Dr. Meniere advises the following treatment for removal of green, red, yellow, or dichroic fog. Soak the negative in plain water for five minutes and then remove it.
Water roo c. c.
Bromide of sodium 3 grains.
Bromine water 3 c. c.
Let it rest in this for ten or fifteen minutes. The bleached image is then well washed and the image redeveloped with an amidol sulphite developer.
Red Fog occurs when the silver nitrate is in excess of the salts with which it should com bine in the emulsion. A careful study of the tables given on pages r8r and 182 will prevent this. The only remedy for this kind of fog is potassium cyanide, but its action is uncertain, and it is highly dangerous to use.
General Fog is produced in development, and is caused by the partial reduction of the sil ver salt all over the film. This may also be due to the decomposition of the gelatine by prolonged heating. The products of the decomposed gelatine in the presence of the developer have a ten deny to react on the silver salt, producing partial reduction and general fog. It has been noted that in stormy weather, when the air is charged with electricity, that this defect is more easily produced. The remedy is to add a few drops of a carbolic acid solution to the gelatine solution during the cooking process. This will usually prevent decomposition.
Light Fog is caused by the action of light upon the whole or a portion of the film. The only actinic light that should reach the plate before fixing should be that light which enters through the lens and forms the image. Light fog may be caused by the light of the room in which the plates are made (or placed in the plate-holders) being too actinic. It very often happens, however, that the camera or plate-holder is at fault. If there is any defect in the camera produc ing fog the edges of the plate which are covered by the rabbet of the slide will remain clear.
To examine the camera take it out into strong sunlight, place the cap on the lens, remove the focusing screen, and, with the black cloth covered over the head, make a thorough examination of the inside of the camera (racked out to its full extent), and see if there are any holes or other places where light is admitted. If the dark side be at fault the fogging will usually be in streaks.
The inside of the camera and slides should be painted a dull black, as fog is easily produced by reflecting surfaces.
Fogged plates will be produced if white light be not excluded from the dark room, or if the light employed be too actinic.
A peculiar kind of fogging sometimes makes its appearance round the edges of plates, and is due to some impurities in the strips of paper placed at the edge of the plates to separate them.
In the collodion process the principal causes of fog are—the alkalinity of the silver sensi tizing bath, or the neutrality of the same if a bromo-iodized collodion be employed. Other causes may be in over-acidity of the bath, diffused light in the camera, darkroom, etc.
FORMALIN.—Formalin is the trade name given by the manufacturers to a saturated aqueous solution of formic aldehyd (CH the latter being a gaseous body prepared by subjecting methyl-alcohol to oxidization. The saturated solution contains about 4o per cent. of the aldehyd. It mixes with water in all proportions. If it be not kept properly corked, formalin gas evaporates. The evaporation is accelerated by heat, and the formalin in the form of a vapor can thus be utilized.
It appears that formalin is largely used as a disinfectant and as a deodorizing agent, it possessing an extraordinary acti ve microbicide power similar to that of corrosive sublimate, but is comparatively non-poisonous.
The chief use that it is apparently destined to be put to by photographers is, however, due to its hardening effect upon gelatine. This, according to our experiments, appears to be far greater than and superior to alum or sulphite. We treated some plates to a one per cent. solution of formalin for about five minutes and under this treatment they become so hard that they were uninfluenced by boiling water. Some gelatine prints were also subjected to its influ ence. A very strong solution slightly changed their color, but apparently no advantageous effect is produced by using a concentrated solution. In a mixture of one per cent. of formalin in water no change in color was produced, and the prints became so hard that they would stand almost any amount of handling and the film could not be dissolved.