Printing

There are various methods of making overlays, the usual being to pull three impressions from the block on thin, hard paper. On the first sheet, which is the foundation sheet, all the high lights are cut out ; on the next sheet all the solids are carefully cut out and pasted accurately in a corresponding position on to the first sheet. The third sheet has the high lights and intermediate tones cut out, leaving the three-quarter tones and solids; this is then pasted on to the foundation sheet on which the solids have already been fixed. When the overlay is completed and held up to the light "a picture" of the illustration in its various tones will be seen. The overlay is then pasted on to the impression surface in a position to correspond exactly with the block on the bed of the machine; thus the varying tones in the plate will receive graduated degrees of pressure, and by this means a true representation of the tones in the block is obtained when inked and impressed on to a sheet of paper.

There are mechanical means of making overlays. The most popular form is known as the "chalk" overlay. An inked im pression of the illustration is taken on both sides of a piece of paper, which is treated with a chalk preparation. The ink, according to its density, acts as a resist when the prepared paper is immersed in a solution of chloride of lime, the result being that a graduated surface in hardened chalk is obtained in exact relation to the tones contained in the plate, the solids being highest and the other tones correspondingly lower. When the etching is completed the paper is allowed to dry and then pasted on to the impression surface. Another method (a photo-mechan ical one) is that of a thin piece of zinc etched in relief, with the tones in correct relation to each other.

When printing newspapers there is no time for making ready and when blocks are printed they are prepared mechanically by putting the printing surface on different planes. This is generally called "bumping" and is done by making a metal "interlay" which is forced into the back of the plate, with the result that the tones in the plate are slightly raised in accordance with the pressure required to give a correct impression, the solids being highest and the high lights lowest. This method of obtaining regulated pressure is likewise frequently resorted to in magazine printing and in a particular method of colour printing.

Duplicate Plates.

To secure economical and expeditious production of large editions it is necessary to print at the same time a number of duplicate plates identical with the original page of type or blocks. This is done by two processes, known as stereotyping and electrotyping.

Stereotyping.

Stereotyping is a process whereby duplicate plates of pages of type and relief printing blocks are obtained. This is done by taking a mould of the forme in plaster of paris or papier mache, placing it in a "casting box" and pouring molten metal into the box and over the face of the mould.

Like others of the crafts included in the printing industry, there is some obscurity about the date of the invention of this process, but it can be safely dated as about the beginning of the 18th century (1700-25). It is attributed to William Ged, an Edinburgh goldsmith. It is believed that he made a mould by pouring gypsum (plaster of paris) over a page of type, and allowing it to set. In 1730 it is known that he went to Cambridge and made plates for a bible for the university, but it is said that these plates were spoiled by the printers and the process aban doned. Ged returned to Edinburgh in 1733, where he died in 1749. About 1779, Tilloch and Foulis seemed to have "re invented" the process of stereotyping, since they took out patents in April 1784 for a method of making plates for printing. Earl Stanhope is likewise credited with having reintroduced Ged's process early in the 19th century. In America, stereotyping was introduced by David Bruce (1813) who served his apprenticeship in Edinburgh. The first book stereotyped in the United States was the Westminster Catechism, printed by John Watts in 1813.

The Plaster Process.

This method of making stereotype moulds is not now in general use, but certain firms in England employ it for duplicating half-tone plates, the usual method being to spread on a sheet of stout paper a coating of about -A- in. of fine plaster of paris which, when about to set, is placed in contact with a forme (the imposed type matter or blocks), and subjected to pressure. When the plaster is set, the mould is dried by heat.

The Paper Proces.

A plate by the papier mache method is obtained by making a "flong" composed of blotting and tissue papers pasted together. The particular method of making a flong is largely an individual opinion of the stereotyper, but generally it is made up of a sheet of strong, coarse blotting called a backing sheet. On this a fine sheet of blotting is pasted, followed by three sheets of tissue one on top of the other. It being necessary to keep the flongs damp, they are placed between damp blankets. The type from which stereotypes are to be made is placed within bevelled type-high bearers and locked up in a strong chase, the bearers serving the purpose of protecting the edges of the type and facilitating the moulding.