The duguerrotype was invented by Frenchman Louis-Jacques-Mandé Daguerre in 1839. The image is made by exposing a sheet of copper coated in silver halides to light. After the exposure (anywhere between 3 and 30 minutes) the plate would then be developed using heated mercury fumes. To fix the image, the plate was then immersed in a solution of salt or sodium thiosulfate. Gold chloride was then used to tone it. This process created a positive photographic image (no negatives). This made them impossible to exactly reproduce. The process also made the plate quite fragile so Daguerrotypes were often kept in glass cases.
Images and Information from:
http://en.wikipedia.org/wiki/Daguerreotype
http://memory.loc.gov/ammem/daghtml/dagdag.html
http://www.rleggat.com/photohistory/history/daguerro.htm
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