Knowledge of the manufacture and structure of daguerreotypes is important in order to understand the problems of their conservation. Deterioration of the image is due to environmental acids and to physical handling. A daguerreotype treated with thiourea and acid (Missouri method) later developed a mold like growth, containing, upon analysis, Ag, Hg, traces of Cl, S, and Si. The crystalline growth could lead to further tarnish and could possibly entrap the cleaning solution in subsequent treatments. Caution is advised.