PROCESSES IN ILLUSTRATIONS 155 



water till all the unaltered albumen is sufficiently softened to 

 allow it to be easily wiped away, and nothing is left but the 

 hardened ink-covered lines. Asphalt is then dusted on them, 

 and gently heated till it melts and forms a protective covering 

 for the lines of the design. The plate is then varnished on its 

 back and sides and immersed in acid, which eats away its surface 

 except where it is protected by the film. During this process 

 ink is continually being added to the lines to protect their sides 

 from being undercut by the acid. When a sufficient depth of 

 zinc has been removed, the lines of the drawing stand high above 

 the surface, and form the block, which is then mounted on wood 

 or metal to raise it to the level of the type with which it will 

 be printed. 



A drawing for reproduction in this manner should be made 

 with a good black ink on white paper. It is essential that the 

 ink should be of the same colour throughout. Chinese white 

 should never be used to correct the drawing, as it comes out 

 grey in the photograph : a "process" white for use in its place 

 is sold by colourmen. The cost of an ordinary block is from 

 2\d. to 6d. the square inch, with a minimum price of 2s. 6d. to 6s. 

 The variations in price correspond to the difficulty of the 

 subject and the technical skill of the workman employed in its 

 reproduction : the lower prices only applying to the very 

 roughest work. 



Half-tone Blocks 



An ordinary photograph contains, besides its pure white high 

 lights and deep black shadows, a very large number of inter- 

 mediate shades of light, called technically " half-tones," which 

 cannot be adequately reproduced in a line block, and for which 

 the half-tone process is used if cheapness is desired, while, if 

 better results are necessary, photogravure or collotype processes 

 should be employed. The fundamental difference between these 

 methods is that in the first the difference of tone is obtained by 

 the varying size of the white spaces scattered over the reproduc- 

 tion as seen by a powerful glass, in the second by the amount of 

 ink deposited in each part of it from the plate. 



An ordinary illustration in a magazine, looked at through 

 a magnifying glass, is seen to be made up of a large num- 

 ber of black dots of varying size but uniform distribution. 

 When the dots are large and run into each other, leaving 



