CANCELING INKS HAVING AN OIL BASE. 47 



bottom of the cylinder, expressing results in millimeters. During the first hour 

 observations should be made at intervals of fifteen minutes ; later, each hour for 

 several hours successively, and then twice daily for a week to ten days. 



After the settling of the top of the layer of sediment has entirely ceased, 

 the height of the sediment should equal or exceed 16 mm, the amount of ink 

 taken for the test. The rate of sedimentation is an index of the state of division 

 of the carbon, some inks showing no appreciable layer at the expiration of a 

 ten-day test. 



In the case of some inks the supernatant liquid above the sediment is of 

 such a dark color that there is difficulty in locating the top of the sediment, 

 even when the cylinder is inspected by light reflected at various angles. In this 

 event the use of a dark room with a light placed so as to give a strong ray 

 through a small aperture will locate the top of the layer of sediment in all 

 cases except when the ink contains a very large percentage of a dense dye. 



Frequently the layer may be located by holding an incandescent electric light 

 back of the cylinder and noting where the carbon filament can not be seen. 

 The test is somewhat crude and only approximate, but it serves to give an idea 

 of the fineness and amount of carbon, and, as a rule, agrees fairly well with the 

 carbon determinations. 



5. Determination of Lampblack. 



Load a porcelain gooch with asbestos, using a felt about one-fourth of an inch 

 thick. After washing the felt thoroughly with water to remove fine particles, 

 finally wash with alcohol and ether, dry, and weigh. Weigh out about 5 grams 

 of ink in a small beaker, dilute with a suitable solvent (alcohol is used in case 

 of rosin-oil inks), transfer to the gooch and wash until all oil and soluble color 

 is removed. Finally, wash with alcohol and ether, dry, and weigh. 



6. Determination of Ash. 



The lampblacks prepared for the manufacture of canceling ink yield less than 

 0.5 per cent of ash when burned, and the coal-tar dyes employed should contain 

 no mineral matter other than that which is an essential part of the molecules 

 of the substances to which the tinctorial power of these dyes is due. 



For the determination of the ash, place 2 to 3 grams of the ink in a porcelain 

 dish, which must be of such size as to avoid loss of ink due to the foaming 

 which is likely -to attend the beginning of the incineration. Heat the dishes 

 thus charged in a muffle at a low red heat, until all organic matter and uncom- 

 bined carbon have been burned. Cool and weigh. 



If an excessive percentage of ash is found, the percentage of mineral matter 

 contained in the alcohol extract should be determined by incineration of the 

 residue obtained after evaporation of this extract. If either the total ash or 

 the ash of the alcoholic extract is high, a qualitative examination should be 

 made. 



7. Resistance of Pigments and Dyes to Light and Reagents. 



It is necessary in the case of canceling inks, and important, if not necessary, 

 in the case of many stamping inks, that the pigments and dyes employed in their 

 manufacture be as resistant as possible to means which may be employed for 

 the erasure of marks made with them on paper. Under this heading may be men- 

 tioned also the importance of the use of dyes which possess considerable affinity 

 for vegetable fibers. It is not practicable to enumerate the agents which should 



