Illustration: LITHOGRAPHIC PRESS.
This is the art of producing designs upon stone in such a manner
that impressions may be printed from them. It is often miscalled
“engraving on stone;” properly speaking, it is not engraving, but a
process depending on the want of affinity between watery and greasy
matters, the design being made with a greasy substance, either in
the form of ink or crayon called “lithographic chalk,” as follows.
A close-grained stone called lithographic stone (a kind of magnesian
limestone) is ground perfectly even on the surface, and (if for being
written upon) polished with pumice-stone, or (if for being drawn upon
with chalk) grained with fine sand; the design is drawn with chalk
or ink, each of which is made up of greasy materials--wax, soap,
and asphaltum, colored with lamp-black. When the design is made,
the stone is etched--washed over with a weak mixture of nitric acid
and water--which answers three purposes; it very slightly bites away
the stone in all places except where it is covered with the ink or
chalk, it smoothes the grain in the same places, and it converts the
soap of the ink or chalk into grease. We have now the surface of the
stone quite wet, except where the design is, which, being greasy,
keeps quite dry. The stone, placed in a press, is rolled over with a
roller smeared with greasy printing ink, which will not adhere to the
wet stone, but it readily does to all parts of the design; a piece
of paper is placed over the stone, and a flap of leather covers it;
along this leather the scraper of the press is brought very forcibly
when it is worked, and on removing the paper an impression is found
to be transferred to it from the stone. This process is repeated
again and again, many hundred times, without much injury to the
original design.
Colored lithography, called chromo-lithography, is produced by using
several stones consecutively (each transferring a separate color), so
that the design shall be complete when the whole are printed.