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Application of resistance measurements of black gel ink to identify altered documents

Identification of altered documents written in black gel ink is widely recognized as problematic, especially if both the original and altered strokes contain carbon. In this work, we used a black handwriting resistance measurement instrument to measure the resistance values of ink strokes. Because black gel inks are made with different types of pigments, their resistance values could be used to differentiate between different pen models from the same brand and among brands. In this study, 30 black gel inks were classified by their resistance values. In a pair-wise comparison analysis of the results, there were 435 possible pairs and 347 (i.e., 79.8% of the total) of these were able to be effectively distinguished. This is a nondestructive, convenient, and effective method to identify altered documents.