Key security features of genuine banknotes..
Key security features of genuine banknotes.
Shaposhnikov Yu.I.
Paper
Banknotes are printed on high-quality durable paper specially designed for printing banknotes. As an example, let's briefly consider the process of making paper used to print US dollars.
The raw material used for its production is scraps of cotton and linen fabrics. Colored fibers for introduction into paper are supplied in skeins, with fibers of each color purchased from different companies. The fibers are cut according to technical requirements. Then the raw material is manually sorted, and foreign elements are removed from it, after which it is sent for cutting. The resulting mass is sent to a rotary boiler, where it is converted into paper pulp when treated with superheated steam. After cooling and squeezing, the mass is sent to a washing machine, where it is repeatedly passed through special shafts equipped with steel knives, and abundantly washed with artesian water. In this case, foreign inclusions are removed from the paper pulp, and the length of the fibers is reduced. Next, bleach is added to the resulting mass, and the raw material is placed on a porous surface that allows water to pass through and left in this form for several days. The pulp is then fed into a milling machine where coloured fibres and dye are added to give the paper a creamy colour. The pulp is placed in a cleaning machine called a 'Jordan' and then passed through a filter which removes unmilled fibres. The resulting pulp contains up to 99% water. To remove it, the pulp is repeatedly rolled over a rotating wire mesh, intertwining the fibres to form paper. The resulting paper fibre undergoes additional processing to remove residual water and compact the fibres (special suction cups, vacuum rollers, etc.). Finally, the paper is dried by passing it through a series of rollers consisting of large, hollow steel cylinders heated with steam. The result of this process is paper similar to blotting paper. To give the paper the required density, it is impregnated with animal glue and glycerin, passed through rigid rollers and dried. The finished paper arrives at the Bureau of Engraving and Printing in Washington, cut into 32-plot sheets in packs of 10,000 sheets. The resulting paper can withstand repeated bending (up to 4,000 times), is resistant to tearing and crushing, and has a characteristic crunch.
Usually, banknote paper contains a two- or multi-tone watermark — alternating darker and lighter areas that differ from the rest of the banknote. Well visible when held up to the light, it must necessarily have slightly blurred, fuzzy contours. This is due to the fact that the thickness of the paper changes smoothly. Note that the watermark, which originated as a stamp of the paper manufacturer, has now become an integral part of the security of banknotes. A distinction is made between a local watermark — a drawing located in a certain place on the banknote (usually on the coupon field), and a general watermark — a continuously repeating drawing located across the entire field of the banknote.
Security fibers of various colors are added to the banknote paper. The fibers are randomly located on the paper and are found both in the thickness and on the surface of the paper. Sometimes, also at the casting stage, colored inclusions made of polymer film in the form of circles or polygons — the so-called confetti — can be introduced into the banknote paper.The paper of banknotes does not contain optical brightener and therefore appears dark in filtered ultraviolet light (wavelength 366 nm). General-purpose paper will luminesce with blue or bright blue light. This is clearly visible even if you illuminate the side of the paper pack. However, it should be noted that if a banknote gets into a solution of washing powder (for example, during accidental washing), the paper will absorb the powder and will luminesce in UV light.
Security thread
Often plastic, metallized or metallic threads are introduced into the paper of banknotes, sometimes coming out on the surface of the banknote from the front side, the so-called «floating». The thread can have magnetic properties or fluoresce under the influence of UV radiation, and the glow can be either single-color or multi-color. Usually the thread contains repeating microtext.
Paints
The paints used to make banknotes differ from conventional printing paints. They are more resistant to various chemicals and do not change color. Sometimes a ferromagnetic pigment is added to the paints, which triggers various testers. Magnetic paint is often used to print serial numbers (Russian rubles) or fragments of metallographic images (US dollars). A single-color drawing can be made with two paints that have the same color but different magnetic properties. The use of pigments that luminesce under ultraviolet light (red, green, and yellow glow) is very common. When printing modern banknotes, paints are sometimes used that have the same color but different reflection in the infrared region of the spectrum — IR metameric paints. There are special devices to detect all of the above-described features.
In recent years, optically variable ink (OVI) has often been used for printing on large denomination banknotes. Such inks have a metallic sheen, and their color can change when the viewing angle changes. Optically variable inks are manufactured by the Swiss company SICPA, and their manufacturing technology is very complex and expensive.
Under the influence of various chemicals (washing powders, solutions used in dry cleaning, solvents) paints can partially change the original color, and sometimes components that glow under the influence of UV radiation are washed out.
Printing methods
The following types of printing are used to produce banknotes:
- Letterpress printing. Letterpress printing forms are designed in such a way that the relief printing elements are located in the same plane and higher than the non-printing elements. During printing, a sheet of paper is pressed against the printing form and the ink on the printing elements is squeezed out to the edges of the elements. In this case, a characteristic «border» of ink is formed along the edges of the resulting images and a slight deformation of the paper is created. The diagram of the letterpress printing process is shown in Fig. 31. This is how the images of series and numbers of banknotes are made on most currencies of the world, including Russian rubles.
- Intaglio printing. Intaglio printing forms are designed in the opposite way to letterpress printing forms. The image elements are recessed in the printing form. During printing, the paint from the forms sticks to the paper and when dry forms a layer of paint protruding above the surface of the paper of sufficient thickness, which can be easily felt by touch. The diagram of the intaglio printing process is shown in Fig. 32. Intaglio printing achieves high accuracy and clarity of reproduction of the image. The smallest elements of the images on the banknotes are made using this method.
- Offset (flat) printing. In offset printing forms, the printing and non-printing elements are located in the same plane. The printing process from such forms is based on selective wetting of the non-printing elements with water, and the printing elements with oily paint. The production of a printing form comes down to obtaining stable hydrophobic (fat-sensitive) and hydrophilic (moisture-sensitive) films on the surface of the form material. To obtain flat printing forms, it is necessary to create stable printing and non-printing elements on the surface of the form material (form base). This can be achieved in various ways. The most widely used are monometallic and bimetallic printing forms. Aluminum or steel plates are used as the form base. The surface of the plate remains unchanged when producing a monometallic printing form, and when obtaining bimetallic printing forms, a copper layer is applied to it (it forms the printing elements), and a chromium or nickel layer is applied on top of it (to form non-printing elements). In both cases, a negative or positive copying layer is applied to the plate. Photoforms are exposed to the copying layer by contact: a negative or a diapositive. On monometallic forms, the unexposed areas of the copying layer serve as the basis for the printing elements; for non-printing forms, it is an aluminum plate that is exposed when the image is developed. The process of forming a bimetallic printing form is more complex. The copying layer loses its solubility in illuminated areas and, after development, the upper layer of metal on the form is etched under its protection. As a result, the next layer of metal is exposed in areas corresponding to the dark places of the photoform. After this, the copying layer is removed and simultaneous chemical treatment of the non-printing and printing elements is carried out. During printing, the paint from the form is first transferred to an intermediate elastic rubber web, and from it to the paper. The paint lies on the print in a thin, even layer, through which the structure of the paper is clearly visible. Let us remember that this is the method used to print magazines, booklets, calendars, books, etc.
- Orlov printing. It is a type of offset printing, in which the elements of the resulting images have a transition from one color to another. In this case, the transition boundary is clear, there are no distortions and breaks in strokes, or overlapping of one color on another. The method is named after the technician of I. I. Orlov's expedition, who invented a method of multi-color printing at the end of the 19th century. Its essence consisted in the simultaneous application of various tones and colors on paper in one impression from one printing form. In 1891, he designed the machine, and two years later it was built and operating. Orlov printing was first used in the production of 25-ruble banknotes of 1894.
The most complex high-precision equipment is used for Orlov printing. It is impossible to reproduce the effect of Orlov printing using conventional classical printing methods. - Iris printing. It is also a type of offset printing. With iris printing, the elements of the resulting images have a smooth transition from one color to another. At the same time, there is no clear transition boundary.
Having considered the general elements of banknote protection, we will examine specific currencies in more detail, focusing on Russian rubles and US dollars, as the most widely circulated currencies in our country.