Casting Footwear Impressions in Soil

Pouring Dental Stone

Casting footwear and tire impressions in soil is a fairly common practice at crime scenes. The purpose of course it to make a reproduction of the impression that can later be compared to the class and individual characteristics of the suspect shoe or tire.The most common casting material is called dental stone. It comes in a powder form that is mixed with water to create a “pancake” like batter that is poured into the soil impression.  There are a number of dental stone products in the marketplace but a common formula is to mix two pounds of dental stone with 12 ounces of water. This will provide enough material for one shoe impression.

Every once in a while I’ll see a reference to Plaster of Paris being used for casting crime scene impression. In truth, Plaster of Paris hasn’t been used for decades by law enforcement. It is a much weaker product requiring forms (a frame to contain the liquid while setting up) and reinforcement devices (usually sticks or tongue depressors) to provide extra protection from breaking. It also lacks the level of detail provided by modern dental stones.

Cleaning dental stone cast

Dental stone is most commonly mixed in a gallon sized Ziploc bag that also makes a handy way to pour it out.  The dental stone is poured just outside the impression and allowed to flow into the impression.  Never pour the stone directly onto the impression as that may cause damage to it as the “batter” strikes the details of the impression. It takes about thirty minutes to set up before it can be removed from the soil. Once removed your character won’t be able to see any detail because the bottom of the cast will be coated with soil. The cast is allowed to dry for several hours at least before a footwear examiner cleans the soil off.

This information probably won’t figure greatly into a storyline. I suppose you could have a private detective (retired police) character that still used Plaster of Paris in some scene where he loses credibility or something.  The terminology is the main thing I wanted to convey so your storyline is accurate.

Examination of a dental stone tire cast with a known tire

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About forensics4fiction

Hi there. Thank you for visiting my BLOG for crime writers. I hope you will find it interesting. I would love to hear your questions and thoughts regarding forensics and criminal investigations. I hope that the information here will help answer your questions or ignite your imagination. I am a retired senior criminalist with 15 years of forensic experience. I have served as the president of the Association for Crime Scene Reconstruction, Rocky Mountain Association of Bloodstain Pattern Analysts, and the Rocky Mountain Division of the International Association for Identification. I am triple board certified in forensic related fields and one of only 40 board-certified bloodstain pattern analysts and 80 board-certified footwear examiners worldwide In addition to writing over 60 scientific papers, I have worked as the editor of the Journal of the Association for Crime Scene Reconstruction, been interviewed by and consulted for television, books, magazines, and newspaper articles including documentaries on the Discovery Channel and National Geographic.

Posted on June 5, 2011, in The Crime Scene and tagged , , , , , , , . Bookmark the permalink. 1 Comment.

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