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How did mercury vapor help produce a daguerreotype image?
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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.
In the daguerreotype process, mercury vapor served as the essential developing agent that made the latent image visible on an iodized silver plate.12
After the plate was exposed to light in a camera, the image remained invisible to the naked eye. By fuming the plate with mercury vapor, the mercury particles settled onto the areas of the plate that had been exposed to light. This chemical reaction effectively "developed" the image, significantly reducing the required exposure time from hours to approximately 20 to 30 minutes. Once developed, the image was made permanent by using a solution of common salt to remove the unexposed silver iodide.312