WRITING INKS. 43 



with reagents and preserve some for final comparisons. If the ink is a copying 

 Ink, make press copies of some of the strips as soon as they are thoroughly dry 

 and treat the press copies and the copied original as in the other cases. In 

 testing with reagents, immerse the strips in the reagents and observe the effect 

 at the end of fifteen minutes, of one hour, and of twenty-four hours. Test, 

 using the following reagents, though others may also be employed : 



Water ; 95 per cent alcohol ; 90 volumes of water and 10 volumes of ammonium 

 hydroxid (0.90 sp. gr.) ; 90 volumes of 95 per cent alcohol and 10 volumes of 

 ammonium hydroxid (0.90 sp. gr.) ; 2 per cent hydrochloric acid; 2 per cent 

 sodium hydroxid; bleaching powder solution, N/200 available chlorin. 



The tests used are relative, and it is well to have a standard ink for com- 

 parison. Such a standard may be prepared by the following formula: 23.4 

 grams of pure dry tannic acid ; 7.7 grams of pure crystallized gallic acid ; 30.0 

 grams of pure crystallized ferrous sulphate ; 10.0 grams of gum arable ; dilute 

 hydrochloric acid in sufficient amount to contain 2.5 grams of hydrochloric 

 acid (25 grams dilute hydrochloric acid U. S. P.), and 1.0 gram of crystallized 

 carbolic acid. 



All of these chemicals should be of U. S. P. quality, and in addition the purity 

 of the tannin should be determined by the hide powder method. 



Dissolve the tannin and gallic acid together in about 50 cc of warm water, 

 dissolve the ferrous sulphate in about 150 cc of cold water, dissolve the gum 

 arabic in about 150 cc of warm water. Allow the warm solutions to cool, add 

 the hydrochloric acid to the ferrous sulphate and immediately mix all the solu- 

 tions and make up with distilled water to 1,000 cc. Mix thoroughly and allow 

 to stand for at least four days at room temperature. Without shaking the 

 bottle draw out the ink to be used in making comparisons. This standard ink 

 will not be of a good color but will make a dirty gray green mark which will 

 finally turn black. It may be colored to match the ink under examination by 

 addition of a small amount of soluble dyes. 



11. Bating. 



The rating of a number of samples of ink is entirely relative, and any sys- 

 tem may be adopted. The exposure to sunlight is, of course, the most impor- 

 tant test, and the following scheme answers very well for record inks. Rate 

 the standard ink as follows : 



Exposure to sunlight 70 



Exposure to reagents 10 



Keeping quality, penetration, stickiness, fluidity, and action 



on steel pens - 15 



Composition 5 



Total 100 



Other record inks are given values above or below the figures for the 

 standard ink, as the judgment of the analyst may indicate, so the total for a 

 very good ink may be over 100. 



Rate copying inks as record inks and make a separate rating on the copy, 

 comparing one with another, and giving a weight of 50 per cent to clearness 

 and depth of copy, and 50 per cent to resistance to reagents and sunlight. For 



U. S. Dept Agr., Bureau of Chemistry, Bui. 107, Revised, p. .'iu. 



