The US Library of Congress is brimming with flawless daguerreotype photographs that give us a pristine look into the state of things over the past two centuries. Now that's great and all, but perhaps even more incredible are the ones that succumbed to the 200 years of damage and decay.
According to The Public Domain Review:
[The plates were] 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.
These plates all come from none other than the father of photojournalism himself, Mathew Brady, a 19th century American photographer best know for his portraits of celebrities and documentation of the Civil War -- and his is a work that has unquestionably improved with age. The rest of the decayed daguerreotype collection can be found at the Public Domain Review.
Portrait of James J. Mapes [between 1844 and 1860], by Mathew Brady's studio.
Portrait of Emma Gillingham Bostwick [between 1851 and 1860], by Mathew Brady's studio.
Portrait of an unidentified man about 40 years of age and a somewhat younger woman, both in Oriental costume, between them is a hookah, the stem of which both are holding [between 1850 and 1860], by Mathew Brady's studio.
Portrait of unidentified woman [between 1844 and 1860], by Mathew Brady's studio.
Portrait of William C. Bouck [between 1844 and 1859], by Mathew Brady's studio.
Portrait of unidentified man [between 1844 and 1860], by Mathew Brady's studio.