It is very probable, too, that tests applied to the freshly-written entries would have shown that the ink in this entry was of different composition from that of the inks in the other entries.
Lovibond’s tintometer, an instrument which enables slight differences of colour to be distinguished more accurately than is possible with the naked eye, has been used in matching the colour obtained in chemical reactions with those given by the colour scales prepared from known or suspected inks.
For recording colour, strips of glass graduated so as to form a series of colour scales are employed in this instrument, and in this way a note can be taken of any given tint.
The Tintometer
The first occasion upon which this instrument was employed in criminal work was in the Brinkley poisoning case, in which the colours of the different inks upon the will and other documents were examined by its means.
The problem of determining the age of an ink in writing is much more difficult than that of deciding whether two writings are in the same or in a different kind of ink.
It is, as a rule, possible to distinguish, with the aid of the microscope and tintometer, between freshly-written and old writing up to about the sixth day, after which the black pigment has attained sufficient intensity to prevent further differentiation until after the lapse of two or three years or more, when the provisional pigment will have faded or have become fixed by the iron tannate.
In most cases the provisional pigments used offer greater resistance to the action of chemicals, but are infinitely less stable than the iron tannate when exposed to the action of light and air, and eloquent testimony to this difference is given by the comparison of certain manuscripts of the seventh and eighth centuries with typewritten matter in aniline ink, which has been put aside for a few years.
Thus it happens that when characters written in blue-black ink are kept, the blue pigment will gradually fade out, leaving the black pigment; and when this stage is reached the ink in old writing is readily distinguished from ink that has been freshly put upon paper.