Practical Lithography - lightnovelgate.com
You’re reading novel Practical Lithography Part 3 online at Lightnovelgate.com. Please use the follow button to get notification about the latest chapter next time when you visit Lightnovelgate.com. Use F11 button to read novel in full-screen(PC only). Drop by anytime you want to read free – fast – latest novel. It’s great if you could leave a comment, share your opinion about the new chapters, new novel with others on the internet. We’ll do our best to bring you the finest, latest novel everyday. Enjoy
LITHOGRAPHIC PRESS WORK
Preparing the Design--Treatment of an Ink Drawing--Chalk Drawings--Alterations--Value of Impressions--Offsets--The Lithographic Hand-roller--Proving--Registration--General Features--Transferring--A Commercial Necessity-- Arrangement--Choice of Paper--Transference to Stone-- Preparing the Forme.
The operations directly associated with lithographic press work are more or less of a preparatory character. The preparation of a design, in its progressive stages, from the lithographic draughtsman to the printing machine, is usually carried out in conjunction with the press. Only under exceptional conditions or for some particular class of work is the lithographic press actually employed for printing purposes. Its ready adaptability to the ever-varying thickness of lithographic stones, and the manner in which pressure can be applied at will, as well as the intense sharpness of such pressure, render it peculiarly suitable for the work now under discussion. Such operations will be better understood and probably more easily remembered if they are described in a sequence such as might be presented under average commercial conditions. Taking a design as it leaves the lithographic draughtsman, _i.e._ in the form of a greasy drawing on stone, the first object of the printer is to so prepare it as to preserve the conditions described in Chap. I. page 2.
This he may accomplish in the following manner.
Cover the whole stone with fresh strong gum and allow it to dry. Then if it be an ink drawing, wash off the gum with water, and remove the drawing ink from the surface of the design with a few drops of turpentine and a piece of clean rag. Proceed to roll up with a lithographic hand-roller charged with good black printing ink. The consistency of this printing ink can only be determined by the character of the work under treatment. It is therefore a matter of experience rather than rule. Heavy designs covering large areas can be worked up with moderately thin ink, while work of a finer description will most probably require a stronger ink for its successful treatment. Between these two extremes there is a variety of conditions and effects which will require a ready recognition and an intelligent adaptation or modification of any operation which may be described. It may even be advisable to _rub up_ the work with a piece of soft rag and printing ink, but the clearness and crispness of the drawing can best be preserved by a complete removal of the greasy ink with which the drawing was originally made. More particularly is this desirable when heavy, solid work is in close contact with work of a finer description, for the excessively greasy character of the artist's drawing ink has a dangerous tendency to smear or spread and to thicken the design, unless a reasonable amount of care is exercised. After rolling up the work as well as possible, and having decided that it is firm and strong and is fully charged with ink, dry the stone perfectly and dust over the design with finely powdered resin or French chalk. With a piece of water of Ayr stone polish away any scum or dirt which may surround the work, and etch it quickly with a weak solution of nitric acid. Cover up with strong gum and dry it. The design is now ready either for proving or transferring.
The treatment of chalk drawings, grained stones, or transfers from grained paper needs a slight variation of the operations already described. The preliminary etching is generally carried out by the draughtsman by flooding the stone with a mixture of gum and acid, after which the gum solution is allowed to dry. The chemical change which takes place during this etching is often described as one in which the soap present in lithographic chalks is changed to an insoluble compound.
This chemical change is perhaps a somewhat contentious matter, but the effect and not the principle involved is to us the matter of primary importance, and this effect is such as to actually prevent any spreading of the design on the stone beyond the lines of the original drawing.
Returning once more to the operation, wash off the gum, and, having removed the excess of water in the usual way, roll up firmly with a strong black ink. Instead of washing out the drawing with turpentine immediately, work off the original chalk by rolling up with a good nap roller and taking frequent impressions. In this way the grain of the drawing will be gradually developed and rendered fit for further operations. The stone can then be passed to the prover or transferrer.
A French writer, in referring to the importance of really good _chalk_ drawing and printing, as well as to its artistic and technical value, once said: "The printer requires a fair appreciation of that subtle suggestiveness which gradations of tone can impart to a chalk drawing before he can hope to successfully reproduce the artist's original conception. A good printer handles his roller over a chalk drawing with the same feeling as that with which a violin player handles his bow. By movements rapid or slow, and by greater or less pressure over certain parts, he charges the drawing to the proper tone."
If at any time the original work requires alterations, they may be executed in the following manner. Roll up the design firmly in strong, black ink, and, after fanning the surface dry, dust it over with French chalk. Make the necessary erasures with water of Ayr stone and etch with fairly strong nitric acid. Polish slightly, and wash well with a plentiful supply of clean water. Pour over the stone a very weak solution of alum, and again wash thoroughly with hot water, so that its rapid evaporation may leave the work ready for immediate manipulation.
Alterations may be made by transferring or drawing. In either case it is advisable to gum up the work with strong gum and allow it to stand until dry. The subsequent treatment of any alteration will, of course, depend upon their character and extent. As new work, they should be carefully handled.
It is most important that a very _weak_ solution of alum should be used.
Being an alkali, a strong solution would have a tendency to dissolve the greasy particles of the drawing and cause them to spread and thicken.
It is always advisable to take an impression from each design, whether it be in ink or crayon, before it is laid aside for subsequent manipulation. These impressions will not only reveal any inaccuracies or weaknesses which might otherwise pass unnoticed, but also serve as a useful record and for comparison with other transfers or impressions which may be required.
There are other phases of preparatory work which come within the scope of the lithographic pressman, and as they frequently constitute an intermediary stage between the first drawing of the draughtsman on stone and the making of transfer impressions to facilitate reproduction, a description at this point will be appropriate.
It may be that a key forme only has been prepared, or perhaps an outline forme with sufficient detail. In either case a number of offsets equivalent to the number of colours necessary for the completion of the design will be required. These are made by taking good, solid impressions in stiff black ink from the key or outline forme. Dust these over with a mixture of three parts Venetian red and one part lamp-black.
Lay them in convenient positions on a well-polished dry stone, and run them through the lithographic press with a light yet firm pressure. The result will be faint yet sufficiently clear offsets of an outline which will enable the lithographic draughtsman to prepare any number of formes, and these will register or fit each other and the original drawing with perfect accuracy. Such outlines will in no way affect the work of the draughtsman, and will disappear at the first application of the gum sponge or moisture in any form.
[Illustration: FIG. 9.]
A lithographic _nap_-roller (Fig. 9) facilitates the work of the pressman in the preparation and development of original drawings on stone, and becomes an absolute necessity when crayon drawings on grained stones are operated upon. The preparation and preservation of a roller of this description requires a more than average amount of care and attention. The best rollers are covered with French calf-skin with a soft, velvet-like nap, and may be prepared as follows. Run the roller in crude castor oil for a short time until the leather becomes soft and pliable, then work out the superfluous oil by repeated rolling in medium varnish, occasionally scraping off the varnish with a broad blunt knife.
Continue this for a day or two, then gradually work into the skin some good non-drying black printing ink. The roller thus prepared may be somewhat harsh, but a few days' use will bring it into condition. An occasional application of tallow or lard, say about once a week, will keep the roller skin soft and pliable, and counteract the hardening effect of constant contact with the damp surface of the lithographic stone and the oxidisation of the printing ink.
Proving the work of the lithographic artist, though not always an absolute necessity, is a helpful and most important function. In its progressive stage it enables both designer and lithographer to observe the realisation of their colour schemes, and to amplify or minimise if necessary the effects they desire to produce. Errors of judgment or of detail can be rectified before the work reaches a more advanced stage.
Again, a finished proof offers something of a tangible character for an expression of approval or disapproval, and serves as a useful and helpful guide to the printer throughout the subsequent operations.
This will show clearly the importance of the prover's work, and though it is not by any means an unusual proceeding to _prove up_ even the most elaborate designs in the lithographic printing machines, it is, for obvious reasons, more convenient to confine such work to the press. It may therefore be regarded as an intermediate operation, distinctly apart from the preparation of the original drawing which precedes it, and the arrangement for machine printing which follows. The distinctive and pre-eminently the most important feature of proving is the manner in which one colour is registered with another; and although the methods usually adopted are of the simplest possible character, the most scrupulous care is requisite for their successful application. It appears to be an almost ridiculous plan, so simple is it, to cut away the angles formed by the register lines after the first printing (Fig.
10A), and then to place them to corresponding lines on each colour forme, or to pierce the register lines as in Fig. 10B, passing a fine needle through each puncture into corresponding holes drilled in the stones and allowing the sheets to fall into position,--yet these operations demand constant care and attention.
[Illustration: FIG. 10A.]
[Illustration: FIG. 10B.]
The mixing of colours for proving, and the general principle of their application, are matters which are almost entirely under the control of the printer. Their selection and the manner in which they are employed are both determined by the individual character of the work. It is impossible to indicate any "rule of thumb" guide for their application or manipulation. The individual fancy of the artist, or the wish of a customer, are the only probable complications which may have to be considered. Then again, many phases of the work are more or less experimental, when the resourcefulness of the printer may be tested, and the mechanical features of his work be relieved by the exercise of intelligent application, if not of artistic perception. Very rarely is it possible to print from the litho-draughtsman's original drawing, and even when it may be convenient to do so, it is, in the majority of cases, unadvisable on account of the element of risk involved. There is an ever-present danger of the stone breaking,--a catastrophe which would necessitate an entire reproduction of the design, and even under the most favourable conditions the constant attrition produced by the rollers, etc., would have an appreciable effect on the work, and in course of time destroy its value for graphic reproduction.
Many other equally cogent reasons why duplicates of the original should be made for printing purposes present themselves. The chief of these is an essentially commercial one. To reproduce half a million impressions from a single small drawing would obviously depreciate the commercial value of lithographic printing very considerably, and although there is no record of the circumstances under which the duplicating of original work by means of transfers was first evolved, it is only reasonable to suppose that it was the direct outcome of a necessity which was as peremptory in its demands as it has been far-reaching in its effects.
The method is one by which any number of impressions can be made on a suitably prepared paper, and with a sufficiently greasy pigment. These can be re-transferred to a lithographic stone, and in this way facsimiles of the original may be secured and arranged in the manner most convenient for machine printing. Great care is necessary in making these transfer impressions. They must be perfectly solid, yet not overcharged with ink, _i.e._ they must be clean and sharp, and as nearly an exact replica of the original work as it is possible to make them.
That the further description of these operations may be as lucid and practical as possible, we will apply it to ordinary work-a-day conditions, and suppose that a design in three workings has been lithographed and prepared for transferring as already described. The size of the work is 7-1/4 in. by 4-1/2 in., then the paper on which it is to be printed ought to be double crown, 20 in. by 30 in. This will allow 1-1/2 in. for the gripper and 1/2 in. margin at the back and sides. Sixteen transfers can be pulled from each colour forme on a thin, transparent transfer paper. Mark out a sheet of stout paper as in Fig.
11, and arrange the transfers in the position indicated by the dotted lines. The gripper margins A A are determined by the construction of the machines, and may be varied accordingly.
Varnished transfer papers may be laid down on a slightly warmed dry stone, and if French transfer paper be used the stone must be slightly damped. If the sheet of transfers is laid down to a board--Fig.
12--uniformity of gripper margin will be assured throughout the series, and the work of the machine printer facilitated.
[Illustration: FIG. 11.]
Pull it through the lithographic press with a gradually increasing pressure in the usual way. The varnish transfer paper will be sufficiently tacky to adhere slightly to the surface of the stone, so that the pressure may be repeated again and again with perfect safety.
It may not be possible to remove the base upon which they were arranged, as it is usual to secure them in position with paste. At this point the manipulation of the two varieties of paper differs slightly. The thin, transparent variety is usually fastened down to its paper base with syrup, glucose, or some sticky composition of a similar character. This paper backing can be removed immediately after sufficient pressure has been applied to fix the transfers to the stone. As this transfer paper is adhesive it is necessary to damp the surface of the litho-stone before it is laid down, when, of course, it will readily adhere, even under a moderately light pressure.
[Illustration: FIG. 12.]
The further preparation of work, after being transferred in this manner, is in many respects similar to the treatment of new work, but with this important difference. A new transfer should almost invariably be worked up with a soft rag and black ink, the latter being thinned down with turpentine and varnish. Gum up the work, and allow the gum to dry. Roll a piece of soft rag into a pad, and charge it with printing ink which has been thinned down. Wipe off the gum on the surface of the stone, leaving only a thin film over the work. Rub up the transfers with the rag already prepared, and when fully charged with ink cover them with fresh gum. If possible they should stand for one or two hours, when the rolling up and etching may be proceeded with.
Accuracy of register can be ensured by the second and third sets of transfers being patched up to the first forme in the following manner.
Make two fairly strong black impressions of this forme on a stout unstretchable paper. Fix these up on a glass frame in such a position as to allow the light to pass through them, and carefully place each transfer in its exact position. They can then be laid down on separate stones in the same way as the first set. These are the simple outlines of the transferring process. In detail they may, of course, be modified to meet the exigencies of peculiar conditions, which in lithography are frequently the controlling powers, and at all times are matters of vital importance.
The Printing Machine--The Halligan--Some Mechanical Phases--Speed--Pressure--Levelling the Stones--Cylinder Brake--Inking Rollers--Damping.
Concerning the structural qualities of the various types of lithographic printing machines now in use, much might be written and divers opinions expressed. In this respect, however, it would be invidious to suggest that one maker's machines were better than another's, and such would be the natural trend of a discussion on these lines. The machines all have, it is true, many points in common where comparisons would be legitimate and easy. Yet, on the other hand, they each possess distinct advantages which will no doubt appeal to the printer individually, in proportion to their suitability or otherwise for his particular work. Conviction will follow experience in these matters, and any decision arrived at after this fashion may be regarded as a useful and valuable acquisition.
The illustration on page 35 (Fig. 13) gives a fair general idea of the modern lithographic machine.
Fig. 14 illustrates a somewhat novel type of lithographic printing machine, in which the gripper is entirely dispensed with, the sheet being held to gauges by the operator until caught between the small cylinder and the stone, when pressure is immediately applied. The stone is simply blocked up in the bed of the machine and the position of the print on the paper assured by moving the gauges. This useful little jobbing machine is a decided innovation, and the simplicity of its construction is only equalled by the precision of its movements.
[Illustration: FIG. 13.]
Lithographic machine printing presents many peculiar features, each one of which requires careful and constant attention for their successful operation. Some of its purely mechanical aspects--the care of the machine and its accessories, together with their various functions and applications--offer a wide scope for resourcefulness and ability of a high order. The primary purpose of the machine itself was undoubtedly to accelerate the reproductive power of lithography from a commercial point of view; and throughout the entire course of its development the aim of the engineer has been to produce a printing machine with an ever-increasing capacity for reproduction. It does not follow, however, that the printer's responsibility has been proportionately increased.
Mechanical appliances have now so far superseded hand labour that, apart from a thorough knowledge of the principles of lithography, which is in itself essential, successful lithographic machine printing is largely due to resourcefulness, alert perception, and a skilful blending of mechanical and technical knowledge.
[Illustration: FIG. 14.]
Passing over the vast amount of detail which is usually and almost invariably associated with machine printing, but which offers little that is new to the practical worker, it might be advantageous to discuss a few points which are too often overlooked.
Speed, as has already been pointed out, is a very important factor in lithographic machine printing. It has become quite a necessity, and everything which conduces to it should receive the most careful consideration. Economy of power is too seldom regarded as a standard of efficiency in the printer. At any rate, as far as this is concerned it is doubtful if he fully realises the effect of what may appear to him as insignificant matters. A little pressure more or less on the stone may be in itself a mere trifle, so also would be a careless arrangement of the inking rollers or indiscriminate damping of the stones, yet, when taken together, what a considerable waste of power they might cause;--a waste which is altogether unnecessary and could easily be obviated by care and forethought. Excessive pressure is frequently resorted to in order to "bring up" an impression which is defective owing to some error of judgment in its preparation. It undoubtedly secures the desired effect, but at what a cost! There is a proportionately heavier drag on the machine and a greater strain on its most vital parts. The following view of this matter may be regarded as somewhat exaggerated, but it is by no means an uncommon state of affairs, and will at least serve to emphasise the importance of this point. It is a popular fallacy to suppose that in adjusting the litho-stone to the bed of the printing machine it should be made _perfectly level_. As a matter of fact a much easier and more satisfactory impression can be made from a stone which is worked just a little higher at the front or gripper edge than at the back, and for this reason. The drag on the cylinder as it makes the impression is appreciably greater at the back than at the front, and when the pressure is heavy it has a tendency to leave the back edge with a very decided jerk. The remedy is obvious and simple. As already suggested, the stone should be set in the machine with the least possible inclination towards the front. This adjustment is easily effected by a judicious arrangement of a few layers of brown paper. Just think for a moment of the effect likely to be produced by such a jerk or jar, which would under ordinary working conditions occur from twelve to fourteen times per minute whilst the machine was in motion! Abnormal pressure would of course intensify the strain, and sooner or later produce results of a decidedly disastrous character. Under the most favourable conditions this continual springing would tend to move the stone out of position, and thus affect the register of one forme with another.