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.
As I have indicated in other articles, "CasTips" (casting shortcut tips) deal with crime scene investigation tips explaining the various methods and materials available for recovering impression evidence at crime scenes.
Many 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. It's always best to experiment before being faced with the real thing.
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 stream-bed. Sure enough, impressions were visible. Here's the procedure we used:
"CasTips" for recovering impressions in standing water:
1. Gather up some rocks along the stream-bed 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 allowing 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 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 pre-measured casting formula, known as Shake-N-Cast, 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.
Visit the CSI TECH BLOG for more Crime Scene Investigation Tips
Expert Author Don Penven is a freelance writer and professional photographer based in Raleigh and Morehead City, NC