Surface preparation in serial number recovery: Its effect on accuracy and recovery times

A common recommendation when undertaking serial number restorations is to prepare the materials surface to a 'mirror-like' finish through various means including polishing. However, altered serial numbers are rarely defaced uniformly. Hence there are instances of forensic examiners not able to adequately level (polish) the materials surface without also potentially removing deformation detail that is needed to successfully restore the pre-defaced marking. To assess the importance of polishing in serial number restorations, 118 sample plates were stamped, obliterated, and then chemically etched. 59 plates were left with the coarse grinding marks from the obliteration process then restored, whilst the remaining 59 were treated with Brasso® polish and increasing grit from 120 to 1200 grit sandpaper, prior to restoration. Recovery times and accuracies of participant interpretations were recorded. The polished plates were found to take significantly less time to restore compared to the unpolished, but also appeared to provide no difference with the accuracy of interpretations. This study found that surface preparation (polishing) prior to serial number restoration (chemical etching) did not provide any increase in participant accuracy. This work further provided surface topographies of both polished and unpolished surfaces, which may be of assistance for practitioners undertaking laboratory based serial number restorations.