An impression can tell a lot about the shoe that made it. Every shoe is unique, with its own pattern of wear and tear. Therefore an impression of footwear can be compared to a particular pair of shoes to help connect those shoes to the crime scene. Impressions can also tell something about the height and weight of the wearer. Heavy individuals make deeper impressions than light ones. In addition, a tall person has a longer stride than a short person. There are several techniques for making casts of shoe impressions.
Today, in this short article we will learn the basic procedure of casting shoe print Impression by using POP or Dental Stone.
- 45 minutes on day 1
- 45 minutes on day 2
- 2 pounds (lbs)/(0.91 kilograms [kg]) of dental stone in a large Ziploc bag
- 3 lbs (1.36 kg) of Plaster of Paris in a large zip lock bag
- 3 to 5 cups of water in a bottle 2 5 to 10 twigs or craft (Popsicle™) sticks soaked in water
- 3 or 4 long strips of cardboard (about 4 x 24 inches or 10.2 x 61 cm)
- Large spoon
- Talcum powder
- Magnifying glass
- Scrub brush
- Paper towels
The procedure, Day 1
- Visit to an outdoor area where there are some shoe impressions.
- Closely examine shoeprint. Carefully remove any rocks, twigs, or other debris from the footprints.
- Sprinkle a little talcum powder over these two footprints to help hold the soil in place and secure the impressions.
- Build a retaining wall around each impression using strips of cardboard. (This wall will prevent the Plaster of Paris or dental stone from escaping when it is poured into the impression.)
Figure 1- Footwear Impression
Figure 2- Cardboard Strip Wall Around the Impression to Prevent the Escape of Casting Material
Casting of Footwear Impression Using Dental Stone
- Pour about 1 cup of water into the Ziploc bag of dental stone.
- Knead the bag of dental stone for 2 to 3 minutes (min) to mix thoroughly.
- Examine the consistency of the dental stone. Ideally, it should be similar to pancake batter. If it is too thick, add a little more water. (Be careful; if you add too much water, your mixture will be too thin.)
- The dental stone becomes firm very quickly, so work rapidly to pour the mixture. Avoid pouring the material directly into the impression, which can distort it. Instead, hold the spoon just above the impression and slowly pour the mixture into the spoon so that it falls into the impression. When the impression is about half full, remove the spoon and pour the remaining mixture directly. The poured cast should be 2 to 3 in. (about 5 to 7.5 cm) thick.
- Use a stick or pencil to etch your name and the date in the upper surface of dental stone.
- Let the cast sit for 30 min to harden. During this time, carry out step 6 below.
- After 30 min or more, insert the handle of the spoon into the ground about 1/2 in. (1.2 cm) from the impression and gently pry up the cast.
- The cast may have soil or grass on it; do not remove it at this time.
- Take the cast indoors and allow it to harden for 24 hours.
Casting of Footwear Impression Using Plaster of Paris
- Pour about 1 cup of water into the Ziploc™ bag of plaster of paris.
- Knead the bag of plaster of paris and water for 2 to 3 min to mix thoroughly.
- Examine the consistency of the mixture. Ideally, it should be similar to pancake batter. If it is too thick, add a little more water. (Be careful; if you add too much water, your mixture will be too thin.)
- Pour about 1/2 in. (1.2 cm) of plaster of paris into the impression, using the spoon as you did earlier to avoid damaging the impression. Add some reinforcing materials such as twigs or craft sticks that have been soaked in water (see Figure). (Dry sticks will absorb water from the plaster of paris and make it brittle.) Quickly finish pouring the plaster of paris into the impression to a thickness of at least 3 in.
- Follow the remaining steps as mentioned in the Dental Stone Casting.
Figure 3 - Cast Preparation by Plaster of Paris Method
The procedure, Day 2
- Examine each shoe cast. Note any significant differences in the casts.
- Using a scrub brush, gently clean each cast with water. Pat dry each cast with paper towels.
- Examine each shoe cast again, note whether or not washing damaged either cast.
- Closely examine each shoe cast with a magnifying glass. Search for nicks, cuts, or scrapes in the casts that indicate identifying markings on the shoes. Record these in your science notebook.
Figure 4 - Footwear Impression Cast Prepared at SIFS
What’s Going On?
Ideally, footprints at a crime scene can lead investigators to the person who made them. A clear shoe impression can yield a distinctive and easy-to-read cast. The quality of impressions varies with weather conditions, amount of moisture in the soil, and the type of soil. Clay-based soils, which are fine-grained and sticky, produce better impressions than sand, a loose soil made of relatively large grains.
For investigators, there are many benefits to taking a cast. Casts provide lifelike, actual-size molds of prints. Close examination of a cast can reveal microscopic nicks and scratches on the shoe as well as unique patterns of wear.
In the past, plaster of paris was the most commonly used material for making casts. Now, the dental stone is generally recommended. Dental stone is harder than plaster, is more durable, and gives finer impressions. In addition, it does not have to be supported by reinforcing material and it lasts longer. With few exceptions, the dental stone is the first choice of crime scene investigators working in the field.
SIFS India in their offline training uses this method to make students learn about the peculiarities of the casting of shoe print impressions found in the crime scene. To learn more and to get in hand exposure to such practical procedures for different aspects of forensic science, you can visit the institute page and get enroll yourself for the Certificate course or Training program of your choice.
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