422 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



screens, exposed twice to create cross-lines, rather than the cross-line 

 screens now in use. Saalburg's firm, the Van Dyck Gravure Company, 

 also made great advances in color, preparing a four-color rotogravure 

 page for the Printing Art of June 1908, which is said to be the first 

 example of machine-printed rotogravure in full color ever used in a 

 periodical. 



immm 



Figure 2. — Detail of rotogravure halftone screen, greatly enlarged. 



In the rotogravure process a special halftone screen is used, one 

 with opaque squares and transparent lines, a reversal, that is to say, 

 of the usual screen. This screen is placed in contact with the carbon 

 tissue, previously sensitized with potassium bichromate. Upon ex- 

 posure to light the line structure of the screen is hardened on the 

 tissue, while the dark areas, which prevent the light from reaching 

 the gelatin, remain soft. A positive of the subject is then placed in 

 contact with the carbon tissue and exposed again. This time the 

 light penetrates the previously unexposed gelatin in the square areas 

 and hardens it in proportion to the amount of light received. The 

 tissue is then squeegeed to the copper cylinder, developed, and the 

 unhardened areas and the tissue backing are washed away, after which 

 the cylinder is etched through the varying levels of the gelatin. The 

 resulting image is etched beneath the level of the cylinder to varying 

 depths, with the hardened and unetched lines of the screen forming 

 walls that remain level with the surface. The cylinder is rotated in 

 a trough of thin ink, after which a flexible steel blade held in contact 

 with the cylinder wipes the ink from the surface, allowing the ink to 

 remain in the wells below. The cylinder rotates through the ink, 

 the steel "doctor" blade wipes the excess from the surface, and the 

 image is impressed on paper fed from a cylinder either in separate 

 sheets or from a continuous web of paper. The entire picture is a 



