Grant Romer, director of conservation and museum studies at the George Eastman House, Rochester, New York, says the challenge of dating photographs can often stump the experts. With some background and practice, however, even amateurs can narrow assumptions enough to identify people or places. One suggestion Romer makes is to think of the history of photography in 20- year increments. You can start at 1840 and attempt to date photographs to within a decade or so based on process or information in the photo itself, such as style of dress. Romer's model follows.
Most photos are on glass or metal support including daguerreotypes, tintypes and ambrotypes.
Paper photography began to appear in the 1850s. The most common photographic process was the use of wet collodion glass negatives and albumen print positives. This period is distinguished by introduction of gelatin dry plates and papers that use gelatin or collodion emulsions. Analysis of prints can positively identify whether a photograph is an albumen, gelatin or collodion print. But amateurs can also date photos by looking at people's dress and hair styles.