Explain how photogenic drawing differs from daugerreotype photography.

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Photogenic drawing was developed by Talbot. Using silver chloride paper and a stabilizing exposure bath of potassium chloride, Talbot could make negative image pictures from objects in contact with the silvered paper. Exposures of an image took from several minutes up to two hours (depending on the subject) of exposure to sunlight. The key role of sunlight in the exposure process is why these negative image prints are called photo (sun, solar) -genic drawing. Talbot discovered that by placing the image in contact with a second piece of silvered paper (treated with silver chloride), a positive image could be produced, but only once. The potassium iodide (or potassium bromide) stabilized photogenic images as long as they were not displayed in daylight and produced colors of lilac, brown, green, orange, yellow and red. Fuzzy and limited in functionability, photogenic drawing--though a primitive form of today's photography--was less appealing than daguerreotypes (Michael R. Peres, The Focal Encyclopedia of Photography).  

Daguerreotyping was developed, originally in partnership with Niepce, by Daguerre. Using the same silver chloride used by Talbot for photogenic drawing, Daguerre's latent images were exposed with, first, mercury fumes (later iodide fumes), then stabilized on the same thin silver iodide treated copper plates used in the camera that were exposed for 20 to 30 minutes. The resulting fixed image was a daguerreotype and combined a positive and negative image depending upon angle and light while viewing. Permanently fixing (not just stabilizing it) the image was accomplished with sodium thiosulfate. Daguerreotypes were less time consuming than other methods, had "infinitesimal" resolution with extremely fine detail and permanence. Nonetheless, the practicality of a 20 to 30 minute exposure limited usefulness to still-lifes and landscapes until bromine fumes replaced mercury fumes in the initial step of plate sensitizing. Eventually the fuming process changed to sequential fuming with iodine, then bromine, then iodine. The heightened copper plate sensitivity reduced exposure to seconds instead of minutes thus allowing for daguerreotype portraiture in studios under skylights (Michael R. Peres, The Focal Encyclopedia of Photography).   

[Developed by] Daguerre, who invented the process in the 1830s, daguerreotypes were created by exposing silver-plated copper slides to natural light in a camera for several minutes before fuming the plate with [originally] mercury vapor [later to iodine and bromine] to develop the image on the [copper] plate. ... the first photograph of Abraham Lincoln is a daguerreotype. (Ted Gregory, Chicago Tribune)

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