INKS. 249 



writing will not readily run on ordinary slightly glazed sized 

 writing paper ; most of the " aniline inks " and ordinary red ink 

 are of this character. The old fashioned black inks, however, are 

 of a somewhat different nature, being essentially black lakes pro- 

 duced by the combination of iron with certain kinds of colouring 

 matters and allied products, these lakes being in an excessively 

 fine state of division, so as not to settle readily in the bottle, 

 somewhat like Faraday's ruby gold described in Expt. 136. The 

 colouring matter developed in Expt. 292 from logwood and sul- 

 phate of iron is one of this class ; gallnuts boiled with water, and 

 the infusion similarly treated with sulphate of iron and thickened 

 with gum or dextrin, forms the basis of one of the best kinds of 

 ordinary writing ink. Iron inks of this class often write pale, 

 but darken greatly after a while ; this is because the iron com- 

 pound present absorbs oxygen from the air, and develops a much 

 darker tint in so doing, the action being somewhat analogous to 

 that occurring when white indigo solution is exposed to the air, 

 whereby oxygen is absorbed and insoluble blue indigo precipitated 

 in the pores of the cloth (Expt. 151) ; the ink, as at first used, is 

 chiefly a mixture of various substances all truly in solution ; but 

 under the influence of the oxidising action of the air an insoluble 

 lake is formed and precipitated in the body of the paper, so that 

 water will not wash out the tinting substance produced. 



A good black ink may be thus prepared. Finely powder some 

 gallnuts and boil the powder with water for three hours, using 

 about 7 parts by weight of water to 1 of galls (or about 3 ounces 

 of galls per pint of water), and adding more water from time to 

 time to supply that lost by evaporation ; strain the decoction 

 tolerably clear, and then add to it a solution of sulphate of iron 

 dissolved in three or four times its weight of hot water, and 

 another of gum of the same strength; 5 parts of sulphate of iron 

 and 5 of gum being used for every 1 2 of galls ; so that in all the 

 proportions will be about 



Gallnuts, 12 parts by weight. 



, Sulphate of iroD, .... 5 ,, 



Gum, ...... 5 ,, 



Total water, 120 ,, 



Copying ink chiefly differs from ordinary writing ink in con- 

 taining sugar or glycerine intermixed, which prevents the ink from 

 drying in so completely, and enables some of it to be transferred 

 more readily to damp tissue paper by pressure in a copying press. 



Pencils are sometimes used, consisting of some soluble colouring 

 matter in the solid state mixed with gum and other materials, so 

 that when the writing is moistened a solution of the colouring 



