Impression evidence is simply where several objects are pressed or stamped against one another allowing the objects to transfer and retain characteristics from one another. Footwear, tire tracks, and tool marks may be some of the most overlooked types of physical evidence left at a crime scene. Extreme patience and common sense are needed to find and recover these impressions.
In other impressions there are two categories or types of impressions likely to be deposited at a crime scene. This impression evidence would be deposited either on top of a hard surface or into a softer surface. Two (2D) dimensional impressions illustrating length and width, are usually found indoors on surfaces like counter tops, glass, paper, cardboard, or ceramic and waxed floors. Three (3D) dimensional impressions that have 3 measurements, length, width and depth, are usually discovered outdoors in surfaces like dirt, sand, clay, mud, tar, or snow. I can not explain to you how to recover 3D impressions in snow. It is something that I have never had the opportunity to do. The only white that is encountered down here comes in a block referred to as a kilo.
Tool marks are usually found at areas of forced entry. Regardless of whether the force was by cutting, prying, pinching, or sheering. The investigator has an ability to recover some of these items of evidence.
A rubber base or silicone impression material is used to recover the tool mark impression. There are several such items on the market that will duplicate the fine material detail of the tool mark.
A large latent backing card can be used as a mixing surface. A line of the base material is laid on the card. On the card, beside the rubber base, a line of the hardening agent or catalyst can be laid. A tongue depressor can be used as a stirring instrument. The compounds can be thoroughly mixed until it is a single color. The mixed compound can then be scooped up with the tongue depressor and pressed against the tool mark. The tongue depressor can be left to harden in the mix. The tongue depressor becomes a handle to remove the casting compound with when it is dry. It also gives the investigator somewhere to label the compound with all of the information needed to identify the item with. The mixed casting compound will dry in approximately 15-30 minutes depending on the climate.
The impression cast is packaged in a small plastic container or small box for its safety and protection during transporting to the lab.
The investigator should try each of the various brand of commercial compounds to see which item works best for his/her recovery task. In a 4-hour work shop that I teach the student takes a quarter and after documenting the quarter with photography techniques he/she uses all three (3) compounds, available on the market, to recover 3 individual cast of the quarter to simulate the fine tool marking detail duplicated with the casting compounds. The student also develops and recovers latent fingerprints from light bulbs, door knobs, and other irregular shaped surfaces to see the versatility of the products and their many uses.
Developing latent footwear (2D) impressions may be as simple as in developing latent fingerprints. The same equipment and enhancement techniques can be used in recovery of both. Mechanical processing, using a brush and powder is one method used to enhance the impression for recovery. These impressions are found indoors on tile floors, glossy finished wood floors, counter tops, chair bottoms, or cabinets. After the impression is mechanically developed it is lifted with tape or a latent lifting medium and placed on a piece of sketching paper or a shirt box top
The lifted item is then slid into plastic to protect the lift from being wadded or folded.
Consideration as to comfort is needed during the development stages, since the investigator will be moving about continuously and in odd positions. The suggestion of a kneeling or knee pads would be a consideration.
Just like any other technique used in recovering evidence, the investigator should familiarize himself/herself with the process to assure skills are up to standard in performing the task.
The impression is recovered by casting. Casting is simply filling the 3-dimensional form. This is done by using a material that will duplicate and retain the characteristics of the impression. Fixatives such as spray paints, talc and a retaining wall are not necessary unless the impression is in an uneven, elevated or extremely soft surface.
The water is simply poured into the zip lock bag containing the compound. The bag is sealed, a fold is put at the top sealed portion of the bag for added stability and the compound is mixed by gently squeezing the outside bottom portion of the bag. This allows the water and powdered compound to mix into a liquid form. The mixing should continue until the compound has the feel and consistency of a pancake batter. Then it is ready to pour. Just a note; some of the compounds that are available will start the hardening process as soon as the water is poured in. If you are not aware of this the mix could start setting and began to harden before it is thoroughly mixed to its consistency. This will make the mix lumpy and extremely hard to remove from the bag. Once the mix is at the right consistency it can be poured right from the bag. This keeps the process from becoming a messy chore.
The mixture dries in 30 to 45 minutes depending on the climate. If to much water is put into the mixture then either more of the compound can be added or it will take a tad bit longer for the mix to dry. A light touch on the top of the cast will tell the investigator/technician when the cast is dry. If it is tacky feeling to the touch then it is still in the drying process.
To mark the cast with the appropriate information for identification, the investigator has a choice. While the cast is still tacky a sharp stick or pencil can be used to mark directly into the cast. I prefer to wait until the cast is dry and use a permanent marker to write across the top of the cast.
Once the cast is hard and smooth to the touch it is ready to remove. Take a putty knife or long chisel and gently work the entire surrounding edge of the cast loose from the surface. Once the entire edge of the cast is loosened, the cast will lift from the surface easily. If a mistake is made and the cast begins to crack as you are attempting to loosen the edge, just cautiously allow the cast to rest back to the surface. Mix another bag of the casting material and pour it on top of the existing cast. The new casting mixture will flow into the cracked area and reseal itself.
Once the cast is lifted do not attempt to clean the surface material from the cast. The exterior layer of the cast will be dry. It takes approximately 48 hours for the entire cast to dry throughout. If you attempt to wash and clean the cast you will possibly wash away some of the fine detail from the cast.
The cast would be packaged in a sturdy box to protect it from cracking or breaking. Do not stack any items on top of the box containing the cast impression during transportation.
Mike Byrd (1955-2005) joined the Miami-Dade County Police Department in 1983 and started with the Crime Scene Investigations Bureau in 1987. He took an exceptionally active part in the science of forensic crime scene investigations, including development of new techniques, publishing methodology of crime scene procedures, and teaching. Mike developed new techniques for gathering and cataloging crime scene evidence including the lifting of fingerprints, vehicle tire impressions, and footwear impressions.
Mike's methods and analysis withstood the scrutiny of the criminal justice process. He published more than thirty crime scene articles on crime scene evidence collection and for the International Association for Identification and was awarded The Good of the Association Award in 2002 for his innovative identification methodology and techniques. He taught crime scene investigation procedures and techniques at police departments around the country and took great pride at instructing smaller Florida police departments in the latest techniques in evidence gathering.
Mike performed the tough detailed oriented forensic work at many major crime scenes and disasters over two-decades. He gathered, processed, and identified the DNA evidence used to convict the Tamiami Strangler for a string of heinous murders in 1994. His thoughtful gathering of evidence at the Valujet crash allowed families to reach closure for the deaths of loved ones.
Mike Byrd died after a more than two year battle with multiple myeloma cancer. Annually, the Police Officer Assistance Trust awards the Mike Byrd Crime Scene Investigation Scholarship in his honor.
Article submitted by the author.
Article posted March 2, 2000