The daguerreotype, invented by Louis Jacques Mandé Daguerre in 1839, was the first commercially successful photographic process. The image forms directly on a silver iodide plate which is placed in a camera obscura. During exposure, the sunlight dries the silver iodide whilst leaving unexposed shadow areas still wet. The image is then developed by exposing the plate to mercury vapour to fix the dry areas before the unreduced iodide is removed with salt. The resultant image is a mirrory silver and exquisitely sharp. There was a brief craze for daguerrotype portraits amongst those able to afford them in the mid-1800s but they quickly fell out of use when cheaper and simpler methods were introduced.