Glass Fracture Evidence

• Forensic analysis of glass fractures reveals clues about projectiles, weapons and direction of force. For example, projectiles such as bullets leave telltale holes with radial and concentric cracks, notes Richard Saferstein in the book "Criminalistics: An Introduction to Forensic Science." Radial cracks look like spokes in a bicycle tire, while concentric cracks resemble ripples in a pond. Add gunpowder residue, and there is no doubt someone fired a gun. In addition, bullet holes in glass are always wider on the exit side, revealing the direction of the bullet, according to Saferstein.
  
  

Glass Fragment Evidence

• Glass fragments from a crime or accident scene sometimes stick to weapons, tools, automobiles or clothing, notes Saferstein. Forensic glass examiners compare the density of glass fragments--typically, one sample from a suspect and one from the scene of the accident or crime. If the densities fail to match, police may have enough information to exclude fragments as coming from the crime scene source, states Saferstein.
  
  However, identical densities do not indicate a match, since many types and samples of glass may share the same density, notes Saferstein. Therefore, forensic glass examiners also analyze the refractive index of both samples. Refractive index measures the ratio of the velocity of light as it passes through a medium versus through a vacuum, according to Saferstein. A match in refractive index may mean that two glass samples came from the same source, although no glass expert can proclaim that with 100 percent certainty.
  
  

Considerations

• Although forensic glass analysis attempts to match glass samples, jurors should not consider it as reliable as DNA. As of 2010, little data exists to demonstrate the statistic probabilities of particular glass traits, according to Terrence F. Kiely in the book "Forensic Evidence: Science and the Criminal Law." In fact, Kiely notes that forensic glass examiners must carefully choose their words in court, using phrases such as "consistent with" or "not dissimilar."
  
  

Misconceptions

• Although forensic glass analysis typically results in matches of class characteristics--in other words, traits that many other glass samples may possess--one type of glass evidence may result in an individual match. When glass breaks into fragments or shards that a forensic glass analyst can reassemble, it results in a true match, according to Saferstein. Very rarely would any two glass fragments from different sources fit together like puzzle pieces.
  
  

Significance

• Glass is such a common material that it shows up on shoe soles and clothing for almost anyone, notes Kiely. With so much glass picked up from casual contact, courts and investigators need some way to determine which fragments actually have relevance in cases. This makes forensic glass analysis essential to the criminal justice system.