LITHOGRAPHIC COLOUR PRINTING
A Commercial Value--Peculiar Features--Colour Sequence--
Controlling Elements--A Question of Register--Suitable
Paper.
As a commercial phase of lithographic printing, colour printing offers a
vast and ever-widening field of usefulness. Nor is it altogether
deficient in these artistic qualities which are pre-eminently
suggestive, as well as attractive and artistic. Colour printing, in its
application to lithography, is in many respects peculiar. It is not what
might be described as a self-contained process; for its successful
realisation depends as much upon the harmonious and skilful combination
of colours in the design as upon the manipulation of the printing inks,
the sequence of the colour formes, and their accurate fit or register
during the actual printing. The most excellent printing would produce
barely passable results unless the design was effectively arranged, and
prepared with some consideration for the conditions under which it might
be printed. Nor is it at all unlikely that a design, however smart and
artistic it might appear in its original form, would be irretrievably
spoiled by clumsy handling or careless printing. The subject for
immediate consideration is the practical employment of printing inks for
the reproduction of coloured designs, their qualities, peculiarities,
and relative values, as well as the means employed to make them amenable
to commercial conditions. An intelligent appreciation of these points
will not only extend the possibilities of printing inks, but will also
enable the machineman to accentuate their attractive and suggestive
power.
"Colour is to design what salt is to food," and successful colour
printing has been very aptly described as the adaptation of printing ink
to the ever-varying character of work and conditions of employment. This
very practical definition will form the keynote of a chapter which, by
the very nature of things, must be to some extent authoritative and
comprehensive. The colour sequence, _i.e._ the order in which the
colours must be employed to secure the best and most economical results,
is of primary importance in colour printing. On broad lines, the
principle usually followed is one in which the opaque colours are
printed first, and upon these all secondary effects are built up. This
building up of colours plays also a most important part. Its relation to
colour sequence is a necessary and influential one. For example, it
might not be absolutely essential that even a yellow should be printed
first, if it did not form the base for the building up of a green by the
super-position of blues, of an orange effect in conjunction with red, or
as a secondary flesh tone under the buff.
The difference between printing a blue over a red or _vice versâ_ is
also very striking. One produces a purplish-black brown, and the other a
rich chocolate-brown. Other complications of a similar character are
common, but these will indicate with sufficient clearness the possible
modifications of colour sequence.
Another feature upon which colour sequence in printing largely depends
is the point at which the outline forme can be most effectively
introduced. It is advisable to print the outline forme at as early a
stage as possible for obvious reasons. Perfect registration is far from
easy to secure. Red in the lips, blues in the eyes, and isolated touches
of colour in various parts of the design must fit the browns, and
therefore fit each other, and yet they may have no direct relation to
each other in the printing. A remedy has been already suggested, but
once an outline forme is printed the cause of bad registration is to
some extent removed, and a remedy quite unnecessary. When worked on
reasonable lines it is frequently an advantage to make the outline one
of the earlier printings, so that any harshness of contour, etc., may be
toned down by the succeeding greys. It is often a matter of personal
opinion, or perhaps of circumstance, which decides the final printings.
The pink may be reserved to impart brilliancy and warmth to the prints,
or it may be equally suitable to hold back a grey, and, by regulating
its tone and strength, soften down any tendency to hardness, pick out
the darker prints, and emphasise the shadows. Even these suggestions,
although usually regarded as standard ideas, must be subjected to
modifications under certain conditions.
Here is a practical instance. Unless paper is unusually well seasoned
and of first-rate quality, the temperature of the workroom equable, and
the printing machine in good order--a combination of excellences which
is unfortunately rarely met with--the colour sequence must be of a
fairly elastic nature. To print a gold first is quite usual, because the
bronze powder will persistently adhere to any preceding printings. From
that standpoint alone such a procedure would be eminently practical and
convenient, but suppose for a moment that the gold must fit a later
printing with absolute accuracy, _e.g._ an outline forme, or as forming
the base for some ornamental scheme, then the difficulties which arise
are somewhat trying, and for this reason. The paper being new, the most
serious distortion of any kind is likely to occur during the first
printings, and so long as yellows, fleshes, or other colours of a
similar character are printed first, no serious difficulty is likely to
arise; but with the gold printing it may be altogether different. It is
quite possible to make both yellow and flesh dry dead, _i.e._ without
even sufficient tack to catch the almost impalpable bronze powder. At
the same time, care must be exercised that the colouring matter is not
left dry on the surface of the paper owing to its separation from the
reducing medium. This plan has been adopted under actual commercial
conditions and with conspicuous success, and it is therefore offered as
a preventive measure which is free from many drawbacks which are the
frequent accompaniment of novel ideas and operations. Here then is a
simple practical summary of the idea. The yellow and flesh, or
equivalent colours, are printed first, so that they will dry free from
gloss or _tack_. The fit required between such colours and subsequent
printings is generally a matter of minor importance, and at this stage
distortion of the paper, whether it be by stretching or contracting,
will not seriously depreciate the value of the print when completed.
Register between the gold and an outline is frequently of an entirely
different character, and in many cases the slightest variation will be
readily discernible, and have a decidedly bad effect on the finished
work. Apart from this, the questions which decide or control the colour
sequence have been clearly indicated previously.
This matter may be one of convenience also, for unless otherwise
predetermined it would be unwise and far from economical to print a blue
before a yellow, or a black before a red, etc. The amount of cleaning up
thereby involved would become a serious and distinctly disagreeable
item, and purity of tone in the lighter colours would be conspicuous by
its absence.
The matter of well seasoned printing paper has been already referred to.
For effective colour printing the paper must also possess several other
essential qualities. It should be firm in substance, sufficiently
absorbent to carry the successive layers of printing ink, as far as
possible unstretchable, and should present a smooth surface though not a
glazed one. The chalky, dull, enamelled papers offer many recognised
features of value to the colour printer. They assist in the absorption
of the ink as well as afford a suitable surface for their impression.
Friction-glazed and other prepared papers are also excellent for colour
printing by lithographic methods.