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

 

To learn when new post are made to this blog: Signup for Email Alerts

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

Keep up to date on the CSI Tech Blog posts: Register for Email Alerts HERE