The so-called Pettenkofer process involves the exposure of blanched paintings to alcohol vapor and copaiva balsam. Approximately 80 paintings in the Bayerische Staatsgemäldesammlungen can be documented and are recorded as having been treated in 1863 by Pettenkofer or his assistant. From various documents relating to these paintings, many not studied until now, it is possible to understand the genesis of this treatment and its development. Paintings known to have been treated with the Pettenkofer method and others likely to have been treated in a similar manner have been examined under the microscope in order to assess their condition. All of these paintings exhibit related defects in the varnish and the paint layers and particularly in the interaction between the two. These defects appear to be the result of extreme swelling and the consequent migration of components. Further investigation must be undertaken to determine if these irregularities are a direct consequence of the process.