READ: Glass and Soil Evidence

Introduction

Introduction

 Evidence found at crime scenes is found in all shapes and sizes. Whether it is a fingerprint, a blood spatter, glass fragment or pair of muddy boots, a Forensic Investigator must understand the value of the evidence and how to collect and preserve it for analysis in the lab. Two substances most commonly examined by forensic scientists for their physical properties in a crime lab are glass and soil. Both are so common that the forensic scientist must determine properties that distinguish characteristics in an effort to find a unique identity that matches the evidence to the victim, the crime scene, or the perpetrator.

Essential Questions

1. What are the various types of evidence and how do they differ?

2. What types of crimes involve glass and soil?

3. In what ways do glass and soil evidence aid in solving a crime?

4. How are glass and soil evidence collected and preserved?

Module Minute

Evidence can make or break a case in court, so it is very important that a Forensic Investigator follow proper evidence collection procedure. If proper procedure for collection of evidence or chain of custody is not maintained, the evidence may be considered as tampered and inadmissible to court. Not all evidence has the same probative value, or value in court. Some types of evidence can be traced to a single source or person and are considered individual evidence, whereas other evidence known as class evidence is more general in nature. Individual evidence is preferred, but class evidence can also be used with mathematical probability to place a suspect or source at the scene of the crime. Glass and soil are two types of physical evidence that are commonly found in crime scenes and can tell an investigator many things including the geological features of a crime scene when the body has been moved, how an intruder entered a residence, and from which direction a bullet was fired.

Georgia Virtual, Forensic Properties of Glass and Soil Evidence, CC BY-NC-SA 3.0

Glass Evidence

Properties of Glass Evidence

Glass evidence is commonly encountered at a crime scene; especially those involving a burglary or a hit and run car accident. For example, broken glass at a crime scene can be used to place a suspect at the crime scene or may become lodged in the shoes or garments of a victim of a hit and run and matched to a certain vehicle. When glass evidence is available, it is typically examined for its physical properties. While glass itself can't be linked to an individual source in most cases, it can contain other types of evidence which are individual evidence such as fingerprints, blood or hair! Glass evidence can also often be linked back to a common source by a combination of density, refractive index and any production or other irregularities on the surface of the glass.

Types of Glass Interactivity

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Forensic Comparison

 The use of glass evidence relies on the investigator's ability to match glass from an unknown source to a crime scene. Chemical analysis is usually not unique enough to accomplish this task so the focus is to associate one kind of glass with another while minimizing or eliminating other possible sources. The easiest way to accomplish this is when glass fragments can be pieced together with matching irregularities; similar to putting pieces of a puzzle together. If the pieces can be put back together; the original shape and dimensions of the piece of glass may be determined.

Glass is also compared on the basis of certain physical characteristics; namely those are glass color, thickness, fluorescence, curvature, surface characteristics, density and index of refraction.

Color

Color is typically observed visually with the glass fragments against a white background in natural light. Glass fragments of the same size are typically placed side by side for color comparison.

Thickness

Thickness is measured when glass fragments have both sides of their original surfaces. A small tool such as a caliper or micrometer is used to measure the thickness of the glass. Glass thicknesses are kept in a database for Forensic comparisons and are generally the same in various types of glass within one thousandth of an inch! The thickness of the glass is due to the materials used to make the glass and the method in which it is made. If it is not kept at a uniform thickness, the glass will be irregular with ripples which lessens the value of the glass when sold.

Fluorescence

 Fluorescence of glass is caused by either materials added to the glass such as Uranium in a certain type of green glass, or the process in which it is produced such as the method of producing float glass in which liquid glass is poured into molten tin. The tin will cause the glass to fluoresce in UV light on the surfaces in which the tin touched the liquid glass. Fluorescence of glass is evaluated by shining UV light on the glass.

Curvature

 Curvature of glass refers to whether or not the glass is flat or curved into shapes such as containers or glass lenses. The curvature of a lens can be evaluated using low-power magnification.

The surface characteristics of glass can vary greatly and be quite useful in an examination of glass evidence. Some examples of surface characteristics include scratches, decorative etching, roller marks from production, polish marks, frosting of the glass and coatings on the glass. Most of these surface characteristics can be observed and evaluated with the naked eye or stereoscopic Microscope. Some evaluations, such as those involving coatings on glass, require more intensive methods such as Transmission Electron Microscopy.

Density

Density of glass, as discussed earlier, can be determined in several ways including calculation or a special method known as the flotation method. The flotation method is a quick and easy method for comparing glass densities. Glass particles are immersed in a liquid; the density of the liquid is adjusted until one or more glass particles remain suspended in the liquid. When the glass is suspended, it has the same density as the liquid and the density can then be inferred from the liquid. The comparison pieces of glass will either suspend, float or sink depending on their density relative to the liquid. One form of analysis that uses the principles of the floatation method is the Density Gradient Column that was mentioned earlier. Remember that density is an intensive property, so the density of a glass fragment will remain the same no matter how many times it is broken into pieces. This is why glass fragments can be reliably matched using their densities.

Glass Fractures

When a force is applied to one side of glass, the elastic property of the glass will allow it to bend to a certain degree. When the force surpasses the limits the glass can withstand, cracks will form. These cracks are characterized as radial fractures or concentric fractures.

Radial, or primary fractures are cracks in the glass which radiate outward from the point of impact. These cracks form on the opposite side of the impact and radiate outward like spokes on a wheel.

Concentric, or secondary fractures, are fractures that form an approximately circular pattern around the point of impact. These fractures always form on the same side as the point of impact.

Examining the edges of radial and concentric fractures can help investigators determine the rate of impact and the direction that a projectile was travelling as it passed through the glass.

Radial fractures follow the 3R Rule, which means that Radial Cracks form a Right Angle on the Reverse side of the force.

When more than one projectile or object has impacted the glass, it is often possible to determine which impact occurred first as well as the order of the subsequent impacts. New fractures will always terminate at an existing line of fracture because the stress placed on the glass (causing it to crack) will be transferred along the existing crack rather than across it. The crack line of the first impact will, in effect, stop the crack line of the second impact from crossing. Finding where this occurs will give you information as to which impact occurred first. In the image you can see in the red circle that impact A occurred first because the radial fracture from impact B was stopped by the radial fracture in impact A.

It is hard to determine if glass was broken by a bullet or some other projectile. One thing that can be determined is the direction the projectile traveled. This is because the hole that forms in the glass will be cone shaped with the exit side always appearing larger than the entry side.

Georgia Virtual, Forensic Properties of Glass and Soil Evidence, CC BY-NC-SA 3.0

Collection of Glass Evidence

Collection and Preservation of Glass Evidence

Because of the prevalence and usefulness of glass evidence, it is important to properly collect and preserve it for analysis. When glass shatters, it travels in all directions up to four meters away from the site where it shattered! In addition, glass can be transferred to a person, animal or object near the object when it shattered. Anyone who traveled through the area could also pick up a glass particle in a shoe tread, pant cuff, fur or other article. Once it is transferred to a person, object or animal, it can then be transferred much larger distances from the site of the glass shattering. This is why glass analysis is so crucial; if the investigator can state within acceptable limits that a glass fragment from a crime scene matches a glass fragment found on or in the possession of a suspect, it is a good indicator that the suspect was present at the scene. Research has shown that the closer a person or object is to the source of the breaking glass, the more glass particles they are likely to have on them. This idea can be used in cases such as when there is no consensus within a group of suspects on who actually broke a glass window in a burglary. The person found with more glass on them is most likely the person that broke the window! In the case of glass particles transferring to clothing, there are a few things to keep in mind:

• Particles are less likely to transfer and stay on slick clothing such as Nylon.

• Particles are more likely to transfer and stay on rougher textured clothing such as woven sweaters of wool.

• Time matters; glass particles tend to fall off over time. Larger pieces will fall off before smaller pieces; thus examining clothing for glass evidence should be done as soon as possible.

• Glass tends to stick to wet clothing longer than dry clothing.

• Glass falls off clothing faster if the person is running or more active.

Glass Collection

Glass evidence should be collected from the scene and packaged to prevent any further breakage. The direction of impact should be determined and the orientation of the glass (inside/outside) should be noted when collected.

 Here are the basic glass collection procedures:

• Collect as many of the glass particles that you can near an obvious broken glass object. In windows, collect broken glass pieces from the window itself so that the identity of the glass source is ensured.

• If there is glass from multiple sources in a crime scene, make sure to package them in separate boxes so that they don't get mixed together.

• All packages of glass evidence should be labeled with the location and description of the glass source (if known).

• Large pieces of glass should be packaged in sturdy cardboard boxes. Small pieces of glass fragments can be packaged in envelopes or bags as indicated.

• Ensure the package has no holes through which glass evidence could fall out. Secure corners of boxes with tape.

• Collect clothing items suspected to have glass evidence on them and packaged them whole to send to the lab for examination. It is important that the clothing isn't shaken or handled excessively to avoid dropping small glass particles.

• When collecting glass evidence from hair: Comb the hair over a large clean piece of paper, such as butcher paper so that the glass particles are captured on the surface of the paper. Fold and tape the paper so that the glass pieces are securely held inside the paper bundle.

• If an object is too large to collect (such as carpet), the area can be vacuumed and the contents of the vacuum cylinder can be submitted to the lab.

Georgia Virtual, Forensic Properties of Glass and Soil Evidence, CC BY-NC-SA 3.0

Soil Evidence

Properties of Soil Evidence

Forensic Characteristics of Soil

The legal application of Earth and soil science is known as Forensic Geology. Forensic Geologists are often called in to review soil evidence because of the complexity of soil content. Soil is the top layer of the earth's surface, consisting of rock and mineral particles mixed with organic matter. Minerals are a very important part of soil analysis because of the variety of minerals found in specific geographic regions. Soil samples that contain more rare minerals are often very useful in forensic investigations. A mineral is essentially any of a class of naturally occurring solid inorganic substances with a characteristic crystalline form and a homogeneous chemical composition.

 When soil evidence is collected in a Forensic investigation, it includes other items normally found on the ground such as asphalt, brick fragments, and cinders in addition to the dirt, rocks and minerals. Soil often varies in composition when digging down vertically into the various layers of soil as well as surface areas in lateral areas surrounding from the original site. The value of soil evidence is due to its transferability between crime scenes and suspect and victim and the uniqueness of soil samples in small areas. While soil often contains some of the same things in many samples, the smaller elements such as rare rocks or minerals, fossils, trace elements, or debris such as brick pieces can pinpoint an area much more specifically. Some examples of this include dried mud on a suspect's clothing that may link them to a particular crime scene, or a soil sample found with a body that has been moved. The more unique soil elements, such as rare minerals, in the soil from the body may help investigators trace the location the body was moved from. While it is considered class evidence, soil can be very useful in an investigation to generate leads or guide the focus of the investigation.

 Types of soil typically encountered in a Forensic Investigation:

1. Sand - A loose granular substance, typically pale yellowish brown, resulting from the erosion of siliceous and other rocks.

2. Clay- A stiff, sticky fine-grained earth, typically yellow, red, or bluish-gray in color and often forming an impermeable layer in the soil.

3. Silt- Fine sand, clay, or other material carried by running water and deposited as a sediment, especially in a channel or harbor.

Examining Soil

Soil can be examined for both physical and chemical properties. The first step in examining physical properties of a soil sample is to observe it under low power with a microscope. This process will reveal any plant material, animal material, artificial materials, and specific rocks or minerals that may be present but not visible or clear to the naked eye.

Some specific physical characteristics that are observed in soil analysis include:

1. Color - It is estimated that there are more than 1,100 different colors of soil!

2. Type of soil - Sand, clay, and silt all have different properties!

3. Presence of unique materials - Materials such as magnetic particles can give important clues!

4. Fluorescence - Observing the soil sample for fluorescence using UV light can give clues to the type of soil or originating location.

Specific chemical characteristics observed in soil analysis include:

1. pH - pH varies by soil type and composition

2. Composition and presence of various compounds - Presence of Nitrates, Phosphates, Potassium, Carbonates, Iron, Chloride, Copper are generally examined.

While most soil can be differentiated by observation, it must be visualized when dry and when moist because it usually appears darker when wet. Minerals can also be observed in soil samples. Minerals are naturally occurring crystalline solids. There are more 2200 minerals known to exist but most are rare. Only about 20 of those minerals are common and frequently encountered by forensic geologists. Thus, finding a mineral outside of those common twenty is quite rare and could provide information of great evidentiary value. The probability of finding two soils that are indistinguishable in both color and mineral properties in two locations separated by more than 1000 feet is about 1 in 50. 

Testing Soil Samples

Besides microscopy, scientists can compare soil specimens using several other techniques to analyze the physical and chemical characteristics of the sample. 

• The density of soil can be tested using a density-gradient column which is filled from the bottom to top with liquids of successfully lighter densities so when soil is added it "sorts" based on the different densities of its components.

• The size of the particles of soil can be assessed using different sizes of sieves to "sift" the sample.

• Soil samples are tested for the presence of magnetic materials using a magnet.

• Depending on the location and soil type, chemical analysis can be done to compare soil samples.

Georgia Virtual, Forensic Properties of Glass and Soil Evidence, CC BY-NC-SA 3.0

Collection of Soil Evidence

Collection and Preservation of Soil

Because of the likelihood of soil samples varying even within a small radius of the area of interest of the crime scene, it is important that multiple samples be collected for the purpose of soil analysis and comparison. The soil samples should be collected at various intervals within a 100 yard radius of crime scene as well as from all possible alibi locations. In general, the soil samples needed for soil analysis are taken from the top layers of soil as that is where soil is almost always picked up from and tracked to other locations. An exception to this is if a body was buried; deeper soil samples will need to be taken for a soil comparison in this context.

Soil Analysis Collection Guidelines

• Always allow soil samples to air dry before packaging them to avoid growth of mold in the sample.

• All samples should be labeled with the date, time, location, person who collected the sample and any other pertinent identifying information.

• Each suspect soil sample should contain about 1 tablespoon of soil and comparison soil samples from the areas of interest should contain about 3 tablespoons of representative soil. Comparison samples should be taken from the crime scene, as well as all areas indicated in the suspect's alibi.

• Loose samples of dirt/sand can be swept into a clean piece of paper, folded, taped and labeled for analysis at the lab.

• Soil should not be removed from clothing or shoes of a victim until they are in the crime lab. When soil is present in tires, shoes, or other objects, the layering should be preserved as much as possible to help establish a timeline.

• When collecting impressions from shoes or tire treads, the area must first be photographed, then sprayed with a fixative to harden the impression. The final step is to obtain a plaster cast of the impression for lab examination.

Georgia Virtual, Forensic Properties of Glass and Soil Evidence, CC BY-NC-SA 3.0