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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 t.i.tle 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 a.s.sumes 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 r.e.t.a.r.ded; 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 quant.i.ty used. In all probability 1 oz. of a solid oil composition, otherwise known as lithographic reducing medium, would soften down a quant.i.ty 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 "sc.u.mmy" 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 "const.i.tutional 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 quant.i.ties 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.
SUBSt.i.tUTES FOR LITHOGRAPHIC STONES
Metal Plates--Preparation--Manipulation--Descriptive Details--Machine Printing--The Printing Bed--Rotary Printing Machine.
Metal plate, as a subst.i.tute 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. .h.i.therto 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 substantially practical 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:--
18 oz. Water.
1/4 oz. Alum.
1 dram Nitric acid.
Keep this liquid in constant motion over the face of the plate until it a.s.sumes 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 demonstrated by 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.
[Ill.u.s.tration: 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.
[Ill.u.s.tration: Plate-graining machine, showing interior of trough.]
Roll the plate up _solid_ in 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, and _thinly_ gummed 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 quant.i.ty 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 left _to 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 on the _clean dry rag_ till 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 sc.u.m.
"When alterations are necessary the work should be rolled up with a strong black ink, and dusted over with French chalk (powdered talc).