Abstract3
Lab Four: Casting Footwear and Tire Impressions..4
Method.5
Material
Procedure
Results7
Discussion7
Appendix..9
Abstract
The following lab reports gives an account of experiementation with the casting of two different types of impression, using plaster of paris.
Keywords: footwear, tire, impressions, Plaster of Paris, casting
Lab Four: Casting Footwear and Tire Impressions
In the experiment conducted we took a casting from the track made by our footwear, as well as tire impressions, in ordeto determine which left more detail, or allowed for the simpler casting. For this experiment we used plaster of paris in order to create the needed casts, understanding that it is in many ways inferior to the dental stone used by professional forensic teams.
Method
In order to cast the footprint the following materials were used.
Gallon ziplock bag, preferable freezer weight.
A flathead screwdriver (Small to medium)
2 lbs. Plaster of Paris
A butter knife
Measuring Cup
Footwear
Hairspray
Similarly, the materials used to cast the tire impression include:
In order to cast the footprint the following materials were used.
Gallon ziplock bag, preferable freezer weight.
A flathead screwdriver (Small to medium)
2 lbs. Plaster of Paris
A butter knife
Measuring Cup
A tire
Hairspray
Procedure:
Experiment One: Footwear Casting
We created a footwear impression in the dirt by moistening the soil with water and stepping into the moistend area in order to make an imprint of the sole. We were wise to keep all materials close at hand for ease of use after the impression is made, and because certain aspects of the experiment must be completed quickly. Spray the shoe impression and the surrounding area with the aerosol hairspray, from a distance, so that it seals the impression without blowing any of the details away. While the hairspray is drying, mix the Plaster of Paris by adding 1 cup of water and 2 cups of plaster together in the ziplock bag. Seal the bag, and shake until evenly mixed, or for a period of 2 to 3 minutes. The mixed plaster was poured into the impression at an oblique angle, slowly, so that it also does not obscure the original impression. Allow it to set for a period of 30 minutes, or until solidly dry. Exact drying time was dependant on the temperature, wind, and humidity. Level of drying can be indicated by the sound that the plaster makes when tapped. In our experiment, there was not a lot of sun, so drying took about 60 minutes. When the casting was fully dry, it was still well stuck in the dirt, so the butter knife was used to lift and separate the edges sot that it could be removed without damage. The cast was then rinsed in water to remove any remaining dirt and debris, to display the clear impression.
Experiment Two (Casting tire impression)
We created a tire impression in the dirt by moistening the soil with water and rolling the tire over the moistend area in order to make an imprint of the tred. We again kept materials close at hand for ease of use after the impression was made, and because certain aspects of the experiment must be completed quickly. The hairspray was again used to set the impression, by spraying it and the surrounding area with the aerosol hairspray, taking care not to damage the impression. While the hairspray was drying, 1 cup of water and 2 cups of plaster were added together in the ziplock bag. We sealed the bag, and shook until evenly mixed, or for a period of 2 to 3 minutes. The mixed plaster was poured into the impression at an oblique angle, slowly, so that it also does not obscure the original impression. Instructions indivated to allow it to set for a period of 30 minutes, or until solidly dry. Exact drying time was dependant on the temperature, wind, and humidity. Level of drying can be indicated by the sound that the plaster makes when tapped. In our experiment, the sun had come out so expected drying time was accurate. When the casting was fully dry, it was still well stuck in the dirt, so the butter knife was used to lift and separate the edges sot that it could be removed without damage. The cast was then rinsed in water to remove any remaining dirt and debris, to display the clear impression.
Results
Experiment One: Footwear Impression
The footwear impression was relatively positive, the impression was clear, however parts of the tred seemed to be missing. Adding more water to the dust, or adding more pressure when leaving the impression might have created a clearer impression for the plaster to be poured into, and to take shape around. It was, however a successful casting with well set plaster, and with no damage to the actual impression. My only regreat was that it was not deeper.
Experiment Two: Tire Impression
The tire impression was less successful. Though the parts of the casting that turned out well were approparite and clear, the ground where the impression was made was not level enough and the plaster tried to run off of the surface of the impression. As a result some sections were too thin, and it broke while lifting it.
Discussion
In terms of understanding forensics, this experiment was valuable. Crime scene investigators use a variety of investigatory techniques to try and lift information from a scene in order to piece together the story it tells, and to try to determine who was invlolved. Casting is one of those approaches, or skills, which can be used to preserve evidence that cannot otherwise be collected, like tire and footprints. I thought that the process of creating and preserving the impression would be simple, but getting the plaster the right thickness, pouring it on so that it coverd the whole imprint evenly, and lifting it off the soil without damaging it were all challenges. This is a concern, because investigators only get one chance to get it right and the information can be integral to solving the case.
