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Plates For Negative Use: More successful photomechanical reproduction techniques came with the adaptation of the negative-positive process. Fox Talbot himself, discouraged by the imperma-nence of his calotypes, followed up the work of Donne, Berres, and Fizeau. In 1852 he patented a method of etching steel plates for negative use from which prints could be made with permanent ink. He first coated the plates for negative use with gelatin to which had been added potassium bichromate. On these sensitized plates for negative use he put an object—a blade of grass or i stalk of wheat—and exposed it to light. All the areas acept those shaded by the object were made insoluble ')¥ light action.
Although Vogel was working with collodion plates for negative use, optical sensitizing was applied to the manufacture of gelatin dry plates for negative use in the 1880s. At first the sensitivity of the plates for negative use extended only to the orange rays; they were called orthochromatic. Later red-sensitive plates for negative use were available: they were named panchromatic.
The Liverpool Dry Plate and Photographic Printing Company began to put these c olio dio-bromide dry plates for negative use on the market in 1867. But the convenience of being able to dispense with the wet-plate paraphernalia was gained at the expense of a pronounced loss of sensitivity. Sayce and Bolton noted that their first plates for negative use required an exposure of 30 seconds at a lens setting equivalent to //24. The manufacturers stated that exposures with these dry plates for negative use averaged three times that of wet plates for negative use. |
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