In 1905 the painter Harald Brun received a permanent position as a conservator at the National Gallery three years prior to the appointment of the museum's first director, Jens Thiis. In 1917, Brun accused Thiis of letting the caretaker clean the paintings in the collection, and thereby destroying several of them. Thiis accused Brun of the destruction of several other paintings by overcleaning. This cleaning controversy lasted for four years. Thiis claimed complete control in the matter of conservation and, unless he was successful in this, he would resign his position as a director. Brun pleaded not guilty and stated that he had taken an oath not to reveal any information about his material and method. With written statements from colleagues in Germany, Denmark and Sweden, we get an insight into the phenomenon of keeping secrets in the field of conservation.