Applications of annealing in forensic glass examinations

Abstract

Aim: In this paper, our primary aim is to present the results of our experiments, carried out at the Hungarian Institute for Forensic Sciences, as well as the possible applications of annealing in the forensic investigation of glasses.
Methodology: 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 garment can originate from any of the broken samples found on the scene of the crime. A common method for forensic investigation of glass microfragments is the refractive index measurement. In cases when the refractive index of the recovered fragments and the reference is the same, 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. In this work we present this method via examples, comparing the results with previous literature data.
Findings: As a result of annealing, a structural rearrangement takes place inside the glass fragments and the refractive index changes. The extent of this change is characteristic of the glass types due to the different internal stress present in the different types. Besides 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 direct comparison of the recovered fragments with the reference sample would have given false results due to the significant heat effect (e. g. fire) on the control sample after the crime.
Value: We have successfully applied annealing to determine the type of glasses, to examine the possibilities of origin, and to analyze glasses exposed to heat, thus helping to answer forensic questions in criminal cases.