Casting for Evidence: casts of footwear marks solve crime

Taking casts of footwear marks to be used as evidence

The police find a footprint in the soil of the flowerbed beneath the window. In crime novels it  turns out to be that of the gardener and not related to the crime. How did the police discover the gardener’s boot made the print? What we don’t see in the Mystery Books is the Crime Scene Investigator (CSI) making casts of footwear marks to help the police solve the crime.

As with fingerprints vs fingermarks (https://mozartcultures.com/en/fingerprints-vs-fingermarks/ ), the CSI calls what is found at the scene Footwear Marks.

Types of Marks

For all marks at a crime scene, it is important to photograph first. Sometimes the method of recovery damages the evidence.

When considering footwear marks, determine whether they are 2D or 3D. If the scene of a crime is reached quickly, then a 2D wet foot marks to be photographed or a faint impression on a carpet can be captured. For 3D marks, we get the footprint in the flower bed or a boot print in the snow. From these 3D prints we get impression evidence that can be cast.

What use are they?

A random print found in our example flower bed can be helpful. The police take a cast of it, allowing it to be checked to see if it matches our hypothetical gardener’s boots. In addition, the sole of a modern shoe can also be read like a story. Different manufacturers insist on individual sole patterns for the grip of their shoes. Wearing shoes leaves an imprint of the way you walk. These unique wear patterns are yet another clue and help to link an individual to a crime scene (The Forensics Library).

Let us focus on 3D

Returning to the damp soil in the flower bed. This is the perfect condition to preserve a footwear mark. The CSI pours casting material poured into the mark. Once set, the cast is cleaned and there is a clear impression of the sole of a shoe.

Materials used for Casting Impressions

What is used to cast footwear mark that the police use to solve crime?

Plaster of Paris

Plaster of Paris an excellent material for casting. It does not shrink as it dries – vital if we are to match shoe sizes to a known piece of footwear. It is gypsum, also known as Calcium Sulfate Hemihydrate, and was first found near Paris, France (Britannica Ed, 2018). It sets with an exothermic reaction (“Ken” 2013).

 Dental Stone

Dental stone is another gypsum-based product. As the name suggests it is often used to make dental casts. Similar to Plaster of Paris, it requires less water to mix. The gypsum is steam heated to 120-150ºC (Oxford Reference). This heating process makes dental stone stronger, making thinner casts (Forensic Magazine, 2011). Again it is exothermic as it sets (Dental Stone Casting Material, 2022).

Exothermic? Did you mention snow earlier?

Image 1 footprints in snow. Footprints,_Omagh_-_geograph.org.uk_-_1151479.jpg

Snow presents a clear image of a footwear mark. However, casting material placed directly onto the snow will cause it to melt. This adds extra water to the casting material making the impression useless. To prevent this, pre-treating the snow with a beeswax-based product. This protects the cast from dilution, giving a clear impression for evidence (Hammer et al, 2003). An older method of casting snow prints uses Sulfur cement (Wolfe, 2008). A newer method for capturing prints in the snow is biofoam blocks. These blocks require great care when using. The CSI must press the foam into the imprint, which could damage the evidence, but they give very clear impressions (Petraco et al, 2016).

Comparison of Cast Evidence

Image 2 photographs of footwear mark, laboratory impression, and the footwear. https://commons.wikimedia.org/wiki/File:Comparison_of_crime_scene_print_Q2_with_suspects_shoes_K1_right_shoe_with_labels.jpg

Once you have created your cast of the footwear mark compare it must be compared against the sole of a show taken in evidence.  Placing your cast from the crime scene next to the shoe means that important evidence could be missed. Comparisons must be made like for like. For example, if you have a photograph of a wet shoe print on pavement, then you make a wet shoe print and take a photograph of it (see image 2). If you have a mark on paper then a mark must be made on paper to compare. With this in mind, if you have a shoe to compare with then a cast must be made of the sole.

Casting Evidence in a lab

Casting in a laboratory environment is more controlled. The investigator presses the shoe into sand. With a fine material like sand it is necessary to add a fixative to prevent the collapse of the print (SWGTREA, 2020). This is the case in the field or the laboratory.  The best fixative is ordinary hairspray (Battiest et al, 2016).  Once the sand is stabilized then the casting can take place with the usual materials.

What if there is nothing to compare with?

Many countries keep a database of footwear marks. The investigator uploads their recovered mark. A mark may turn out to match one from a different crime scene. This links crimes committed in separate crime scenes that are being investigated by another team. When two crime scenes are linked, shared evidence makes catching a criminal more likely (Footwear, 2022).

Conclusion

Footwear marks can be useful tools for investigating a crime. It is possible to link a criminal to a crime, by the sole print of their shoes. Alternatively, an impression mark can link two disparate crimes together allowing the sharing of evidence between teams. All of these are important in solving crime.

Take a quiz on your footwear mark evidence.

https://www.opinionstage.com/vanessa-knipe/following-in-their-footsteps

References

Battiest, T., Clutter, S. W., & McGill, D. (2016). A Comparison of Various Fixatives for Casting Footwear Impressions in Sand at Crime Scenes. Journal of forensic sciences, 61(3), 782–786.

Britannica, T. Editors of Encyclopaedia (2018, February 9). Plaster of Paris. Encyclopedia Britannica. https://www.britannica.com/technology/plaster-of-paris

Dental Stone Casting Material. (2022) Bureau of Criminal Apprehension. https://dps.mn.gov/divisions/bca/bca-divisions/forensic-science/Pages/forensic-programs-crime-scene-dentalstone.aspx

Footwear. (2022) Lancashire Constabulary. https://www.lancashire.police.uk/about-us/our-organisation/scientific-support/footwear/

Forensic Magazine (2011)  Impression Evidence: The Right Materials for the Job available at https://www.forensicmag.com/article/2011/06/impression-evidence-right-materials-job

Hammer, L. & Wolfe, J. (2003) Shoe and Tire Impressions in Snow: Photography and Casting. Journal of Forensic Identification, 53 (6,) 647-655. https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1046.339&rep=rep1&type=pdf

“Ken” (2013, July 15). Plaster of Paris Mixing Guide. Observations Blog. http://www.observationsblog.com/sciencetechnologyexperiments/plaster-of-paris-mixing-guide

Oxford Reference. Stone, Dental. Oxford University Press. https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100534531?rskey=Vepbne&result=2

Petraco, N., Sherman, H., Dumitra, A., & Roberts, M. (2016). Casting of 3-dimensional footwear prints in snow with foam blocks. Forensic science international, 263, 147–151. https://doi.org/10.1016/j.forsciint.2016.03.033

SWGTREA, (2020). Guide for Casting Footwear and Tire Impression Evidence. Crime Scene Investigator Network. https://www.crime-scene-investigator.net/guide-for-casting-footwear-and-tire-impression-evidence.html

The Forensics Library Impression Evidence. https://aboutforensics.co.uk/impression-evidence/

Wolfe, J.R.. (2008). Sulfur cement: A new material for casting snow impression evidence. 58. 485-498.