The Calotype proper is a negative image (along with its offshoot the waxed paper negative), although its positive counterpart, the salted paper print, is the more common form in which it is encountered. Calotypes are made by brushing the best quality drawing or writing paper with a solution of silver nitrate, drying the paper, and then immersing it in a solution of potassium iodide to form a light-sensitive layer of silver iodide. Immediately before use the surface it treated with 'gallo-nitrate of silver' (a mixture of silver nitrate solution and gallic acid) to act as an accelerator. Exposure in a camera, where the paper must be held in a dark slide, produces a latent (invisible) image which is developed by washing in gallo-nitrate of silver, fixed in hypo and thoroughly washed. The translucency of Calotypes can be improved by waxing, and a positive can be made by repeating the original process or by 'printing out' the image in much the same way as making a Photogenic Drawing. When toned, in, for instance, gold chloride solution (to give it a purpleish tone), a positive produced in this way is known as a 'salted paper print'.
With the exception perhaps of the waxed paper process, which was invented in 1851 by Gustave Le Gray (1820-1882) and extended the life of paper negatives in the 1870s, the first generation processes - the Daguerreotype, Photogenic Drawing and Calotype - were all extinct by the end of the 1850s, having given way to their own offspring: the wet collodion glass negative and the albumen print.