Ink Chemistry

[This is taken from David N. Carvalho's Forty Centuries of Ink, originally published in 1904.]

SOME OBSERVATIONS AS TO CHEMICAL EXAMINATION OF INK MARKS BY ALLEN—ERASING OF INKS BY CHEMICAL MEANS—APPROVED CHEMICAL TESTS IN THE ASCERTAINMENT OF INK CONSTITUENTS.

A COMPILATION of the methods of Robertson, W. Thompson (Lord Kelvin), Irvine, Wislar, Hoffman and others, relative to the chemical examination of ink marks, is to be found in “Allen’s Commercial Organic Analysis.” Their experiments, however, date back many years ago, a few of them before the time of the use of the “anilines” for added color. The so-called “alizarin” ink referred to has now become obsolete.

The following is the citation in part:

“In chemico-legal cases it is sometimes of importance to ascertain the nature of the ink used, to compare it with specimens of writing of known history, and to ascertain the relative ages of the writings. A minute inspection should first be made with a magnifying power of about 10 diameters, and any peculiarities of color, lustre, shade, etc., duly noted, and where lines cross each other which lie uppermost. The examination is often facilitated by moistening the paper with benzine or petroleum spirit, whereby it is rendered semi-transparent. The use of alcohol or water is inadmissible.

“Valuable information is often obtainable by treating writing or other ink-marks with reagents.  Some inks are affected much more rapidly than others, though the rate of change depends greatly on the age of the writing. Normal oxalic acid (63 grammes per litre), or hydrochloric acid of corresponding strength, should be applied to a part of the ink marked with a feather or camel-hair brush (or the writing may be traced over with a quill pen), and the action observed by means of a lens, the reagent being allowed to dry on the paper.  Recent writing (one or two days old) in gallic inks is changed by one application of oxalic acid to a light gray, or by hydrochloric acid to yellow.  Older stains resist longer, in proportion to their age, and a deeper color remains. Log-wood ink marks are mostly reddened by oxalic acid, and alizarin marks become bluish, but aniline inks are unaffected. With hydrochloric acid, logwood ink marks turn reddish or reddish-gray, alizarin marks greenish, and aniline ink marks reddish or brownish-gray. The treatment with acid should be followed by exposure to ammonia vapors, or blotting paper wet with ammonia may be applied.  Thus treated, marks in logwood ink turn dark violet or violet-black. The age of ink marks very greatly affects the rate of their fading when treated with dilute ammonia, the old marks being more refractory. The behavior of ink marks when treated with solution of bleaching powder is often characteristic, the older writings resisting longer; but unless the reagent be extremely dilute, writings of all ages are removed almost simultaneously.  Hydrogen peroxide acts more slowly than bleaching solution, but gives more definite results. After bleaching the mark by either reagent, the iron of the ink remains mordanted on the paper, and the mark may be restored by treatment with a dilute solution of galls, tannic acid, or acidulated ferro-cyanide. The same reagents may be used for restoring writing which has been faded from age alone.

“When ink marks have been erased or discharged by chemical means, traces of the treatment are often recognizable. After effecting the erasure the spot is often rubbed over with a powdered alum or gum sandarac, or coated with gelatin or size.  The bleaching agents most likely to have been used are oxalic, citric, or hydrochloric acid, bleaching powder solution, or acid sulphite of sodium.  Moistened litmus paper will indicate the presence of a free acid, and in some cases treatment with ammonia fumes will restore the color. The presence of calcium, chlorides, or sulphates in the water in which the paper is soaked will afford some indication of bleaching powder or a sulphite having been used. Potassium ferro-cyanide will detect any iron remaining in the paper. Exposure to iodine vapor often affords evidence of chemical treatment, and other methods of examination readily suggest themselves.”

M. Piesse, in the Scientific American, is authority for a method of removing ink, found on “patent” check paper:

“Alternately wash the paper with a camel’s-hair brush dipped in a solution of cyanide of potassium and oxalic acid; then when the ink has disappeared wash the paper with pure water.”

Inks of the tanno-gallate of iron family, whether containing “added” color or not, can be more or less “erased” by chlorinate of lime or soda, in the presence of a weak acid. These chemicals do not, however, materially affect the prussian blue inks, which require solutions of hydrate of potash or soda.  Real indigo can be removed by chloroform, morphine or an aniline salt (indigo and aniline both owe their names to the same Portuguese source), which possess the rare property of dissolving pure indigo. Such combination, if refractory in the presence of permanganate of potash with sulphuric acid, must be followed by an application of sulphurous acid. In like manner, inks composed of by-products of coal tar, can be effectively treated, when irradicable with plain water or soap and water.

The erasure and removal of most inks from paper can be accomplished by the application of the chemicals heretofore enumerated. The requirements in this direction of some inks, however, though of rare occurrence, are to be met by the employment of other and particular reagents.

Many of the tests specified in the Allen citation to determine the character of ink constituents, if made alone are practically valueless, because the same behavior occurs with different materials employed in the admixture of ink. To avoid error in judgment the operator should verify if possible by confirmatory tests. Thus, in the one for logwood, sulphurous acid will cause a logwood ink mark to turn yellow; mercuric chloride, orange; tartar-emetic, red; and if the marks are faded ones, solutions of sulphate of iron or bichromate of potash will restore them respectively to a violet or blue-black color.

Prussian blue, aniline blue and indigo blue are to be tested as follows: Solution of chloride of lime, no change of color for prussian blue; decoloration or faint yellow for aniline blue or indigo. To discriminate between the two latter, test with solution of caustic soda, when decoloration or change of color will indicate aniline blue and permanence will indicate presence of indigo blue.

In the manufacture of the blue-black inks, a variety of violets have been and are still employed. Among them are aniline violet, iodine violet, madder, alkanet, orchil and logwood.

(a)  Apply chloride of lime solution: 1. No change of color indicates alkanet. 2. Any change, one of the other five.

(b)  Apply lemon juice: 1. The violet becomes brighter if it is one of the aniline violets, to be distinguished from each other by applying one part of hydrochloric acid to three parts of water, when it will become violet-blue, changing to red if it is common aniline-violet, but blue changing to a green hue and upon adding plain water to a lilac or pearl gray if it is iodine-violet (Hoffman’s). It will also turn from red to yellow in lemon juice. To test for the other three violets: (a) Apply chloride of lime, to be followed by a solution of yellow prussiate of potash: absence of a blue coloration leaves orchil and logwood to be considered. To distinguish between them apply solution of hydrate of lime, whereby a change to gray, followed by complete decoloration indicates logwood, and a change to violet-blue, orchil.

The substances utilized with but few exceptions for red ink are the “eosins,” possessing different names like erythrosine, as well as different hues. Antecedent to about thirty-five years ago, cochineal (known as “carmine”), madder, Brazil wood and saffron formed the basis of most of the red inks.

Make a soap solution adding a small quantity of ammonia, lemon juice, muriate of tin, all in water:

1.   No change upon application indicates madder.

2.   Any change, the presence of one of the three other reds: (a) thus a complete decoloration with a return of the color indicates saffron; (b) reappearance of the red color though weaker, aniline-red: © production of a yellowish red or light yellow color, cochineal or Brazil wood, to be distinguished from each other by the application of concentrated sulphuric acid, when Brazil wood will at once give a bright cherry-red, and cochineal a yellowish orange.

No yellow inks are in commercial use. Documents do, however, often contain yellow marks about which information is required as to their origin. As a rule they are iron rust, picric acid, turmeric, fustic, weld, Persian berries or quercitron. In order to recognize the different colors, the presence or absence of iron rust and picric acid must first be determined.

Apply a warm sample of a slightly acid solution of yellow prussiate of potash; iron rust will be indicated by a blue coloration.

Apply a weak solution of cyanide of potassium; picric acid will yield a blood-red coloration.

If picric acid and iron rust are both absent, apply a bit of ordinary wetted soap: 1. It turns reddish-brown and becomes yellow again with hydrochloric acid— turmeric; 2. It turns quite dark—fustic; 3. It is unaffected—weld, Persian berries or quercitron. To distinguish between these three, apply sulphuric acid, the color of weld will disappear, and of the others remaining apply tin-salt solution, when a change to orange indicates Persian berries, and no change or a very slight one, quercitron.

Inks containing also logwood, fustic, Brazil wood, or madder, were all of them more or less employed some years ago. Their color phenomena, following long periods of time, is much the same. Tests as prescribed in the accompanying table for such inks will serve to classify them preliminary to subsequent and more certain ones.