Zinco

Photozincography, sometimes referred to as heliozincography but essentially the same process, known commercially as zinco, is the photographic process developed by Sir Henry James FRS (1803–1877) in the mid-nineteenth century.

This method enabled the accurate reproduction of images, manuscript text and outline engravings, which proved invaluable when originally used to create maps during the Ordnance Survey of Great Britain during the 1850s, carried out by the government's Topographical Department, headed by Colonel Sir Henry James.

The foundation of this method is the insolubility of bichromate of potash upon exposure to light, allowing the printing of images on to zinc from photographic negatives.

At this time, high-contrast negatives were made using the wet plate collodion method (a solution of nitrocellulose in ether or acetone on glass). Once the negative had been made, a sheet of thin tracing paper was coated in a mixture of saturated potassium bichromate solution and gum water, and dried. This was then placed under the photographic negative and exposed to light for 2–3 minutes. The bichromate/gum mixture remained soluble on the parts of the tracing paper that were shielded from light by the opaque areas of the negative, allowing it to be removed, leaving an insoluble ‘positive’ image. This bichromate positive was then placed on a sheet of zinc covered in lithographic ink, and put through a printing press three or four times. After removal of the paper, the zinc plate was washed in a tray of hot water (containing a small amount of gum), using a camel-hair brush to remove all the soluble bichromate combined with ink. What remained on the zinc plate was a perfect representation in ink of the original composition, by virtue of the ink binding to the insoluble potassium bichromate.

The main advantage and innovation of this process over lithography was the use of zinc plates rather than stone ones. Zinc plates were lighter and easier to transport, could produce more prints, and were far less brittle than the stone plates originally used. The use of zinc plates was also the origin of the name photozincography, which Sir Henry James claims to have invented.

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