beginner (and sometimes even an experienced photographer) meets with a failure he immediately blames the camera, the plates, the chemicals and their respective purveyors ; almost always he forgets to ask himself what blunder he has committed.
Nobody is infallible, and, in spite of strict control, a manufacturer (of whom one cannot ask that he should test all his plates and films before issuing them for sale) will send out— but very seldom—a plate or film showing some slight defect. Long experience shows, however, that the great majority of failures are due to faulty working of which the photographer is often unconscious, and which he will not hesitate in good faith to deny.
If, after careful investigation, a fault appears to be due to manufacture, and is repeated on several plates from the same box or from the same emulsion, replacement may be demanded from the manufacturer. This will always be courteously furnished if it be courteously asked. In addition to the faulty negatives, some other plates (exposed or otherwise) which have not been developed should be returned, packed in their original wrappings. In order to identify plates sent back for testing, some distinctive mark should be made on them, so that the sender may have no doubts when, as frequently happens, faultless negatives are returned to him.
431. Faults Appearing During Development.' The Image Does Not Develop. Anti-halation plate exposed through the back; plate not ex posed ; absence of one of the essential ingredients from the developing solution. Put the plate aside for the time being in pure water, and then try to develop it later in another bath.'
The Emulsion Darkens Before any Image Appears. The sensitive surface has been ex posed to light outside the camera.
The Image Appears Almost Lost by Uniform Fog. If those parts of the sensitive emulsion which were protected by the rebates, etc., of the dark slide are not fogged, the trouble is probably due to excessive exposure ; to sunlight on the lens during exposure (absence of lens hood) ; or to the use of a lens of which some parts are dirty or covered with mist.
If the fog also covers the protected parts of the surface, several causes may be suspected : long storage of the plate or film under unfavour able conditions ; wrongly-mixed developer (ex cess of alkali, insufficient bromide), or developer contaminated with hyposulphite or metallic salts from the materials of which dishes and accessories are made, or with sulphide formed in old developing solutions which have been kept for a long time. In either of these cases of fogging the cause may be (§ 200) the action of certain materials used in the construction of the dark slide (resinous wood, varnish, drying oils ; zinc, aluminium, and their alloys), or of paper used for re-packing the plates or films between exposure and development ; any printed matter on this paper may then leave an image more or less plainly visible than the image proper. Except in the latter case, prolong development as for a normal negative, adding a fairly large amount of bromide. In spite of being very dense, the negative will yield passable prints.
Fog Appears After the Image is Distinctly Visible. Development is being done too close to a dark-room lamp which is fitted with an unsuitable filter'; or a very little diffused light is penetrating to the inside of the dark-room. In these cases the sensitive surface in the shadow cast by the sides of the dish remains clear of fog. In the case of a developer contain ing hydroquinone, the fog may be due to oxida tion of the developer as a result of too prolonged withdrawals of the negative for examination (§ 339). These failures may be avoided by desensitizing.