TIN-PLATE PRINTING
Its Evolution--Transfer and Direct Transfer Printing--The
Coated Paper--Reversed Designs--Sequence of Printing--
Printing Inks--Purity of Tone--Drying.
Tin-plate printing or decoration is probably the most remarkable
development of modern lithography. From a most unpretentious and
unpromising beginning its evolution has been an unbroken record of
phenomenal progress. Owing to its intrinsic merits and peculiarly
assertive character it has created an enormous demand for its own
productions.
Difficulties innumerable were encountered and surmounted, whilst a whole
host of trade prejudices had to be removed before anything approaching a
commercial success could be claimed. The original idea was to transfer a
printed design from a suitably prepared paper to the metal plates in
much the same manner as the children of to-day use transfer prints for
decorative and other purposes. Though this method is now to some extent
superseded, it is by no means obsolete. It is extensively used for
certain classes of work, and so long as the present type of direct
tin-printing machine is used this transfer method of metal decoration
will more or less be retained.
It is difficult, and in some cases impossible, to print unusually large
or exceptionally thick metal plates by the direct process. Hence the
value of Transfer Printing as a useful auxiliary process which can be
resorted to without the expense of special machinery. The ordinary
lithographic paper-printing machine, without any modification whatever,
will fulfil every requirement.
Almost any paper which has been coated with a starchy composition will
be suitable for Transfer Printing. Though not absolutely essential, it
is decidedly an advantage to use a paper which is somewhat porous, not
too hard. A brief description of the principles involved will probably
lead to a clearer understanding of the points already indicated.
The print is made on the starchy composition with which the paper is
coated, and never actually penetrates to the paper itself. This
composition, being readily soluble in water, can be transferred from its
paper base to any other surface, and will carry with it any print or
impression previously made thereupon. In theory, then, this process is
exceedingly simple, nor is it likely to present any insuperable
difficulties in practice.
One of the most important features of Transfer Printing, which affects
not only the colour sequence but the draughtsmanship, is the inversion
of the design when printed. To the printer who is accustomed to chromo
work on paper, this matter will probably offer many awkward situations.
The control over colour effects is somewhat restricted, and consequently
not only care and skill, but a certain amount of special training and
actual experience, will be necessary for the production of really
successful work. In theory the outline forme should be printed first,
but in practice it is frequently advantageous to introduce some of the
transparent tints as first printings. In the first place, such prints
rarely affect the design to any great extent as far as register or fit
is concerned. Consequently, any distortion of the paper from atmospheric
or other causes has no very serious consequences, and such distortion,
_i.e._ expansion or contraction, would be most likely to occur during
the initial printings. There is yet another and equally important
reason for this suggested modification of the colour sequence. The
light, tacky tints considerably improve the printing surface offered by
the mucilaginous coating of the paper, and the stronger inks can be
printed on it with a full body of colour yet without any tendency to
smash and smear. The lighter tints, then, are printed first, and these
are followed by the opaque colours, reds and yellows being last. The
peculiar feature of such an arrangement is that, after the first few
printings, the design becomes partly obscured, and the relative value of
different colours, together with the effect of their super-position, are
frequently questions of skilful calculation rather than visual
appreciation. Thus successful manipulation is therefore more or less the
result of careful observation and wide experience. In certain classes of
work it may, of course, be absolutely essential to ascertain the effects
of colours as they are printed. This can be done by transferring a
progressive print to a sheet of dull enamel surface paper, using gum
water as a transferring medium. In all work where specific colour
effects are required it will be advisable to make such test transfers
again and again at different stages of the work.
Concerning the printing inks themselves but little need be said. They
must be of good quality, varnishable, and of intense colouring power.
Purity of tone is requisite, not only under ordinary circumstances, but
under what may be regarded as exceptional conditions. The print, when
subsequently transferred to the metal plate, will be placed in a stove
having a temperature of 140° Fahr. Few colours remain quite unaffected
by this stoving, but with carefully selected printing inks the effect is
scarcely perceptible. It can, moreover, usually be anticipated, and to
some extent minimised, either by judicious super-position or skilful
preparation of the colours themselves before printing. The following
example will suggest a number of expedients for the preservation of tone
of colour in tin-plate decoration.
Pale blues will frequently become of a decidedly greenish _hue_ when
varnished or stoved, and this may be modified by an exaggerated
intensity of colour, even to hardness, when printing. At the same time,
it must be remembered that "blues" are seldom, if ever, affected in
_tone_. Pinks of the lighter shades, on the other hand, may suffer both
in _hue_ and _tone_. "Reds" may be mixed with a more liberal amount of
"lake" than they apparently need, and then backed up with solid masses
of "yellow" and "flesh." These will restore the bright vermilion hue,
and likewise give a greater amount of density or covering power to the
colour. Black, in the same manner, may be intensified by an underlay of
blue. As far as purity and density are concerned, the "white" printing
is by far the most important. The primary function of this printing in
tin-plate decoration is the formation of an opaque ground on the metal
plate--equivalent to the white paper. Without it the colours of a design
would lose their brilliancy and effect, for their purity and density
would be affected by the lustrous sheen of the bright metal plate.
It is a printing, then, of some, importance, and the most scrupulous
care should be exercised during its manipulation. In consequence of the
exceptional density required it is usual to repeat the "white" printing
either two or three times. The first printing especially should be made
with a smooth, well-mixed ink, which should be worked with the intention
of securing a perfectly flat and solid impression rather than a heavy
body of colour. The requisite amount of density can be obtained by the
second or third impressions.
A slight tinge of "Oriental blue" given to the "white" will improve its
appearance and counteract the discoloration produced by the varnishing
and stoving.
Another point with which the average printer is not familiar is the
unusually rapid drying of each colour. The printing inks must dry on the
surface, and not even in the slightest degree be allowed to permeate the
paper. This is, in fact, the _crux_ of the whole matter; and, assuming
that the coating of the paper is suitable and perfect, the drying cannot
be otherwise than on the surface. Dryers, preferably liquid, must be
mixed with the printing inks in liberal quantities. Here, again,
experience alone can be relied upon for guidance. Printing inks have
their peculiar characteristics, and atmospheric conditions are ever
varying. It would therefore be unwise to indicate any hard-and-fast
lines upon which to work. There is at least one danger accompanying the
use of dryers in any form which must not be overlooked. Colours may dry
too hard and present for the succeeding printings a surface similar to a
varnished sheet, which will offer no _grip_ to the printing ink, and
upon which it will probably run, and, consequently, produce a defective
impression. This is by no means an infrequent occurrence. Experience
alone can prevent it, but the remedy is as simple as it is effective.
Briskly rub each printed sheet with a pad of soft rag and a little
magnesia or French chalk. Magnesia is best. It is an almost impalpable
powder, and has no ill effect even upon the most delicate tints. This
rubbing will produce a slightly matt surface on the hard glossy ink, and
on this a good solid impression may be made without further trouble.
The printed sheets should be exposed to the air for a few hours to
accelerate drying. This may be done in any convenient manner, either by
laying them out in frames or hanging them up. A "set off" in Transfer
Printing should be sedulously avoided; not that the soiling of the back
of a sheet is of any moment, but because the matter set off on the back
of one sheet must be pulled off the face of another, and that very often
in liberal proportions.