A novel, non-invasive, multi-purpose and comprehensive method to date inks in real handwritten documents based on the monitoring of the dye ageing processes

Blue ink strokes belonging to 11 types of writing tools from 7 different brands were naturally aged in the darkness for 2 years and characterised by Vis-microspectrophotometry (Vis-MSP). Ink clustering and classification was performed by applying principal component analysis (PCA) and hierarchical cluster analysis (HCA) in order to subsequently assign an optimal orthogonal partial least squares (OPLS) model capable to predict the age of different inks. This method was found applicable to the exact dating of forged documents with ink strokes of up to 2 years old. The method was tested with blind samples supplied by the Forensic Science Unit of the Basque Country Police Department. Age predictions (p ​