To cast plastic impressions of footprint

Biological

Shoe imprint on sand or dirt.

Chemicals

A. Plaster of Paris

B. Fixative such as hair spray or dust hardeners.

C. Water

Glassware/Plastic ware

All glassware and plastic ware used should be sterilized:

1. Gloves
2. Beaker
3. Spatula

Casting plastic impressions

Any three-dimensional impression created by our feet or the footwear we wear can be collected by casting and preserved as physical evidence for forensic records.

Theory

The imprints left by a person walking are referred to as footprints. They are extremely important in the forensic investigation of a crime scene. They aid in the identification of all the people who were present during the conduct of a crime. Footprint traces are collected and 3D images of the footprints are used for further analysis.

There are three basic categories of impressions present in footprints:

• Latent impressions.
• Patent impressions.
• Plastic impressions.

Latent impressions

These are two-dimensional impressions that are invisible to the naked eye, produced when minute debris such as oil, fine soil, etc. is transferred from an object and deposited onto a clean floor. They can be made visible by dusting, electrostatic techniques or chemicals.

Patent impressions

These are two-dimensional visible impressions produced when materials are transferred from an object and deposited onto a hard flat surface.

Plastic impressions

Plastic impressions are three-dimensional, visible imprints produced when an object, in this case the sole of a shoe, is pressed into soft materials such as mud, sand or snow.
Depending on where an imprint has been left in, the material used for their casting varies as follows:


• Plaster of Paris is used if the imprint is on sand or dirt.
• Dental stone, a gypsum-based product, is used when the imprint is on snow. Dental stone hardens faster than Plaster of Paris. It has minimal shrinkage and captures minute details.

Plaster of Paris

Plaster of Paris chemically known as calcium sulphate hemihydrate is a fine white powdery, quick setting gypsum plaster. When moistened and allowed to dry, gypsum crystals are formed causing it to achieve a solid state. While hardening plaster expands then contracts slightly before solidifying completely. This property of plaster of Paris makes it ideal for making moulds and casts.
Before casting of the impressions is performed, fixatives like dust hardeners and hair spray, are sprayed into the impression to stabilize it during the pouring of the casting material.

1. Identify a footprint on soft sand.
2. Mark and create a slightly raised boundary around it, to prevent the casting material to flow out.
3. Fix the shoe imprint by spraying dust hardener or hair spray. Leave for about 5 minutes.
4. Make a thick but flowy paste of plaster of Paris by slowly mixing water with it. Make sure there are no lumps.
5. Pour the Plaster of Paris on the shoe imprint from one end gradually till the entire print is covered.
6. Leave it for two to three hours to dry completely.
7. Carefully lift up the casting and place it in a cardboard box.
8. Leave it for the next 72 hours to allow it to set and harden firmly.
9. Rinse off the dirt from the cast after 72 hours.
10. The footprint is now ready for analysing and even preserving.

Shoe imprints are very handy in forensic analysis. Much information can be obtained from a shoe imprint, such as:

• Size of a person's foot which is equivalent to the size of shoe imprint.
• Person's weight depending on depth of the show or foot imprint.
• Information on the job or personality of the person depending on the type and brand of the shoe.
• We can also gather information about the brand or store from where the shoe was bought.

1. Make sure there are no lumps while mixing the Plaster of Paris with water.
2. Pour the Plaster of Paris from one end and gradually so as not to disturb the imprint.
3. Do not rinse/brush off the dirt of the cast before letting it to set for about 72 hours else it can hamper the details of the imprint.