Track Impressions in Snow

Topics - Track Impressions in Snow

Four Acceptable Techniques
Highlighting with Colored Aerosol Sprays
Grey Primer with Dental Stone combination
The addition of a 5% solution of Potassium sulfate
Snowprint Wax Method
Snowprint Wax Ordering
Assist the setting of cold-weather casts
Time to allow cast to harden
Sulfur Casting
Articles - The Recovery of Track Impressions in Snow

Historical Difficulties in Recovering Track Impressions in Snow

Normally, impressions in snow display fine, clear and distinct detail. The difficulty has been to accurately and effectively record this valuable detail through casting and photography.

Substandard methods used to cast snow impressions assume a "porous" appearance, due to partial melting of the original details. This is due to the heat generated by the setting of the casting material.
Viscous casting material has a tendency to penetrate the base of the snow impression, and flow through the impression. This yields limited results with a corresponding loss of details.

Other difficulties encountered during casting include snow from the "walls" of the impression falling into the impression, and, trapped air creating voids in the cast. The freezing of the casting material before the cast has set is a common problem.

Photography of snow impressions also has its drawbacks. Photography without the use of highlighting techniques results in a monochromatic complexion of the snow background and depicted details.

Outline of Techniques - Track Impressions in Snow

Four Acceptable Techniques:

• Highlighting with colored aerosol sprays.

• Combination of grey primer with dental stone.

• Combination of Snowprint Wax with dental stone.

• Sulfur Casting techniques.

Highlighting with Colored Aerosol Sprays

Lightly spray the impression with the colored aerosol, at a low angle. Photograph the impression before any deterioration due to melting from sunlight. Choose one of the following colored aerosol sprays:

• Grey Auto Primer
• Rust Auto Primer
• Snowprint Wax

Gray Primer with Dental Stone combination:

For more detail regarding this technique, please refer to the article: "Dental Stone Casting of Snow Impressions", Beheim, C.W. and Wolfe, J.R., presented at the International Symposium on the Forensic Aspects of Footwear and Tire Impression Evidence, FBI Academy.

This technique accomplishes better results by cooling the dental stone-water mixture:

• Addition of snow.
• Addition of cold water.
• This adjusts the viscosity of the mixture to "thicker", not "slushy".
• The net effect is to reduce the heat generated by the setting mixture.

The addition of a 5% solution of Potassium sulfate:

• Prevents the formation of ice crystals on the cast
• Lowers the freezing point of the water
• Decreases the setting time, so that the cast sets before the water freezes

Snowprint Wax Method:

• Observe the 'Important' notice.
  * Can should be at room temperature before use.
  * If the can is too cold before use, then the pressure of the propellent will be too low.
  
  
• Observe the "Caution" notice.
  * The pressurized can should not be submitted to temperatures over +50 Degrees C.
  * Very inflammable propellent.
  
  
• Lightly spray the impression to highlight the impression's detail.
  Don't let the "blast" of the spray out from the nozzle damage any detail.
  
  
• Photograph the highlighted result.
  
  
• Completely seal the impression, including the walls of the impression and surrounding area with at least three generous applications of the wax spray.
  
  
• Let the wax set for 10 minutes.
  
  
• Prepare, then pour, a slightly thickened mixture of dental stone/water.
  
  
• When hardened, lift the cast/wax shell without damaging the wax shell.

Assist the setting of cold-weather casts:

• Take procedures to allow the cast to retain heat during setting.
• Cover cast with sheets of newspaper.
• Cover cast with cardboard box.

Time to allow cast to harden:

• Minimum setting time of 60 minutes.
• Place cast aside for 48 hours, to allow any frozen stone material to thaw without surface damage.

Sulfur Casting:

Three sources of reading material:

• "Sulfur Casting", Thompson, R.W., AFTE Journal, 12:2 (1980).
• Bodziak's book, pages 92 to 95.
• Cassidy's book, pages 28 to 31.
• Be alert to the decompositon of sulfur during heating. The decompositon product is toxic fumes of sulfur oxides, which are classified as an irritant. Causes irritation to eye, skin, mucous membrane and upper respiratory tract.

Articles - Track Impressions in Snow

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