Daguerreotypes, ambrotypes and early tintypes were usually sold in small folding cases. The cases were designed to keep the fragile surfaces of these images safe behind glass. Over the years, the cover glass in the cases can crack or become dirty. Some researchers report the glass itself can deteriorate, causing damage to the surface of daguerreotypes.
Collectors and conservators choose to replace the old cover glass. This is an operation that requires EXTREME CAUTION. The fragile surface of a daguerreotype was compared to the delicate wings of a butterfly--one inadvertent touch and it will be marred forever. Removing the image from its case also requires care, because a slip can permanently bend a daguerreotype or tintype...or crack an ambrotype. One other potential drawback to replacing the glass: some collectors put a premium on daguerreotypes that have their original paper seals intact. If the seal is present, breaking it to replace the glass may have an impact on the image's market value.
When in doubt, consult an expert.
N.B.Roughly 80% of all daguerreotypes have had their original seals broken. Those images have been resealed with paper tape, or left unsealed. Many times the glass was changed. Underneath that glass about 95% of the time is a layer of hazy film that both diminishes the contrast and softens the appearance of the image. Often, it can't be seen until the old piece of glass has been removed. If moisture is also present, very detrimental problems can occur on the actual silvered surface.
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