HALFTONE SCREEN — KAINEN 421 



The most importtint improvement in the process was the contribu- 

 tion of Karl KHq, of Bohemia, who in about 1875 employed carbon 

 tissue, a paper coated with pigmented gelatin, originally invented by 

 Swan, which was sensitized with potassium bichromate and exposed 

 under a positive. It was then placed face down on a copper plate 

 coated with minute rosin granules which had been made to adiiere by 

 heat, and washed with warm water to dissolve the unhardened gela- 

 tin and remove the paper backing. The image was thus fixed on the 

 plate with various densities of bichromated gelatin corresponding to 

 the variations of tone in the original. When placed in an acid bath 

 the acid, generally perchloride of iron, etched through the resist to 

 the degree that the levels of gelatin permitted. The result was a 

 plate etched in varying depths, a photoaquatint, that when inked and 

 printed on a copper-plate press reproduced the subject in varying 

 depths of ink. Klig's improvement on Talbot's process was in the use 

 of carbon tissue, which showed the image clearly in pigmented gela- 

 tin and which could be firmly fastened to the grains of resin. Klig 

 did not patent this process, which he kept secret for a few years, but 

 it soon became known and was generally used by gravure platemakers 

 from that time on. 



The greatest improvement in photogi'avure, from the standpoint of 

 commerce, was KHq's invention of the rotogravure process so univer- 

 sally used today. In about 1894 Klic etched copper cylinders, prob- 

 ably using a variety of halftone screen and sheets of carbon tissue, 

 which, after exposure under a positive, were squeegeed to the copper 

 cylinder, developed, and etched. Klig's idea was to etch square dots 

 of equal size but varying depths into the cylinder, which, when filled 

 with ink and the surface wiped clean with a steel blade, would create 

 tones depending upon the thicknesses of ink deposited upon the paper. 

 Letterpress halftone, on the contrary, made use of dots of unequal size 

 but of uniform color. In rotogravure a thin ink was used so that the 

 paper showed through in the light tones but less and less as the ink 

 deposits became heavier. 



At a single leap rotogravure became a practical, high-speed process. 

 It took some years for others to work out a process comparable to 

 KHq's, which was employed by the firm he set up in England, the 

 Rembrandt Intaglio Company. This organization was the first to 

 make commercial use of the rotogravure process. 



Although KHq kept his methods secret, his results clearly indicated 

 the nature of his general conception of platemaking and printing. 

 Between 1901 and 1908 Dr. Edouard Mertens, of Berlin, and Charles 

 W. Saalburg, of East Orange, N. J., succeeded in perfecting the meth- 

 ods now in use. Saalburg, in 1909, patented the process, which, in its 

 essentials, is in present use, although he originally used single-line 



