Chapter 14: Trace Evidence II: Paint, Glass, & Soil

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Paint Paint spread onto a surface will dry into a hard film that can best be described as consisting of pigments and additives suspended in the binder. One of the most common types of paint examined in the crime laboratory involves finishes emanating from automobiles. Automobile manufacturers normally apply a variety of coatings to the body of an automobile. These coatings may include electrocoat primer, primer surfacer, basecoat, and clearcoat.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Methods for Paint Comparison The wide diversity of automotive paint contributes to the forensic significance of an automobile paint comparison. Questioned and known specimens are best compared side by side under a stereoscopic microscope for color, surface texture, and color layer sequence. However, paints can only be individualized if they have a sufficiently complex layer structure. Pyrolysis gas chromatography and infrared spectrophotometry are invaluable techniques for distinguishing most paint binder formulations, adding further significance to a forensic paint comparison.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Methods for Paint Comparison Solid materials, such as paint, may be heated or pryolyzed to high temperatures so that they will decompose into numerous gaseous products to flow through the GC column. Crime laboratories are often asked to identify the make and model of a car from a small amount of paint and will make use of color charts for automobile finishes or the PDQ database.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Collection and Preservation Paint chips are most likely found on or near persons or objects involved in hit-and-run incidents. Paper druggist folds and glass or plastic vials make excellent containers for paint. Paint smeared or embedded in garments or objects require the whole item to be packaged and sent to the laboratory. Uncontaminated standard/reference paint must always be collected. Tools used to gain entry into buildings or safes often contain traces of paint, requiring the tool be collected, along with reference paint samples.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Glass Composition Glass is a hard, brittle substance that is composed of silicon oxides (sand) mixed with various metal oxides. Sodium carbonate (soda) is normally added to the sand to lower its melting point. Calcium carbonate(lime) is also added to the sand mix to prevent the glass from dissolving in water. Forensic analysts usually examine “soda-lime” used to manufacture most window and bottle glass.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Glass Composition When molten glass is cooled on a bath of molten tin, this is called float glass. Tempered glass is stronger than normal glass due to rapid heating and cooling of the glass surfaces. Laminated glass found in car windshields has a layer of plastic between two pieces of ordinary window glass.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Forensic Glass Comparison For the forensic scientist, the problem of glass comparison depends on the need to find and measure those properties that will associate one glass fragment with another while minimizing or eliminating other sources. To compare glass fragments, a forensic scientist evaluates two important physical properties: density and refractive index. The FBI has created databases to correlate refractive index and density values of glass to their frequency of occurrence in the glass population of the United States.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Glass Flotation Method Density is defined as mass per unit volume. The flotation method is a rather precise and rapid method for comparing glass densities. In the flotation method, a glass particle is immersed in a liquid. The density of the liquid is carefully adjusted by the addition of small amounts of an appropriate liquid until the glass chip remains suspended in the liquid medium. At this point, the glass will have the same density as the liquid medium and can be compared to other relevant pieces of glass which will remain suspended, sink, or float.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Glass Immersion Method The immersion method is best used to determine a glass fragment’s refractive index. The bending of a light wave occurs because of a change in its speed is called refraction. The ratio of the velocity of light in a vacuum to its speed in any medium determines the refractive index of that medium. This method involves immersing a glass particle in a liquid medium whose refractive index is varied until it is equal to that of the glass particle. The Becke line is a bright halo near the border of a particle that is immersed in a liquid of a different refractive index. At this point, known as the match point, the Becke line disappears and minimum contrast between liquid and particle is observed.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Analyzing Cracks in Glass The penetration of window glass by a projectile, whether it is a bullet or a stone, produces cracks which radiate outward (radial fractures) and encircle the hole (concentric fractures). By analyzing the radial and concentric fracture patterns in glass, the forensic scientist can determine the direction of impact.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Analyzing Cracks in Glass A high-velocity projectile, such as a bullet, often leaves a hole that is wider at the exit side; hence its examination is important in determining the direction of impact. The direction of impact can also be accomplished by applying the 3R Rule: Radial cracks form a Right angle on the Reverse side of the force. The sequence of impacts when there have been successive penetrations of glass is frequently possible to determine because a fracture always terminates at an existing line of fracture.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved

Collection of Glass If even the remotest possibility exists that glass fragments may be pieced together, every effort must be made to collect all the glass found. When an individual fit is thought improbable, the evidence collector must submit all glass evidence found in the possession of the suspect, along with a representative sample of broken glass remaining at the crime scene.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Collection of Glass The glass fragments should be packaged in solid containers to avoid further breakage. If the suspect’s shoes and/or clothing are to be examined for the presence of glass fragments, they should be individually wrapped in paper and transmitted to the laboratory.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Soil The value of soil as evidence rests with its prevalence at crime scenes and its transferability between the scene and the criminal. Most soils can be differentiated by their gross appearance. A side-by-side visual comparison of the color and texture of soil specimens is easy to perform and provides a sensitive property for distinguishing soils that originate from different locations.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Comparison of Soil In many forensic laboratories, forensic geologists will characterize and compare the mineral content of soils. Considering the vast variety of minerals and rocks and the possible presence of artificial debris in soil, the forensic geologist is presented with many points of comparison between two or more specimens.

© 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Collection of Soil Standard/reference soils are to be collected at various intervals within a 100-yard radius of the crime scene, as well as the site of the crime, for comparison to the questioned soil. Soil found on the suspect, such as adhering to a shoe or garments, must not be removed. Instead, each object should be individually wrapped in paper and transmitted to the laboratory.