Different corrosion theories and methods of conservation apparently of Scandinavian and German origin are described and analysed. The methods in question are: impregnation/coating as the only treatment, soaking in water, annealing/heat treatment, acid stripping (sulphuric acid) and electrochemical and electrolytic reduction. Some of the methods have not been published in the original version, and some, as a whole, only in the original language. As some methods are still in use, unchanged or modified, it is considered that their origin and the causes for their rise in popularity would be of some interest for modern conservators, especially because some of these original theories are not entirely valid any more. All the methods mentioned were used at the National Museum in Denmark at different times during the 19th century. Examples of the differently treated objects have been examined and their present condition estimated. The conclusion is that the old methods as a whole were surprisingly efficient and the reasons for their use were well-considered and often in accordance with the level of scientific understanding of the time.