Refractive index measurement of the smallest bulk and surface glass microfragments in a model case

The refractive index measurement is a widely used investigation method of glass microtraces in forensic laboratories. The advantage of the method, in addition to its cost efficiency and repeatability, is that it is suitable for examining small fragments. In the present study, a real case was simulated, and fragments smaller than 250 micrometers obtained from five pieces of clothing, after breaking a pane of float glass, were investigated. In addition to the known (control) sample, altogether 25 bulk and 83 recovered surface fragments were also examined. In the first step, only one measurement was performed for each fragment to simulate the investigation of small fragments and test their measurability without crushing. All the bulk fragments met with the matching criterion K_min  ≤ Q ≤ K_max , in which K_min and K_max are the minimum/maximum value of the measured refractive indices of the known sample, while Q is the measured (mean) value of the questioned fragment. However, the matching ratio was only approximately 60% for the surface fragments according to the one-edge measurements. After crushing, the false exclusion rate was significantly reduced, and 79 of the 83 recovered surface fragments met with the matching criterion. As a result of crushing, not only the probability of matching, but also the edge count values improved significantly. Although the refractive index is only one property examined in the totality of glass analysis, the results of the present work can significantly contribute to the determination of the origin of small glass particles in real forensic cases.