There are two groups of printing blocks, originals and duplicates, but in these groups there are different kinds of each.
ORIGINAL blocks are produced by means of an etching process and may be either a line or a halftone block. These may be reproduced from drawings, photographs or actual objects. Lino cuts and wood engravings (cuts) are also original blocks.
DUPLICATE blocks are produced by moulding from original blocks or type formes and may be either stereos or electros. The method of identification is as follows:
Line Blocks are made from copper or zinc and are approximately 6 points thick. Possess only black lines and solids, therefore have no intermediate tones.
Halftone Blocks are made from zinc or copper and are approximately 6 points thick. The image is broken up into small dots.
Electrotypes have a copper printing surface backed with stereo metal and are approximately 12 points thick.
Stereotypes are made from plastic, rubber or an alloy of lead antimony and tin and are approximately 12 points thick.
Stereos and electros can be nickel faced for longer life. To identify each, compare the non-printing areas. The stereos usually are rough in these areas and electros are usually quite smooth.
Manufacture of a Line Block
Line blocks/engravings are made by a photo-mechanical process and are usually made from zinc. There are no intermediate tones in this kind of printing plate, except by means of lines or dots. Copy for line blocks should be black lines on white background. This copy is placed on the copy board of a camera and a negative made from it. The negative is made of either glass or film. The negative is then placed on a sensitized piece of zinc in a vacuum frame and is printed down with the aid of powerful lights. The light passing through the transparent portions of the negative acts on the light sensitive solution on the zinc thereby rendering that portion insoluble in water. The zinc plate is then washed under a tap and the soluble parts are washed away. The image is then "burnt in" in readiness for etching by being heated over a gas flame. Etching is done by immercing the zinc plate in diluted nitric acid and those portions which are not protected by the coating are etched away. The etching is done in stages to safeguard "undercutting" the printing surface. Line blocks can also be made without acid etching by the use of hand and machine tools.
Manufacture of a Halftone Block
The halftone process is a photo-mechanical method of reproducing in a printing stroke the details of a photograph, drawing or an object including all the graduations of tone. It receives its name from the fact that it not only duplicates the black and whites but also the intermediate shades or halftones, thereby representing the actual photograph. The basic principle of the process is the photographing of the copy through a cross-lined screen. The screen breaks up the image into a series of fine dots, the dark lines of the screen producing transparent lines on the negative which when printed down on the unsensitized metal plate makes these lines insoluble and therefore they become the printing portion.
A halftone negative is made by placing a screen in front of the negative glass or film. The screen is made of two pieces of glass which have parallel lines engraved on them and are filled with black pigment to make them opaque. These glasses are then cemented together face to face with the lines running at right angles to one another. Screens are designated by the number of lines to the inch. They may be 60, 85, 100, 120, 133 or 150. The metal plate after etching is then made, having minute pin points in the highlights to larger dots in the mid-tones and solids in the darker portions. The plate thus made is mounted to .918."
Manufacture of a Stereo Block
The reason for making stereo blocks are:
1. For printing more than one up.
2. To preserve the original forme.
3. Easier to store than type formes.
4. Makes standing type formes unnecessary.
5. Plates can be curved for rotary printing.
The stereotype is a duplicate of a block or type forme and is made by making a mould from the original, and from this mould casting a plate of metal, rubber, or plastic. It is essential that before a mould can be taken all printing surfaces are type-high. Any damaged lines in the type or block will be duplicated in the stereo, so it is wise to replace them before making the mould.
The moulding material is known as either wet or dry flong. Wet flong is made from layers of blotting paper and tissue. It is prepared for use prior to the mould being made and is beaten into the forme with a stiff brush. Dry flong is a machine-made sheet and is moulded in a press. The mould is formed by either of these two methods and is then prepared for casting by trimming all the excess mould away then pasting a "lip" of brown paper and packing out the mould.
Normal stereotype metal comprises 78% lead, 16% antimony and 6% tin.
The plate is cast by placing the prepared mould into a casting box and molten stereo metal is either poured or pumped into the box. The cast plate is then trimmed and mounted for use. Stereos may be nickel or chrome-faced for longer life.
Manufacture of an Electrotype
Electrotyping is the process of producing duplicates of different kinds of blocks and type formes by means of an electro-deposition of copper upon a mould taken from the original. A forme for electrotyping must have all printing surfaces type-high and all spacing material should be high and the forme surrounded by type-high bearers.
For making the mould, the three main moulding ingredients are wax, lead and vinylite - although vinylite has almost displaced wax. An electro is made by taking an impression in one of these substances to forme a mould which is then placed in a bath of copper sulphate solution. An electric current passing through the solution causes the solution to be decomposed thereby depositing its copper on the mould to form a shell. When this copper shell has reached the required thickness (about .006") it is removed from the bath. The copper shell is then separated from the mould and is backed by soft metal to the thickness of approximately 12 points. The plate is then trimmed and mounted.
This process is used primarily in the making of diagrams and ruled formes etc. They are engraved in the wax on a ruling machine with the aid of hand tools, thereby forming a mould which is then treated as an electrotype.