Technically, glass is described as “an inorganic fusion product that has cooled to a rigid state without crystallizing.” Glass is a hard, brittle, amorphous substance produced by melting sand (silica dioxide, SiO2), lime (calcium oxide, CaO), and soda (Na2CO3) at extremely high temperatures. Despite the fact that glass is a liquid, it appears to be solid. It does not appear viscous like other liquids, but it does appear hard. The viscosity of the amorphous substance is so great that it behaves like a solid. The lime (CaO) is used to keep the glass from becoming water soluble. The soda (Na2CO3) is added to make silica (sand) simpler to deal with by lowering its melting point.
Glass or glass fragments play a major role as an evidence in cases of arson, Hit and run, burglary, assault, shooting etc.
FORENSIC SIGNIFICANCE OF GLASS
✔ Glass that has been broken or fragmented at a crime scene is crucial forensic evidence.
✔ At a hit-and-run crime scene, glass fragments from headlights might leave evidence about the unknown car.
✔ Window pieces or glass fractures can also assist investigators to determine the direction, angle and first impact of a gunshot. These hints can be discovered by examining even the tiniest glass particles.
✔ If glass pieces are found on the clothing of an alleged burglar in a case involving entrance through a broken window, this might be an important evidence.
✔ The source of origin can also be determined. If the density of two glass samples are unequal, both the glass samples come from different sources.
DIFFERENT TYPES OF GLASSES
👉 Soda-lime Glass
It is the most common form of glass, used in the manufacture of windowpanes and glass containers (bottles and jars) for beverages, food, and certain commodities. Soda-lime glass, rather than the more prevalent borosilicate glass, is used in certain bake ware. Approximately 90% of all produced glass is soda–lime glass. Approximately 90% of all produced glass is soda–lime glass. It’s great for glass recycling since it can be resoftened and re-melted several times. It’s easy to clean soda-lime glass since it’s light permeable and has a smooth, fine-pored surface. It also expands rapidly when exposed to heat, thus caution should be exercised when pouring hot water into a soda-lime glass container.
👉 Borosilicate Glass
It is a kind of glass in which the major glass-forming components are silica and boron trioxide. Borosilicate glasses are noted for having extremely low thermal expansion coefficients, making them more resistant to thermal shock than any other common glass. This type of glass is extremely resistant to chemicals and temperature changes. As a result, it’s mostly utilised in chemical manufacturing, labs, for pharmaceutical ampoules and bottles, to package injectables, and as very resistant lamp covers. In addition to baking and soufflé dishes, borosilicate glass is used in the kitchen for various “heatproof” kitchenware. Borosilicate glass is marketed under a number of brand names, including Borosil, Duran, and Pyrex.
👉 Lead Glass
A type of glass in which lead substitutes the calcium component of a standard potash glass is known as crystal glass. Lead glass generally comprises 18–40 percent lead (II) oxide by weight, but modern lead crystal, also known as flint glass due to the original silica source, contains at least 24 percent PbO. Lead oxide, on the other hand, is no longer widely utilised in the glass industry. It’s made up of 54 to 65 percent sand, 13 to 15% alkali oxide, and a variety of other oxides. Because of its high refractive index, crystal glass cuts well. Its density is much higher than that of soda-lime glass. We utilize crystal glass to produce drinking glasses, vases, bowls, ashtrays, and ornamental decorations in our daily life.
👉 Coloured Glass
These glasses are also known as tinted glass. Glass that has been tinted with metallic salts during the manufacturing process. Small pieces of coloured glass are placed to form patterns or pictures in stained glass windows, which are held together (traditionally) by strips of lead and supported by a solid frame. To accentuate the design, painted features and golden stain are frequently employed.
👉 Safety Glass
It’s a type of glass with extra safety features that make it less likely to break or pose a hazard if it does. Special technological and scientific applications need the usage of safety glasses.
There are few types of safety glasses, which include:
✔ Tempered Glass/ Toughened Glass:
The most frequent type of glass used in balustrades and other structural applications is this one. Conduction, convection, and radiation are used to heat annealed glass to around 700 degrees Celsius. A uniform and simultaneous blast of air on both surfaces speeds up the cooling process. Distinct cooling speeds generate different physical characteristics in the surface and interior of the glass, resulting in compressive stresses in the surface balanced by tensile stresses in the body of the glass. The glass is four to five times stronger and safer as a result of this procedure than annealed or untreated glass. Toughened glass’s enhanced mechanical resistance to breaking is due to counteracting stresses or surface compression, which allows it to shatter into small, regular, generally square fragments rather than lengthy, hazardous shards that are considerably more prone to inflict injuries.
✔ Laminated Glass:
Two sheets of toughened glass are bonded together with a 1.52mm thick Polyvinyl Butyral (PVB) interlayer in the most typical final product. Laminated glass has a number of advantages. Because safety and security are the most well-known of these, laminated glass is held together by the interlayer rather than breaking on impact. Two sheets of toughened glass are bonded together with a 1.52mm thick Polyvinyl Butyral (PVB) interlayer in the most typical final product. Laminated glass has a number of advantages. Because safety and security are the most well-known of these, laminated glass is held together by the interlayer rather than breaking on impact.
✔ Annealed Glass/ Float Glass/ Clear Glass:
It is a type of glass that does not need to be quenched once it has been heated; instead, it is allowed to cool slowly. The molten glass is gently cooled in the ‘annealing lehr,’ where it is exposed to a regulated chilling procedure that helps relieve internal stress. Tabletops, cabinet doors, and basement windows are all examples of annealed glass.
👉 Wire-Mesh glass:
It is also known as Georgian wired glass. The glass is inlaid with a grid or mesh of thin metal wire. Because the wire mesh is metallic, it looks to be a reinforcing component, and it conjures up images of rebar in reinforced concrete and other similar examples. Despite popular perception, wired glass is actually weaker than unwired glass due to cable intrusions into the glass structure. In compared to unwired glass, wired glass is more likely to inflict damage since the wire magnifies the irregularity of any fractures. As a result, its institutional use, notably in schools, has decreased.
DIFFERENT TYPES OF GLASS FRACTURES
🔎 Radial Fractures
A crack or fracture in a gloss that spreads outward or radiates from the place where the glass was struck (Point of impact), like the spoke of a wheel. These fractures originate from the surface opposite to that of the force applied. These fractures are known as primary fractures.
🔎 Concentric Fractures
Rough concentric circles that form around the point of contact in a glass break. These fractures are formed on the opposite side of radial fractures, i.e. on the same side of impact. These fractures are known as secondary fractures.
🔎 Cone Fractures
When a high velocity projectile, like a bullet strikes the surface of glass, a round crater shaped hole is formed on the glass. The exit side has a wider hole than the entrance. It is shaped in the form of a cone. Cone fractures help in determining the direction and point of impact.
🔎 Stress lines/ Heckle marks/ Rib marks
The edges of broken pieces of glass contain curved lines which are known as stress lines. These are always perpendicular to the side on which blow is struck. Heckle marks are small straight lines which can be observed in the broke edge of glass.
DETERMINATION OF DIRECTION OF FORCE
The 4R Rule is used to determine the direction of force: 4R rule states that “Ridge lines on Radial fractures are at Right Angles to the Reverse side of impact.”
Finding radial fractures within the initial concentric fracture is the first stage in this approach. The spokes of a wheel are comparable to radial fractures. Concentric fractures form a spider web structure that connects the radial fractures. The following step is to determine which side of the fragment faces in and which side faces out. Contaminants or residues on the inside surface will feel different than on the exterior surface, making it easier to determine the side.
After locating the radial fracture and determining which side of the glass faced which direction, the technician must examine the fractured edge of the glass. When a bullet hits glass, it generates ridges along the edge known as conchoidal fractures, which are evident in profiling. These conchoidal fractures are roughly parallel to the force’s application side (the direction which the projectile came from). The back of the glass is the side of the glass opposing the force; it is this side of the glass where the conchoidal fractures are at right angles.
SEQUENCE OF IMPACT
Examiners can determine the shot sequence by looking at the radial fracture’s termination sites. The radial fractures in the initial shot will fully expand, but those in future shots will be halted or terminated when they come in contact with previous fractures.
COLLECTION AND PRESERVATION OF GLASS
💡 Glass pieces on clothes, an individual, or at a crime scene can be found and extracted using tweezers by trace examiners utilizing magnification and illumination. Glass samples can also be collected with tape, although the adhesive residue makes this a less attractive collecting method.
💡 Suspects’ shoes and clothes, as well as any other glass-contaminated materials, should be wrapped in paper and sent to the Laboratory for testing.
💡 At hit-and-run incidents, all glass should be retrieved. Because headlight glass might be dropped off at a distance when the automobile leaves the crime scene, the search should not be confined to the site of contact. Glass from various locations should be stored in separate containers. Because more than one sort of glass may be present, all of it should be gathered.
💡 Small glass fragments should be placed in paper bindles, then into coin envelopes, pill boxes, or film cans that can be labelled and completely sealed. Fill boxes with big glass bits. To avoid breakage and damaged edges while shipping, separate individual pieces with cotton or paper. The package should be sealed and labelled.
💡 Standards for comparison also must be collected. For example: Send the entire window or all leftover glass to the Laboratory if the damaged window is small. If the window is big, take many samples from different parts of it. The entire shattered window is required if the evidence glass is large enough for physically matching the broken edges or comparing the fracture lines, hackle marks or contamination.
SOURCES AND REFERENCES
Ø Glass as Forensic Evidence: Purpose, Collection & Preservation, Study.com, https://study.com/academy/lesson/glass-as-forensic-evidence-purpose-collection-preservation.html
Ø Forensic science, glass, fractures, http://www.santoshraut.com/forensic/glass.fractures.htm
Ø Intro to Glass, Crime scene, https://sites.google.com/site/crimesceneanalysis/intro-to-glass
Ø Glass analysis, Crime museum, https://www.crimemuseum.org/crime-library/forensic-investigation/glass-analysis/
Ø A simplified guide to trace evidence, http://www.forensicsciencesimplified.org/trace/how.html
Ø Evidence Collection Guidelines, https://www.crime-scene-investigator.net/collect.html
Ø A guide to the 4 main types of glass, https://www.basystems.co.uk/blog/2016/10/glass-types/
Ø Forensic Interpretation of Glass Evidence, James Michael Curran, Tacha Natalie Hicks Champod, John S. Buckleton
Ø Forensic Science in Criminal Investigation & Trials, BR Sharma, Glass and fractures
BSc Forensic Science