Forensic and Crime Scene Investigation: Glass Items - Essay Example

The 'Scientific Working Group for Material Analysis' (SWGMAT) documents several investigative techniques that can be applied to a comprehensive range of glass types commonly found. These types are - flat glass used for windows, doors, display cases and mirrors; container glass; tableware glass; optical glass; decorative glass; and specialty glass used for headlamps, cookware and others (SWGMAT, 2004). Known Sample: 'This is a subset of a larger population or sample originating from an identifiable source, collected as representative of that larger group' (SWGMAT, 2004).

(Example: Fragments of glass from the display window of a case inside a room where a crime has been committed). Questioned Sample: 'This is a material of unknown source collected from a known location either as, or from, items of evidence' (SWGMAT, 2004). (Example: Fragments found sticking to the cloth of a bag suspected to belong to the perpetrator). Refractive Index: For a particular transparent medium, this is the ratio of the speed of light in a reference medium (in the case of forensic science a vacuum) and the speed of light in that transparent medium.

Mathematically, it is expressed as - =. Here, is the refractive index of the medium at specific wavelength 'i' of light, is the velocity of light in the reference medium (in this case vacuum) and is the velocity of light in the medium of investigation. There is a clear-cut logic for or. Logic for Initiation of AnalysisThere is a clear-cut logic for organizing a forensic analytical scheme for locating, collecting, identifying and analyzing glass. This logic is most influenced by the information required and the size and condition of the samples available, sample availability itself and the laboratory equipment available.

It is also emphasized that since glass is such a material that items possess both physical and chemical heterogeneity even when taken from a single source it is essential to procure samples from a known source for comparative analysis with collected evidence (SWGMAT, 2004). Collecting Glass SamplesThe collection guidelines as stipulated by 'Scientific Working Group for Material Analysis' (SWGMAT) are succinctly enumerated below.1) The largest possible number of samples should be collected from each broken object and packaged separately. 2) The samples should be carefully removed from the structures (window panes and car window glass).

If multiple panes are broken a diagram is recommended to allow the individual samples to be identifiable by the source panes.If reconstruction of the direction of breakage or the pane is required the inside and outside of the samples should be marked to allow that (Fig. 1). 3) For laminated sources both sides should be sampled. 4) Samples from all parts of the broken source should be collected to be as representative as possible. 5) Each sample should be collected taking into account the possibility that there may be other material evidence on them.

(SWGMAT, Standards and Guidelines, 2004)Figure 1:Fragment