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.
Proper Light For Developing. While gaslight papers are not nearly as sensitive to actinic or white light as dry plates, still it is necessary that a certain amount of precaution be exercised. A very strong yellow light or a dim lamp light is perfectly safe, providing the direct rays from the light do not strike the sensitive paper. It is not necessary to develop by a dark room (ruby) lamp. The printing frame may be loaded and the development carried on in the same light.
Disposition Of Outfit, Management Of Light, Etc. To insure perfect results the place where you intend working should be far enough from the source of light to prevent the possibility of the paper becoming fogged. Six to eight feet from an ordinary lamp or gas jet is far enough, or if the space is limited work nearer to the source of light, interposing a piece of cardboard between the light and the place where you are developing and loading the plate holder. The safety of your position can easily be determined by taking a small strip of the paper, covering a part of it and exposing the remainder for about 30 seconds in the light in which you intend working. If upon development there is a difference between the exposed and the unexposed portions of the strip, the light is too strong, and you should either move farther away or shade your light more.
Printing Light. For exposure, artificial light is preferable to daylight, because it varies less in strength. When you have obtained the proper distance and time of exposure on a test print from a given negative, it is easier to make the remaining prints from the same negative of uniform quality.
Length Of Exposure. If a kerosene lamp is used for printing we would advise the placing of a piece of camphor about the size of a walnut in the oil. This will have a tendency to make your light of a whiter color and naturally its strength will be greatly increased. With a kerosene lamp you can place your negative, while printing, quite close to the lamp, but keep it revolving and moving so that the light will be distributed evenly over the entire negative. If there are parts of the negative that print slower expose longer on these parts, holding them closer to the light. As a general thing the frame placed about 10 or 12 inches from the light gives the best result.
685. The length of exposure is naturally governed by the strength of negative and the kind of light employed. If a 32 candle-power incandescent light is used with a fairly good negative, from 6 to 8 seconds exposure will be plenty if the negative is held about 10 inches from the light. A thin or soft negative would probably print in 4 seconds. The same negative held 20 inches from the light would probably take 16 seconds. One advantage of holding the negative at least 10 inches from the light is that the light is more evenly distributed. If held very close to the light there is danger of printing one part of the negative more than the other, thereby producing an uneven print. The length of exposure during printing depends entirely on the density of the negative and the strength of the light employed.
686. To find the proper distance from the light at which the negative should be placed during printing, take a piece of white cardboard, actual size of negative to be printed from, and move to and from the light until you find the shortest distance at which the entire card is evenly illuminated. You will find that the regular papers will need from four to five times more exposure than the specials, if the negative is placed at the same distance from the light.
687. The following table will give you a slight idea of exposure, but only practice and close observance of your results can teach you the exact exposure: With special papers, and size of negative 4x5, or smaller, held 7 inches from the light of a kerosene lamp, it will require about 30 seconds; Welsbach Burner, 10 seconds; 16 candle-power electric light, 20 seconds; 32 candle-power, 10 seconds; ordinary oil lamps, from 30 to 50 seconds. Regular Carbon papers, 7 inches from the light, with a kerosene lamp, will require about 1 1/2 minutes to print; Welsbach Burner, 40 seconds; 16 candle-power electric light, 50 seconds; 32 candle-power, 40 seconds; average oil lamp, anywhere from 2 to 3 minutes. If a 4-foot gas burner is used, give the same exposure as with a 16 candle-power electric light. If a 6-foot gas burner is used, give the same exposure as used for the 32 candle-power.
688. A good rule to follow is to first sort the negatives. Place your strong negatives in one pile, the thin ones in another, and when about to print from the heavy ones use the special papers of any surface you prefer. In printing from the thinner negatives use the carbon, or regular paper. While making the exposure be sure that the box containing paper or exposed prints is covered. Being quite sensitive to light, it is easy to ruin an entire box of paper by allowing it to remain uncovered during a single exposure.