The 16th century chased helmet had been placed in a closed glass showcase. When the relative air humidity is accidentally elevated, temperature changes can induce water condensation. The preservative used in this case -- a mixture of lanolin, vegetable fats and water -- formed an emulsion which made a favorable environment for microorganisms. These microorganisms produced some lipolytic enzymes which hydrolized the fats of the preservative into free fatty acids. This lowered the pH value, which in turn considerably increased the corrosion of the helmet. In the first stage of the corrosion process, the tin surface coating was damaged. Once this coating was penetrated, the corrosion began to damage the underlying steel. In this way a galvanic cell arose, with the steel functioning as an anode and the tin coating as a cathode. Thus, it was found that the greatest damage had occurred on surfaces that had been preserved by means of a mixture of vegetable and animal fats, which forms an excellent substrate for microorganisms. Therefore, the most important preservative method is to remove all fats and decay products completely prior to restoration with preservatives containing neither lanolin nor vegetable fats. -- AATA