As to the inking-in color, I am not able yet to lay down a strict rule. All that I can say, as a result of my experiments and experiences, is:—
(1) The firmer the varnish in a color is, the cleaner is the work of inking-in.
(2) The same is true the more lampblack it contains.
But in both cases the finer parts of the work are easily rubbed away, and too much lampblack makes the lines squash the impression.
(3) The toughness or fluidity of the color must bear correct proportion to the power of the press. The harder the varnish, the more power is required in the press.
(4) Tough varnish is not so liable to squash under pressure, but if it has once been pressed into the spaces between the lines of the design it is not readily removed by the mere action of the inking-roller, and this causes more and more smutting and, finally, total ruin to the stone. Generally when a tough color has adhered too much, there is no other remedy than to clean the stone well with gum and oil of turpentine; and this, if done too often, damages the preparation and makes the impressions continuously poorer.
(5) Soft color spreads more readily under pressure, but is removable after each impression by merely dampening the plate.
(6) In using soft color, the paper may be kept damper than with hard colors.
(7) Soft as well as hard printing-color, if not mixed with the proper amount of varnish, has the property of producing poor, sooty impressions because of a defect called shading. Shading is caused as follows: If a drop of oil falls into a basin of clean water, a part of the oil will spread immediately. Now, a stone is wetted before inking-in. After the inking a considerable portion of dampness remains. If the ink is very fluid, it will happen often that a part of it will spread away from the design to the surrounding moisture, producing something that looks like a shadow around every part of the design. This does not occur instantly, as in the case of the pure oil, but gradually, so that it is not as noticeable when the swifter lever press is used as with the slower cylinder press or if the workmen are slow. If a stone can be dampened so exactly that with the last touch of the ink-roller the last vestige of dampness is removed, this is not likely to happen. But it is difficult to arrive at such accuracy. It is better to add enough lampblack gradually to the varnish to make it lose its elasticity, when the shading effect will cease.
(8) While shading is obviated largely through enough intermixture of lampblack or other coloring substances to take away the fluidity of the printing-color, this intermixture will cause other troubles. The finer places will not take the harder color so well, whereas at other places too much will be taken. Also an impression made with much lampblack will off-set more than one made with color in which varnish predominates. Neither will the impressions be so black. Experience teaches that a printing-color that has less lampblack will be blacker, because the sheen of the varnish will make the color strong and lacquer-like. I have tried to invent a kind of varnish that would not be so liable to shading and thus would permit a greater fluidity with safety, but lack of time has prevented me from exhausting the possibilities. I am sure, however, that it can be done, for I have found that the common linseed oil varnish can be made to lose its property of shading by admixture of fatty and resinous bodies. For instance, the addition of a slight amount of Venetian turpentine permits a greater fluidity. Very good is the following composition: Six parts linseed oil, two parts tallow, one part wax, melted together and thickened by boiling down and burning like the ordinary linseed oil varnish.