The black ink used on modern lines is a solvent based ink that uses MEK as a solvent to deliver the ink, printing being done with a continuous jet printer. The ink is quite durable on almost any surface except glass, where it only had a fair resistance to removal, but on a lamp base it is pretty much permanent. It is an expensive ink, but the amount used per lamp is nearly nothing, and it is not thrown away like in an inkjet printer, but is recycled. Nice printers, you can print on anything, even a raw egg yolk ( though you need an alternative ink that is edible for that).