 
The use of photographs in book and magazine production has long since
supplanted hand-engraving, for which a considerable amount of copper
was used. Nevertheless the use of copper has increased. Today, by
far the larger part of book illustration, except in the case of line
blocks which are generally zinc, is by means of 'half-tone blocks'
or their copies, 'electros'. These are produced by photo-engraving,
which is also known as process engraving.
A half-tone printed picture is made up, as a magnifying glass will
show, of thousands of dots of ink large and small, large dots in the
lightest areas, grading down into fine dots in the shadows. A copper
plate with the polished surface sensitized by an emulsion, usually
bichromated fish glue, has the image of the photograph impressed upon
it in a camera; and a glass screen, ruled to a suitable mesh, is inserted
between the negative and the plate, so as to regulate the sizes of
the dots and thereby the quality of the half-tone. The sensitized
plate is then heated or 'burnt in' until the glue becomes a hard enamel.
To avoid softening the copper on heating, process plates always contain
a little silver or arsenic, which raises its softening temperature.
The plate is now etched, so as to make the dots stand up in relief
and thereby provide a printing surface.
One interesting application of copper in printing is in the making
of matrices for 'Linotype' and other automatic type-setting machines.
The compositor presses a keyboard on the machine, rather like a typewriter
keyboard, and as each key is depressed a small piece of brass called
a matrix is released from an inclined storage rack or magazine. The
matrices are each impressed with a letter or figure; leaded brass
is usually employed, although copper or bronze may also be used. A
belt conveyor carries the released matrix to a box, where it joins
others to form all the words in a line. The operator then presses
a small lever which moves the line opposite the mould wheel, and the
whole line is cast in one piece or slug by pumping in molten type-metal.
After trimming, the finished slug is delivered on to the galley; meanwhile,
the matrices are automatically returned to their magazines.
Another very interesting use for copper which has come to the fore
during recent years is the printed circuit, now so widely employed
by electrical engineers everywhere, especially in the electronics
industry. This shows the value of applying the techniques established
in one industry to another completely different in structure and outlook.
Wherever elaborate wiring layouts are involved, as in radio and television
sets, computers, etc., an immense amount of labour in wiring and fixing
the circuits may be saved by simply photographing the wiring diagram
on to a copper sheet, which can then be etched so as to leave the
diagram in relief.
By punching holes through the points of connexion, through which terminal
wires or tags from valves or other parts pass, and soldering these
tags to the plate, the circuits are established; thus, by the aid
of dip-soldering, all the joints can be fixed in a single operation.
For the circuits themselves a laminated sheet is employed, having
a face and sometimes also a base of copper foil which is bonded to
a sheet of plastic. |
