Retouching glass plate negatives. What do old manuals say?

This paper aims to present a study of old photographic retouching manuals which is part of a research project on the retouching of gelatin silver glass plate negatives from Portuguese photographic collections from the first half of the twentieth century. Several contemporary authors of various nationalities were studied. Differences in approach to retouching have been explored in terms of techniques and materials, particularly as the authors addressed issues such as ethics and the legitimacy of photo retouching, as well as the relationship of photography to the other arts.