The Invention of Lithography Part 13

The varnish must be prepared in the open, far from buildings, because of its combustibility. The best utensils and skilled workmen are required, because otherwise terrible accidents may occur, and even life be lost through explosion of the copper receptacle. Whoever does not require as much as one or more hundredweight of varnish in a year, would better buy it from printers or make only a small quant.i.ty, one or two pounds, and in an open vessel. For this purpose I will describe the process.

One, or at most two pounds of good old but not rancid linseed oil are poured into a clean iron pan which has a long, strong handle and is so large that the oil takes up only one half or, better, one third of the s.p.a.ce. This is heated over a good fire till it burns, which is facilitated by applying flame to it. Oil that is too new has much water and other impurities that make it froth and run over. In that case the oil must be poured into the pan only in small quant.i.ties, when one must take great care to avoid spattering. As soon as the oil burns, the pan is removed from the fire and placed in a safe spot. If it is hot enough, it will continue to burn. It must now be stirred from time to time with an iron rod. Usually the flame increases under this stirring, but sinks again immediately at its cessation. So long as it does this, there is no danger that the flame cannot be easily extinguished if need be. But when it begins to continue burning with a great flame after the stirring stops, and at the same time to bubble and froth, it is high time to cover the pan with a close lid and leave it covered till the oil no longer takes fire when exposed again to the air. Then a dry knife is introduced and as much oil removed as will adhere to its point. If it does not permit itself to be pulled into long threads when cool, but is too thin, it must be heated again until it gets the required consistency.

A good varnish dries very readily of itself, and it is not only unnecessary but inadvisable to mix a drier with it, as varnish so treated is too likely to off-set on the stone.

Several strengths of printing-varnish are needed for the various methods of lithography. Therefore a stock of thin, medium, and thick varnish is needed.

In making the thin, the oil has been reduced to about two thirds through combustion. It is somewhat like fluid honey and does not pull into threads.

Only a little more than half the oil is left in the case of medium varnish. It is thick as old honey and can be pulled into threads a foot long.

In the thick varnish the ma.s.s is not much less, but it can be pulled into threads of a yard in length; and further boiling makes it thick and tough like gum elastic. In the latter case it can be used with advantage when rubbed down with oil and properly thinned. But as soon as it has obtained the last-mentioned degree of thickness and toughness, it must be cooled quickly, for then it is not far from hardening completely and becoming worthless. In the beginning it requires a long while for the oil to reach the first degree of thickness, an hour or more for a pound.

But after that period the thickening progresses rapidly, so that a quarter of the time will bring it to the point of total toughness.

To make printing color of the varnish, the proper amount of lampblack must be mixed in. The roasted or burned-out is best in this also, because the ordinary lampblack delays the drying and turns yellow with time.

The more lampblack is mixed in, and the more thoroughly they are combined by rubbing down, the better will be the color. But lampblack must not be added in such quant.i.ties that the color becomes dough-like.

In describing the various styles of printing I will describe the best printing-inks also. I will merely make the general note here that designs on stone take the ink best when it is thin and fluid, but that there is less danger of off-set on the parts of the stone that are to remain white, if the ink is tougher or contains more lampblack.

Too much lampblack and too tough a varnish endanger the finer strokes and dots, however, so that they will not take ink, being, as lithographers say, rubbed out. The rubbing or grinding effect of too tough an ink is like that of pumice or other grinding material. With tougher varnish, clearer imprints can be made and they do not become yellow easily. But the inking is more difficult and demands greater skill, as well as heavier pressure in the press.

The varnish can be mixed not only with lampblack but with many other colors, which will be described when I reach color printing in this essay. Sometimes black lacquer is used with advantage instead of lampblack; and Frankfurter black is successful in the intaglio and aquatint methods.

X

RUBBING-UP INK

It happens often that weak parts of a design cannot withstand the etching fluid and are cut away; also, that fine lines are rubbed away through unskilled treatment during printing. Then frequently a very simple remedy is to ink the plate with the so-called rubbing-up ink.

This color consists of a thin varnish in which a portion of litharge of silver or mennig or white lead has been dissolved thoroughly over the fire, and a proper amount of lampblack added. Often it is good to add some finely powdered sand or powdered pumice stone.

To prepare this, a portion of the thinnest varnish is heated in a pan till it burns. Then about an ounce of finely powdered mennig (or another lead oxide) is stirred in to each sixteen ounces of varnish till all is thoroughly mixed.

A rubbing-up ink can be made also by mixing common printer's ink with vegetable oil, tallow, and a very little soap.

Each of these colors adheres to all those places that have a trace of fat and thus gradually makes faint places in a design receptive again.

Later I will describe how to use care in applying this color, so that the entire stone shall not be s.m.u.tted and spoiled.

CHAPTER III

CONCERNING ACIDS AND OTHER MATERIALS

I

GENERAL PROPERTIES OF ACIDS

Probably most lithographers still believe, as I did once, that the etching with acids prepares the stone, and that the succeeding application of gum merely increases this preparation. Countless experiments have taught me that the exact reverse is true. Gum arabic and a few other similar bodies are the true factors in preparation, and the acids simply make the stone more receptive for them. Only sulphuric acid, which changes the surface of the stone into gypsum, prepares it without gum; but this is available only for a few intaglio methods.

The stone used for lithography consists mostly of limestone sated with carbonic acid. Most acids, and even the salts, possess more affinity for limestone than the carbonic acid, which latter is freed and escapes in gaseous form as soon as another acid touches the stone. If aquafortis, muriatic acid, vinegar, etc., is poured on the stone, there rise a number of air blisters, which are nothing except the escaping carbonic acid, and the applied fluid seems to boil, in degree according to its strength. The boiling and bubbling last till the fluid has sated itself with lime, after which it becomes still, and is impotent for further etching.

The direct effect is the solution and destruction of parts of the surface of the stone. If it has been coated in parts with a fatty substance that resists the etching fluid, the places so coated are left untouched, so that, when the stone is cleaned, all the fat-coated lines and dots are in relief.

If the stone is coated with fatty matter, but not so thickly that the acid is entirely resisted, it will pierce the covering and eat away more or less of the stone. If the etching is continued or if the acid is strong, the fatty coat will be destroyed entirely, the surface of the stone will be clean, and ready for the ensuing preparation. The preparation of the stone for pen drawings with oil or soap-water and several aquatint methods, is based on this principle, that a very thin coating of grease can be etched away partly or wholly, at will.

After eating away the surface of the stone the acids have the property of giving it a fine polish.

Therefore if the stone has been covered with a design, and then etched with an acid, it could be inked and printed many times, as long as it is kept properly dampened and not too much pressure is used in applying the ink. However, this could be done also with a thoroughly clean stone, using only water, though the polish obtained from etching makes it much easier. But this apparent preparation is not by any means sufficient to print with certainty; and it becomes perfect only if the stone is coated with a solution of gum arabic in water after being etched. If a plate that has been merely etched and not treated with gum becomes dry during printing, or even if too much pressure be used in applying ink or in cleaning with the more or less s.m.u.tty cleaning rags, it generally takes color and s.m.u.t which are extremely hard to remove.

We may a.s.sume, therefore, that the acids have the following effects on the stone:--

(1) They will not attack the parts coated with grease.

(2) They will penetrate more or less if the fatty coating is only thin.

(3) Where they touch the stone they dissolve it and eat it away.

(4) They give it a polish that facilitates printing. This polish disappears after a time on account of the cleaning with sponge or rag, but is replaced by a new polish produced by this very means.

(5) They do not prevent the adherence of fatty material later, as soon as the stone is dry, for which reason the parts prepared in the beginning with acid and gum arabic must be prepared again by renewed etching, to take the ink.

(6) Finally the acids have the property of giving to prepared stones that have been used for impressions, a rough surface instead of a polish when they are applied again, because they attack some parts more than others, producing little pores with sharp edges which catch the ink.

This fact, as I will show more clearly later, makes necessary extraordinary care if one wishes to clean prepared plates or correct defects with new etching, because unskilled handling will often make them worse.

II

THE ACIDS SPECIFICALLY

Nitric acid or aquafortis, muriatic acid, vinegar, tartaric acid, and acid of wood sorrel, all have nearly similar effects, but aquafortis and muriatic acid are used because of their greater cheapness.

Oil of vitriol or sulphuric acid, very much diluted with water, is available for light but not for extensive etching, because it transforms the surface of the stone into gypsum and deposits it again, so that after that the acid cannot penetrate at all, or only partially. If a part of vitriol, say diluted with twelve parts of water, is poured on a cleanly ground stone, there ensues a violent action which, however, is only brief. It might be supposed that the acid is sated with lime when it ceases to act, but if it is moved to another part of the stone it etches anew.

If the acid is washed from the stone and a woolen rag be used to rub it after it is dry, it takes on a mirror-like polish. In this dry condition it can be cleansed of color as easily as a copper plate, and if a stone thus polished is engraved with a steel tool, it is possible to make several impressions from it just as from copper. The polish is not lasting, however, because the skin of gypsum is very thin. But it is a useful method if it is desired to engrave the stone and ink it frequently to see the effect.

All the acids named have the property, previously mentioned, of etching the stone rough if it has been prepared before or used for impressions.

It seems that the gum unites more strongly with some parts of the stone than with others, admitting the acid in these latter places. Possibly, also, the bubbles caused by etching may help to produce this roughness by hindering the uniform action of the acid. This seems to be confirmed by the fact that an etched stone, prepared with gum, does not get nearly so rough when etched again with very weak acid as it does when stronger fluid is used.

In still greater degree does this appear when using citric acid or a solution of alum in water. Take a finely ground stone, pour diluted aquafortis over it, prepare it with the gum solution, and then dry it thoroughly with a clean rag. Now pour a little citric acid or alum solution on parts of it and let it dry. Then paint the parts so treated with a fat or printing-ink. If the color is rubbed off with a wet rag, it will be seen that the stone has become white again in all places except those where the citric acid or alum are. Those parts will have taken the color exactly as if they had been painted with chemical ink.

The same occurs when applying other acids, but in a lesser degree. This effect will be mentioned in future for many methods. Here I will remark only:--

It happens often that the stone takes color on places where it should remain clean. This is caused by clumsy handling, unclean rags, etc., and occurs particularly at the ends, because they dry first and are more exposed to careless manipulation. These s.m.u.tted places usually can be cleansed with a clean woolen rag and gum solution or even with a wetted clean finger. But sometimes the defect will not yield so easily, especially if the printing-color is soft. Then the only remedy is to prepare the stone over again, and that is the time when one must have regard to the roughening that ensues, if the stone is not to be rendered worse instead of better.