Offset.—In most printing processes the sheet of paper is brought into direct contact with the plate on which the design is placed. This is known as direct printing, but in the offset method an intermediate cylinder covered with a sheet of rubber is included. The same method of printing has been applied to the printing of tin for a number of years, but it is only com paratively recently that it has been adopted for printing upon paper.
The construction of the offset machines mostly in use may be likened to that of the direct rotary, with the addition of a third cylinder. As the machine operates, an inked impression of the design on the plate cylinders is transferred on to the rubber sheet and this, in turn, is transferred to the sheet of paper. A flat-bed offset machine is sometimes used. This is similar to an ordinary stone litho printing machine, but fitted with an addi tional drum for pressing the sheet against the main cylinder covered with a rubber blanket, which takes the impression from the stone.
The advantage of this method lies in the fact that a printer can otherwise only get the best results when a paper with a smooth surface is used, but by the offset method, a paper with a rough surface, or material such as fabric or leather, can be printed with remarkable results. The reason is that a sheet of paper has an undulating surface and, when printing by the direct method, the highest "points" on the paper receive the impression; but in offset, the resiliency of the rubber does not only allow for the printing of the highest points but will go down into the hollows, which are naturally more pronounced in a rough paper than in that which has been calendered or coated. Offset printing is used not only for the highest grade of work—such as art subjects, show-cards, calendars—but also for the production of news papers and magazines. In the latter instance the paper is fed into the machine from a reel instead of a single sheet at a time.
The development of this section of lithographic printing is still in its infancy and its possibilities have not yet been fully investi gated. In time, however, with a fuller knowledge of the capabili ties of the process, difficulties will be surmounted, making it pos sible to produce illustrated papers with a result equal to that of other processes.
Tin Printing.—This is a branch of the lithographic art in which a few firms specialize. The beautifully decorated tin con tainers used for confectionery and household requisites are printed by this method. The method is similar to offset printing, but in stead of a sheet of paper, a sheet of prepared tin is "fed" into the machine which receives an impression of the design from a rubber blanket. This sheet of tin is then "stoved," i.e., dried in a high temperature, this being done for each printing until all the colours necessary to complete the design have been printed : a separate stone or plate is required for each colour.
Remarkable developments have taken place since 1910 in the lithographic branch of the printing industry. Reproductions by lithographic draughtsmen of original drawings in monochrome and colour are rapidly being superseded by photographic methods; whilst the printing by slow-running flat-bed machines is being supplanted by high-speed rotaries fitted with automatic feeders and the latest mechanical devices, ensuring better and quicker production. It was not uncommon under the old method, when reproducing a colour printing, to have as many as 20 printings, each of a different colour or tint. The modern method (a photo mechanical one) whereby colour negatives are secured by scientific photographic dissection of the colours of an original, has been made possible by the development of photography and the progress of mechanical science.
The rapid growth of planographic printing has put printing engineers on their mettle, and it is generally acknowledged that available processes and machines are far in advance of the crafts men. Machines are now produced to deal with every phase of lithographic printing, even to the printing of two, three or four colours simultaneously, and the perfecting of the sheet, i.e., print ing on both sides before delivery. (See PHOTO-ENGRAVING; COLOUR PRINTING; COLLOTYPE.)