The colourless writing lines resulting from the application of
heat could be observed by their sheen differing from that of the
paper surface when the page was observed at a shallow angle
opposite a desk-lamp or other extended light source. (A technique
or observation known as “specular reflection” in theUK“refractive
index” in the USA.) Under ultraviolet light the colourless lines
could be seen clearly as dark lines against the fluorescent
background of standard office paper. Under conditions for red
and infrared luminescence the colourless ink-lines luminesced in a
fashion similar to that of the original ink-lines. Examination with
electrostatic detection apparatus of a page bearing original inklines
and ink-lines decolourised by the application of heat revealed
both as white lines against the grey background.
Placing the sheet with the decolourised writing on it in a
domestic freezer at about −23 °C restored the writing to full
visibility within 1 h.