Property of the United States Government. 



CIRCULAR No. 12, Revised. 



United States Department of Agriculture, 



BUREAU OF CHEMISTRY, 

 H. W. WILEY, Chief of Bureau. 



METHODS FOR THE INVESTIGATION OF CANCELING INKS AND 

 OTHER STAMPING INKS. 



The following methods have been devised for the purpose of ascertaining the suit- 

 ability of canceling inks for the use of the Post-Office Department. Many of these 

 methods will be found of assistance in passing upon the quality of stamping inks for 

 miscellaneous uses. 



It is important that the ink used by the Post-Office Department for postmarking 

 possess in the highest possible degree certain properties. The ink, first of all, 

 must produce an indelible cancellation; that is, it must be relatively indelible as 

 compared with the ink used for printing the postage stamps. The postmark made with 

 the ink must dry quickly in order that the mail matter may be handled immediately 

 without any blurring or smearing of the postmark. Both this property and the prop- 

 erty of indelibility involve the question of the rate at which the ink penetrates or is 

 absorbed by the fiber of the paper. A satisfactory ink does not harden or form a crust 

 on the ink pad on exposure to the air. There must be no deposition of solid matter on 

 the bottom of the vessel in which the ink is stored, and the pigments, on which the 

 indelibility of the ink depends, if insoluble, must not settle out in such a way as to 

 make it possible to pour off from the top of the container a portion of the ink which con- 

 tains little or none of the insoluble pigment or pigments. The following methods have 

 been found of value for the purpose of ascertaining the quality of a given sample of ink as 

 well as the appropriateness of certain materials used for the manufacture of canceling 

 inks 



I. METHODS FOR THE INVESTIGATION OF INKS MADE WITH 



AN OIL BASE. 



1. PREPARATION AND CARE or THE SAMPLE. 



Since canceling inks contain more or less insoluble and volatile matter, special atten- 

 tion must be given to the preparation and care of the sample. It must be carefully 

 mixed by shaking before each portion is removed for analysis, and the container must 

 be left open no more than is absolutely necessary for the removal of portions of the ink. 



2. DETERMINATION OF MATTER VOLATILE AT ORDINARY TEMPERATURES. 



Place a carefully weighed quantity (between 5 and 5.2 grams) of the ink in a flat- 

 bottomed aluminum dish 102 mm (4 inches) in diameter. Distribute the ink com- 

 pletely over the surface of the bottom of the dish by gently tilting the same. This 

 quantity of ink should be sufficient to completely cover the bottom of the dish. Place 

 the dish on a horizontal shelf or table where air will have free access to it and where it 

 will be screened in such a way that no dust can fall into it. 



Reweigh the dish at the end of 18 hours, 24 hours, 2 days, 3 days, 4 days, 5 days, 

 6 days, 7 days, 14 days, 21 days, and 28 days. Calculate the total percentage loss of 

 weight at the end of each period of drying. The loss of weight should be gradual and 

 should not exceed 15 per cent during the first 7 days, nor 25 per cent during 28 days. 

 This test shows the absence of highly volatile ingredients and the absence of an t-xrrss 

 of matter volatile at the ordinary room temperatures. The constituents of a canceling 



22073 No. 1207 



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