The Invention of Lithography Part 18

(4) It must combine the greatest possible speed with this power.

(5) It must be easily operated, to save the workman.

All these qualities combined are not to be found in any press. .h.i.therto applied to lithography.

II

APPLICATION OF BOOK- AND COPPER-PLATE PRESSES TO LITHOGRAPHY

If we consider the peculiarities of book and copper print, we find a decided difference between them that affects printing importantly.

The letters of book-type are raised, the engraving in copper is depressed. It is evident that the former requires no such power for making impressions as the latter. Therefore the presses are so different that copper plates cannot be printed on a book-press and vice versa.

Now, as the stone combines both the elevated and the depressed principles, the natural idea would be to combine the fundamental principles of both presses as nearly as possible for stone-printing. In book-print, only the types are exposed to the pressure, and in the average printed sheet these are only one fourth part of the entire surface. The remaining white s.p.a.ce is not affected at all by the press.

In the stone, however, the elevation of any part of a design is so slight that the entire surface is affected, and consequently a stone plate offers four times as much resistance. A book-press therefore would print a stone only if it were arranged for a pressure four times greater. Now, for a stone of the size of a letter-sheet the power required to print with one vertical pressure would be five or six hundred hundredweight, a pressure that could be supported only by a thick stone laid very exactly on a perfect foundation.

An ordinary copper-plate press increases the pulling of the paper so much in the case of a stone plate that the impression would be worthless. This pulling is not caused, as in the case of the sc.r.a.per, during the impression itself, as already described, but it is caused before the impression through the endeavor of the cylinder to force the plate along under it. Once the stone is under the cylinder, the paper is not pulled noticeably, because the cylinder glides over the leather much more gently and with much less friction than the sc.r.a.per.

This defect might be corrected:--

(_a_) By supporting the cylinder so that it would come down on the stone only at the point where the print is to begin. But as the stone must be drawn pretty well forward for convenience in inking, this would demand that the cylinder be revolved forward and backward again as far as is needed for the impression, which means a great demand on the strength of the printers, not to count the loss of time.

(_b_) A second way would be to plane off a piece two inches wide from the cylinder at the point where the impression is to begin. The stone could be forced under this s.p.a.ce readily, and when the cylinder revolves, it presses forcibly at once without pulling the paper very much.

(_c_) The press might be fitted with iron wheels with cog teeth to engage similar cogs on the cylinder. This would prevent pulling, but the mechanical work would need to be very accurate.

(_d_) The best arrangement will be the following: Set the upper cylinder so high that the stone can be brought under it without touching. Then bring it down with a screw, or better still, with a lever that can be operated by the foot.

The first figure in the plate showing presses represents about how a copper-plate press is to be fitted for this work. On the whole, this is an ordinary copper-plate press, but the upper roller is set with its two axles or spindles in two iron levers, each of which is fastened to a piece of wood with iron screws one inch thick. Each of these pieces of wood is covered with strong sheet iron and can be adjusted higher or lower with two screws or with underlay of pasteboard. This is necessary that the press may be adjusted to varying pressures. The two other ends of the two levers, in which the cylinder sits, can be raised or lowered, so that the cylinder also can rise or sink. Now two springs or two weights are so adjusted that the cylinder with the levers always remains elevated. To force it down on the stone, an iron beam enters both sides of the press with two pegs so adjusted that when the beam is turned ninety degrees the levers are depressed at least two inches. As the cylinder is about in the middle of the two levers, it will thus be depressed one inch, which suffices to permit the stone to pa.s.s under it freely while it is elevated and gives the greatest pressure when it is depressed. However, the upper cylinder must not be one inch distant from the stone, but at the most only one fourth inch, for the remaining s.p.a.ce of three fourths inch is required to provide margin for the elasticity of the various materials, and also to give margin for increased pressure whenever demanded.

On one end of the iron beam with the two pegs is an arm or lever which is joined to a thin stick with a treadle. This tread is so arranged that it remains elevated of itself. If the pressure is to reach sixty or more hundredweight, it must not be fastened directly to the treadle, but a second lever is required which is affixed to the side of the press.

Without going into tedious detail I cannot further describe this press.

Mechanicians will understand me readily and perhaps be able to add many improvements. My belief is that a copper press so arranged would diminish all danger of squas.h.i.+ng and pulling the impression, furnish powerful pressure, permit overlays of felt or fine cloth, and make possible considerable facility and celerity, which is a great advantage, because impressions always are better if too much time is not lost between inking and printing.

To safeguard the stone against cracking in such a press, the following points are to be noted:--

(1) The stone must be ground very true on the under side as well as the upper.

(2) Both cylinders must be perfectly true, and care is to be taken particularly that one cylinder is not thin toward the middle and the other thick, as this would easily crack the stone lengthwise.

The board on which the stone rests must be equally true and uniformly thick. At the same time it must be very thin, only one half inch thick at most. It will get very heavily squeezed during the printing, and the more the impression approaches the centre, the more concave will it become. The parts farthest from the point of pressure then resist unduly if the board is thick, and thus become the chief cause of cracking the stone. If the rollers are very true and the stone is very uniform, it is almost impossible to crack it if it is pa.s.sed between the two rollers without a board underneath. If the board is thin, it is as if it were not there.

I believe that competent mechanicians can improve the present presses greatly.

III

LITHOGRAPHIC PRESSES USED HITHERTO

Most owners of lithographic printeries have tried their hands at inventing presses, but in the end it has always been something based on the sc.r.a.per or the cylinder principle. I myself have made more than twenty designs. Some were very useful and had advantages either in power or convenience, but generally were handicapped by some defect, so that I cannot even say with certainty which was the best of them all. So much depends on the mechanic's execution of one's plans, and a perfect design can be so spoiled by a workman that it is worthless.

I will, however, recount the best that has been done so far for lithography.

In Munich two kinds of stone presses are mostly used. They are:--

(1) The lever press, or, as the workmen generally call it because of its form, the Gallows Press.

(2) The Cylinder or so-called Star Press, the latter term being used because a star-shaped lever is commonly used instead of a crank to turn the rollers.

I have tried and found good the following:--

(3) A press with double levers.

(4) A gyrating or sliding press.

I know also--

(5) The roller press used by Herr Andre.

(6) And the press of Herr Steiner in Vienna.

Herr Muller in Karlsruhe and Herr Ackermann in London have a press with paper cylinders the construction of which is unknown to me.

IV

THE LEVER PRESS

This was the first press that I used with advantage, and it is used still in Munich in all important establishments for work that demands speed particularly. It would be an excellent printing-machine in all respects if it did not have the defect that its power cannot be increased much more than six hundredweight without forcing the workmen to undue exertions. Therefore it is no longer available for large plates or for works that require immense power. It is very good for pen designs not larger than a letter-sheet, and two workmen, one to ink-in and the other to print, can produce twelve hundred impressions in a day without hards.h.i.+p.

The pressure is produced by a lever six to twelve feet long, fastened to the sc.r.a.per below and to a spring (an elastic board) above. It is connected with a tread, and when forced down, presses with the desired force on the sc.r.a.per and so on the plate. The board holding the lever overhead must be partially movable like a spring because the lever describes a part of a circle on the plate below. Hence the pressure at the beginning and end of the impression is not so great as in the middle, and great care in choice of wood and manufacture is demanded to give the spring board the necessary elasticity and power combined. I have found a board of young dried pine the best, the dimensions being six feet long, eight inches wide, and two inches thick, provided that the fibres all ran lengthwise. It is not always possible to find a good board at once. Often I have found that the difference between two boards made a great difference in the effectiveness of two presses otherwise exactly the same.

The sc.r.a.per arm consists of two parts, of which the shorter one, to which the sc.r.a.per is fastened with a screw, is only one and one quarter feet long. The other part is as long as the height of the press permits.

The higher a lever press is, the better is it, because then the circular motion described by the sc.r.a.per wood approaches a straight line more and more, so that the press exercises a more uniform pressure during all stages of the impression and is easier to handle. The second ill.u.s.tration shows this kind of press in the moment when the impression has been finished, the printing-frame opened, and the sc.r.a.per arm swung back again.

The printing-frame is much like a book-printing frame, and is furnished inside with a second small frame which holds the paper, being furnished with small springs or strings. When the frame has been turned over the stone, the paper must be at least half an inch from the stone to avoid s.m.u.tting, which will occur if it touches. The paper must not touch the stone till pressure is applied, and then only on the spot pressed downward by the sc.r.a.per.

As soon as both parts of the sc.r.a.per arm are in a straight line, so that they form practically one piece, the sc.r.a.per wood is pulled down and the printer draws it toward himself over the printing-frame and the stone plate. At this time the following is to be observed:--

(1) Both parts of the arm must be so fastened to each other that they may be bent like a knee, but once they are straight in line, they must stay in that position. It is well, therefore, so to adjust the parts that they will not be directly over each other, but rather exceed a straight line under pressure, and bend a little inward. The position of the sc.r.a.per must be considered also. On the whole the following rule holds good: the point where both parts are united with a nail or a screw must not be in a perfectly straight line between the point where the sc.r.a.per rests and the point where the arm is fastened above, but should be at least two and a half inches forward of that point. Otherwise the arm may spring outwards toward the workman and injure him severely. The third ill.u.s.tration shows the construction of the sc.r.a.per arm and the sc.r.a.per.

(2) The arm must be grasped as low as possible when being drawn toward one's self, in order to diminish the danger of springing outward.

(3) The workman must press his body tightly to the table of the press to get proper leverage. Standing free, a man of moderate strength could not move the sc.r.a.per at all when the pressure is on, but a man standing in correct position can do it without difficulty.

(4) Under very heavy pressure the inker-in, who stands on the other side of the press, can help by pus.h.i.+ng.