. Discovery. Science. 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.\, INK FIVE 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
. Discovery. Science. 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.\, INK FIVE 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 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
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