There are several main manifestations of ink deterioration.

First, the appearance changes. Normal ink is a uniform liquid. After deterioration, precipitation may occur, such as solid particles accumulating at the bottom; there will also be suspended matter floating in the ink; stratification may also occur, such as the upper layer becoming clear and the lower layer becoming thicker.

Second, the smell changes. Ink that has not deteriorated usually has a light ink fragrance. After deterioration, it may produce a pungent smell, such as a sour smell, which is caused by the growth of bacteria or mold.

Third, the color changes. The original color of the ink may become lighter, darker or discolored. For example, black ink may have a gray or brown tone, which is caused by changes in the ingredients in the ink.

Fourth, abnormalities occur during use. Deteriorated ink may cause writing tools to clog, such as the nib of a fountain pen not flowing smoothly, or the print head to be clogged, resulting in unsmooth printing and writing.