By Don Penven

Over the years I have worked with virtually every crime scene casting medium available. My first casts were of tire tracks in soft sand behind a business that had experienced a burglary during the previous night. Plaster-of-Paris was the casting medium because that’s all we could get from the local hardware store. And way back then, dental stone hadn’t even been heard of. Four 18” casts were submitted to the NJ State Police Crime lab, and all four were matched against the tires of a suspect vehicle.

Since then I’ve worked with several silicone compounds for toolmark and bite mark recovery. I’ve had good success with liquid silicone rubber, Mikrosil and Durocast, and I figured that nothing better was likely.

But remember that old saying, something about building a better mousetrap. Well someone did just that, but this mousetrap has become a valued addition to many crime scene investigation kits. And Polyvinylsiloxane is now my all-time favorite for most toolmark recoveries.

Polyvinylsiloxane (PVS) differs from the many silicone-based toolmark recovery methods in that it uses equal parts of base material and catalyst. Most of us who have used Mikrosil or Durocast have messed up casts because we used too little or too much catalyst when mixing the material. But the guesswork is eliminated when using PVS because of its unique delivery system.

PVS is supplied in cartridge form. Two side-by-side cartridges (base and catalyst) are hooked onto the delivery end of a device that resembles a caulking gun. A specially manufactured tip mixes the two compounds as the trigger is pulled on the applicator. Setup time for the cast is about 4-6 minutes.

What I’ve found most valuable with this delivery system is that mixing the correct amount of catalyst and base material is automatic—no guesswork or stop-watch needed.

PVS is the ideal medium for lifting powdered latent prints from textured surfaces. The base material is available in white brown and transparent. The technical manual explains it all HERE

 

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Point of Interest: It hasn’t changed much in over 100 years.

The first spring-loaded mouse trap was invented by William C. Hooker of Abingdon Illinois, who received US patent 528671 for his design in 1894. James Henry Atkinson, a British inventor who in 1897 invented a prototype called the "Little Nipper", probably had seen the Hooker trap in the shops or in advertisements and used it as the basis of his model.

   

Top Photo: Quarter with developed latent print

Bottom Photo: PVS Cast of developed latent print

 

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:

1. 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.
2. 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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By: Don Penven

An Introduction to Impression Evidence

It’s been more years than I can remember when I first heard the term “Moulage.” It was an old detective movie and one of the officers at a crime scene suggested making a moulage of footprints near a body. We certainly don’t hear this term used much anymore.

Moulage in French means “casting,” and today the term is used by some to describe a form of makeup used to simulate wounds on the body for EMT training purposes.

Casting is the accepted term used at crime scenes and it refers to making casts of 3-dimensional evidence like toolmarks, footprints and tire tracks. But I’m getting ahead of this story…

In his book, “Footwear…The Missed Evidence,” Dwayne S. Hilderbrand, M.Ed., states: In almost every criminal investigation it is necessary to determine and prove that a particular person or persons may or may not have been present at the scene of a crime. For this reason, the collection, preservation and analysis of physical evidence has become more frequent in the law enforcement community.

Around 1910, a criminologist by the name of Edmond Locard arrived at a theory that every time something comes into contact with another it either takes or leaves a portion of itself or another. This theory is called the Edmond Locard Theory, which simply states "Every contact leaves its trace." This theory is continually used today in crime scene investigations and the analysis of physical evidence.

Since criminals must enter and exit crime scene areas it should therefore, be reasonably assumed that they may leave traces of their footwear. Criminals have become smarter and wiser by beginning to frequently wear protection over their hands to avoid leaving fingerprints, and masks over their faces to avoid eyewitness identification. However, they are rarely aware of, or make little attempt to conceal footwear. During an every- day routine it is normal to see an individual wearing gloves, but it's not normal to see individuals wearing protection over their shoes.

The CasTips-series of articles on this blog is an attempt to bring more crime scene investigators under the awareness that impression evidence can serve as silent testimony in criminal cases, but the biggest problem is that all too often it is overlooked or accidentally damaged or destroyed.

Impression evidence can become apparent in two forms:

• 2-Dimensional: This type of evidence includes dust or dirt prints, blood prints (footwear and fingerprints) or shoe prints in some foreign substance such as oil or grease on a garage floor.
• 3-Dimensional:  This type is usually the most visible form of 3-D impressions that are left behind in soft or wet soil, sand and snow. Touching wet paint or soft foods like butter or cheese are forms of evidence left behind by the perpetrator.

The effectiveness of crime scene security often determines if impression evidence will be available. The most fragile form, the type most often overlooked or destroyed, is dust print evidence. This is due largely to the intrusion of the first responders including emergency medical as well as law enforcement personnel. What is often lacking is formal training and an awareness of the fact that dust impression are often found in the tidiest of structures, let alone some of the hovels you may encounter.

The initial officer on the scene is the key to impression evidence survival, and these individuals must be given the awareness of just how valuable impression evidence can be. Protecting the crime scene is vital, but all too often it becomes impractical when serious injuries of victims are apparent. Still, keep in mind that the perpetrator will most often follow a distinctive, direct course through a structure so it behooves the initial responders to avoid stepping in areas along the probable course taken by the perp. Simply put, walk along the walls and avoid open areas of flooring—especially non-carpeted flooring.

A series of articles will follow this post covering the most popular—yet the most effective means of recording and recovering impression evidence.

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