The earliest salted paper prints were made in the 1840s and 1850s from calotype paper negatives, waxed paper, and albumen-on-glass negatives. Salt prints were also sometimes called “Crystalotypes.” After the albumen print became popular, salt prints were often referred to as “plain” prints. This may have been because the albumen print had a glossy appearance, a much desired aesthetic of the period. Salted paper prints were also used as a base for over-painted solar enlargements from around 1859–1870. These required an exposure of at least an hour for the solar projected image to be printed deep enough as a reference for the application of chalk, charcoal, or pastel. Photographic printing frames are fitted with a hinged back to allow inspection of the print, without changing the registration of the paper to the negative. The most effective way to protect a salt print is to coat the surface with wax, thereby protecting the silver from harmful atmospheric effects.