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, paste and putty, and I figured that nothing better was likely to come along soon.
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 silicone paste or silicone putty 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.
At one time or other we have all encountered difficulty in developing and lifting latent prints from textured or just plain rough surfaces. Surfaces that also present challenges are those with multi-colored backgrounds that defy conventional photography-since basic crime scene protocol suggests photographing latent prints (with a scale) prior to any attempts are made at lifting them.
PVS is a likely choice for bite mark recovery too. While I haven't had firsthand experience in doing this, (I no longer am obliged to attend post mortems) feedback from field crime scene technicians indicates that PVS does perform very well. In brief, PVS is an ideal casting medium for just about any 3-dimensional impression.
PVS is offered commercially in three colors: white, brown and transparent. The applicator tips are offered in a small aperture and a much wider tip for covering larger surfaces.
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. (Wikipedia)
If crime scene investigation is your current field, or you hope that one day it will be, then please visit our CSI Tech Blog for free training lessons in crime scene technology: Crime Scene Investigator Training
Expert Author Don Penven is a freelance writer and professional photographer based in Raleigh and Morehead City, NC,
Article Source:http://EzineArticles.com/?expert
Source: http://newsandsociety-crimenews.blogspot.com/2012/07/crime-scene-investigation-casting-with.html
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