GLASS AND PAINT AS FORENSIC TRACE EVIDENCE

Glass

inorganic product of fusion that has cooled to a rigid condition without crystallization

Forensic examiner role in glass analysis

• Recognize the possibility that the transfer of glass fragments may have occurred during the event

• Ensure sample is seized and packaged appropriately

  • Examination of glass objects for other evidence types–Fire inves roken glass surfaces can determine which side window was broken from (rib or hackle marks)

Packaging for glass

Paper bags or plastic bottles

Types of glass

• Building glass

• Vehicle glass

• Container glass

• Glass fibers (insulation)

• Domestic glass (tableware)

• Automotive headlamps

• Optical glass

Most common type of glass

Soda lime glass –manufactured by melting silica from sand (SiO2), soda ash, limestone and other components such as magnesium oxide, aluminum oxide, potassium oxide, barium oxide

Other types of glass

Borosilicate and lead silicate

Steps in glass manufacturing

1. Storage, weighing, mixing of raw materials

2. Melting of raw materials, including refining and homogenizing

3. Forming the melt into the required shape

4. Annealing of glass

5. Warehousing and/or secondary processing of product

Flat glass is manufactured through what process

float or rolled

Principal method of producing flat glass

Float

Float process

• Homogenized glass passing onto bath of molten tin in float chamber. Glass forms a ribbon, which is drawn continuously along tin bath

• As glass exits chamber, pulled by series of rollers

• speed determined thickness of glass

Secondary processing

Toughening, coating, and coloring or decolorizing can be carried out

Laminated glass

formed by placing sheet of plastic between 2 or more layers of flat glass

Minor variations may occur in glass because of

slight impurities or variations in raw materials

backscatter

Transfer of glass

Factors affecting number of glass fragments transferred are:

• Manner glass was broken

• Type and amount of glass broken

• Distance between person and breaking glass object

Main factors that affect persistence:

• Type of clothing worn (coarseness of fabric and construction of garments)

• Length of time between when glass object broken and when clothes seized

• Activity of wearer

• Whether clothing was damp

Steps in glass analysis

1. Recovery of glass fragments from submitted items (i.e. clothing, linens)

2. Comparison of recovered glass fragments to the alleged source of broken glass (control sample)

3. Assessment of evidential value of results

What is analyzed in larger glass samples

Color, thickness, fluorescence characteristics, curvature, surface features

What is analyzed in smaller glass samples

Compared using RI and elemental techniques

Most commonly used technique for comparing glass fragments

RI

Refractive Index

When light passes through from one medium to another, its velocity changes as well as its direction (it is refracted). Will vary with wavelength and temperature

Beckeline

By altering the RI of medium with addition of a miscible liquid with different RI, possible to reach a point where this line is no longer visible, RI of both media is the SAME

Examination of Tempered glass by reannealing

• First measuring the RI of glass, then reannealing glass sample in lab

• Refractive index of reannealed glass is measured and change in RI recorded

• Large positive differences between RI measurements and before reannealing support hypothesis that glass is tempered

• Reannealing also suggested in order to reduce internal variation of glass = enables greater discrimination

Composition of glass can vary as a result of

• Deliberate choice by the manufacturer

• Variations in trace and minor components in raw materials

• Interaction of molten glass with surfaces of furnace during manufacture

Variety of elemental techniques available, for forensic purposes they should:

• Be able to analyze multiple elements over wide concentration range

• Be accurate and precise

• Able to analyze small fragments nondestructively

• Cost effective

Purposes of elemental analysis of glass

1. Classification according to its end use, help evaluate if recovered glass originated

   from a window or container for example

   1. Based on composition of major elements

2. Discrimination, to differentiate glass with the same end use, for example: glass from 2 different windows

   1. Relies on minor or trace elements

Elemental Techniques

• spectrometry (XRF)

• Micro-XRF

• Energy-dispersive X-ray spectroscopy

• Inductively coupled plasma mass spectrometry (ICP-MS) (requires dissolution of glass fragment)

• Laser ablation ICP-MS (can be performed on solid sample)

If “matching” glass found, final step is to _____ evidential value

assess

Factors to consider when glass is your trace evidence…

• Availability of physical fits

• Presence of original surfaces

• Number of glass items allegedly broken during offense

• Number of glass fragments recovered from suspect that match controls

• Number of distinctly different sources of glass recovered from suspect that do NOT match controls

• Distribution of fragments of glass on clothing, hair and footwear

• Time interval between alleged incident and the seizing of the items

• Frequency of the “matching” recovered glass fragments in relation to database

Reasons why substrates are painted

Protection or decoration

Powder coatings do not include solvents but just ____

resin and pigments

Clear finishes contain a

resin and a solvent but no pigments

Resin portion of paint

the part that holds everything together and imparts most of the physical characteristics as well as curing mechanism & durability qualities to final coating

Curing mechanism

way the wet paint or powder particle becomes a solid, continuous film. Done via polymerization or evaporation

Polymerization

used to form covalent bonds in the resin resulting in cross linked film, not easily put back into solution

Types of resin coating

solvent-borne, waterborne, emulsion, powder

Resin Types

• Acrylic

• Amino

• Alkyd

• Epoxy

• Polyester

• Urethane

• Vinyl

T/F solvents provide coating with protection qualities

F

Solvent purpose

Permits ease of application to the surface or substrate it is designed to protect. solubilize resin and adjust viscosity of the paint

Other terms fie solvent

thinner or reducer

Solvents are classified into what groups

• Hydrocarbons

• Terpenes

• Oxygenated solvents

• Furans

• Nitroparaffins

• Chlorinated solvents

Solvents are chosen for

Solvency, volatility, odor, toxicity

Additives

Introduced into a coating to enhance the performance of the paint

Pigments

Impart color, add bulk to liquid paint, or impart desired physical quality to wet & final film

T/F All coatings contain pigments

F

Color pigments are divided into

organic and inorganic pigments

Inorganic pigments

white and some black called “hiding pigments” (hide any color variation in previous coating or surface). Lower in cost

Organic pigments

provide brighter, purer and richer color than inorganic pigments. More susceptible to destructive influences of UV radiation, chemical damage, color bleeding. Do not provide hiding ability

Other pigments

• Extended pigments –first added to increase volume or bulk thereby lower cost when compared to hiding pigments

• Metallic pigments –aluminum flake most widely used metal flake in automotive paint–Add sparkle or subtle sheen

• Nacreous pigments –give substrate a pearl-like finish

Architectural Paint

Any coating that imparts color and/or protection to surfaces on or in buildings. Can be pigmented or clear

Pigmented liquid paints or coatings consist of:

• Resin or binder

• Particle portion (color pigments and extender pigments)

• Solvents

Architectural paints divided into 2 major categories depending on solvents used

Solvent-thinned coatings and water-thinned coatings

Resin portion of paint conveys properties to

final film: resistance to acids, water or any material that would mar the surface of coating

T/F In architectural paints: ratio of binder to pigments affects the gloss of coating

T

Interior paints come in:

gloss, semigloss, flat

Gloss finish

• Binder-to-pigment ratio is higher than in semigloss

• Hard, easily cleaned and reflects light

• Adhesion of subsequent coating minimal

Semigloss

• Binder-to-pigment ration higher than flat paint

• More pigment than gloss

• Silky appearance

Flat finish

• No gloss

• Adhesion is good between flat film and successive coating, no need for primer

Resins found in architectural coatings

• Acrylics

• Alkyds

• Urethane–Mainly used for floors, highly durable film

• Vinyl–Encountered as latex paints

• Styrene-butadiene–Used primarily for interiors

• Epoxy–Used where chemical resistance is required

• Varnish, lacquer & shellac

Pigments used in architectural coatings include:

• Black and white pigments

• Absorptive color pigments

• Extender pigments

Additives in architectural coatings

• Antisettlingagents : used to prevent pigments from settling or separating from surface in wet coating

• Antiskininngagents : prevent premature oxidation in the wet film, which causes a “skin” to form on surface

• Mildewcides : prevent growth of mildew on the surface of paint

Forensic chemists receive automobile paint most often in

hit-and-run cases

Typical case with automobile paint

Unknown paint is collected from the clothing of an individual, another vehicle, or structure that has been struck by vehicle which has left scene. Also in insurance fraud cases. In breaking and entering where vehicle used as break-in tool

Resins used in automotive paints

• Acrylics

• Alkyd

• Amino

• Epoxy

• Urethane

• Carbamates (used in clearcoat resin systems, acid-resistant quality)

Pigments used in automotive paints

• Color

• Hiding

• Organic/inorganic pigments

• Extender pigments

• Nacreous pigments

T/F Automotive paint in solution form generally not encountered by forensic examiners

T

T/F Solvents is not important when formulating the paint

F

Solvents in automobiles

• Hydrocarbon solvents

• Oxygenated solvents

• water

Automobiles application sequence

• Closest to substrate –primer layer

• Basecoat –layer that contains the color and any effect pigments

• Clearcoat is applied over wet or uncured basecoat

If layer system consists of series of basecoats and clearcoats with no additional primer

assume recoating done in factory

If there is primer over basecoat or clearcoat

painted “aftermarket”

Transfer

When contact occurs between coated object and substrate

Quantity and nature of the paint transferred will depend on

• Properties of the paint

• Nature of substrate surface

• Intensity of contact

In the absence of control material

Trace sample may be characterized and/or analyzed to obtain intelligence information to investigative team

Paint database

Paint Data Query

control sample is available and forensic examiner compares trace and control samples in terms of physical and chemical characteristics to reach one of the following conclusions:

2 samples are distinguishable or 2 samples are indistinguishable

What provides accurate information concerning morphology of paint traces

Stereomicroscopy and microscopy

What is specific to type of paint

Surface characteristics, number of layers, colors, size and appearance

Bright field

enhances contrast of red layers

Dark field

enhances color contrast between opaque layers

Crossed polarizers

allow observation of birefringent material

UV

allows good observation of clearcoat layers based on their fluorescence

IR spectroscopy

Technique of choice for characterizing main organic and inorganic components of a paint system –including mainly the binder, extenders, and some organic/inorganic pigments

IR spectroscopy is used because

• Fast

• Repeatable

• Semidestructive

• When coupled to IR microscope, excellent microsampling capabilities

• High discriminating power

• Existence of databases

FTIR

Provides molecular structure information on organic/inorganic components of coating

Raman Spectroscopy

Measures inelastic (Raman) scattering of molecule irradiated by monochromatic light (laser)

Raman Spectroscopy advantages and disadvantages

Adv. no sample prep, high spatial resolution

Disa. possible fluorescence of sample which will totally mask Raman spectrum

Pyrolysis –GC/MS

Paint binders will decompose into smaller volatile fragment. Each paint flake must be isolated prior to analysis

Pyrolysis –GC/MS advantages and disadvantages

Advantage: high discrimination power, ability to detect and compare minor constituents

Disadvantage: destructive and time consuming

SEM/EDS X-ray analysis

elemental analyses. Provides indirect ID of inorganic content of paint. Technique will characterize extenders and inorganic pigments. Rapid analysis and sensitive, complimentary as it analyzes inorganic component

Known sample should:

• Include all layers of paint down to substrate

• Taken from area as close as possible to say toolmark

• In automobile cases:

  • Damage occurs to vehicle, usually not limited to one body part

  • 2 adjoining body parts (i.e. door and panel) may have same color and layer structure but binder system in one of coating may differ

• Certainty that paint from ALL damaged parts of vehicle is submitted

T/F Paint is heterogeneous substance

T

Variations between known and questioned samples can mean

samples are different or that there is contribution from another source

Level 1

Identification. The questioned paint positively came from the known source. This is usually determined by a physical match

Level 2

Evidence contains unusual characteristics. nclude a paint-layering system that is so unusual or complex that only intentional duplication could produce similar product. This level could also include double paint transfers. These associations include not only conventional characteristics in common but also unusual characteristics

Level 3

Conventional Association.

A single paint transfer found on a hit-and-run victim’s clothing or transfer of green architectural paint onto break-in tool. This is an association where all the measurable examination and analyses are consistent between the questioned and known paint, but there is nothing extraordinary or unusual to what is found in the standard population of that paint.

Level 4

Limited Association

Due to sample size or condition, only certain aspects of the paint could be examined. A small particle of heavily filled architectural paint would be an example due to size constraints and heavy pigment load. Information from such a sample would be limited

Level 5

Inconclusive

There are similarities between the questioned and known paints, however, due to either the condition of the paint (severely comingled with another substance) or aging/exposure issues, the paint cannot be confidently associated with the known paint from a source that was not exposed to these conditions. In this level, the paints cannot be eliminated, because the differences can be explained, but they cannot be associated because the differences cannot be duplicated.

Level 6

Elimination/Exclusion

There is a fundamental difference between the questioned and known paint. Significant differences in color, appearance, polymer type, or elemental composition will result in elimination.

Levels of association

1. Identification

2. High degree of association in which the evidence contains unusual characteristics

3. Conventional Association

4. Limited Association

5. Inconclusive

6. Elimination/Exclusion