412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



with bichloride of platinum. A coating of acid-resisting resin powder, 

 as used in the aquatint process, was also used occasionally to provide 

 a stronger ink-holding tooth. 



In his 1852 patent he mentioned the use of two or three sheets of 

 black gauze, laid across each other obliquely to break up the photo- 

 graphic image, as well as a suggestion "to manufacture on purpose 

 some pieces of more delicately woven fabrics, or to cover a sheet of 

 glass by any convenient method with fine opaque lines to intercept 

 the light, or with a powder adhering to the glass, consisting of distinct 

 opaque particles, and very uniformly diffused over the surface. These 

 things, which I believe have not been heretofore used in the fine arts, 

 I would denominate photogi'aphic screens or veils." This was the 

 first known proposal of the modern halftone screen. The division of 

 graphic arts of the United States National Museum has on display 

 a glass screen made by Talbot, a positive, in which regular circular 

 openings appear on a dark ground. The date of this experimental 

 screen is unknown, but it is likely that it was made about 1860, antici- 

 pating by three-quarters of a century the Grennell patent of 1935, 

 which proposed a screen on a similar principle. 



The first photomechanical halftones for relief printing (Talbot's 

 work was primarily in gravure) were made by Paul Pretsch of Vienna 

 in 1853 and patented in England in 1854. Pretsch exposed a glass 

 or metal plate coated with bichromated gelatin in the camera or copy- 

 ing frame and washed the resulting image in cold water, which 

 caused the gelatin to swell. Where light rays had exercised hardening 

 action the gelatin swelled least, and where the gelatin was least ex- 

 posed to light and remained softest it swelled most. In this manner a 

 delicate relief was obtained, graduated in height and depth according 

 to the amount of light absorbed. The reticulated gelatin was then 

 chemically treated to create a more pronounced grain, after which 

 an electrotype was made either directly from the swelled gelatin relief 

 or from a gutta-percha mold. Electrotype plates could be prepared 

 for either relief or intaglio (gravure) printing. The gravure plates 

 were extremely good for this early stage. Results in letterpress, while 

 coarse and far from uniformly successful, were quite creditable as 

 the first examples of relief printing to be achieved through the use of 

 photography. 



Pretsch's use of swelled-gelatin molds was widely adopted by suc- 

 ceeding practitioners in all phases of photomechanical printing up to 

 the perfection of the modern screen. Alphonse Poitevin of Paris 

 claimed priority in making use of a swelled reticulated gelatin mold 

 as a printing surface although there is no evidence to show that he 

 antedated Pretsch. Poitevin also made use of reticulated gelatin in his 

 discovery of the collotype process in 1855. 



