This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Temperature. The temperature of the developer is of the utmost importance, as high temperature hastens the action, and lower temperature retards the action. Sixty-five degrees Fahr. should be a normal temperature, and all developing solutions, in order to be accurate, should be of the same temperature. A few degrees one way or the other, however, will do little or no harm, but one should strive to retain the normal temperature.
453. One of the chief difficulties in obtaining a factor lies in the accurate judgment by different persons of the time of appearance of the image, and not only is this true of different persons, but the same person at different times when developing exposures under varying conditions, for instance plates with strong highlights and plates with soft highlights as in misty scenes, etc., may estimate the appearance of the image as being shorter in some cases and longer in others. The only real accurate way to obtain the factor for a certain developer which you wish to employ, would be to make an exposure on some object with medium highlights, and develop it, and obtain your factor from this plate; then all other plates under any and all conditions can be developed by the same factor.
454. It is argued that with a long factor the multiplication of any error, engaging the exact time of appearance, will greatly increase the total length of development, thus producing negatives of undue density, and probably incorrect gradations, but the longer the factor the greater is the latitude, and this makes up, to a certain extent, any variation in judging the appearance of the image.
Developing Light. It is essential that you always have a liberal allowance of light in the dark room (of course non-actinic), for this is of the highest importance while watching for the first appearance of the image. If average care is used the slight variation of judging the first appearance of the image is of comparatively small importance and, in fact, is much less likely to cause variations in the results than with the old system of judging density by inspection. Some individuals are occasionally quicker or slower in noting the appearance of the highlights than others, but as a rule this variation is uniform, and may be allowed for by adopting a proportionately higher or lower factor for the same developer.
456. When you have a given factor for a certain developer, you may proceed to pour the developer on your exposed plate, keeping exact time required for the image to appear, then multiply this time by your factor, and the result will be the number of seconds required for the plate to remain in the developer. If you wish to change this time to minutes, simply divide your result by sixty (60).
Effect Of Different Developing Agents. Pyro and amidol are different from all other developing agents, when considering the factorial system. With all other developing agents the factor does not alter, to any great extent, with strength or dilution, but with pyro or amidol the factor varies with the strength in grains to the ounce in solution. The use of bromide of potassium (or its omission), also alters the factor greatly with pyro and other short factor developers, such as hydroquinone, etc., while with the longer factor developers, such as metol, etc., the bromide has but very little effect. In pyro, and other short factor developers, the addition of bromide in the proportion of one-fourth grain to each grain of pyro will cut the factor in half.
458. Variations in the amount of alkali (carbonate of soda or potash) in the developer, does not alter the factor. As a general rule the factor for different brands of plates or films, which is right for one brand of plate, or film, is also correct for another make of plate or film. It is true that some plates develop much more quickly than others, but the time of appearance, which is the key to the total time required for development, makes due allowance for this.
459. For orthochromatic, or any double coated plates, the factorial method is almost indispensable, for they, above all plates, are the most difficult to judge when they are completely developed, owing to their special coating; but as the factor is the same for all plates, all you need do is to note the time of appearance of the highlights and multiply that time by your factor, and the result is the time at which your plate is completely developed.
Table of Factors.
Amidol (2 grains per ounce).
Metol - hydroquinone
1 grain Pyro per ounce solution . . .
2 grains Pyro per ounce solution...
3 grains Pyro per ounce solution. . .
4 grains Pyro per ounce solution
Pyro with Bromide.
1/4 gr. Bromide to 1 gr. Pyro per oz. solution .
1/2 gr. Bromide to 2 grs. Pyro per oz. solution .
1 gr. Bromide to 4 grs. Pyro per oz. solution .