Seal ink is a mixture of (commonly) red pigment, most often vermillion, mixed with oil and bound together with fibers—usually silk or [moxa] plant fiber. Each manufacturer may also include other ingredients in lesser quantities—all closely guarded secrets. Top quality ingredients ensure a bright color impression when stamped with less color fade over time, and with less oil separation and eventual oil bleed/stain on the finished artwork. Good seal ink will also withstand the wetting required when mounting an artwork in the traditional method. Poor seal ink will run or blur during this process and leave an unsatisfactory appearance to the finished mounted artwork.
Since seal ink is an oil-based ink you need to be careful not to get it on anything you don’t want colored with permanent red ink. Also be sure to give it plenty of time to dry—a minimum of a few days—otherwise you run the risk of smudging the ink.