Unit 6: Glass 

  • Types of Glass

    • Borosilicate (pyrex) → 5% borax is added to resist breaking when heated or cooled

    • Colored → metal oxides or colloidal iron and sulfer are added to change its colo

    • Lead → Pb increases refractive index and density - used for “crystal”

    • Flat/Float/Annealed → made by a “float glass process”; molten glass is floated on a pool tin while cooling - commonly found in doors and windows, just regular glass is not heat strengthened

    • Laminated → used in windshields, two sheets of glass with plastic between them

    • Tempered → used in car side windows and designed to break into tiny squares; potassium (K) replaces sodium (Na) on the surface - made stronger by rapid heating and cooling

  • How do Glass Windows Break?

    • Window first bends after being hit, then it breaks on the far side 

  • How Glass Breaks

    • Radial → is a straight line that extends from the point of impact

      • Radial cracks form first and are propagated in short segments on the side opposite the force

    • Concentric → a circular line of broken glass around the point of impact 

      • Concentric cracks come later from continued pressure on the same side as the force applied 

  • What Side is the Force?

    • Entry → smaller hole, smooth surface

    • Exit → wider hole, crater shape, rough surface


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  • How Does Glass Break?

  • Edges of broken pieces of glass will show rib (“stress”)

  • 3R Rule:

    • Radial cracks give rib marks

  • Glass Transfer Evidence

    • When glass objects are broken, glass flies backward from all parts of the object where cracks appear not just from point of impact

    • This creates a shower of minute glass particles and transfer evidence

  • Collection of Glass Samples

    • The glass fragments should be packaged in boxes to avoid further breakage

    • If evidence is to be examined for glass fragments, it should be taken whole and each item individually wrapped in paper and boxed 

    • If even the remotest possibility exists that glass fragments may be pierced together, ever effort must be made to collect all glass fragments 

    • Submit glass evidence along with a representative sample of each type of glass from the crime scene 

  • Trying to Individualize Glass to a Source

    • Compare physical and chemical characteristics:

      • Optical properties: color and refractive index

      • Non-optical properties: surface wear, straiations from manufacturing, thickness, surface film or dirt, hardness, density

      • Chemical properties: additives or trace elements

  • Optical Properties of Glass

    • Make side-by-side comparisons using similar-sized fragments 

    • Place samples on a white surface using natural light

    • Use both fluorescent and incandescent light to determine the glass’s color

    • Visual color analysis is very subjective

    • Dyes and pigments can be almost impossible to extract

  • Nonoptical Physical Properties of Glass

    • Surface striations and markings:

      • Rollers leave parallel ream marks on sheet glass

      • Markings may indicate the glass’s orientation when pieces are missing

      • Surface scratches, etchings, and other markings may also be used to individualize evidence

  • Quantative Properties of Glass

    • Density

    • Refractive index

  • Glass Density

    • Directly determining mass and volume (usually by displacement)

    • Gradient column method

      • Fragments of different densities settle at different levels in the colum of liquid of varying density

      • Not accurate for cracked fragments

    • Floatation method

      • Put glass in rubbing alcohol while slowlying adding water. Once the glass is in the middle, you can calculate the density

    • Refraction

      • Light travels at a different speed in air vs. water, an object in water appears to bend 

  • Why Measure Refractive Index?

    • Ratio of the velocity of light in a vacuum to the velocity of light in any other medium

  • Refractive Index By Immersion 

    • Glass particles immersed in a liquid medium whose refraction index is varied 

    • Becke line

      • bright halo that is observed near the border of a particle immersed in a liquid of a different refraction index

      • ↗↖Glass has higher refractive index-note white line

      • ↖↗Glass has lower refractive index-note white line outside 


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  • Other Ways to Analyze Glass

    • Fluorescnece

    • Scanning electron microscopy energy dispersive x-ray analysis

    • Atomic absorption spectroscopy