Last chapter completed the subject of chalk drawing, as far as putting the subject on the stone is concerned; but there is an intermediate process to be gone through before it is ready to be printed from, which is very important, as affecting, in a very marked degree, the good quality of the impressions. Our present chapter is therefore devoted to this operation, which is called—

91. Etching The Drawing.—This operation is usually performed by the foreman-printer ; but as it does not essentially belong to printing, and might with advantage be done by the artist, it will be described in this place.

The term “etching,” in lithography, is no doubt borrowed from the practice of etching on copper, but it is somewhat improperly applied. The etching process on copper consists in producing an effect by drawing with a point through a wax surface spread upon a metal plate, and afterwards fixing or deepening such work by “biting in” with dilute nitric acid. In Lithography the term “etching” is applied only to the acidulation of the stone by dilute nitric or other acid, the effect of which is rather to make the work lighter than stronger, and is thus diametrically opposed to similar operations on metal.

When nitric and most other acids are brought into contact with the carbonate of lime, of which the lithographic stone principally consists, decomposition ensues; the nitric acid seizes upon the lime, and sets the carbonic acid free, which then passes off rapidly in minute bubbles, producing the phenomenon known as effervescence.