Detailed Procedure:

Three-dimensional impressions in a substrate may be left in a soft medium, such as mud, snow, or sand. Footwear and tire impressions may also include other evidence transferred from the tread that can be analyzed, such as soil, botanicals, glass, or paint.

Because these impressions can be fragile, they should be carefully protected and processed as soon as possible to prevent degradation. All impressions must be protected from water, wind, equipment, and foot and vehicle traffic until they can be recovered. This can be accomplished by covering with boxes, marking areas with cones or crime scene banner tape, or by posting a guard.

It should be noted that other objects, such as fabrics, can also leave three-dimensional impressions. If another type of object leaves a three-dimensional impression, the collection techniques would be the same as with footwear and tire impressions.

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To collect questioned 3-D impressions:

1. Photograph the impression in normal illumination for reference purposes and then again using oblique illumination to cast shadows that highlight the 3-D features. Take contextual wide-angle photos as well as closeups with a right-angle scale (or ruler or tape measure) alongside and parallel to the long axis of the impression, not across the middle. The scale should be next to the impression and positioned so that the scale is in the same plane as the bottom of the impression or as close to it as possible to avoid depth of field issues. Take closeups of any evident alterations to the tread, such as worn areas, cuts or divots.

2. When taking photographs of the questioned impression, the plane of the image-bearing surface of the camera (film or LCD panel for a digital camera) should be parallel to the plane of the questioned impression. A tripod and a small level are useful tools when photographing questioned impressions as they help ensure that the image-bearing surface is level. If the questioned impression is not on a level surface (eg: on a hillside), then attempt to make the plane of the impression and the plane of the image-bearing surface parallel. When photographing a tire impression, take photographs in 1-foot intervals.

3. Assemble the necessary tools (fixative, casting medium, and collection container) to cast the impression. Dental stone is the recommended casting medium. This can be obtained online from any dental supply store or commercially available kits sold by evidence collection supply companies. Make sure you have sufficient training and practice in the collection technique because casting may destroy the impression when removed and you will not get a second chance. Contact the laboratory if you need assistance.

4. Wear new, unused, clean latex or nitrile gloves.

5. If necessary, erect a dam around the impression using a casting frame or other material so that the casting material stays in the impression. A dam is usually not necessary unless the impression is on a steep surface.

6. Use tweezers to carefully remove loose debris, such as leaves, that has fallen on the impression. Do not attempt to pull out any debris that is embedded in the impression. Standing water in an impression in soils can be removed with an eyedropper. Or, dry dental stone powder can be sprinkled into the standing water portion to act as a fixative.

7. A fixative can be applied before the dental stone is poured into the impression. Acrylic spray paint and hair spray are examples of fixatives for sand, soil, or other dry or semi-dry materials. The fixative is gently sprayed over (not directly into) the impression. This will firm up the impression before the dental stone is poured in.                SnowPrint Wax (TM)  or automotive primer sprays are examples of fixatives for snow. The fixative is gently sprayed over (not directly into) the impression. These fixatives will not only firm up the impression, they will also provide contrast in the snow. Additional photographs may be taken after the fixative is sprayed onto the snow impression. These fixatives also slightly insulate the snow from the dental stone, which generates some heat as it cures. If the weather is particularly cold, the addition of potassium sulfate to the dental stone mixture will accelerate the curing process before the water in the mixture has a chance to freeze. Also, cooling the mixture by substituting some snow for water or laying the bag of dental stone mixture in the snow for a few minutes before pouring in into the impression will cool the mixture, preventing any melting of the impression.

8. Mix the casting medium according to the directions. If using bulk dental stone, the mixture is about 2 pounds of dental stone to 10 ounces water. It is easiest if it is mixed in a plastic Ziploc bag. Once the mixing is complete, cut a hole in the corner of the bag to enable you to dispense the mixture into the impression easily and with control.

9. Hold a mixing spoon, spatula, or another flat surface just under the flow of casting medium out of the hole in the bag. This will deflect some of the force and prevent pouring the dental stone directly into the impression. Hold the deflecting tool above and just outside the impression and slowly pour the medium out of the hole in the bag, directing the flow of the casting medium onto the tool first, and letting it fall over the tool and onto the ground. Once the flow has started, move the bag and the deflecting tool over the impression, covering it completely with casting medium. There should be a small margin of casting medium outside the impression. This ensures that the medium does not destroy any features of the impression with the force of the pour.

10. Allow the cast to cure until hard. The time needed to cure will vary based on temperature. When dry, scratch or write identifying information onto the back of the cast, including your initials, date, exhibit number, and a directional arrow pointing north.

11. Remove the cast gently with adhering soil and place the cast flat in a rigid box. Air dry the cast thoroughly before storage to minimize mold growth or damage to other stored materials. When possible, casts should be cleaned in the laboratory by the examiner. If it is necessary to remove adhering soils, do NOT attempt to do so until the cast is fully cured (this will take about 48 hours). Adhering soil can then be collected as a reference sample for soil comparisons. Use the proper procedure for soil collection.

12. If casting tire track impressions, a cast should be done every 1 foot until you reach a length of 7 feet or you come to the end of the impression.

13. When ready, place the cast in a suitable container, such as a rigid box. The container should protect the cast from chipping or breaking. The cast should be secured in the container.

14. Seal the container with evidence tape. Initial and date the tape.

15. Label the container with identifying information, including case number, date, exhibit number, a brief description, and your name.

16. Store the item in a secure location, such as a locked evidence van or your vehicle, until you are able to transport it to the laboratory.

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Laboratory examination of impressions:

• Type

• Manufacturer identification

• Size

• Unusual wear features

• Trace evidence

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Known footwear and tires can be compared to questioned impressions left at a scene. Associations may be made by comparing tread design and tread design orientation. A positive match may be made if the questioned impression has sufficient individualizing or unique features (eg: cuts, nicks, wear pattern) in common with the known footwear or tire.

These unique features may change if the footwear or tire is used frequently over a period of time, possibly to the point of preventing identification.

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Source:

CFITrainer.Net®. "Physical Evidence at the Fire Scene." Website, video, 2013.

https://www.cfitrainer.net/Training_Programs/Physical_Evidence.aspx

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"SWGMAT Web Site." Scientific Working Group for Materials Analysis, Website, July 1, 2013.