A plate is said to be fogged or foggy when the portions of the film which have not been exposed to light in the camera become dark or black in the developer. Every plate on very prolonged development shows some signs of fog, and quick plates are liable to become fogged more readily than slow ones, though with normal development both should work quite cleanly.
Fog is caused, then by the developer acting upon the film even when unexposed, and the action of the developer in producing fog is always accelerated by forcing, by warmth, or by unsuitableness of character. All plates would fog if left long enough in any developer, so that normal time is always to be desired.
Sometimes too much exposure to an unsafe dark-room lamp will fog the plates, or a stray streak of white light which has no business in the dark-room ; but this is " light-fog," and can only be caused by carelessness. The other kind of fog may be termed chemical fog, and this the kind which can almost always be obviated.
Now when a plate is very fast, besides being very sensitive to light, it is also sensitive to the developer. Any developer will fog if allowed time, as I said, but some developers are much worse than others, and some are much more suitable for certain brands of plates than others.
Metol and amidol are the developers most likely to produce fog, because they are the most energetic. Pyrogallic acid and hydroquiuone are "clean-working developers, because they are less energetic and take longer.
Potassium bromide, sodium sulphite, and potassium metalbisulphite are three chemicals with which we can enable the developer to work cleanly. But for every improvement we make in a developer, there is sure to be some counteracting disadvantage, and so it is that the more bromide we have the slower will development take place and the less we shall get out of our exposure. If your plate is rather under-exposed, you naturally do not want to lose any of the effect of exposure, and this is why I think you will prefer, after an experiment or two, to dilute your developer with water and give the plate plenty of time, rather than use a vigorous developer with bromide in it to prevent fogging.
A fairly dilute pyro soda developer " wants a lot of beating," especially for the beginner, but at the same time any developer which the makers of a plate recommend is sure t<5 be all right for that plate. Not necessarily for other makes, though! There is a certain class of photographers who try every make of plate on the market with the same developer, without thinking that it may be unsuitable for some brands, and they express an opinion on different makes which is neither correct nor reliable. So when you try a new brand of plate, use the developer recommended for it and give it every possible opportunity of showing its good qualities.
Prolonged development is, of course, sometimes necessary, and an impurity sometimes gets into the developer; both these circumstances may cause fogging of the clear portions of the negative. In such cases as these, try soaking the plate for a minute in four ounces of fixing solution, to which has been added a few drops of a one in five solution of potassium ferricyanide. Do not leave the plate in this too long, or it may reduce the negative itself. - 'Amateur Photographer."