Metallographic Etching of Aluminium and Its Alloys for Restoration of Obliterated Marks in Forensic Science Practice and Investigations

Abstract

1.1 The problem A problem of common occurrence in forensic science is the restoration of obliterated serial numbers on the chassis and engine of stolen motor vehicles, firearms, jewellery, valuable tools, and machinery (Nickols, 1956; Wolfer & Lee, 1960; Jackson, 1962; Cunliffe & Piazaa, 1980; De Forest & Gaensslen, 1983; Schaefer, 1987; O’Hara & O’Hara, 1994; Moenssens et al (1995); Heard, 1997; Petterd, 2000; Lyle, 2004; Katterwe, 2006; Seigel, 2007; Mozayani & Noziglia, 2006; Jackson et al (2008); Levin, 2010). Serial numbers or other markings, which are unique to that particular item, are usually marked on the above metal surfaces during the manufacturing process. Criminals alter or obliterate these identification marks during thefts or other illegal uses in order to prevent their identity. On many occasions a fraudulent number would be introduced after removing the original one. In abandoned vehicles all serial numbers are verified in order to detect alterations in the identity of the vehicle (Svensson et al, 1981). Sometimes the serial numbers on firearms are removed more professionally making it hard to distinguish whether the numbers are original or not (Shoshani et al, 2001). Restoration of the original obliterated numbers provides important forensic evidence in order to return the items to the owner and also to follow up the criminal proceedings against the perpetrator. In cases involving firearms the recovered numbers provide an important investigative lead. The history and the ownership of the firearm can be obtained only from the serial number. In forensic practice a few experimental techniques are being used successfully to restore the obliterated serial numbers on the above surfaces. The methods include: chemical etching, heat treatment, magnetic particle, ultrasonic cavitation and relief polishing (Nickols, 1956; Polk & Giessen, 1989; Maxwell, 1993; Heard, 1997; Hogan et al, 2006; Katterwe, 2006). Nondestructive methods such as hardness testing, magnetic imaging, x-ray imaging, eddy current, infrared imaging, scanning acoustic microscopy, electron channeling contrast and thermal wave imaging are also proposed (Hogan et al, 2005; Katterwe, 2006; Klees, 2009). However, chemical etching is most popular because it is sensitive, easy to apply on any size and also kind of object and reproducible. Etching can be applied on all metal surfaces including precious ones (Crowe & Smith, 2005). This chapter presents background information on serial number restoration and etching techniques applied to recover the obliterated markings on aluminium and especially two of