Fixative or Finish? Identifying the Surface Coating on John Constable’s Drawings Using Peptide Mass Fingerprinting

A survey of graphite drawings by the English painter John Constable revealed that in many cases the artist applied a coating overtop of the media. This thin, clear, glossy coating was characterized on over thirty drawings where it appeared to be the same material although employed in different ways. Several attempts were made to identify the material with minimal success until peptide mass fingerprinting (PMF) was used, and a minimally invasive sampling method involving vinyl eraser cubes proved to be a very easy and effective technique to collect sufficient sample material for the analysis. The results of testing samples from seven drawings conclusively identified the coating as isinglass, which Constable was using in other preparations of support materials for painting. As part of this work, putative markers for the identification of isinglass by PMF were developed.