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Copper plates were, until about 1820, the only metal on which intaglio prints, either engravings or etchings, were made. As Bamber Gascoigne explains in his excellent book How to Identify Prints , steel plates were first used in the 1820s "to print long runs of banknotes which would remain identical throughout the run and which would be so finely engraved as to make forgery difficult." The harder metal was more difficult to work with, but enabled very fine detailing, and the plates never wore out. Steel plates became popular for various kinds of subjects, notably topographical views, which might be printed in large numbers to appeal to tourists. From the 1850s it was possible for artists to work on the more forgiving copper plates, which were then subsequently "steel-faced", a process in which a thin layer of iron is electro-plated to the surface of the copper, thereby extending the life of the plate. Steel plates remained the choice for banknotes and postage stamps