For those or you who may own or curate daguerreotype collections, there is an interesting article in the current, December 2012 issue of Scientific American
concerning recently discovered nano-molecular processes that are deteriorating daguerreotypes worldwide. Apparently the molecular nature of the original daguerreotype
process makes these images particularly unstable after approximately 150 years. The images literally can re-develop themselves with atmospheric contact and
produce a haze over the metallic image. Apparently there are now efforts to produce affordable argon-gas frames that can keep these valuable images stable.
The link below is to a summary of the current Scientific American article. Good luck: