An albumen print was developed in 1850 by Louis Desire Blanquart-Evrard. It was the first commercially exploitable method of producing a print on a paper base from a negative. It used the albumen found in egg whites to bind photographic chemical to the paper. First, the paper, which is usually 100% cotton, is coated with an emulsion of egg white and salt (sodium chloride or ammonium chloride), then dried. The albumen seals the paper and creates a slightly glossy surface for sensitizer to rest on. Then, the paper is dripped in silver nitrate and water, which renders the surface sensitive to UV light. After drying, the paper is placed in a frame in the direct contact under a negative. The paper and negative are exposed to light until the image achieves the desired level of darkness. Sodium thiosulfate is used to fix the print's exposure preventing further darkening. Optional gold or selenium toning improves the photograph's tone and stabilizing against fading.
Carbon print, a photographic printing process that used lampblack (carbon black), was introduced in 1855 by Alphonse Poitevin. It was the popular printing method for the first half of the 20th century. A sheet of paper was coated with a layer of light-sensitive gelatin which contained a carbon permanent pigment. It was exposed to daylight under a negative. Originally, carbon tissue (a temporary support sheet coated with a layer of gelatin mix with a pigment) is bathed in a potassium dichromate sensitizing solution, dried then exposed to a strong ultraviolet light through photographic negative, hardening the gelatin in proportion to the amount of light reaching it. The tissue was developed with a warm water treatment, which dissolved the unharmed gelatin. The resulting pigment image is physically transferred to a final support surface. The process can produce high quality images and was adapted for color printing in 1868 by Louis Arthur Ducos du Hauron.
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Week 5
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