2004 年 9 巻 2 号 p. 135-142
The near infrared light with 0.7-1.0 μm wavelength is widely used for nondestructive decipherment of obliterated writings. However, the near-infrared light is absorbed by oil-based nigrosine dye black marking-pen ink. On the other hand, the near-infrared light is not absorbed by oil-based dye black ballpoint pen ink. For this reason, when a letter, which is written using an oil-based dye black ballpoint pen, is obliterated with an oil-based nigrosine dye black marking pen, the letter is not detected in 0.7-1.0 μm wavelength region.
In the present report we describe the decipherment of obliterated writings by middle infrared light (2.5-14 μm wavelength). In a preliminary measurement using Fourier transform infrared (FT-IR) spectroscopy, we found that the absorption spectra of all oil-based dye black ballpoint pen ink had a specific peak of 1585 cm−1, and that the spectra of any oil-based nigrosine dye black marking-pen ink did not have the peak at 1585 cm−1.
For the preparation of simulated samples of obliterated writing on a piece of PPC paper, a Japanese character [よ] was written using an oil-based dye black ballpoint pen and the character was obliterated using an oil-based nigrosine dye black marking-pen. The obliterated writing was measured by the FTIR-ATR method. The obliterated writing could be deciphered by mapping the peak area at 1585 cm−1.
As a result of this experiment， we conclude that the FTIR-ATR mapping with the middle infrared light will be useful to decipher obliterated writings.