Part 1 (2/2)
By an automatic action they are almost imperceptibly lowered at each traverse of the machine, when they lightly cut away the surface of the stone until the old work is completely removed and a smooth level face is a.s.sured. The chief objection to this type of machine is that in course of time the knives become worn and slightly irregular, and it is but reasonable to suppose that when a number of small stones have been operated upon and immediately afterwards a full-sized stone is planed, such irregularities will be very p.r.o.nounced and detrimental.
[Ill.u.s.tration: FIG. 1.]
Many machines have been designed on the simple grinding principle, but one type differs from its contemporaries and offers several distinct advantages over them (Fig. 2).
This machine is constructed on hand-polis.h.i.+ng lines, _i.e._ the movements are to some extent mechanical arrangements of hand-polis.h.i.+ng principles. The size of stone makes no difference whatever, and the results are in the main uniform and satisfactory. The inconveniently sharp edges, such as are produced by the planing machine, are unknown,--the wear and tear on the stone is perceptibly lessened, and the power required to drive such a machine is not by any means a serious matter.
When a planing machine does not enter into the operation, and grinding by hand is therefore necessary, sharp, clean sand should be used as a grinding medium. To secure some degree of uniformity in the grain, and at the same time remove all the larger particles of grit, pa.s.s the sand first through a fine sieve. The harder qualities of sand have, of course, the greatest cutting power, and therefore are the most suitable for this purpose.
[Ill.u.s.tration: FIG. 2.]
When hand-grinding is resorted to, a continuous elliptical motion of one stone over the other with a slight twist from the wrist will prove most effective.
To finish the grinding, and as far as possible remove the deeper sand scratches, work off the sand in the form of a _sludge_. Unless this operation is carefully and patiently performed, scratches of considerable depth may appear on various parts of the stone's surface.
These, in the subsequent polis.h.i.+ng, may offer a strong temptation to the operator to work over one part of the stone more than another, so as to effect the removal of such scratches with greater rapidity. The almost certain result of this would be an uneven surface, which would in many ways prove troublesome to the printer.
_The graining of stones._--This is a matter concerning the preparation of stones which must not be overlooked. The introduction of shading mediums and other contrivances of a similar character has considerably minimised the importance of the grained stone, inasmuch as it cannot now be regarded as an indispensable feature of lithography. It is, however, still of inestimable value, and will probably always find a place and purpose in the practice of lithography, despite its depreciation owing to present-day limitations.
Recognising, then, the possibility of its retention, at least for some time to come, as a suitable printing surface upon which the lithographic draughtsman can work with undoubted facility of execution and effect, we must perforce include a brief description of its preparation in this chapter.
First of all, level the stone and to some extent polish it, after which the graining may be proceeded with. A gla.s.s muller about 6 or 7 inches in diameter makes an excellent ”grainer.” Failing this, a handy subst.i.tute will be found in the form of a small litho-stone, hard in texture, and with a smooth, level surface. Use as a graining medium sharp, clean silver sand only, pa.s.sing it carefully through a sieve according to the size or depth of the grain required. Sprinkle a little of this sand uniformly over the stone under treatment, together with a few drops of clean water. With a continuous circular movement pa.s.s the graining muller from end to end of the stone, exerting a firm and uniform pressure. Repeat this operation again and again, adding sand and water as required. Considerable time coupled with intelligent application will be necessary to carry out this work successfully.
Should the sand become too much worn before its renewal the grain will in proportion lose its ”tooth” or sharpness. On the other hand lies the danger of producing a grain which is too harsh or p.r.o.nounced. Therefore much depends upon the skill and judgment of the operator. A safe plan is to ascertain the progress at any time when a satisfactory result might be reasonably expected. The best way to accomplish this is by was.h.i.+ng from the face of the stone any acc.u.mulation of sand, and drying it, so that a test can be made with the actual grade of crayon to be used in the subsequent drawing. A powerful current of clean, cold water affords the best means for removing every trace of sand from the finished stone, and will leave it in a condition of almost complete readiness for the draughtsman; a good drying is then all that is necessary. Good results have been claimed for a method of graining which was introduced by the Americans a few years ago. In this process the grain is produced by sprinkling the surface of the stone with sand and rolling it with small gla.s.s b.a.l.l.s. These b.a.l.l.s having a limited area in which to work exert a continuous cutting power without any tendency to produce scratches. Some mechanical arrangement is necessary to impart this continuous rolling movement to the gla.s.s b.a.l.l.s and to maintain a uniform speed. It is quite easy to understand that with such a process, carried out under favourable conditions, very fine results might be produced with great rapidity.
CHAPTER II
LITHOGRAPHIC TRANSFER INKS
Various Forms--Distinguis.h.i.+ng Features--Formulae--Writing Transfer Ink--Stone-to-stone Transfer Ink--Copperplate Transfer Ink--A Modification.
Given a perfectly clean and smooth polished stone as described in the previous chapter, the next important point is the composition of the fatty matter wherewith a design or drawing is applied.
As already stated, the active principle in any suitable transfer medium of this character is invariably the same, no matter what form its composition takes. For applying with pen or brush it must possess soluble properties, and of necessity be reduced to a liquid form. Such soluble properties, however, must not interfere with its fatty properties, these latter being essential features.
In crayon or _chalk_ drawing the composition must be employed in a concrete form, as a crayon, the hardness and texture of which will be controlled (1) by the character of the work to be carried out; (2) by the character of surface to be operated upon.
Transfers from other printing surfaces can only be made when the composition used is in the form of a pigment, and reduced to a convenient working consistency.
These, then, comprise the varieties of transfer mediums which are likely to be required in most phases of commercial lithography. Others are, of course, employed for specific purposes and under peculiar conditions; but these, again, are more or less modifications of existing formulae, prepared to meet particular requirements.
The ink used for transferring impressions from one printing surface to another, _e.g._, the re-transferring of work from stone to stone, may with a very slight alteration serve for type to stone transfer; but a considerable departure must be effected to produce a satisfactory photo-litho transfer ink, while a composition of a peculiarly distinctive character will be requisite for the successful production of transfers from copperplate engravings, as well as for a transfer ink for writing and drawing on stone or transfer paper. This writing transfer ink must be soluble in water, yet without becoming slimy; otherwise it will not work freely with the pen or brush. It must also dry quickly, and without any tendency to smear.
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