The examination of the shape, size and type of damage that pieces of broken glass have sustained, plays an important role when answering the questions: How was the glass originally broken and is it associated with a scene of crime? The analysis of glass fragmentation patterns can be so precise, that the type, direction and sequence of impact can all be determined by forensic investigation, even down to discovering what tool caused the impact. Additional information regarding the source of the glass, the variety of glass and who had contact with the glass can also be identified.
Many techniques are available for answering the myriad of raised questions. Some can be used alone, others in conjunction with alternative methods. Priority would be to use those least destructive to the evidential sample first. Each process has advantages and disadvantages. I aim to examine four specific techniques, these are: Microscopic examination and physical fit, density and refractive index, the use of the comparison microscope and the use of the scanning electron microscope.
I will explore the advantages and disadvantages encountered when using these approaches to analyse glass in the laboratory. When glass is examined at the laboratory, it should initially be macroscopically examined. This means to inspect visually using the naked eye or a magnifying glass. Ideally a sample/s should be freshly broken with clean and sharp edges which are easy to examine. Fluorescing and chemical properties of a substance, manufacturing marks and features that were possibly obtained during the incident should be closely inspected.
This should be done over a sterile surface or collection paper to prevent loss of evidence and documented fully. Macroscopic examination is non destructive and can be conducted on large pieces of glass or fragments of microscopic size, to determine basic characteristics. Thickness, colour, texture, uniformity and surface appearance of the glass sample recovered are all examples of the type of properties that would be examined. Depending on the crime in question, the forensic scientist will probably want to attempt a physical fit.
This ‘jigsaw’ method can take time but proves successful when it eliminates the need for other more costly and technical procedures. Physical fit involves matching up the individual recovered fragments of glass to create a perfect fit (much like the pieces of a jigsaw). Each shard of glass will have distinct physical traits and random breaking fractures. This will cause uneven surface qualities, including stress marks and fracture lines. If under close inspection, these lines and pieces can be reunited then a positive match can be made.
The physical process of sifting through varying sized pieces of broken glass in an attempt to reconstruct the physical appearance is extremely fiddly and tedious. It requires much patience, care to prevent injury and is very time consuming and exhausts resources. On the other hand, this method is cost effective and has high evidential value owing to the unique characteristics of individual glass samples. The markings on a piece of glass will only match up to its original source. For this reason physical fit is an ideal way to prove a positive association with the scene, item or person in question.
This form of evidence cannot be argued if queried during criminal proceedings in court, therefore it rules out the need for further investigation which may prove fruitless and drain financial resources. In situations when physical fit is not successful or viable as an option, other scientific approaches can be used. Two non-destructive methods which are routinely conducted, focus more on the physical and chemical composition of glass. They investigate the density and refractive index of glass. Any other method used will require specific apparatus and equipment and will be more technical and less cost effective.
The use of technology provides faster results than other manual methods. The simplest technique to perform is density analysis, as this property varies within glass composition. This method easily differentiates various samples of glass to discover if they originate from the same source. The specimen of glass must be suitably sized as it is difficult to conduct on tiny fragments. The sample is immersed in a container filled with a known volume and density of liquid. If the sample neither floats or sinks but is suspended in the liquid this indicates the glass and liquid density are equal.
The results can then be cross referenced with known samples. This method can be conducted in conjunction with measuring the refractive index of a glass sample using a microscope. The refractive index (RI) is easier to determine than the density. It accurately measures the degree of how much glass bends light. Different glass has a different refractive index and can provide information regarding chemical elements that are associated with colours of specific glass types thus making it possible to identify groups of glass.
The result can be compared with a known sample from a scene of crime or person associated with a particular crime and can lead to elimination of potential matches. The result can also be cross matched to a database of reference samples which list various types of glass produced, manufactures’ details and known glass refractive index measurements (GRIM). One way to calculate the refractive index of glass, is to place a sample of the questioned glass onto a heat equipped microscope. Silicone oil is added and heated.
Lines of refraction can be observed at different temperatures until the RI of the glass and the silicone oil match. As this occurs the halo characteristic associated with glass, known as a Becke line will no longer be visible. Then the RI measurement can be calculated, recorded and compared with other glass samples to obtain a glass profile. (Forensic Access Ltd, 2009) (Whiteleys, 2008) After conducting macroscopic examination and preliminary tests that determine the physical properties of a specimen.
The next logical step is microscopic examination using illumination. Although slightly more technical than manual approaches and more costly due to necessity of expensive equipment, the use of a microscope can closely examine, identify and even offer comparison of questioned and known glass samples from the scene, obtained from a suspect or already on file. Various sized shards of glass including fragments minuscule in size that normally would not be visible without the use of optical aids can be viewed easily and effectively.
A microscope can zoom in and view a highly magnified version which enables the user to focus in on minute details and features. Some microscopes can even perform chemical analysis and determine chemical components. Others are limited to measuring a sample of glass, including the thickness, diameter and angle. Stereomicroscopes focus on the examination of the broken edges of a glass sample, in particular the radial fracture for signs of stress marks associated with conchoidal fractures.
The comparison microscope easily and effectively allows two samples, usually a questioned and a control sample to be examined and compared side by side. The instrument itself consists of two identical microscopes next to each other and attached to an optical bridge with a light source to illuminate the specimen. The bridge houses a set of optical lenses each with a different level magnification. A mirror is also housed within the bridge which allows the user to view a single image of both samples merged together, rather than having to observe each specimen back and forth individually.
Some comparison microscopes also have the ability to superimpose the view field of each respective microscope to view the images on top of each other and compare the similarities. This is useful when comparing impressions on glass or identifying foreign matter invisible to the naked eye for example the tiniest speck of dna material. This is only a valid method when the microscopes have been used under identical conditions. They must be precisely calibrated and set to the same level of magnification.
Determining that the pieces originate from the same source depends on how well the markings match. It can be used to identify glass type and match up impressions from manufacture markings. Serial numbers imprinted on car windows and specific fracture detailing for example. (Saferstein, 2004) Another microscope which is an integral instrument in the forensic crime lab is the scanning electron microscope (SEM). Rather than using light to reflect off the sample and produce a magnified image, this type of microscope works using electromagnets to attract electrons on to the surface of a sample.
The electrons bounce off the surface of the sample and the electron emission can be observed. A detector which captures the electrons creates a pattern of deflection and produces a highly magnified three dimensional image of the sample on screen. (Lerner et al, 2006) This microscope is ideal for examining the composition of glass, in particular the surface morphology and texture of a sample including surface coatings. Examination can reveal the finest detail, surface patterns and structure of a sample. This specialist microscope is an asset to forensic science.
Although they are expensive and require a moderate level of technical skill to operate and quickly and relatively easily prepare and process a sample, they have many positive points. It hosts powerful levels of resolution and magnification ranging from 10x up to 100,000x and a depth of field up to 300 times greater than other optical devices such as light microscopes. The real bonus though is that the results are available in less than half an hour! (Langford, 2005) When evaluating the techniques researched it is difficult to reach a foregone conclusion about which approach is best.
Each technique has its own merits e. g. providing fast results. What one method fails on such as being very time consuming, the other excels, but at a higher cost. When examining glass I think that each technique is a valuable source of information. When performed sequentially to minimise destruction to items of evidence they are complementary. All approaches provide a different perspective and highlight various qualities of specific evidential samples. Therefore it is best to use a series of methods to obtain the most significant intelligence and greatest evidential value.