*1.2. Deterioration of the Daguerreotype*

Daguerreotypes are subject to the formation of surface tarnish, which, in the extreme, can completely obscure the image. Surface corrosion will alter the shape and refractivity of the image particles, and hence the direction and intensity of the scattered light. The most frequent tarnishes are silver halides, silver oxides, and sulfur compounds originating from incomplete washing during the original preparation, exposure to atmospheric gases, and deterioration of the cover glass [5,6]. Studies have also focused in great detail on the effects of the daguerreotype storage and exposure conditions. Extreme temperatures and/or humidity can have a negative influence on the integrity of the surface. When a daguerreotype is stored, a glass cover is usually present on the image side of the surface; however, deterioration of the glass cover can be another factor contributing to the degradation of the plate [7]. Many original glass cover plates contained sodium and potassium, which have been shown, over time, to diffuse from the glass and leave deposits on the daguerreotype surface. This leads to highly localized spots of corrosion across the daguerreotype. In addition, the glue that was used to adhere the glass cover to the plate also contributed to corrosion at the perimeter of the daguerreotypes [8,9]. Compounds such as oxides and various sulfides may have been formed because of this practice [10].
