Many printing methods are used in the production of bank notes, engraved printing, color, print registry, serial numbers, micro printing, and latent imaging. Today the most important parts of bank notes are generally printed in a method know as intaglio printing. This starts with a master engraver, engraving the art into steel plates, from which the production plates are made. Then ink is applied to the plates, wiped clean and the remaining ink in the groves and fine lines is apply under enormous pressure to the paper (7,500 to 15,000 psi), this gives the printed surface of the note a raised feeling. The use of circular or symmetrical lines on the engraved plates, called guilloche, are made by machine or laser and Computer Added Design(CAD). This fine line designs are extremely difficult to produce by hand. The American inventor Asa Spencer invented the lathe that was originally use to create these designs on steel plates
Some countries also use this feature of intaglio printing to build up some areas of the note to assist the blind or visually impaired, by placing raised marks on the notes to distinguish which denomination the note is.
Most countries have switched a long time ago to using multiple colored inks to help foil counterfeiting. The use of color increases the expense and time required to produce counterfeits. With the advent of laser printer these two methods are not as effective as the use to be.
Many notes are also produced with perfect registry. This is were a design on the front will align perfectly with the design on the back. The French are true masters of this art and beautiful examples of this can be seen many French notes, the facial parts of the portrait align perfectly front and back (example 5 Franc note). This technique requires great skill. Other examples can be seen on notes where a design on the front and back becomes complete when held up to the light. Example Colombia, 1000 Pesos Oro, 1986, p-424.
Another technique of the printers skill is the serial number. Serial numbers are applied to notes after the intaglio press, using letterpress printing. Many countries have started using serial numbers that may be vertical on one side of the note and horizontal on the other and vary in color. They may also vary the size of the numbers. This seems simple but is difficult and expensive to do, requiring special equipment. Examples can be seen on notes from Cape Verde and Ireland.
Many countries have started using special engraving to create a latent image or optical effect on certain areas of the note. These images or optical effects can be seen when the note is held up to the light at right angles revealing the image or latent effect. These effects are caused by special engraving on the intaglio plates. Examples can be seen on many notes to day such as Argentina 5000 Australs, 1989, p-330a.
Also in use today is micro printing. The use of printing of letters in such a small size that they can only be seen clearly with a magnifying glass. It will usually appear as fine line when viewed by the naked eye. This printing technique is difficult to photocopy because of the high resolution required. Current photocopiers do not have the ability at this time, but may in the near future be able to reproduce this. This feature can be seen in US notes with a fine line around the portrait and on the lapel of Franklin and Grant. Other examples can be seen on Sweden, 20 Kronor and Belize 1 Dollar.
Another printing method, not in wide used, is the tête-bêche method of printing. This is where the same design appears twice on the same note. So no matter which way you look at the note is appears the same from all angles. This can be seen on notes issued by Brazil from 1978 to 1981. Example of this can be seen on Brazil, 200 cruzeiros, 1981, p-199.
The most recent development in the printing skill are the use of color shifting and metallic inks. The color change is caused by multi-layered metallic flakes suspended in the ink. The inks change color when held to the light at different angles, as on the US$100 and new US$50 note (to be issued this fall) turning green to black. Other examples can be seen on Thailand 60 Baht, Argentina 5000 Australs. The metallic inks also help prevent copying, turning black when it is photographed or copied. Because of the special combination elements in the ink, this is very difficult to reproduce.