Magnetic ink

Magnetic ink allows a computer to read characters even if they have been covered with signatures, cancellation marks, bank stamps, or other marks. MICR lines help facilitate automatic check-clearing when banks send their checks to central processing systems at the end of the day.
How it is made.
A ferrofluid is used in the manufacturing of the magnetic ink. A ferrofluid consists of colloidal particles (nanoscale) composed of iron(II) and iron(III) compounds such as FeCl2 and FeCl3. A ferrofluid is attracted by a magnetic field but does not become permanently magnetized.