Estimation of gelatin content in historical papers using NIR spectroscopy

Gelatin sizing was a key ingredient during the handpapermaking era. The gelatin concentration in historical papers has never been well documented, however, because measuring the gelatin content required destructive sampling. In this project we developed a non-destructive method using near infrared (NIR) spectroscopy. Gelatin concentrations of 40 historical papers from the 15^th-18^th centuries were determined from amino acid (AA) concentrations by using gas chromatography/mass spectroscopy. These values were combined with NIR spectra from the same papers to generate a model for predicting concentrations of unknowns. If a NIR measurement predicted a gelatin concentration in the range 0-6 percent then there is a 95% probability that the difference between the NIR model value and a destructive AA measurement would be between -1.6 and +1.3 percentage points. For 6-8 percent there is a 95% probability the difference would be between -2.0 and +1.5 percentage points, and for 8-12 percent the difference is between -3.0 and +2.0 percentage points. In a study of 159 specimens from books, loose leaves, and artworks printed from 1460-1791, the means for all papers were quite high in the 15^th century and dropped an average of 20% every 50 years. Possible explanations for the decline are offered.