PROCESSES IN ILLUSTRATIONS 161 



drawing. The stone is then passed through the press, the ink 

 is printed off on the paper, and the stone is ready for a new 

 impression. Coloured inks may be used. 



A lithograph can be made to give great variety of tone if 

 its surface is roughened, or given a grain, as the grain on the 

 stone repels moisture in proportion to the amount of ink upon 

 it, and so gives accurately all the intermediate tones of the 

 drawing. 



In photo-lithography a negative— line or half-tone — is printed 

 on a transfer paper made sensitive by a coating of bichromatised 

 gelatin. The paper is then rolled with transfer ink and placed 

 in a bath of water, where the unaltered gelatin softens and can 

 be wiped away, leaving the print upon the paper, which can be 

 transferred to stone in the ordinary way. 



In chromo-lithography a separate stone is provided for each 

 colour, and the artist draws each by hand, adding new tints as 

 the work approaches completion. The usual number of stones 

 for posters or commercial work is seven, but as many as fifteen, 

 and sometimes twenty, may be employed in the finest work. 

 The difference between the chromo-lithograph and the three- 

 colour process is that in the former the artist's eye essays to do 

 what is automatically performed by the light-filters in the 

 camera. 



Photo-lithography is especially useful for the reproduction of 

 large work, such as charts, diagrams, architectural drawings, etc., 

 as for any small edition it is cheaper than a line block of the 

 same size. 



Combination Methods of Colour Printing 



Many very pleasing effects are produced by a combination 

 of various methods. It is quite usual to print the colours of a 

 reproduction by lithography and then to print the subject over the 

 colour either from a collotype or from a half-tone block. This 

 process is used in many of the most effective of the coloured 

 postcards of the Continent. Line blocks are sometimes used 

 when a block of solid colour has to be added to a design. 



In cases of special difficulty in three-colour reproduction it is 



not unusual to use a fourth block of a grey or light black tone 



to produce a softening effect, while when it is not desired to 



reproduce natural colours, but to make an effective picture, very 



ii 



