Daguerreotype portraits were made by the model posing (often with head fixed in place with a clamp to keep it still the few minutes required) before an exposed light-sensitive silvered copper plate, which was then developed by mercury fumes and fixed with salts. This fixing however was far from permanent – like the people they captured the images too were subject to change and decay. They were extremely sensitive to scratches, dust, hair, etc, and particularly the rubbing of the glass cover if the glue holding it in place deteriorated. As well as rubbing, the glass itself can also deteriorate and bubbles of solvent explode upon the image.
Eight daguerreotypes from 1848, taken by Charles Fontayne and William Porter, which comprise a panorama of the waterfront of Cincinnati, Ohio, at the city's zenith have recently been restored revealing astonishing detail for photographs of that era, like a legible clocktower 1mm wide in the original, and people walking the streets of the city. [more inside]