Crime Scene Training

CasTips07 – Casting With Polyvinylsiloxane

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

 

CasTips06 – Casting in Standing Water

By Don Penven

How often have you encountered footprints or tire tracks at the bottom of a puddle? Granted, this doesn’t happen often but when it does—desperate measures are needed to recover those impressions.

Most liquid silicone formulations will just float on top of the water’s surface. Casting plaster then seems like the best possible choice to make a useful cast. Here’s a technique I tried several years ago during a training session.

My students were from a state investigative agency and one student asked, “What about a footprint in a stream bed or puddle? How do we salvage that?”

I had no idea, but suggested we try a few techniques—maybe something would work.

At that time I had recently learned about dental stone as a casting medium, and we were using it during this training class. Nearby was a slow moving stream so one jack-booted student walked across the streambed. Sure enough, impressions were visible. Here’s the procedure we used:

1. Gather up some rocks along the streambed and build a protective dam across the line of flow. Don’t try to dam the entire flow—just use enough damming materials to divert the flow.
2. We had several plastic bags with a pre-measured amount of dental stone in each one. About 2 – lbs. per bag.
3. No special preparation was taken. We did place a casting frame around the impression to contain the plaster, and then just opened the bag and begin sprinkling the plaster onto the water’s surface directly above the impression.
4. The dental stone obligingly sank down to the bottom and collected over the footprint. We added a second bag that was mixed with water just to be sure, but that was probably overkill.
5. Naturally it took a while for the plaster to harden. While waiting for this to happen we set up sites to cast several damp footprints along the stream bank.
6. After about an hour or so I reached down and felt the cast and decided it was hard enough to lift it out. It worked!

We set the cast aside and allowed more curing time as we worked on the other impressions.

Since that time, at another training session, I tried out a different casting material. A product marketed as “Shake-N-Cast,” is available in pre-measured bags. Each bag has a built-in water capsule. To use the package merely apply pressure to the capsule until it breaks, and then squeeze the daylights out of the bag until a smooth mixture results.

But since my class wanted to see an underwater footprint recovered, we left the water capsule intact. (Special Note: It may be a good idea to use the water capsule, mix it up, and pour this mixture into the water. This may speed up the process.) Just behind our classroom we found a muddy area (it had rained for 24 hours straight). We found a couple of footprint impressions filled with water. This process provided a couple of excellent casts.

The Shake-N-Cast formula is a variation of dental stone and has an added ingredient—Portland cement. This makes a super-hard cast under most conditions, and it is ideal for casting submerged impressions at crime scenes.

Download the technical Information Sheet for Shake—N-Cast Here: 

http://www.sirchie.com/Assets/Manuals/pdf/UPD/SNC42-SNC100_TI03-190ENG.pdf

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CasTips05 – Toolmark-Bite Mark Recovery

By: Don Penven

Recovering toolmark impression evidence can be a valuable contribution toward successful prosecutions—if it is done properly. 

Toolmarks are frequently apparent at points of entry occurring during a burglary, but this form of impression evidence covers a number of other 3-dimensional impressions uncovered at crime scenes—most notably—bites marks. 

When forced entry into a structure is evident, tool marks may be present. Of course some types of forced entry rely on brute force, i.e. a shoulder or foot against a door, a rock through a window or a motor vehicle rammed into a storefront. We’ll deal with toolmarks and the impressions they leave. Bite-marks are recovered in the same manner as toolmarks. 

Toolmarks are recorded/recovered by three different means:

1. Photography
2. Casting
3. Physically removing the entire toolmark from the structure by cutting it out or submitting the door or window to the crime lab. 

Photography

Regardless of which of the other means of recovery are used, all such impression evidence must be photographed prior to any other recovery method.

1. Take a few overall photos of the impression from several feet away in order to show its relationship to other nearby 0bjects.
2. Place a photographic scale adjacent to the impression, but not obscuring any part of it.
3. Take several close-up shots using different angles of light. (A flash attachment with a remote cable is a must.) 

Casting the Impression

Due to the very nature of tool and bite marks, any sort of plaster or dental stone is not the casting medium of choice. These impressions require a casting medium with a degree of flexibility. Silicone, in its many forms, has proven to be the best possible medium. Most of the commercially available silicone formulas use Polydimethylsiloxane as their basis. Silicone casting materials are available in liquid, paste and putty formulations. Each of these formulas requires a catalyst to activate the hardening or set-up reaction. These three different silicone formulations are covered below.

1. Liquid Silicone Rubber: This casting compound is a mixture of quite a number of ingredients: Get the MSDS Here: This formulation is white in color and may be used on everything from toolmarks to footprints in soil or snow. The catalyst for liquid silicone is clear in color. This formulation should be mixed in a bowl or similar container. The liquid is best used on flat, horizontal surfaces.
2. 2.       Silicone Paste

A. Mikrosil is a product imported from Sweden. It is a paste and is therefore provided in toothpaste-like tubes. Its catalyst is blue in color. Mikrosil is available in white, brown , gray and black. Mikrosil is mixed on any flat surface. The distributor of this product supplies vinyl plastic strips as mixing surfaces. You May Download the MSDS Here.

B. Polyvinylsiloxane is also a paste, but it is applied using a device similar to a caulking gun. It uses a paste for the base compound and the catalyst is also in paste form. The extruder gun employs special tips that mix equal parts of catalyst and base as the material is applied to horizontal or vertical surfaces. Download the MSDS Here:  

This formulation is ideal for use on irregular or vertical surfaces

1. Silicone Putty: Durocast is offered in putty-form and thus, it is effective on virtually any surface. This compound is mixed in the palm of the hand. Even though it is a putty, it produces amazing detail. It has been used to lift the embossed printing from U.S. currency. Download the MSDS Here:  http://www.sirchie.com/Assets/msds/doc1001.pdf

 Technical manuals for these formulations may be found at the links below:

Liquid Silicone Rubber  

Mikrosil 

PVS  

Durocast:  

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CasTips04 Tire and Footprint Casting Techniques

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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CasTips03 – Recovering Dust Impressions

By: Don Penven

Methods for Lifting Dust Print Impressions

Dust impressions are fragile at best, so the utmost care must be taken when attempting to lift them. Some methods work better than others, so it is suggested that you make several trial runs before going after the real thing.

1. Using adhesive materials to lift dust prints. Prior to the invention of the electrostatic dust print lifter, dust prints were recovered using adhesive-backed materials like tape.  Fingerprint lifting tape, especially the wide variety, does a fair job as a lifting medium. The widest latent lifting tape I am aware of is 4-inches. So it will be necessary to put down two lengths, side-by-side in order to retrieve a normal adult shoe print. If using 2-inch wide tape, use three or more strips.
   1. Use the tape in the same manner as when lifting latent fingerprints. Pull off a few inches of tape and tack down the end at one end or the other of the dust print to be lifted.
   2. Press the tape along its length to the surface using every effort avoid any air bubbles.  The tape should extend 2-3 inches beyond the beginning or end of the impression.  When the impression is covered, cut the tape with scissors and puck back about 1-inch (stick it to itself) because you are going to need something to grasp when you lift it from the surface.
   3. Add another length alongside of the tape on the surface and allow it to overlap the first tape by about 1/4 –inch along its length.
   4. Add a third and fourth strip if using a narrow tape (1.5 – 2-inches)
   5. You will need some form of backing material, preferably black in color, for mounting the tape. Most suppliers of fingerprint lifting tape also offer plastic backing materials.
   6. Slowly lift the tape(s) in one continuous motion.
   7. Attach one end of the tape(s) to the end of the backing material and slowly press it into place on the backer.
   8. The electrostatic dust print lifter eliminates the complications of the tape lifting process. A number of these devices have been available for several years. The one depicted here is unlike most competitive models since it does not require any wires or complicated hookup.
      1. Once the impression is located and photographed, a pickup mat is placed over the impression—black side down.
      2. The charging unit is positioned as shown below on the charging mat and metal ground plane
      3. When the charging unit is energized, it imparts an electrostatic charge to the surface, thus charging the dust particles present. These particles are then attracted to the charging mat.
      4. Turn off the charging unit and lift the mat from the surface. Note: Charging mats are supplied in 2’ x 3’ sheets, and they may be cut to smaller sizes when lifting a single dust print.
      5. The impression on the mat is photographed and then may be placed in storage/shipping boxes supplied by the manufacturer.

You may download the Technical Information Manual for the Electrostatic Dustprint Lifter HERE

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CasTips02 –Recording Dust Print Impressions

Crime Scene Investigation-- The Recording Dust Print Impressions

By: Don Penven 

Dust prints occur at crime scenes as a result of two influences:

1. They are tracked into a structure as a result of dust, dirt, pollen or other foreign material adhering to the soles of shoes
2. Once inside the subject may walk through a dust covered area and will leave negative impressions—where the existing dust was removed by the footwear and will leave positive impressions in areas where the transported dust is deposited on floors.

The search for dust impressions must begin prior to any other activity except for rendering aid to injured persons.

The Search for and Recording of Latent Dust Prints

The first challenge is to locate any possible dust print impressions.  The most effective method available is a flashlight. If possible, darken the room (total darkness usually is not necessary).

1. Direct the light beam across the floor at floor level. Dust impression, if present, will become immediately visible. 
2. Mark each visible impression with an evidence marker such as an evidence tent.
3. Once all available impressions are located and marked, photograph them as follows:
   1. Using a tripod, position the camera directly above each impression—at right angles to the surface.
   2. Always include a scale in each photo taken
   3. Assuming you are using a digital camera (and if not, why not) use the flashlight beam as a light source or use a flash that may be extended from the camera via a flash extension cable. The light (or flash) should be aimed at an oblique angle similar to what you used to originally locate the prints. The digital camera will provide an immediate preview of the impression.
   4. Maintain a written log of each photo taken.
   5. Be certain to pin-point the location of the impressions on the crime scene sketch. 

Special Note: If the crime scene is being recorded on video, be certain to get video footage of the location of the prints as well as close-up shots. While video isn’t very useful in the actual comparison of the dust prints with a suspect’s shoes, it will give the jury a glimpse of what you were working with. 

Once you are satisfied with the quality of your photos you may consider recovering the impressions if you are equipped to do so. The next article in this series, PrinTips03 – Recovering Dust Impressions, will cover several methods for dust print recovery. 

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CasTips01 – Introduction to Impression Evidence

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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