AFTE Store - Investigating the Effectiveness of Chemical Etchants on Medium Carbon (0.40% C) Steel

Serial number restoration allows forensic investigators to recover a potentially lost chain of ownership relating to a piece of evidence in an investigation. The ability to restore an obliterated serial number relies on the permanent residual stress left in the metal substrate from a cold working application of the stamped serial number. The application of chemical etching reagents is one method which is used to exploit this stress, and there are a number of different formulations available for this purpose. This research aimed to review and compile historically successful and other proposed etching reagents, to further understand how each formulation can be used more efficiently in the field of serial number restoration. The importance of the composition of the metal substrate being etched is well established, although from the current literature there is a lack of consistent knowledge surrounding which reagent should be used with specific metal compositions. This research intended to aid forensic investigators in determining the most suitable etching reagent for medium carbon (0.40%) steel. The efficacy of reagents at varying depths of obliteration on a consistently stamped steel substrate was tested. The standard Fry’s Reagent, consisting of 90 g copper (II) chloride, 120 mL hydrochloric acid and 100 mL deionized water with 15% nitric acid, was determined to provide the best quality of restored marks and the highest percentage of correctly interpreted restored marks per plate out of the six reagents tested in this study.