Casting Footwear Impressions in Soil
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.
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.