Trimmed Edges.Fig. 19.
Start the machine with a light pressure, for once the stone is locked up a certain amount of danger will always exist if at any time it is necessary to reduce the pressure. The stone may still be held by the blocks, even after the bed of the machine has been lowered, only to come down with a snap when pressure is applied. Such a danger might, of course, be averted by slackening the screws and blocks; but then the stone would almost certainly move out of position and the registration of the forme be altered.
Narrow slips of paper folded two or three times, and inserted between the block and the stone, will often check any tendency the latter may have to lift when the screws are tightened.
Of the many annoyances associated with the lithographic machine printer’s work,gritis probably the most troublesome, inasmuch as its presence is almost imperceptible, while its effect is extensive and often disastrous. Its sharp grains become embedded in the inking-roller skins, and plough tinyfurrows across the printing forme, doing much damage before the printer realises the presence of any foreign matter on the inking-rollers. Prevention is a simple matter enough, but a cure is rarely, if ever, accomplished. Dust the rollers and examine them carefully before commencing operations, and in this way ensure perfect cleanliness. It may seem a trifle, but it is none the less an important one, and perhaps thereader has already realised that “trifles make perfection, and perfection is no trifle.”
Counter Shafting.Fig. 20a.—Patent conical counter shafting.
Counter Shafting—plan view.Fig. 20b.—Patent conical counter shafting.
Inmaking readyon a lithographic printing machine, as in almost every phase of industrial life, method is the great secret of success. Method conquers the most stubborn difficulties,and, though it is not at all times profoundly interesting in its application, yet it more than repays any monotony it may involve. In the matter of lithographic printing, at any rate, a few methods of an essentially simple character might be cultivated with advantage. This chapter is not intended as a complete record of such methods, but a number of items are discussedhereinwhich, though simple, are intensely practical, and likely to suggest more to the reader than is found described in the text.
The question of speed may sometimes seriously handicap progress. It is a self-evident fact that the solid impression of a heavy poster cannot be made at the same speed as a light tint in chromo work. Speed cones are usually fixed to a counter-shaft to regulate the speed of the machine as required. Figs.20Aand20Bshow an improved arrangement of this character, in which tapering drums A A are substituted for cones, the belt being moved and held in any position by the screw and forks B and C. D is the driving pulley which transmits the power to the machine.
Electricity as a motive power for printing machinery is quietly yet irresistibly winning its way into general favour, and for very cogent reasons. It is the most convenient form of motive power, and can be transmitted for long distances without any appreciable loss. It takes up little space, and almost entirely dispenses with belts and shafting. It is also essentially economical, because it can be applied to the smallest press just as easily as to a 60' by 40' poster machine (Fig. 21).
Motor Driving.Fig. 21.
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 thesuper-positionof 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 orvice versâis 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, andtherefore 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 bealtogether 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 ortack. 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 thesuccessive 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.
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 commerciallyas “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 thetackof stiff pigments and enable them to work freely during the printing operations. The drying of inks thus prepared is not seriously retarded; theyliftreadily 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 reducedwith 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 theintelligentapplication 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 astockgrey 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 transmittedlight. 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 registeron 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.
Metal Plates—Preparation—Manipulation—Descriptive Details—Machine Printing—The Printing Bed—Rotary Printing Machine.
Metal plate, as a substitute for stone, is now such an important factor in lithography that the printer who wishes to consider himself thoroughly efficient must possess a fairly comprehensive and practical knowledge of its manipulation and possibilities.
The prejudice which has hitherto checked the progress of this branch of lithography was not altogether of an unreasonable character. The plates themselves were far from reliable, and the difficulties resulting therefrom were a fruitful source of trouble and expense.
Metal, as a printing surface, is even yet a comparatively new factor in lithography, and the majority of printers have been working with lithographic stones from their apprenticeship till the present time. It is not surprising, therefore, that in relation to the use of stones almost every possible contingency has been provided for, but with metal plates a little fresh knowledge must necessarily be acquired before the workman can claim the same familiarity of manipulation which he may feel towards the parent process. This is, in fact, the point upon which the whole question usually turns. Good work can be produced from zinc and aluminium plates,—of that there is not the slightest doubt,—and it is equally certain that the advantages offered by their use are of a substantiallypractical character. They can be handled with ease and with absolutely no fear of breaking. They are much less costly than stone, and require less storage room.
A grain of a finer and sharper texture can be imparted to metal than is the case with stone, and what is even of greater importance, the character of such a grain remains unaffected for a considerable time. Surface inequalities are rarely met with in metal printing surfaces, and consequently uniform pressure is to a certain extent guaranteed. In photo-lithography it is possible to make a print from a negative direct on to the plate (Chap. XVII.page 100). This ensures an original of exceptional clearness and strength, especially in half-tone subjects.
Although an ordinary zinc plate, which has been carefully polished to free it from every trace of grease, can be used for lithographic printing, the best results are obtained from plates which have been specially prepared. A slight de-polishing with pumice sand and a piece of felt may impart the requisite “tooth” to the face of the plate, or the following method may be adopted:—Clean the plate with pumice sand and felt, and immediately immerse it in a hot bath containing:—
Keep this liquid in constant motion over the face of the plate until it assumes an even, silvery-grey appearance, and then wash it thoroughly with a plentiful supply of clean water. Dry at once, and quickly.
These plates can also be sand-grained by specially constructed machinery, or a variety of grains and stipple can be imparted to their surface by etching or sand blast.
A novel yet practical idea, which has met with considerable success, is to electrolytically prepare the surface of the plates. The value of this preparation has been amply demonstratedby its extensive adoption and successful use. Another distinctly progressive feature is a deposit of alumina on the zinc, which for printing purposes gives it all the advantages of an aluminium plate.
Plate-graining Machine.Plate-graining machine, showing oscillating motion.
Transfers can be made on metal plates in much the same manner as on lithographic stones. For press work mount the plate on a piece of cardboard its own size, then, having gummed a sheet of brown paper on the face of the litho-stone, place the mounted plate near the centre and fasten it with paste or gum. This will prevent it moving about, and also raise it sufficiently from the stone to enable the printer to use his damping-cloth and roller freely.Before mounting, the back of the plate should be carefully dusted to free it from grit and dirt.
After the drawing or transfer has been made cover the surface of the plate with strong gum, and while this is still wet add to it a solution of bichromate of potash. Leave a slight film only of the gum bichromate, and then dry it quickly and thoroughly. Wash out the work with turpentine,not with water. The gum, being insoluble in turpentine, remains unaffected.
Plate-graining Machine.Plate-graining machine, showing interior of trough.
Roll the plate upsolidin black ink, sponge it over with water, damp it with a cloth, and then roll it up in the usual way. When the work becomes fully charged with ink, dust it over with a mixture of resin and French chalk, and etch it with the acid etching solution.
Every maker of zinc and aluminium plates supplies this special etching solution, prepared for a certain quality and character of metal. It is advisable, therefore, to use the preparation recommended, and thus avoid the many pitfalls which beset the path of the experimentalist.
“The plate is now well washed over with clean water, andthinlygummed up. When the gum is dry wash out the work on the top of the gum with dry flannel or felt till all the work is removed, sprinkle a few drops of water on the plate, and with a rubbing-up rag and a little ink and turps rub up the work till it is gently charged with ink, then roll up till work or transfer looks strong and sharp, when the plate is ready for printing.”
For etching and rolling up chalk drawings on grained plates: “Pour in a saucer some of the etching solution, and add about the same quantity of fresh, strong gum. This solution is evenly distributed all over the plate with a camel-hair brush, and left to dry. Then the etching is removed with water, and the plate very thinly gummed up and left to dry. Now the work is washed out with turps and a piece of dry, soft felt or flannel,without water, till all the work disappears, and the plate rolled up solid black. Then sponge over with water-sponge, damp over and roll up till work appears nice and sharp. Now dust over with resin and French chalk, and etch again with the etching solution, full strength. After the etching has dried remove the same with water, and gum up again.
“When drawings are to be washed out for proofing or printing, the plate should first be thinly gummed up. This gum layer is leftto dry thoroughly, and is not removed when the work is washed out (without water) with turps and a dry piece of flannel or felt. Next wipe away all the black ink and turps, still using no water for this operation. Should any black work be left, use some more turps ontheclean dry ragtill all the work has been removed, then sprinkle a few drops of water on the plate, and use a clean rag to remove the gum layer and loose black ink left on the plate, then damp over as usual and roll up.
“Plates treated in this way retain the grease contained in the drawing, and the work rolls up easier and quicker, and none of the finer work gets weakened or lost; also, the plate keeps cleaner and free from scum.
“When alterations are necessary the work should be rolled up with a strong black ink, and dusted over with French chalk (powdered talc). Remove the part which requires altering with a mixture of equal parts etching solution and turpentine on a small piece of felt or flannel, and rinse well with water. Pour some special re-preparing solution in a saucer, and apply this solution with a camel-hair brush to the parts where the work has been removed; after the solution has remained for a few seconds rinse well with water. This operation is to be repeated several times till the surface looks clean, no work being visible; and after the plate is dried with clean white blotting paper it is ready to receive the additional work. When the alteration has been made, the rolling up should be done with the bichromate solution, as per general directions.
Plate Bed.Fig. 22.
Aluminium Rotary Machine.Fig. 23.
“Where the alterations involve additional work only without erasures roll up the design first with a good,strong ink, and dust over with French chalk. The special re-preparing solution is used in the same manner as acetic or citric acid is employed on a litho-stone. For this purpose we recommend that the special solution should be diluted with an equal quantity of clean water, and the solution applied with a camel-hair brush. It should remain on for a few seconds, and then be quickly rinsed with clean water. This operation may with advantage be repeated two or three times for securing a clean surface for the additional work. When the plate has been thoroughly dried with clean blotting paper it is ready to receive the additional work. The rolling up should be done with the bichromate solution, as per general directions.”
Machine printing from zinc or aluminium plates requires but a slight modification of well-known methods; and although it may in some respects present entirely new features, the making ready of work in which several printings are employed is simplified, and consequently much time is saved. In the ordinary type of litho-printing machine the printing bed (Fig. 22) is levelled in the machine for the first printing, and, if this is carefully arranged, no alteration will be required throughout the whole series; for, in changing from one colour to another, the printing plate can be slipped from the bed and another substituted in a very few minutes. The most scrupulous care should be exercised at all times to prevent dirt or grit of any kind insinuating itself between the surface of the printing bed and the back of the plate.
It is not in connection with the flat-bed printing machine, however, that the most decided progress is likely to be made in surface printing from metal plates. “The change which is already foreshadowed in the printing mechanism of to-day is shown by the growing demand for the rotary in place of the slow and tedious movements of the flat-bed press.”
The mechanical principle of the rotary machine (Fig. 23) at once suggests an absolute precision of movement which it is scarcely possible to guarantee in the flat-bed press. There is no appreciable lift in the gearing of the cylinders when the impression is made, and practically no risk whatever ofslogger, such as that described in Chap. VII.page 38. The uniform velocity of the cylinders, which is to a great extent due to the points already indicated, considerably reduces friction, produces perfect registration, and enables the machine to be worked at a high rate of speed. The machine shown onpage 63presents many novel and essentially practical features. The side elevation (Fig. 24) gives a fairly accurate idea of its general mechanical principles.
Aluminium Rotary Section.Fig. 24.
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 withoutthe 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 initialprintings. 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 followingexample 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 greenishhuewhen 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 intone. Pinks of the lighter shades, on the other hand, may suffer both inhueandtone. “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 notfamiliar 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, thecruxof 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 nogripto 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.
Direct Tin Printing—The Machine—Peculiarities of Impression—Cylinder Covering—Colour Sequence—Printing Inks—Drying Racks—Air Drying and Stoving.
“Direct tin printing” is not, as the term might suggest, adirectimpression of the printing forme on the metal plate. It is in reality a transfer printing process in which the transferring medium is a cylinder with an elastic covering. This additional cylinder is one of the chief characteristics of a direct tin-printing machine, and, apart from a few accessory movements, it is in this respect alone that it differs from an ordinary lithographic printing machine.Fig. 25gives a sufficiently clear idea of this distinguishing feature. A detailed description of its mechanical principles is unnecessary, but the following points will prove helpful.
The impression is made from the printing forme upon the lower cylinder during the first half of its revolution, and transferred to the metal plate, which is carried by the upper cylinder as the revolution is completed. The reciprocal movement of these two cylinders is therefore a matter which demands the keenest possible attention. Their pitch must be absolutely accurate, the pressure of one against the other nicely adjusted, and the brake arrangement on both such as to ensure a perfectly steady revolution.
The lower cylinder is covered with a three-ply rubber blanket, which provides a sufficiently elastic impression surface.