Step on it – forensic footprint casting
Footprints are often vital evidence in a criminal case, and how they are processed is changing.
Professor Matthew Bennett and his team at Bournemouth University are using geoarchaeological methods to help the police analyse crime scene footprints.
Their technique uses digital photographs taken from different angles around the footprint. The images are combined in a computer program to create a 3D model. It is known as digital photogrammetry.
Measurements of depth and distance can be taken from the 3D model, then an analyst can use that to work out the size and type of shoe. They can also comment on the gait (walking style) and weight of the wearer. It is even possible to tell the difference between two prints of the same size and pattern by looking at the wear patterns on the soles.
These digital techniques are likely to change the way footprints are processed at crime scenes, but many cases still rely on a very traditional way of collecting this information – by taking a cast.
MAKE A CAST
SAFETY:
Plaster of Paris is an irritant. It will make you cough if you breathe in the dust. When you mix it with water it starts an exothermic reaction, this means it gives out heat. If it is in direct contact with your skin it could burn you. DO NOT be tempted to put your hands INTO the mixture to feel it warming up – it WILL get hot and it WILL burn your skin. It is safe to feel the outside of the pot.
YOU WILL NEED
– Apron
– Table covering
– Sheet of A4 paper
– Sticky tape
– Waterproof gloves
– Goggles
– Plaster of Paris (6 level tablespoons) available from art/hobby shops or online
– Water (3 level tablespoons)
– Old container for mixing (e.g. clean yoghurt pot)
– Old spoon or chopstick for mixing
– A footprint or paw print
– A useful adult
STEP 1:
Put on an apron and cover your work surface.
STEP 2:
Make a paper collar for your footprint. Fold your paper in half to make A5, then fold it again in the same direction to give you a piece of paper about 21 cm x 7.5 cm. Obviously, your paper size will need adjusting depending on the size of the print! Secure the ends (inside and out) with sticky tape.
STEP 3:
Find a good print to cast. Look in wet mud – you could use human or animal tracks! Put the collar around an interesting part of the print and push it into the ground a little.
STEP 4:
Put on your gloves and goggles and with a useful adult make your plaster mix: you will need to work quickly but safely. Take care not to make a cloud of plaster of Paris dust. It is very messy and will irritate your lungs if inhaled.
STEP 5:
Put three tablespoons of water into your pot, then slowly add six tablespoons of plaster of Paris. Sprinkle the powder evenly over the surface and allow it to settle before adding more. To make a larger amount of mixture always use two times the amount of powder compared to water (e.g. five tablespoons of water would require ten tablespoons of plaster).
STEP 6:
Stir the mixture gently until smooth. It will only stay in liquid form for about ten minutes. Once you feel the outside of the bowl starting to warm up, it will thicken and start to turn into a solid.
STEP 7:
Make sure a helpful adult is able to assist you in pouring the mixture all over the print inside the collar. If any escapes underneath, push the collar more firmly into the ground. If you get any mixture on your skin WASH IT OFF IMMEDIATELY! Setting plaster of Paris is an exothermic reaction. It gives off heat and can burn your skin if it comes into direct contact with it.
STEP 8:
Allow the cast to set for an hour. You can clean up your work area during this time!
STEP 9:
Carefully lift your cast and remove the collar.
STEP 10:
Allow it to cure (dry and harden) overnight so that it is completely hard.
STEP 11:
Use a paintbrush or old toothbrush to clean it up. You can paint the print if you like!
STEP 12:
Identify your culprit – take a look at the shoe prints or paw prints of your household. This print was left by Nova – she still has mud on her nose from digging the hole!
Why not send a photo of your cast to us via the AQUILA website, and we’ll share as many as we can.
If you enjoyed this brilliant blog, then why not think about a subscription to AQUILA magazine!
Words: Dr Sarah Bearchell. Illustration: Ed J. Brown.
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