L4 - 3D Footwear marks (2)

3D impressions, test impressions

What is 3D footwear impression?

Formed when shoe impressed into soft plastic material:

-Earth

-Sand

-Snow

What is the sequence of processing?

-Photographs

-Cast

What are the type of casting materials available?

-Dental stone

-Crown stone (better in field)

Plaster of paris

What is the sequence of casting?

-Spray impression to stabilise surface

-If necessary place casting frame around impression

-Make up casting material (mix with water)

-Pour

-Allow to harden

What is the stabilisation layer?

-thin layer of snowprint wax or Unifix 16

(Aerosol Spray used for the permanent fixation of powdered fingerprints on paper etc.)

-carefully sprayed from a distance to cover the entire impression lightly.

-coating assists in slightly hardening of surface

-to minimize possible disturbance as main casting material is poured into the substrate.

What is the casting frame?

rigid plastic frame

How is casting material poured on footwear impressions?

•not be poured directly over all areas of the impression, but directed to one side only.

How can casting material be mixed?

• Rubber bowl

• addition of water in plastic bag

Why is mixing casting material in a plastic bag preferred?

• Less mess

• easy to dispose of waste

• Heat from hands helps initiate reaction in cold weather

How long is the setting time for casting material?

20 minutes - 2 hours

How would you make a cast in snow?

• Use crownstone or plaster of paris

• need to use snow print wax

What is snow print wax?

• Waxy spray that prevents loss of detail ​

• colour red give good photographic image ​

What is a test impression?

used for Comparison with scene marks​

• From suspects shoes​

• For elimination of shoe marks from victims etc.

How would you make a 2D test impression?

1. Put on questioned shoe

2. Apply film light oil uniformly to undersole ​

3. By stepping onto oil impregnated foam rubber​

4. Sole pressed onto white paper​ (take normal step)

5. Paper dusted with mix of fine black powder and magnetic iron filings using a magnetic brush​ (yank paper at corners to take excess off)

6. Spray with hair spray to let it set

What is the inkless system for 2D impressions?

• Sole print kit

• Yellow chemical impregnated pad

• Special paper​

• Quicker and cleaner than traditional oil and black powde

How would you make a 3D test impression?

• Either by using similar matrix as crime scene – e.g. tray of soil​

• Then cast or​

• Bio-Foam kits​

• kit comes in a cardboard container ​

• Press the footwear into the bio-foam and remove the shoe.​

• Spray the impressions with a separating agent i.e. hair spray. ​

• cast​

How to cast a footwear impression in soft soil?

• mixture of water & dental stone

300ml water to 800g of dental stone