When did we start using paper money?
With paper money, the materials are as important as the manufacturing process in producing the final product. The paper, also known as the substrate, is a special blend of 75% cotton and 25% linen to give it the proper feel. It contains small segments of red and blue fibers scattered throughout for visual identification. Starting in 1990, the paper for $10 bills and higher denominations was made of two plies with a polymer security thread laminated between them. The thread was added to $5 bills in 1993. This thread is visible only when the bill is held up to a light and cannot be duplicated in photocopiers or printers.
The inks consist of dry color pigments blended with oils and extenders to produce especially thick printing inks. Black ink is used to print the front of the bills, and green ink is used on the backs (thus giving rise to the term greenbacks for paper money). The colored seals and serial numbers on the front of the bill are printed separately using regular printing inks.