By: Don Penven
Two different materials have been popular for the collection of 3-D impressions in soil, sand and snow: plaster-of-paris (POP) and dental stone. POP was the medium of choice until sometime in the early 1990s. POP began to fall out of favor for several reasons; here are just a few:
- Compared to dental stone, POP has a slight shrinkage factor. This drawback will be hardly noticeable in small casts—but it is a factor none the less.
- Compared to dental stone, POP is far more brittle, and thus is easily damaged through handling of the casts.
POP remains popular as a casting medium because it is less expensive than dental stone. But because it is much more fragile—it becomes necessary to add reinforcement to the casts while the plaster is still wet, and you need to use nearly twice as much plaster to ensure a cast that won’t snap in half when you pick it up. So… is the cheaper price really worth it? From this point on, all directions will refer to dental stone (casting medium).
I first began hearing about dental stone as a casting medium way back in the early 1990s. I had contracted to teach an impression evidence seminar for a North Carolina law enforcement agency. At the time, the only source for this material was dental supply distributors. The product was used in dental labs.
So much for history…now let’s get to work at recovering tire and footprints impressions.
Advanced Preparation: In advance of a casting assignment, make up several pre-measured Zip-Top bags of dental stone. Weigh out 2 lbs. of dental stone for each bag. When mixing you will need about 16 oz. of water per bag. This formula produces enough casting material to cover a 12-inch long impression.
STEP 1. Preparing the impression for casting: Remove any debris ON TOP of the impression. Any objects (twigs, leaves, trash) embedded in the impression should be left alone.
STEP 2. Photograph the impression. Using a tripod, position the camera directly above the impression, at a right angle to the surface.
STEP 3. A minimum of four photos are taken of each impression: 1 – without a scale, 3 – with a scale
STEP 4. Using a flash attached to an extension cable, take three photos from the positions shown in the diagram below.
STEP 5. Prepare the impression for casting. Apply 3-4 coats of a clear lacquer (dust & dirt hardener) over the entire impression. Allow 2-3 minutes drying time between coats. (Optional – If available, after the lacquer has dried, apply one thin coating of silicone spray release agent.)
Step 6. Place a casting frame around the impression. If the area to be cast is level, the frame may not be necessary. Using dental stone only requires that the cast be ½” to ¾” thick
STEP 7. Add 16 oz. of water to one of the 2 lb. bags of dental stone, zip it closed, and begin kneading (squeezing) the bag to facilitate mixing the bag contents. Keep working at removing any lumps of casting material.
STEP 8. Once a smooth mixture is apparent, open the bag and begin pouring the mixture onto the impression. Use a large spoon or similar item to break the fall of the casting material. Begin at one end on the impression and continuing pouring across the surface of the full impression, Use all of the casting material in the bag.
STEP 9. After about 15 minutes, lightly scratch identifying marks on the top surface of the cast. Setup time will vary upon the current humidity and temperature. When you cannot make an indentation in the surface of the cast with a fingernail, the cast may be carefully lifted from the surface (35-45 minutes setup time).
STEP 10. Invert the cast and set it aside to allow the bottom surface to dry.
Special Note: Most crime labs caution CSIs not to make any attempt to clean the dirt from the cast. Simply allow the cast to be fully dry—at least 6 to 8-hours, and then package it for delivery to the lab.
Special Note: This same procedure is used when casting tire impressions. Ideally you need to cast one full rotation of the tire. Just remember that one 2-lb. bag of dental stone will cover a 12-inch long footprint. Note: Casts should not exceed 20 to 24-inches in length.
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