DISCOVERY 



317 



Ink Pigments in Writing 



By C. Ainsworth Mitchell, M.A., F.I.C. 



Police reports abundantly prove that the crime of 

 forgery frequently engages the attention of our magis- 

 trates. This is not surprising if we remember that the 

 means are in the hands of all who can write, and that 

 bankers' cheques are a common medium of exchange. 

 The evidence usually demanded by the prosecution is 

 that of the handwriting expert, who bases his opinion 

 on the form and peculiarity of the writing ; but there 

 is evidence of another kind, less known to the public, 

 namely the chemical examination of the ink or pencil 

 marks on the documents, which is often not only more 

 trustworthy but actually throws more light upon the 

 mvstery. Suppose, for example, additional letters have 

 been added to a cheque, it is comparatively easy by 

 chemical means to ascertain whether or not the inter- 

 polated characters have been written with the ink 

 used for the body of the document. There are other 

 methods, of course, which are useful in deciding 

 on the genuineness of documents, photography for 

 example, but in this article we shall confine ourselves 

 to describing what is known about, and what can be 

 learned from, the ink pigments which are used in 

 writing, partly as matters of interest, and partly in 

 their relation to the genuineness of documents. 

 For this purpose some characteristics of ink must 

 nrst be described. 



The principal kinds of inks employed at the present 

 time are known as carbon ink, iron-gall ink. and 

 aniline ink. There are others, but these are the 

 principal three. The ink in common use in this coun- 

 try, the famOiar " blue-black," is a mixture of iron-gall 

 ink and a blue aniline ink ; it is best to discuss it, 

 therefore, after its constituents have been described. 



Carbon ink is not generally used for writing purposes 

 in this country. Such inks are used in Europe only 

 in the form of Indian ink or " safety " inks, to which 

 lampblack, which is the basis of all carbon inks, has 

 been added to render the pigment proof against the 

 action of chemicals. Carbon inks, however, are still 

 used in Egypt and in the East for ordinary writing, 

 and are in general use everywhere for printing. The 

 characteristics of a carbon ink are (i) its colour — a 

 black which does not, in most cases, vary with time, 

 and (2) its resistance to bleaching agents. An ordin- 

 ary blue-black ink, for instance, may be bleached by 

 a solution of the chemical sodium hypochlorite ; not 

 so a carbon ink. The two are consequently easily 

 distinguished. 



Carbon inks are the oldest class, and were known to 

 antiquity. A chemical examination of ancient writing 

 has thrown interesting light on their use in the past. 



Sir Humphry Davy found that the ink on the papyri 

 unearthed at Herculaneum was a carbon ink with no 

 traces of iron in it. He concluded that the Romans 

 had not discovered iron-gall inks. Astle, who was 

 Keeper of the Records at the Tower at the close of 

 the eighteenth century, attributed the blackness of 

 inks upon old manuscripts, dating from the ninth to 

 the fifteenth century a.d., to their being carbon 

 inks ; but Blagden, who examined them chemically 

 in 1783, failed to " find any trace of a black pigment " 

 (due to carbon), but found iron in every case. The 

 conclusion he drew, therefore, was that these ancient 

 writings must have been done with the second class 

 of inks, the iron-gall inks in common use hi his own 

 day. It is probable that the transition from the old 



Fig. I.— IRON-GALI, INK. PERIOD 1723. 

 Magnificatiun 20. 



carbon inks to the modern iron-gall inks occurred 

 in the seventh or eighth century a.d., although for 

 some centuries later both were in use. An Egyptian 

 document recently examined by Mr. Lucas, a portion 

 of the Old Testament in Arabic (a.d. 1312), was 

 found to have been written in two kinds of ink. one 

 a carbon ink, the other an iron. These inks were 

 not mixed, but used separately. Ancient carbon 

 mks are not always as black as might be expected, 

 being occasionally yellow or pale brown. Sometimes 

 this brown colour is due to iron compounds, sometimes 

 to those of other elements, but it is probable that 

 these bodies were not deliberately mixed with the 

 carbon for a specific purpose, but were mere im- 

 purities associated with the grade of carbon used in 

 making the ink. 



Iron-gall ink forms the principal, but not the sole, 

 constituent of the ink in general use in European 

 countries. It is made from a compound of iron, known 



