Characterizing pen strokes produced using various commercially available thermochromic inks

Abstract

The characteristics of commercially available thermochromic ink pens have been studied and described since their appearance in 2006. The wide variety of brands and models now available warrants further study using an expanded sample size, to differentiate the general characteristics from specific characteristics. Herein, the ink strokes of 15 pens purchased in the province of Córdoba, Argentina were studied. First, the initial unaltered strokes were examined. Second, heat-manipulated strokes (with and without friction) were evaluated. Several characterization techniques were employed, such as observation by the naked eye, optical magnification, and light irradiation using different spectral bands. In 100% of the unaltered strokes, the general characteristics of thermochromic inks, such as ink accumulation at the end of the stroke path and a "pasty appearance", were found. It was possible to visualize colourless strokes (i.e. erased strokes) based on their contrast with the paper by shining oblique light at an opposite angle to that of the observation. In addition, the responses to ultraviolet (UV) (365 and 254 nm) and cyan light (505 nm) by infrared techniques were useful for distinguishing the thermochromic ink. Specific characteristics, such as high relief in areas of ink accumulation, less ink accumulation in the internal zone of the stroke, and intensity differences in infrared radiation (IR) luminescence emitted by different brands and models, could be useful in determining the writing tool that was used.

Key points: Thermochromic ink disappears with heat but is reversible with cold.UV techniques and IR photography serve to make thermochromic ink visible.Initially use oblique white light opposite to the angle of observation to make strokes visible.

Keywords: erasable; forensic sciences; ink; pen; thermochromic; thermosensitive.

Figure 1

The thermochromic ink pens used in this study. From left to right. Black colour: Pilot FriXion, Trabi Ghost, Keyroad Erasable Gel Pen, Simball Genio 2G 0.7, Filgo Borrax RT Retractable, and Zuixua Animal School K 1300 0.5 mm. Blue colour: Trabi Ghost, Simball Genio 2G 0.7, Simball Genio Teen 2G 0.7, Filgo Borrax RT Retractable, Filgo Borrax SE 0.7, A+ Plus Erasable GA304205E 0.7 (tube: light blue), A+ Plus Erasable GA304205E 0.7 (tube: pink), Faber Castell Magic, and Sabonis Office Line BR1007 0.7.

Figure 2

Table A, incident white light. Photography: Nikon D5100 camera; exposure time: 1/2 s; F: 9; ISO: 100.

Figure 3

Table B, incident white light. Photography: Nikon D5100 camera; exposure time: 1/4 s; F: 9; ISO: 100.

Figure 4

Unaltered strokes produced using the Pilot FriXion pen, showing the pasty appearance and relief in the areas of ink accumulation. Photography: LG-D722AR camera; exposure time: 1/29 s; F: 2.6; ISO: 100.

Figure 5

Unaltered strokes produced using the Faber Castell Magic pen, showing the pasty appearance and less ink accumulation in the internal zone of the strokes. Photography: LG-D722AR camera; exposure time: 1/16 s; F: 2.6; ISO: 300.

Figure 6

Unaltered strokes produced using the Simball Genio 2G Teen 0.7 pen, showing ink accumulation at the end of the stroke path. Photography: LG-D722AR camera; exposure time: 1/16 s; F: 2.6; ISO: 300.

Figure 7

Table A, unaltered strokes (second column). Photography: LG-D722AR camera; exposure time: 1/16 s; F: 2.6; ISO: 300.

Figure 8

Table A, photograph in a dark room with incident cyan light (505 nm). Photography: Nikon D3400 full spectrum camera and 720 high-pass IR filter.

Figure 9

Table A, photography in a dark room using oblique white light opposite to the angle of observation. Photography: LG-D722 camera; exposure time: 1/29 s; F: 2.6; ISO: 100.

Figure 10

Colourless strokes produced without friction. Thermochromic ink pen: Faber Castell Magic (blue colour); photomicrography (20×): LG-D722AR camera; exposure time: 1/40 s; F: 2.6; ISO: 100.

Figure 11

Colourless terminal stroke produced by friction. Thermochromic ink pen: Sabonis® Office Line BR1007 0.7; photomicrography (20×): LG-D722AR camera; exposure time: 1/24 s; F: 2.6; ISO: 200.

Figure 12

Colourless strokes produced without friction. Thermochromic ink pen: Trabi Ghost (black colour); photomicrography (20×): LG-D722AR camera; exposure time: 1/60 s; F: 2.6; ISO: 100.

Figure 13

Table A, photography captured in a dark room under incident UV light (365 nm). Photography: LG-D722AR camera; exposure time: 8 s; F:9; ISO: 100.

Figure 14

Table A, photography captured in a dark room under incident UV light (254 nm). Photography: LG-D722AR camera; exposure time: 3 s; F: 9; ISO: 100.

Figure 15

Table A, photography in a dark room using incident light of 505 nm. Photography: NIKON D3400 full spectrum camera and high-pass IR filter (720 nm); exposure time: 15 s; F: 9; ISO: 100.

Figure 16

Table B, photography in a dark room using incident light of 505 nm. Photography: Nikon D3400 full spectrum camera and high-pass IR filter (720 nm); exposure time: 15 s; F: 9; ISO: 100.