From Fit to Unfit: How Banknotes become Soiled

Transcription

1 From Fit to Unfit: How Banknotes become Soiled Abstract A banknote is a product with a long tradition. It is well known and commonly used. Yet banknotes still have an unreveiled secrecy, not meaning their security features, but their life! Especially the link between their environment and deterioration remains somewhat a mystery. Four typical deterioration stages of a banknote in circulation in the Netherlands are described in this paper. The study elaborates on earlier work conducted by DNB in the field of banknote soiling. The main principle of banknote deterioration within the Netherlands (Eurosystem) is that human fingers are not only the main soiling source but are also a key contributor to the mechanical deterioration and inhomogeneous soiling behaviour of circulated banknotes. It is the (gentle) touch of the human finger causing soil (sebum) adhesion on the elevated parts (crumble- or fold lines) of the banknote revealing a structural yet inhomogeneous appearance. In addition the touch of the human finger accelerates relaxation of the cotton fibres. The induced crumpling and folding of the banknote causes the stiffness to decrease and increases the porosity, thickness and micro roughness. The small weight increase is due to add up of soil. Keywords Banknotes, Deterioration, Fingerprint, Sebum Peter Balke Payments Division/ Cash Policy Department De Nederlandsche Bank P.Balke@dnb.nl

2 1 Introduction The Nederlandsche Bank (DNB) has shown interest in an efficient use of banknotes since This interest is not only from a cost point of view but cleaner notes are also more secure and more attractive. It appears that soiling is the main reason for classifying banknotes as unfit for circulation (Ref.1,2,3,4). The banknotes which become unfit are extracted from circulation and destroyed at DNB. Since soiling is the main reason for classifying notes as unfit for circulation, research is needed in this field. At the Nederlandsche Bank research has already been performed on soiling of banknotes. In 2008 the colour of soil was revealed (Ref.5). It demonstrated that soil is predominantly visible in the blue part of the spectrum, i.e. soil on banknotes has a distinct yellowish/brownish tint. In 2009 DNB concluded that the main soiling mechanism of euro banknotes is that fingerprint deposits will cumulate and eventually will form a yellow/ brownish layer of aged sebum (soil) (Ref.6). This paper considers the basic mechanisms of the overall deterioration of the banknote with the emphasis on soil application. 2 Structural Banknote Deterioration Stages 2.1 Introduction Since the seventies circulation tests are applied to determine the performance of banknotes in circulation (Ref. 7,8). On the market several empirical soiling tests are present which however cannot replicate a banknote in circulation or at least not accurately and reproducibly enough to be able to predict the performance of a (enhanced) banknote product (Ref.9,10). To replicate the induced stresses on a banknote in circulation, first an evaluation should be performed on banknotes from circulation. This creates a reference for the results to be achieved by the developed performance test. This reference should be an average soiled banknote out of circulation. In this research the following parameters are studied to determine the reference values for: micro roughness, stiffness, thickness, porosity, weight and soil distribution. The classification of the notes was performed using only the visual appearance.

3 Several stages of structural deterioration of a banknote during its life are proposed in this chapter. These deterioration stages are based on the banknotes in circulation in the Netherlands and thereby within the Eurosystem. 2.2 Definition of Stages As already mentioned a reference banknote should be obtained, i.e. an average worn banknote from circulation preferably for all stages of life. These references should only include typical structural deterioration characteristics, like soiling, tears and tape to ensure a proper relation to the stage of life. The more accidental deterioration characteristics, like: dog-ears, stains, graffiti and holes will be put less emphasis on. These phenomena are more or less randomly induced to the banknote in circulation and have a weak relation to the stage of life. The following five deterioration stages are identified (see Figure 1 & 2): 0. New Banknote 1. New but crumpled including some mechanical defects 2. Crumpled banknote with soil on elevated parts (fold lines) 3. Pronounced soiling on former fold lines and detached fibres 4. Soil on the entire surface, worn former fold lines are weak spots for tearing Stage 0 Stage 1 Stage 2 Dog-ear Soil Stage 3 Pronounced soiling including wear Tear Stage 4 Figure 1: Schematic cross sections indicating the structural deterioration stages of banknotes in the Netherlands.

4 Figure 2: Explanation on how to interpret the schematic cross sections in Figure 1 of the entire banknote. This is an example of stage 1. To support the theory images of typical soiled banknotes out of circulation are presented in Appendix A. Due to the fact that crumples or folds are less visible in a composed Red-Green- Blue (RGB) image, transparent infrared images are added. In detail the watermark areas are highlighted as well Stage 1 The deterioration stage 1 is characterised by banknotes that are new but crumpled and may include some mechanical defects, like dog-ears or tears. Still these banknotes retain their new look and feel. In this stage the banknote is still quite stiff. Therefore dog-ears cause quite a lot of damage since they are hard to remove Stage 2 The deterioration stage 2 includes all characteristics of stage 1 with the addition that soil will become apparent on the banknote. Especially on the folded or crumpled parts soil adheres best due to the fact that these are elevated regions. To a lesser extent banknote soiling will take place on other (lower) regions. The result is inhomogeneous soiling on especially fold lines Stage 3 The deterioration stage 3 is characterised by banknotes that are flattened on a macro scale. This is the result of the loss of stiffness and due to (long-term) constrained storage in vaults and/ or wallets. Wear on (former) fold lines causes fibres to detach in these regions which again attracts (more) soil in these regions. Again to a lesser extent banknote soiling will take place on other (lower) regions

5 2.2.4 Stage 4 The deterioration stage 4 is characterised by the add-up of all soil present also on the lower regions. The look of the banknote is soiled on the entire surface. Still the soil in-homogeneity remains, the former fold lines appear darker. As the former fold-lines are weakened by wear and soil, tears may appear in these regions. In the Netherlands notes with the characteristics of stage 4 will be removed from circulation. 3 Physical properties of deteriorated banknotes 3.1 Introduction In this chapter the physical properties will be characterised of an average worn banknote from circulation for different stages of life. Based on literature findings the following properties seemed dignified measuring (Ref.11): 1. (Bendtsen) Porosity (ml/min) 2. Thickness (µm) 3. Weight (gr.) 4. Stiffness (mn) 5. Micro Roughness (µm) The amount of stages of life might differ from the described four stages in chapter two. This is done to obtain a clear trend of the physical properties. In total seventy euro five banknotes were selected in seven fitness classes ranging from Super Fit to Super Unfit, each fitness class contained ten banknotes. 3.2 Porosity To obtain a porosity measurement we used the Bendtsen porosity tester. It measures the (macro) porosity by forcing air through the banknote and measuring the rate of flow. The more open the banknote structure is the more air passes through. The unit in which the porosity is given is ml/min. Two relevant but different areas were identified to measure the porosity (see figure 3): Watermark area (disturbance of the watermark, but no print) Intaglio area (no disturbance of the watermark, but printed)

6 Area 1 Area 2 Figure 3: Areas of interest for the porosity measurements The porosity measurement results are shown in Appendix B. 3.3 Thickness The banknote thickness is measured per single banknote using a contact method. The front left upper corner of the euro five banknotes is used to perform the thickness measurements. The unit in which the thickness is given is in µm. The thickness measurement results are shown in Appendix C. 3.4 Weight The banknote weight is determined using a balance. Each time ten banknotes were measured in one run. An average banknote weight is determined just by dividing by ten. The unit in which the weight is given is in grams. The results are shown in Appendix D. 3.5 Stiffness The stiffness of the banknote is determined by specialized banknote laboratory equipment. The unit in which the stiffness is given is Millinewton. The method uses only a small piece of the banknote (see figure 4). It was decided to use only the cross direction. The Intaglio area and Offset region were chosen since the watermark area will have a weakening effect on the stiffness.

7 PD CD CD Figure 4: Area of interest for the stiffness measurements (PD: paper direction, CD: Cross direction) The results of the stiffness measurements are shown in Appendix E. 3.6 Micro Roughness The micro roughness of the banknote is determined by the TRACEiT system of INNOWEP. It is a contact-free surface profilometer. The area of interest is a window of 5 x 5 mm (see figure 5). In highest resolution mode ( pixels) the system evaluates the roughness parameters over 1536 lines, each 5 mm in length. This summarizes to 7.50 meter independently in x as well as y direction. The instrument enables measurements of several roughness values. In this paper the arithmetic average (R a ) is used. R a is determined by the absolute values of the roughness profile ordinates. It gives a general description of the height variations in the surface. The unit in which the micro roughness is given is Micrometer. Figure 5: 3D Height Map of a small part of the watermark area using the TRACEiT system

8 The results of the micro roughness measurements from the recto and verso of a euro five banknote are presented in Appendix F. 4 Discussions This paper elaborates on earlier work conducted by DNB in the field of banknote soiling. The main findings of that work are that banknote soiling within the Netherlands (Eurosystem) is mainly due to human fingers. While the banknote is transferred the banknote is touched and fingerprint deposits are left behind on the banknote. During the lifetime of the banknote this procedure is frequently repeated, i.e. fingerprint deposits will accumulate and eventually will form a layer of aged sebum (soil) with a distinct yellowish/brownish tint. Four typical deterioration stages of a banknote in circulation in the Netherlands are described. In this chapter these will be linked to the observations of the physical properties. 4.1 Stage 1 The deterioration stage 1 is characterised by banknotes that are new but crumpled and may include some mechanical defects, like dog-ears or tears. The observed physical properties do not show major differences with respect to a new banknote. 4.2 Stage 2 The physical properties of a banknote within this stage are becoming somewhat different with respect to stage 0 and 1. The following observations are made: Increase in porosity from ~20 ml/min to ~40 ml/min stiffness reduction from 40 mn to ~30 mn Micro roughness of recto increases from 1.8 µm to 3.0 µm All these observations can be explained by the relaxation of the cotton fibres within the substrate. Due to banknote handling it is crumpled, folded and touched (massaged). All of these processes do not harm the tough fibres but causes them to reposition in a somewhat more relaxed state (Ref.12, 13). This again results in cavities to form which causes a decrease in stiffness and an increase in porosity and micro roughness.

9 In this stage the (first) soil appears on the elevated parts (crumble- or fold lines) of the banknote. The pronounced soiling on the elevated parts may be explained as follows: The angle of attack of the human finger is about 45 with respect to the banknote surface As the finger hits the surface the affinity between surface and finger contaminants determines the transfer of material towards the banknote When the finger encounters (the edges of) an elevated part, finger contaminants are (also) mechanically scraped off (see figure 6) Figure 6: Schematic representations of the soiling mechanism on elevated parts Since the process of soiling on the elevated parts is both affinity and mechanically driven this will be the dominant soiling mechanism. 4.3 Stage 3 The physical properties of a banknote within this stage are diverging further from the characteristics of a new banknote. The following observations are made: Increase in porosity from ~20 ml/min to ~60 ml/min stiffness reduction from 40 mn to ~25 mn Micro roughness of recto increases from 1.8 µm to 3.5 µm Thickness increase from 90 µm to 95 µm Weight starts to increase

10 Again all these observations can be explained by the relaxation of the cotton fibres within the substrate due to banknote handling. The weight starts to increase due to add up of soil. Induced wear by human fingers on (former) fold lines causes fibres to detach. The detached fibres will trap (more) soil towards these regions. Soiling of the lower regions is visible as well. Soiling on the lower regions can take place due to the fact that the banknote flattens with the loss of stiffness. 4.4 Stage 4 The physical properties are characterised of an average worn banknote from circulation for different stages of life. The following observations are made: Significant increase in porosity from ~20 ml/min to ~150 ml/min Thickness increase of ~10% Weight increase of ~2% Almost linear stiffness reduction from 40 mn to ~15 mn Micro roughness of recto equals that of verso to ~4.0 µm All observations, with the exception of the weight increase, can be explained by the relaxation of the cotton fibres within the substrate. The small weight increase is due to add up of soil. The tint of the banknote is in this stage typically yellowish/ brownish. The discolouring of the banknote is caused by two processes: oxidation of the cellulose fibres and naturally ageing of the sebum (Ref.5). Oxidation of the cellulose causes Oxycelluloses to form. This oxidation reaction results in an increase in oxygen containing groups such as carbonyls and carboxylic acids. Oxycelluloses typically causes a yellowing effect on the paper. The layer of sebum does naturally age in the following way: 1 Tri (or di-) glycerids initially decompose to glycerine and fatty acids 2 The double bonds in unsaturated fatty acids are oxidized into carbonic acids 3 These acids trigger other oxidation processes which often result in brown coloured substances The visual impression of the banknote in this stage is that the entire surface is soiled. Yet the soil in-homogeneity remains, the former fold lines appear (always) darker.

11 5 Recommendations Future work will be conducted on the topic of enhanced soil and performance testing. Since several physical properties, the soil application and distribution are now known for worn notes; these may be used as references. 6 Acknowledgement This research could not have been possible without the support of many, special thanks to Joh. Enschedé Security Print, Oesterreichische Banknoten- und Sicherheitsdruck GMBH (OeBS) and the Bank of England. At Joh. Enschedé the physical properties were determined using their banknote laboratory equipment. At OeBS the single note inspection system, ABCS-2, produced high resolution images of fit and unfit banknotes. The author would kindly thank the Bank of England for always being a patient sparring partner to have fruit- and joyful discussions with. 7 References 1. H. de Heij, Durable banknotes: an overview, Presentation of the BPC/Paper Committee to the BPC/General Meeting, Prague 27 May - 30 May N. Pramojaney, Banknote Durability Assessment by Laboratory Simulation, XVII Pacific Rim Banknote Printers Conference, Quintero, Evaluation of Long Lasting Papers for Venezuelan Banknotes, XVII Pacific Rim Banknote Printers Conference, Bank of Russia Detects Radioactive Banknotes, Banknotes of the world, #9, September Th. Buitelaar, The Colour of Soil, DNB Cash Seminar 2008, Amsterdam, February P. Balke, New Soiling Test Method: Anti-Dirty Fingers, Banknote A.H. Gillieson, Research into the extension of the life of banknotes: Results of 1973, 1975 and 1976 field trials, Bank of Canada Technical Report 10, P. Koeze, An accurate statistical estimation of the life-length of f 100-banknotes: A circulation trial with two qualities of currency paper, Int. Stat. Rev. 47, , N. Aitchison MacNab, Dirty Money, Paper Committee Banknote Printers Conference, March 1993

12 10. T. Meuer et al., The Eurosystem s efforts in the search for a longer lasting banknote, Billetaria issue 9, G. Boutin, $1 Circulation Field Study, Currency Conference 2008, Praque 12. Private communication with Paper and Board Research at Wageningen UR, J.M. Grimal et al, Quality of banknotes in circulation: BdF s experience and strategy, Integraf 2004

13 Appendix A Deterioration Stages Images of Euro Banknotes at different Deterioration stages one up to four, left images are in RGB, right images IR in transmission* * Images are taken by the single note inspection system of OeBS, the ABCS-2 system.

14 Deterioration stages 1 up to 4, watermark area in detail

15 Appendix B Porosity measurements Porosity (ml/min) Cl 1 Area 1 Cl 1 Area 2 Cl 2 Area 1 Cl 2 Area 2 Cl 3 Area 1 Cl 3 Area 2 Cl 4 Area 1 Cl 4 Area 2 Cl 5 Area 1 Fitness level and Area Cl 5 Area 2 Cl 6 Area 1 Cl 6 Area 2 Cl 7 Area 1 Cl 7 Area 2 Cl 1 = Class 1, Super Fit up to, Cl 7 = Class 7, Super Unfit Area 1 Area 2 Areas of interest for the porosity measurements

16 Appendix C Thickness measurements Thickness (micrometers) Cl 1 Cl 4 Cl 7 Fitness Level Cl 1 = Class 1, Super Fit Cl 4 = Class 4, Acceptable Cl 7 = Class 7, Super Unfit

17 Appendix D Weight measurements 0,70 0,69 Weight (grams) 0,68 0,67 0,66 0,65 Cl 1 Cl 2 Cl 3 Cl 4 Cl 5 Cl 6 Cl 7 Fitness level Cl 1 = Class 1, Super Fit up to, Cl 7 = Class 7, Super Unfit

18 Appendix E Stiffness measurements Stiffness (mn) Cl 1 Cl 2 Cl 3 Cl 4 Cl 5 Cl 6 Cl 7 Fitness Levels Cl 1 = Class 1, Super Fit up to, Cl 7 = Class 7, Super Unfit

19 Appendix F Micro Roughness measurements 5,0 Roughness (micrometers) 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 Cl 1 Front Cl 1 Back Cl 3 Front Cl 3 Back Cl 5 Front Cl 5 Back Cl 7 Front Cl 7 Back Fitness level with indication front or back Cl 1 = Class 1, Super Fit with intervals up to, Cl 7 = Class 7, Super Unfit