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Soil Impression Casting with Dental Stone For the Recovery of 3-Dimensional Impressions in Soil

FOR ADDITIONAL EVIDENCE PACKAGING SEE: Evidence Collection - Section 1 Evidence Collection - Section 2 Evidence Collection - Section 3

Soil Impression Casting with Dental Stone
	Upon locating a 3-Dimensional Soil Impression, photographic documentation should be undertaken prior to any casting. A Tripod with an invertible column is ideal. This provides a photographic record of the impression detail. In a rare case of improper dental stone mixing, pouring or retrieval, this photograph could provide sufficient detail to make a comparison.
	A photographic scale is always used to provide reference to size and proper parallax. The EVIDENT Reversible Footwear and Tire Track Scale (#5094) is recommended. A photograph should be taken with and without a photographic scale.
	The camera frame should be completely filled with as much of the impression as possible. This provide the maximum amount of detail for comparison purposes. Available lighting, such as natural sunlight, may produce usable photogrpahic images. However, the use of oblique lighting with a flash from various directions can increase comparitibility of the impression.
	After photographic documentation is complete, a casting frame (#7005) should be carefully placed around the impression in order to contain the dental stone. Containment of the dental stone in this method assures a cast of consistent thickness and strength. When working on a sloped surface the use of a frame will prevent the wet dental stone from flowing out of the impression.
	Add 6 oz. of water to every pound of dry dental stone. EVIDENT recommends 3 lbs. of dental stone for shoe impressions, 4 lbs or more for tire impressions (dependant upon total impression area). Tire impressions of considerable length, may require multiple consecutive casts to record the circumference of the tire. A thicker cast is more resistant to damage during retrieval, transport and examination.
	Thoroughly mix the dental stone in a large resealable plastic bag. EVIDENT provides Pre-Measured Dental Stone in large 4 mil resealable bags. Uninterrupted mixing should continue until an even consistency is achieved. This consistency is often compared to pancake batter. There should be no lumps or unmixed dry dental stone in the mixture.
	Immediately after mixing, carefully pour the dental stone mixture into the impression. Allow the mixture to slowly spread from one end to the other, completely filling the casting frame. Dry and delicate impressions can be damaged or destroyed by aggressive casting. A fixative such as Dust & Dirt Hardener may be used to stablize the soil impression.
	Allow the cast to cure (dry) for 30-45 minutes. An exothermic reaction will take place generating heat within the cast. During the curing process the cast will become warm to the touch. When curing is complete a gentle tap on the back of the cast should resonate with a higher pitched sound, an uncured cast will emit a dull sound. Under normal conditions a cast should assume the ambient temperature prior to retrieval from the soil. To prevent accidental breakage of the cast, the surrounding soil should be cleared away from the edges of the casting frame. Gently lift the cast from the soil by hand only. Do not use mechanical leverage to pry the cast. Package the cast is a breathable container such as a cardboard box or paper evidence bag, never package in plastic.
	Dependant upon agency or laboratory policy the attached soil may be left on the cast until time of examination. The attached soil should be left in place and only removed by a trained examiner. This protects the detail during transport. Loose soil may be accordingly washed off of the cast prior to examination, revealing details such as wear patterns and manufacturer defects.
	Detailed cleaning by a qualified examiner can reveal sufficient uniqueness for positive individualization. Conclusive comparisons with known footwear or tire treads are possible if proper casting techniques have been followed. CLICK HERE FOR A DETAILED IMAGE OF THE CAST SEEN AT LEFT

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