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.
Continued Action Of Light. While it seems very singular, yet it is true, that carbon paper continues printing even after the action of light has ceased. This printing is not always noticeable, yet the printing continues mildly after the prints are withdrawn from the light. As this action ceases as soon as the prints are wet, it is advisable to develop the prints almost as fast as they are printed. This peculiar action of carbon tissue is sometimes taken advantage of by expert workers, when the light is dull and a great many prints have to be made. The prints are removed before they are exposed for the full length of time, and are left in a drawer for several hours, when the continued action will make a full exposure. This, however, requires considerable experience to avoid over-exposure.
230. As the surface of carbon tissue is more sensitive than ordinary printing papers, you must avoid touching any portions of the print before it reaches the water. Even should the hands be perfectly dry, the marks from the fingers will show wherever they have come in contact with the print. After printing, place all prints, before developing them, in a light-tight box, face down, weighted with a piece of glass.
Judging The Exposed From The Unexposed Tissue. There being no visible imprint by which to distinguish the difference between a tissue that is exposed and one that is not exposed, it might happen that the tissue which has been exposed will get mixed up with the unexposed. To avoid this trouble, mark the corner of the back of the sheets exposed "ex." Where several frames are being printed from it is well to mark the frames in a like manner. Should you neglect to place the exposed tissue in the proper box and accidentally mix it with the unexposed, by breathing on the film of the tissue you can judge whether it has been exposed or not. If it has been exposed the image will appear faintly, but will immediately disappear.