Fundamentals of Criminal Investigations
Adam J. McKee, Ph.D.
This content is released as a draft version for comment by the scholarly community. Please do not distribute as is.
Section 4.2: Glass & Soil Evidence
Glass is a material commonly found in our environment. The breakage characteristics of glass under impact forces can produce features that can be used as physical evidence in many types of cases. The various examination techniques described in the other Scientific Working Group for Materials Analysis documents in this issue apply to most types of glass, including the following: flat glass used for windows, doors, display cases, and mirrors; container glass; tableware glass; optical glass; decorative glass; and specialty glass used for headlamps, cookware, and others.
Typically, forensic glass examinations involve a comparison of samples from known and questioned sources to determine if they originated from different sources (e.g., a window from a suspect’s car compared to glass recovered from the victim’s clothing). This comparison involves the recognition and evaluation of class characteristics that associate materials to a group but never to a single source. Conversely, individual characteristics allow the association between two or more items with each other to the exclusion of all other items. For glass examiners, this can only occur when pieces of glass are physically matched.
Due to the inherent heterogeneity of physical and chemical properties within a single source of glass, it is essential to emphasize the need to collect and analyze a sample(s) of the known source for comparison to any recovered fragments.
Rock and Soil Evidence
Forensic soil scientists compare soils from crime scenes with natural soils or soils databases in order to locate the scene of crimes the soil samples are obtained from crime scenes by investigators. The soil may be transported by vehicles, shoes, or shovel. The properties of soil are diverse in nature
Soil samples can be collected in different ways, depending on where the sample is being collected from. If samples are being collected indoors or from a vehicle, vacuuming is generally used. If the sample is outdoors, it is collected by placing a teaspoon of soil into a plastic vial. When found on a tool, it is wrapped in plastic and then sent to the lab for testing. Collecting soil samples from a body is not more difficult than collecting a sample from anywhere else, but it takes more work and care so that evidence is not contaminated. When collecting samples from a body, samples should be taken at regular intervals and different spoons should be used each time.
Once the soil samples are collected, they are sent to the laboratory. The examiner will first do a microscopic analysis so as to perform testing of the mineral content of the soil. Another test used for identification of the origin of soil is a density test. The density test is called the density gradient tube. This tests consists of adding liquid to two glass tubes. The soil sample is added to both. After the soil samples become suspended in the liquid, the separation of the bands can then be analyzed to reveal the profile of the soil. Electron microscopes can be used to examine the structure of the minerals in the soil. During examination, an examiner might find that some soil samples may contain biological evidence such as saliva, semen, or blood. If biological evidence is found in the sample, the whole soil sample should be sent to the laboratory for testing.
Collection of Soils
Always use clean tools and individual bindles when collecting soil or rock samples. If an impression, print, body or other evidence is in the area, photograph and document it in notes before collecting nearby soil or rocks. Collect samples from the known crime scene, any “alibi” site(s) (e.g., a site that the victim or accused claims to have visited), or a “representative” site (uch as a site where prints that match recovered shoes or tires are found). When soil is firmly attached to a movable object, collect and air dry the object before packaging it. If it cannot be collected, gently scrape samples from the object onto clean paper.
Never package soil directly into commercially manufactured envelopes or bags. Always package soil in a sealable container: glass or plastic vials, bottles or jars with screw cap lids, self-made envelope, paper bindle, or other containers. Collect a minimum of three tablespoons of soil from each location. Go a little deeper than at least as deep as the evidence sample appears to have penetrated the ground soil. Usually, the top layer of soil is only disturbed for the evidence sample.
Collect a comparison sample close to the suspected or known evidence sample location and at various locations around the evidential sample up to 100 feet, attempting to include varieties of soil in the scene area. The layering of soil can be important to show recent or historical presence at a location. This is particularly true of vehicle collection. Preserve layering whenever possible.
Ensure that the portion of the object with the soil has been photographed and documented in notes and sketching. Include measurements of collected soil (evidence or control/comparison samples) locations. Marking Evidence Evidence labeling: Label a container for each soil sample with your initials, identification number, the date and time, evidence number, location, and evidence description. Label each bindle, envelope, bottle, or jar with your initials, identification number, evidence number, the date and time, location, and evidence description. Each piece of evidence must have a unique number. This number should correspond to the placard next to the evidence (evidence, control, or comparison) and the evidence log as appropriate.
The evidence description includes:
- Location of each soil sample relative to specific landmarks at the scene
- Whether the soil was wet or dry when collected
- Whether the soil contains any detectable odor or other unusual characteristics or objects
- Estimated amount of sample
When describing soil contents, use the word “apparent” or the phrase “of unknown origin” when the source of the stain/mark is unidentified; e.g., “Soil sample taken from approximately 1 foot north of apparent boot print.”
Determine where to begin sample collection depends on the nature of both the crime and the crime scene. In a homicide case, ff a victim died on the ground, and the body has not been removed, collect the sample from as close to the body as possible without disturbing it. Otherwise, prepare to collect a sample from the center of where the body laid and other appropriate locations of the body.
If collecting a series of samples along the path of an impression, determine the start and end points of the path. The starting point is the place where the first impression is made and the first sample must be collected, then along the path traveled by the subject. Be sure to examine the ground for trace evidence that may not have been collected. Dig straight down into the soil/rock to collect a sample of three tablespoons to one cup of the soil. Be sure to start with clean digging tools. Clean the tools after each sample.
Use the mason’s tool, gardener’s hand trowel, and screwdriver as needed to dig straight into the ground or rocks. Place the soil sample into the container (bottle, jar, vial, bindle). Mix soil as little as possible, keeping in mind the potential for layering of soil. To avoid contamination and leakage, it is critical that each sample is stored in its own sealed container and kept apart from other soil/rock samples and tools that were used. Collect and package the remaining samples in separate bindles. Take additional samples at distances of 1, 10, 25, and 50 feet from the original impression/impact point. The size of area comprising the scene will determine how far out the samples need to be taken.
Take at least four samples at varying compass points each time the distance from the initial evidence area and sampling location is increased. Ensure that your notes and the label on the container include the compass direction and distance from the previous location, or the initial evidence sample, or permanent markers used for measurement in the scene. Whichever reference point is used, consistently use that point throughout. If the sample is wet, place it on a clean piece of paper in a secure location used for evidence drying, such as a drying rack, until it is dry. If drugs or ignitable liquids are suspected, the sample must be frozen. Contact a fire investigator or fire debris analyst if ignitable liquids are suspected. Keep the sample as cold/cool as possible helps to slow degradation. Natural components of some soils may degrade the composition of added chemicals in the sample.
Place each initial container (bottle, jar, vial, bindle) into its labeled container. Do not package evidence with comparison/control samples. Close the container and seal the entire opening with evidence tape. Write your initials, identification number, and the date and time across the evidence tape seal.
References and Further Reading
Modification History File Created: 05/02/2019 Last Modified: 06/03/2019
This work is licensed under an Open Educational Resource-Quality Master Source (OER-QMS) License.