J. Locke

Automatic Refractive Index Measurement of Glass particles

Abstract An automatic instrument for Glass Refractive Index Measurement (GRIM), developed by Foster and Freeman Ltd., has been evaluated and adapted to meet the requirements of the UK Forensic Science Service. The apparatus is microprocessor controlled and uses the signal from a CCTV camera to detect the match temperature of the glass. Prototypes were evaluated at the Metropolitan Police Forensic Science Laboratory, at the Central Research Establishment, Home Office Forensic Science Service and at the Home Office Forensic Science Laboratory, Aldermaston. The performance of GRIM was compared to that of the Mettler apparatus currently in use with special regard to precision and long-term stability. GRIM was found to be well suited for operational use.

Refractive index variations across glass objects and the influence of annealing

Abstract A study was made of the refractive index (RI) variations in glass objects with particular attention to building and vehicle windscreen glass. Substantial variations were found across the thickness of certain specimens which could be attributed to thermal effects during manufacture. RI variations were largest in thick shop windows or toughened glasses but were also detectable in domestic window panes. The application of laboratory annealing greatly reduced variations in RI with the exception of a bottle glass where chemical inhomogeneities may also have been present.

Breaking of flat glass — Part 1: Size and distribution of particles from plain glass windows

Abstract An experimental rig was constructed, for the systematic breaking of windows, where a steel ball struck the pane in a controlled manner. For plain window glass the distance travelled by particles from the resulting debris was determined according to fragment size. The general form of the results agreed well with earlier findings. Replicate breakings were then performed to establish the reproducibility of the method. By combining the data for replicates for particular size ranges, smooth relationships were demonstrated between the number of particles and the distance travelled. The distribution of the glass particles radially, which was size dependent, was also studied.

The examination of paints as thin sections using visible microspectrophotometry and Fourier transform infrared microscopy

Abstract A rapid method for preparing and mounting small paint fragments for visible microspectrophotometry and FT-IR microscopy is described. In both cases spectra are obtained in transmission from thin paint sections. The advantages of using this method of sample presentation are demonstrated.

Breaking of flat glass—part 2: Effect of pane parameters on particle distribution

Abstract The effect of changing the dimensions of window panes on the distribution of glass particles when they are smashed has been investigated. The parameters changed were the size, thickness and whether the pane was plain, patterned or wired. When comparing results from experiments, allowance was made for the inherent variability of the breaking process. No significant trends were identified in the particle distribution which could be clearly associated with a change in the pane parameters.

Breaking of flat glass — Part 3: Surface particles from windows

Abstract The debris recovered after smashing of flat glass windows has been examined to determine the percentage of surface bearing particles. Four ranges of particle size were considered and a distinction was made between particles originating from the front surface, from the rear surface and from the centre of a pane. The factors that influence the proportion of fragments from the two original surfaces versus the proportion from the bulk glass are discussed.

The examination of glass particles using the interference objective. Part 2: A survey of flat and curved surfaces

Abstract The microscopical examination of original surfaces on casework sized glass particles using an interference objective and the interpretation of their interferograms is discussed. To reduce subjectivity, a numerical criterion was devised to assess the curvature of glass particles and this was then applied to a survey of window, container, tableware and vehicle glasses. Objects with curved surfaces produced particles that were distinctly curved while those from surfaces of both modern and old windows were clearly identifiable as flat. Apparently flat surfaces from tableware and containers almost always yielded specimens that were detectably curved. In addition, there was no evidence that a fracture surface would be confused with an original surface from a plain flat glass. In a ‘blind’ trial to assess the interferometric routines, three examiners correctly identified flat and curved surfaces on 15 particles derived from a selection of glass objects.

The design of equipment and thermal routines for annealing glass particles

Abstract Equipment for annealing casework sized glass particles is described. Investigations were carried out using a simple muffle furnace, a temperature programmed furnace and a metal sample holder. Because they produced a uniform and reproducible thermal environment, best results were obtained from a combination of the temperature programmed furnace and the metal sample holder. Refractive index changes for window and toughened glass specimens were investigated and a simple, reproducible annealing routine for casework was devised.

A data collection of vehicle topcoat colours. 3. Practical considerations for using a national database

Abstract An assessment was made of the usefulness of microspectrophotometry for identifying unknown automobile paint samples derived from hit and run accidents. This was achieved by measuring the tristimulus values of a collection of paint samples from 98 vehicles. These values were then compared with similar colorimetric data from a reference collection of standard coloured squares. Close agreement between casework samples and the reference collection was obtained provided the casework samples were cleaned by gentle polishing prior to measurement. Errors associated with the technique were investigated and expressed objectively by means of colour difference units.

An investigation of refractive index anomalies at the surfaces of glass objects and windows

Abstract Results obtained using GRIM equipment show that the refractive index (RI) of surface layers of float and non-float flat glass, glass containers and tableware can vary significantly from that of the bulk glass. The results also suggest that, using the oil immersion method with GRIM, it is not possible to obtain surface RI measurements which are useful for discrimination. Instead, a range of values is obtained for each sample, the individual results probably being dependent upon the thickness and geometry of the particular edge under examination.