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How did Daguerre make daguerreotype images permanent?
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History of photography
Child with Striped Dress (Younger Raab Child), autochrome photograph by Alfred Stieglitz, 1907; in the Art Institute of Chicago.
To make daguerreotype images permanent, Louis-Jacques-Mandé Daguerre utilized a chemical process known as fixing.
After exposing an iodized silver plate in a camera and developing the resulting latent image using mercury vapor, the image remained sensitive to light, causing the unexposed areas to darken and eventually obscure the picture. By 1837, Daguerre discovered that he could stabilize the image by washing the plate in a solution of common salt. This solution dissolved the unexposed silver iodide, effectively stopping the chemical reaction and rendering the image permanent.123