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.
240. Most photographers have learned from experience that it is very essential to have sufficient time on all exposures. There are cases when in doubt as to the exact exposure necessary, and in order to be positive of the sufficient time, they expose a trifle longer than in their judgment they consider necessary, thus over-timing the plate.
241. Such a plate, if developed in normal developer to a finish, would be very dense, but flat. The prints from such plates would be anything but pleasing. With the proper manipulation, however, such plates can be made to yield very good prints. In fact, you can manipulate so as to overcome any reasonable amount of over-exposure, and thus save the negative.
242. Extreme over-exposure is not encouraged by any means; in fact, one should aim at the correct exposure at all times, for you cannot always produce as fine a quality plate by over-exposure as you can by correct exposure, no matter how much you doctor the plate. There are times, however, when plates are over - timed accidently, and these plates must be saved, and the very best possible results secured. To teach you how to do this is the purpose of this Instruction.
243. Over-exposing is more apt to occur in commercial photography than in portrait work. For instance, when making interiors, photographing machinery, furniture, stoves, draperies, etc., such work is usually photographed at the factory, and a large number of plates are exposed before returning to the dark room to develop. As negatives made of all such objects must be fully timed you are apt to overtime. All negatives made under the same conditions are given practically the same exposure, therefore, if one plate is overexposed all are. By developing one plate first you have a key to the rest, and can treat them accordingly.
244. Any reasonable amount of over-exposing can be overcome in the developing, provided you are aware that the plate is over-timed, and know pretty nearly how much, for then you can reduce the sensitometer back to that of a slower plate.
245. Note illustrations, Nos. 7 and 8. In No. 7 we have a plate with a two-second exposure, fully timed. No. 8 was given thirty seconds, or fourteen times more exposure than was necessary. In order to determine exactly how much the plate was over-timed, and to know how much the plate must be restrained, we developed plate No. 7 first. Finding this plate fully timed, we prepared to reduce the sensitometer of plate No. 8 before developing it, by placing in a bromide solution composed of: Water eight ounces, and ten per cent, bromide solution, two drams. We immersed the plate in this bath for four minutes, after which it was transferred to a normal developer with a few drops of bromide added. The results were as you see them in plate No. 8.
246. In all cases of extreme over-exposure, if you are aware that the plates are over-exposed, apply the above method. The longer the exposure, the more you must restrain the plate by immersing in a strong bromide bath.
247. When developing plates of unknown exposure, believing them to be over-exposed, always start either in old developer, or part old and part fresh. Or, if you have no old developer on hand, use normal developer with a few drops ten per cent, solution of bromide added. Place the plate in your normal developer, having a second tray containing a bromide bath made up as follows : To one-half-pint of water add two drams of a ten per cent, solution of bromide of potassium. As soon as the image appears on the plate, examine it, and if you find it flashing up quite quickly, as soon as the shadows are developed, immediately place the plate in your second tray, containing bromide bath, and allow it to remain there for about four minutes, covering the tray to protect it from the light. The bromide at once acts upon the shadows, and restrains them from developing farther. After a few minutes, transfer the plate from the bromide solution to the normal developer.
Illustration No. 7 A Correct Exposure See Paragraph No. 245.
Illustration No. 8. Over - Exposure Corrected by Proper Development See Paragraph No. 245.
248. You may find it necessary to make up a fresh normal developer to complete the developing. The strength of the bromide checking bath must be determined by the appearance of the plate when the image first appears. If the plate is only slightly over-timed, then dilute the bromide bath by adding double the amount of water.
249. Care must be taken when immersing the plate in bromide bath to be sure that the shadows are fully developed, for should you restrain the plate before all detail is developed, then your shadows will lack strength.
250. In cases of very slight over-exposure, we advise carrying the plate a little farther than usual in the normal developer, and then reduce the plate after fixing, with red prussiate of potash. (See Chapter X (Varying Water Conditions. Their Effects Upon The Manipulation Of Sensitized Papers), Reducing.) This solution will clear up the plate very nicely.
251. Caution: - If you had restrained the plate before securing full detail in shadows, when concluding the developing in normal developer, you would have found the shadows were developing very little, owing to the fact that the film was saturated with bromide. In order to secure further detail in the shadows you would have to soak the plate in plain water to eliminate bromide, and make a new developer weak in pyro, containing no bromide. That would give the detail producing chemical (carbonate of soda) a better opportunity to penetrate the film, and open the pores. This method will require the developing of the entire plate farther than ordinarily. Finally reduce the whole plate to the proper density with red prussiate.
252. A few points must be remembered when developing plates that are over-timed. First, - it is the shadows that are over-timed and, therefore, they must be treated, and not the highlights. Second, - you must judge as near as possible by the first appearance of the image on the plate how much it is over-timed, and then restrain the shadows accordingly. Sometimes to simply immerse the plate in a weak bromide solution for only an instant will sufficiently restrain the shadows to supply the desired results.
253. Should the highlights alone appear too strong after the plate is developed and fixed, apply the persulphate of ammonia reducer (See paragraph 274, Instruction Reducing), as it acts on the highlights only. Immerse the plate in this bath, and when the desired reduction is obtained place the plate in the sulphite of soda bath, and finally wash and dry.
254. Should you know in advance that the plate is very much over-timed, then reduce the sensitometer by immersing the plate in a bromide solution before it is developed.