When the development is completed the cylinder, with all that is left of the gelatine film, is removed from the water bath and thoroughly dried. This film acts as a resist to the mordant. All parts of the printing surface which are not required to be etched are protected by an acid-resisting preparation. This is painted on by hand, or, as it is termed, "lacked out." Etching.—The printing surface has then to be etched, which is done by placing the cylinder in baths of varying concentra tions of ferric chloride ranging from to 37° Beaume. The thinner parts of the hardened gelatine acid resist are attacked first by the mordant, the thicker parts requiring further etching by the weaker solutions.
It is possible to use the copper cylinders over and over again, the old work being removed by grinding and polishing. In this way cylinders are prepared to receive the new work. In time, however, the grinding wears the cylinder, but this may be brought back to its original circumference by electrolytic deposition.
Recently, a method has been invented whereby a thin sheet of copper is stretched round a cylinder. There is reason to believe that this will supersede the cumbersome cylinders now in use.
The paper is fed into the machines either one sheet at a time or from a reel. The sheet-fed machine is generally used for small editions of art reproductions, whilst those that are reel-fed are used for long runs of magazines and newspaper supplements and other commercial requirements.
In a rotary press the cylinder revolves in a trough of ink, and a few inches above the trough is a "doctor"—a steel plate with a fine edge which has a slight reciprocating movement that scrapes the surplus ink off the surface, the ink dropping back into the trough. The paper, which is on a reel, passes between the etched cylinder and an impression cylinder. In this way an impression of the etched design is transferred to the paper. The paper then passes over a drying drum, and if the sheet is to be "perfected," i.e., printed on the reverse side, it goes through another pair of cylinders before reaching the delivery end of the machine, where the paper is cut up into sheets or delivered folded according to requirements.
It is possible by the photogravure method to print type matter at the same time as illustrations, and successful results are now being obtained when printing colour supplements by machine photogravure.
Machine printed photogravure is known under various names, usually containing a prefix, such as rotogravure, indicating that it is printed on a rotary machine.