Indelible India Ink.
Draughtsmen are aware that lines drawn on paper with good India ink well prepared cannot be washed out by mere sponging or washing. Now, however, it is proposed to take advantage of the fact that glue or gelatine, when mixed with bichromate of potassa, and exposed to the light, becomes insoluble, and thus renders India ink, which always contains a little gelatine, indelible. Reisenbichler, the discoverer, calls this kind of ink “Harttusche,” or “hard India ink.” It is made by adding to the common India ink of commerce about one per cent, in a very fine powder, of bichromate of potash. This must be mixed with the ink in a dry state; otherwise, it is said, the ink could not be easily ground in water. Those who cannot provide themselves with ink prepared as above in a cake, can use a dilute solution of bichromate of potash in rubbing up the ink. It answers the same purpose, though the ink should be used thick, so that the yellow salt will not spread.