Date determination using a combination of Raman and video spectroscopy for the examination of forged documents containing pre-inked stamp impressions.

The examination of stamp impressions is a routine task for questioned document examiners (QDEs) in determining the authenticity of disputed documents. Some studies have provided methods for date determination of stamp impressions, but each single method has its own limitations, so date determination of stamp impressions remains a challenge in case work. This study reports on the real case of a forged contract, in which pre-inked stamp impressions were made on blank paper without the knowledge of the stamp owner. The combination of Raman and video spectroscopy is demonstrated to be a novel method for the date determination of these impressions. In addition to conventional morphological examination methods, video spectroscopy (VS) images were combined with Raman spectroscopy to identify ink components and the distribution of pre-inked impressions. These characteristics vary with the time of stamping, especially after re-inking, and can thus be used to date forged documents. Specifically, spectral analysis of the ink provided a regular representation of temporal variations based on reference stamp samples, which were mutually confirmed by morphological analysis. This approach combines multiple analysis methods to provide diverse evidence for characterizing the tampering and dating the impressions. The results suggest the included stamp impressions were genuine and made on a blank sheet of paper prior to printing the content of the agreement. The five questioned impressions were not entirely consistent in terms of generation time, and none of them were stamped on the nominated date.