“The dirt ground into the margins of medieval manuscripts is one of their interpretable features, which can help us to understand the desires, fears, and reading habits of the past.”
– explains researcher Dr Kathryn M. Rudy who is a Senior Lecturer at the School of Art History, of the University of St Andrews, Scotland. She points out, however, that :-
“Cleaning or trimming the dirt from them is tantamount to discarding a provocative cultural witness.“
Dr Rudy proposes instead the use of a densitometer – a machine that measures the darkness of a reflecting surface and which can reveal which texts a reader favoured, but without damaging the dirt.
See: Dirty Books: Quantifying Patterns of Use in Medieval Manuscripts Using a Densitometer in the Journal of Historians of Netherlandish Art. Volume 2, Issue 1-2 (Summer 2010).