The plastic master plates are transferred from siderography and used by plate makers to create hundreds of identical printing plates — identical because they are all based on the same master.
This is done in a process called electroplating. The plastic master, which is called the "basso", is sprayed with silver nitrate to act as an electrical conductor. The plate is then placed into a tank filled with a nickel salt solution, and an electric current is generated. Nickel ions leave the solution and deposit themselves on the electrically charged surface of the master. After about 22 hours, a nickel plate, called an "alto" has literally grown. The alto is separated from the plastic master, trimmed, and inspected by the engravers. The plate contains the mirror image of the master in all its intricate detail and is an exact replica of the original engraved die.
Even this plate is not the one destined for the printing press. The actual printing plate comes from a later generation of plates grown from the alto plates in the electroplating baths. That final printing plate is coated with a thin layer of chrome to make it hard and slick. It contains the intaglio image in recessed grooves only 2/1000 (0.002) of an inch deep — but that is deep enough to hold ink for intaglio printing.
Even the thickness of the printing plate is scrutinized; the margin of error is only +/- 0.0003 (three ten-thousandths) of an inch. That's 1/10th the thickness of a human hair.