In treating of Chromolithography, we propose first to point out those conditions and processes which are common to all the various methods of producing coloured effects; then we shall give a description of simple colour-work, suitable for plans, labels, show-cards, &c; next, of tinted lithography, which is used in subjects of more artistic character; and, finally, that combination of the former methods which is usually denominated Chromolithography or Oleography, when applied to the reproduction of artistic subjects. In this manner we shall have occasion to introduce some things in the first part which it will not be necessary to repeat in the other two, of which the methods of making sets-off and registering the sheets may be cited as examples.

149. General Conditions.—It may be stated generally that all lithographs in two or more colours are printed from two or more stones. That being so, it will be seen that some method must be employed, first, to get a correct trace of the subject on the first stone made upon the second; and, second, of being able, in printing, to lay the sheet so correctly in position that the second printing may fall exactly into its place upon the first. To attain the first of these conditions we must have what is called a key-stone.

150. Key-stone.—In the ordinary run of show-card and label-work this stone is used for finishing, but in work of more artistic character it is usual to employ this stone as a means of getting all other colours in their place, and generally to omit it in printing.