Text 6 Origination for letterpress ( Unit 2)

In letterpress, pictures are printed from letterpress 'blocks' or plates which have a raised surface for the image area. Original plates are produced by etching. First, exactly as for offset origination, the original is photographed on a process camera and a negative is produced. The same basic principles of line, halftone and four-colour origination for offset lithography apply for letterpress up to the film stage, except for the fact that letterpress plates are made from negatives rather than positives.

A metal plate (usually zinc or copper) is coated with a light-sensitive emulsion, the negative is placed over it and the whole is exposed to light. The image area, which is clear on the negative, is hardened by the light and therefore becomes resistant to corrosion by acid. When the plate is etched, the acid therefore has the effect of bringing the non-image areas down below the level of the image area, so that they do not print. For flat-bed letterpress printing, the thin plate is mounted on a base of wood or metal to bring it up to the height of the type.

It is also possible to have a letterpress plate that has the text as part of the plate rather than as separate type, so that the whole 'forme' consists of one plate rather than mixed type and blocks. However, this process is quite expensive and normally used only in rotary letterpress, where a curved plate is required. It is, however, the only way to make an original metal letterpress printing surface from filmset type.

Duplicate plates for letterpressDuplicate plates are made by making a mould from the original relief surface, which can be in the form of type, blocks or a mixture of both. A 'stereo' plate is made by placing a 'flong' (sheet made of paper mache) on the original relief surface and using a press to make an indented impression in the flong. Molten metal alloy (similar to that used for metal type) is poured into the flong, and when the metal solidifies it has a relief printing surface that is a duplicate of the original.

This method first came into use in the eighteenth century when all printing was from the original metal type. Long runs meant that the type wore out, so that the whole job had to be completely reset. By contrast, when stereos wore out, new plates could easily be made from the flongs.

Alternatively, stereos can be made from rubber or flexible plastic. Here the moulding material is usually thermosetting plastic (bakelite) and the plates are made from the moulds by the application of heat and pressure. These flexible rubber or plastic plates are still used for printing paperbacks on web-fed rotary presses, although offset is now taking over. They are also used for flexography, although most of this is now done using photopolymer plates.

Another form of duplicate plate is called an 'electro'. The technique was developed particularly for fine-screen halftones, where a stereo cannot give a sufficiently precise result. The mould is made of plastic and its surface is sprayed with silver compound to make it electrically conductive; the mould is then placed in an electrolytic bath, so that a film of copper 'grows' on the recessed surface of the mould. This film is then removed from the mould and filled with molten metal alloy to strengthen it. As a result of the demise of letterpress in most areas, there are now very few companies left making letterpress original or duplicate plates, and then only specialized applications such as stereos for national newspapers.

Exercises:

Date: 2015-12-18; view: 759