I have discovered a wonderful on line resource that you can use to help you identify and conserve your photos. It is called the Graphics Atlas which has been put together by the Image Permanence Institute a New York based research centre.
This online resource includes examples of all different types of early photos, slides and negatives. Information about the construction of different types of photographs, negative and slides, the sort of damage they typically suffer from, cross sections and close up views of the image are provided.
Below is an example of an early retouched photograph from the Atlas, proving that retouching and restoration is not a new, and in fact was used in early photographs.
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