• Scott, G.
• Sherriffs, P. B.
• Evans, C.
• Ohara, J.
• Popov, K. T.
• Jones, Bj
• Gulbrandsen, H.
• Lawson, D.
• Davies, Z.
• Breen, F.
• Cammidge, J. W.
• Andersen, P. M. B.

Abstract

Polymer banknotes entered circulation in the UK from 2015 to 2022.These are designed to last two-and-a-half times longer than their paper or cotton predecessors, and some are now reaching their end-of-life.The type of substrate material affects the fingermark development technique or sequence that will reveal the most and best quality marks, therefore as banks switch to polymer notes, there has been a focus of forensic research on assessing the optimum visualisation technique.Most of this work has necessarily been done on mint notes, supplied by the issuing bank.However, the surface of a banknote changes as it is handled, and this affects the way fingermarks and development processes behave at the surface.<br/>This work examines mint, laboratory handled, and circulated £5 and £10 polymer banknotes, studies the performance of fingermark visualisation processes and relates this to the degradation of the surface structure.A range of studies covered 3296 fingermarks, aged 8 to 56 days, on Bank of England, Bank of Scotland, Royal Bank of Scotland, and Clydesdale Bank polymer notes. <br/>A total of 1856 fingermarks on mint and handled £5 banknotes from the four different issuing banks were visualised with Vacuum Metal Deposition (VMD), Cyanoacrylate Fuming (CAF) and, on Clydesdale Bank notes, fluorescent powder.<br/>1440 fingermarks on mint and circulated £10 polymer banknotes from Scottish banks were visualised with VMD, Power Suspension (PS), one-step CAF technique, LumicyanoTM, as well as sequences of these techniques, including visualisation with reflected infra-red (IR) illumination.<br/>As a single technique, VMD was significantly more effective than other processes in developing fingermarks on handled or older circulated banknotes, across all the banks studied, although effectiveness varied with issuer. For example, on handled Bank of England notes 45% of marks showed ridge detail with VMD development and 28% with CAF; for circulated Royal Bank of Scotland 2016 issue notes success rates were 49% with VMD and 25% with Lumicyano.Although for the 2021 issue, which are less degraded, Lumicyano produced better quality marks, with 59% development compared to 56 % for VMD.<br/>The surface structure of the banknotes was studied to examine the texture at a macro and micro level.Circulation of banknotes causes degradation of the surface structure, with different features observable by eye and with simple light microscopy, such as folds and cracks, loss of intaglio and pearlescent surfaces, and contamination.At a higher resolution the formation of a micro-cracked surface structure can be seen in the handled and circulated notes.The wettability of the surface is also increased.These micro features can lead to the trapping of powder, or contaminants, increasing quantity of development agent in fingermark background between the ridges, decreasing contrast and decreasing performance of powder-based fingermark development techniques.For example, circulation of Bank of Scotland notes reduced the number of marks developed with PS showing ridge detail from 63% to 29%.These same features can restrict the migration of components of the fingermark, lessening fingermarks degrading through spread of material and thus reducing potential formation of empty prints, so that VMD development is not adversely affected to the same extent by the surface degradation, showing a reduction from 92% to 70% with circulation.<br/>The separate areas of the banknote surface, such as opaque, transparent and pearlescent regions, as well as the raised text, complicate aging of the note and fingermark development.Some fingermarks were partially visualised due to crossing different regions of the banknote, and a sequence of techniques is beneficial.Lumicyano followed by VMD, or VMD followed by powder suspension, give improved results over single techniques; for example, the second technique increasing observed marks from 25% to 57% and 49% to 77% respectively on Royal Bank of Scotland 2016 issue notes.Reflected IR imaging enables suppression of background features and improves visualisation, regardless of technique or banknote issue or condition, for example on 56-day-old fingermarks on circulated Bank of Scotland note IR imaging increases VMD developed marks with visible ridge detail from 31% to 61%.IR imaging is also beneficial for sequences of techniques, on Clydesdale Bank notes for example increasing VMD + PS developed marks from 66% to 75%.<br/>Different designs and manufacture leading to different ageing behaviour and distribution of material and textures across the banknote, suggest that studies on one issuer such as Bank of England are not necessarily directly applicable to notes issued by Scottish banks.The condition of the banknote needs to be considered when deciding on the optimum fingermark visualisation technique(s).

Topics

• Deposition
• impedance spectroscopy
• surface
• polymer
• laser emission spectroscopy
• crack
• texture
• aging
• aging
• microscopy