LITHOGRAPHIC COLOUR PRINTING--_continued_
Printing Inks--Varnish--Reducing Medium--Relative Values--
Some useful Hints--Bronze Blue--Vermilion--Ink Mixing--
Ceramic Transfers--Colour Transparencies.
For the successful manipulation of printing inks of any description it
will be necessary to know something of their composition, or at any rate
of such features as render them peculiarly suitable for printing
purposes. From a printer's point of view the most important of these
features is what may be described as the reducing medium, _i.e._ the
medium which holds together the various colours so as to produce
pigments of suitable working consistency. The most useful and the
commonest form of reducing medium is a linseed oil product, known in its
prepared state as a lithographic varnish, with a supplementary title
indicating its specific character. Its value to the lithographic printer
lies mainly in the fact that when it is fully matured it possesses a
good full body along with fair drying properties and freedom from any
excess of greasy matter. This varnish is used in three or four
consistencies between which any degree of strength may be arranged by
mixing. A brief outline of the manner in which they are prepared may
still further emphasise their usefulness in lithographic printing.
Raw linseed oil is matured and oxidised until its consistency is
considerably reduced. It is still further reduced by being boiled at a
high temperature, and is known commercially as "boiled linseed oil." As
this boiling is continued the fumes which quickly rise can be ignited,
and the liquid soon assumes a syrupy or stringy character, according to
the length of time during which it is subjected to the action of fire.
By extending or discontinuing the burning the varnish is produced in
three grades--thin, medium, or strong.
Lithographic varnish is a good servant but a bad master, and it is a
generally recognised fact that, beyond a certain point, lithographic
varnishes as a reducing medium will depreciate the value of colour.
The chief requisite in colour printing is the production of a solid flat
impression, and for this purpose almost all printing inks must be
reduced to a suitable working consistency. To accomplish this, and at
the same time retain the full colour strength of printing ink, a soft,
free-working composition will be useful and desirable.
There are several commercial varieties of solid oil from which the
excess of grease has been extracted. These form excellent reducing
mediums. They break down the _tack_ of stiff pigments and enable them to
work freely during the printing operations. The drying of inks thus
prepared is not seriously retarded; they _lift_ readily and usually
produce brilliant impressions. There is a reasonable and logical
explanation of these peculiarities which is both interesting and
suggestive. Whatever the character of a reducing medium may be, its
effect on the strength of colour will of course be in proportion to the
quantity used. In all probability 1 oz. of a solid oil composition,
otherwise known as lithographic reducing medium, would soften down a
quantity of printing ink for which at least three times its bulk of
varnish would be required. Consequently, the depth of colour and
covering power of an ink reduced with "litho medium" would be
proportionately greater than that reduced with varnish. Vaseline in
some of its commercial forms is frequently used by American printers,
and even in this country its use is being tardily, though none the less
surely, recognised.
A few remarks anent the _intelligent_ application of a softening medium
may not be inopportune. Considerable care and judgment must always be
exercised or there will be a loss of cohesion in the colour pigments
which cannot fail to prove disastrous. The tendency of lithographic
varnish is to bind the colour pigments together, and this should not be
entirely counteracted by the addition of fatty compositions, lest the
printing inks run "scummy" during printing operations, and in drying
leave the colouring matter, from which they have been detached, on the
surface of the paper in the form of a dry powder.
Such lack of cohesion may, however, be an inherent feature of the ink
itself, and not be produced in the manner just indicated. Bronze blue
affords a striking example of a printing ink of this character. It is,
in fact, a "constitutional weakness" which cannot apparently be
prevented, but which is fortunately not incurable. The addition of a
little Canada balsam to bronze-blue ink will add considerably to its
working qualities. The loose particles of the pigment appear to be held
together without becoming harsh or stringy, as might easily happen if
varnish of sufficient strength was added to produce the same effect.
Other colours, again, such as vermilion and yellow, owing to their
weight and texture, will always require a fair percentage of varnish in
their composition. At the same time, a little reducing medium might also
prove beneficial. Referring once more to the fact that vermilion, as
distinguished from its imitation, is unusually heavy, etc., it may be
useful to know that for "blocking out" work it has no equal in all the
range of printing inks. It possesses unrivalled opacity, and as a
"blocking-out" agent frequently plays an important part in colour
printing.
Of the other printing inks, few possess characteristics of a
sufficiently striking character to require special mention. Their
working qualities present no exceptional difficulties, and their
employment either under primary or secondary conditions is almost
invariably determined either by the character of the work or some such
conditions as have been already indicated.
When the strength of a colour is problematical, or its effect more or
less a question of experiment, it is a safe plan to mix it a little
lighter than will be required. For obvious reasons it is much easier to
alter the line or tone of a light colour than that of a darker one.
The arrangements for extensive and economical ink mixing need not be of
a very elaborate character. Standard colours might with advantage be
mixed in large quantities and kept as stock shades. Fleshes, pink,
blues, greys, etc., are all useful colours which are in constant use. A
warm or cold tone could be imparted to a _stock_ grey as required, and a
similar method adopted with regard to the other colours. Other peculiar
conditions could be met in a similar manner, and many economies thereby
effected.
Although the foregoing remarks refer mainly to lithographic colour
printing generally, they may with equal effect be applied to many of its
more specific branches.
Colour printing for tin-plate decoration will be fully discussed in a
subsequent chapter.
Transfer printing for ceramic decoration presents many features in
common with transfer printing for metal decoration. Printing colour
transparencies is a commercial phase of colour work which is productive
of many curious and attractive effects. Unlike the ordinary colour
prints, the transparency is intended for exhibition both by reflected
and transmitted light. The paper is of a thin, tough quality, and the
first printing is usually a white of good covering power and exceptional
opacity. It is not necessary to print a solid white groundwork for the
coloured design. If an intense brilliant colour is required by
transmitted light the omission of part of the white printing will add
considerably to the effective character of the design. A brilliancy
secured in such a manner may be still further intensified by printing
the colour or colours in register _on both sides of the paper_. This can
be easily accomplished by first allowing the cylinder covering to take
an impression from the stone, and then, with the sheet laid in the
gripper, make a second impression in the usual way immediately after.
The first print, which might be termed the transfer, will then be made
in accurate register on the back of the sheet.
Semi-transparent, or even transparent, effects can be obtained with any
of the colours by an omission of the white printing from the parts
affected. Complete opacity may be secured by its introduction.
This print is afterwards rendered more or less transparent by coating it
with a suitable varnish.