Possible Applications of Annealing in Forensic Glass Examinations

Abstract

The question that typically arises and requires forensic examinations in connection with crimes involving the breaking of glasses (e.g. burglary, vandalism) is whether the glass fragments recovered from the suspect’s clothing can originate from any of the broken objects found on the scene of the crime. A widely used method in the forensic investigation of glass is the refractive index measurement, which – in contrast to the elemental analytical methods e.g. ?-XRF or LA-ICP-MS – can also be applied to fragments in the size range of a few tens of micrometers. If the recovered fragments and the reference have the same refractive index, it is probable that they originate from the same source. These investigations can be supplemented by the heat treatment of the glass samples under controlled conditions followed by the measurement of the refractive index. As a result of annealing, a structural rearrangement takes place in the glass fragments and the refractive index changes. The extent of this change is typical of the glass type due to the different internal stress present in different types of glass. In addition to establishing a more precise relationship of origin, this method also makes it possible to determine glasses by type. Annealing has also been successfully applied in cases where the reference sample has previously been altered by heat or fire during the crime, hence the direct comparison would have given false negative results. The primary aim of this work is to present the results of our experiments carried out at the Hungarian Institute for Forensic Sciences, as well as the possible applications of the annealing of glass fragments in real forensic cases.