Recovery of Trace Evidence in Forensic Archaeology and the use of Alternate Light Sources (ALS)

Abstract
The effectiveness of alternate light source (ALS) to fluoresce bone and other materials is well-attested to in a laboratory setting but rarely, if ever, has it been used in field excavation. This study examined the recovery rates of fragmentary bone, fabric, and metal, both with and without the use of an ALS, through practical and controlled excavation experiments with multiple users. All archaeology, including forensic archaeology and crime scene investigation more generally, should account for trace evidence. Currently, there is limited empirical data for the recovery of evidence from excavation, and those studies that do exist, highlight the short-comings in current methods.
Six comparable test pits were created, representing empty graves in which only trace evidence remained. Each contained 20 fragments of bone (≤10mm), 16 hair fibres, two pieces of fabric and two lead pieces, which were back-filled and left for over 15 weeks. Three excavators were each tasked with excavating two test pits: one using ALS, one in daylight conditions.
The results of the experiment identified some critical aspects of using blue 455nm wavelength ALS in the field, and the importance of experienced practitioners. Sample evidence was small in size and recovery rates were low. In daylight conditions, an average of 46% of trace evidence was identified, while just 40% was recovered using ALS. This excludes hair fibres which were almost undetectable in all conditions. When using ALS, smaller bone fragments were more than twice as likely to be recovered, but less non-fluorescent materials were found. The experience of each excavator had a positive correlation with excavation results. Excavation error rates were calculated, demonstrating that excavation is comparable using either technique, but daylight conditions lead to greater accuracy.
The findings suggest that ALS can be used to increase recovery of some evidence types. Test pits provided none of the usual primary evidence associated with graves and excavators had no prior experience of ALS. While retrieval rates were low, almost all recovered items were found in situ and an accurate records maintained. Error rates in forensic archaeology are essential and it is hoped that the method outlined here can be developed towards the establishment of acceptable error rates. While ALS use in forensic archaeology should not be considered a panacea to issues of trace evidence recovery, a combination of well-tested archaeological excavation methods, alongside the implementation of such proven forensic techniques, would likely lead to improved recovery of evidence.