318 



DISCOVERY 



as ferrous sulphate or copperas, an extract of nut 

 galJs which contains tannin, and a solution of gum. 

 This mixture when freshly made gives only a very 

 faint colour on paper, but it has the property of 



Fig. -.—BUTE-BLACK IROX-G.\I.I, INK FIVE YE.A.RS OLD. 



SKOWIXG D.\RK M.\RGIXS CAUSED BY PEN NIB. 



Masiiil]cation 20. 



gradually combining with the oxygen of the air to 

 form a compound which is coloured black. It first 

 forms a tannate of iron containing about 5-5 per 

 cent, of iron, and, until this tannate has been formed, 

 the writing is to some extent soluble in water, and 

 so capable of yielding a copy when pressed with moist 

 absorbed paper. Some inks, indeed, are capable of 

 yielding a copy in this way for a week or more. After 

 this insoluble tannate has been formed, it combines 

 further, but slowly, with the oxygen of the air to form 

 a second tannate containing about 8 per cent, of 

 iron. The writing then becomes stable. This body 

 is relatively insoluble in water and dilute acids, and 

 is of a resinous nature. Consequently an iron-gall 

 ink after, say, six months (before the second tannate 

 has been formed) will not bclia\-e in the same way, 

 when treated with certain chemicals, as the same mk 

 which has had eight or ten years in which to become 

 stable. This property, as will be described later, is 

 employed in e.xamining suspected documents. 



There are changes, too, which may be seen with the 

 naked eye, or, more closely, with the microscope. At 

 first, as has been said, the writing is almost colourless, 

 but in a week or two if the written page be left open 

 to the air and strong light, or in a year or more if it 

 be written in a book and the book kept shut, the writing 

 appears black. This progressive change of colour is 

 entirely absent in pure carbon inks. Iron-gall ink 

 is also easily distinguished- from carbon inks in that 

 it is easily bleached by some bleaching agents. Its 



characterictics, therefore, are three : (i) its change of 

 solubility in acids and water with time ; (2) its gradual 

 change of colour to black with time ; and (3) its property 

 of being bleached by suitable bleaching agents. 



The class of aniline inks is that most recently 

 discovered. These inks are made from aniline dyes, 

 the first of which, mauve, was discovered in 1856. 

 These dyes now provide us with inks of all the colours 

 of the rainbow. Eosine is the commonest basis of 

 red ink, methyl violet of violet ink. But for ordinary 

 purposes of writmg, aniline dyes by themselves are not 

 used in this country very much. In the East, on the 

 other hand, they are largely used even for important 

 documents like wills, promissory notes, deeds of sale, 

 and receipts. They have the demerit of fading on 

 exposure, but this change with time is not a progressive 

 one like that of iron-gall inks ; the nature and the 

 extent of the exposure conditions the appearance of 

 the ink much more than mere age. Many aniline inks 

 are easily bleached when treated with suitable chemical 

 reagents, and others change colour. The character- 

 istics, then, of an aniline ink are : (i) it is of modern 

 manufacture ; (2) its age cannot be told from its 

 colour ; (3) it is easily bleached or its colour is easily 

 changed by certam chemical reagents. 



A few words now about blue-black writing ink. In 

 former days iron-gall ink sufficed for the purpose of 

 writing, the ink being e.xposed to the air before use 

 to darken by the formation of the black tannate of 

 iron. In modern ink this preliminary change due to 

 oxidation is omitted, and a colouring matter, such as 

 indigo, is added to give an immediate coloration so 

 that the writer may see what he is doing. The indigo 

 or an aniline dye immediately provides the blue ; 

 the iron -gall ultimately the black. As the iron- 

 gall constituent oxidises gradually, becoming black, 

 the colour changes from the bright blue of the 

 aniline colour to a deep violet which results when 

 the black pigment has formed. This change in colour 

 proceeds fairly rapidly, as common experience shows, 

 in the light, and more gradually in the dark, but 

 in any case is usually complete in the course of 

 a few months. Now if the ink upon a document 

 supposed to be two or three years old be of a bright 

 blue colour, and if chemical analysis prove it to 

 be an iron-gaU and not an aniline ink, the fact is 

 suspicious. If, further, the ink subsequent to examina- 

 tion becomes progressively more violet on exposure 

 to light and air, it is almost certain that the document 

 is not so old as it is said to be, for, if it were two years 

 old, the maximum intensity of colour would long before 

 then have been reached. After writing has attained 

 its maximum intensity of colour, a further gradual 

 change due to oxidation takes place, which is not 

 usually complete until after about four years. 



