Shoe/Tire Impression Casting

Introduction

Shoe and tire impressions in dirt, sand or snow can be recovered by a combination of photographic techniques and making a cast of the impression. Dental stone, a simple, inexpensive casting material, can record sufficient detail for comparison purposes. It is always recommended that photographs be taken before a cast is made. When dealing with impressions in snow or sand, it is recommended to use Snow Print Wax™ to "fix" the impression.

Preparation

After sufficient photographs are taken but before mixing the casting material, it may be necessary to remove leaves, twigs, or other loose debris which may have fallen into the impression. However, in no instance should an attempt be made to remove debris if that debris is part of the impression or if there is any possibility of destroying part of the impression by removing it.

The first step is to place a form around the impression. This will contain the casting material and also help give the cast extra thickness. Two sizes of forms are available. The shoe impression frame is 7 1/4" wide and expands from 10" to 18". The tire impression frame is 12" wide and expands from 15" to 28".

Mixing Instructions

Dental stone may be mixed in a bowl with a spatula or other stirring device. The mixing ratio is approximately 12 ounces of water to two pounds of dental stone. This should cast an average shoe impression. For tire track impressions, larger quantities will be needed.

An easier method for shoe print impressions, which can be prepared ahead of time is to measure approximately two pounds of dental stone into an 9" x 12" zip-top plastic baggie. Measure water into a 12-ounce plastic bottle. Clearly label each baggie and bottle as to its contents and the date it was prepared. A few baggies and bottles can be carried in a crime scene response vehicle until needed. When needed, add about two-thirds of the water into the baggie and mix it by kneading the bag. If the powder seems too thick, it needs more water. Add a little water and knead it again. Repeat until the desired consistency is reached. If the whole 12 oz. bottle is added initially, it may be determined that a little less water was needed as the mixture is now too thin. It is better to have to add more water than to have to add more dental stone, which may not available at the scene.

Application

When the mixture approaches the consistency of thin pancake batter it is ready and should be poured before it becomes thicker. Pour the mixture into the form by pouring it onto a flat stick or spoon held close to the surface of the ground and slightly to the side of the impression so as not to wash away portions of the impression. The mixture must be allowed to flow evenly over the impression. In approximately 30 minutes, the cast may be checked. If the cast is firm, carefully lift it from the surface. If soil or debris from the impression area is adhering to the cast, no attempt should be made to clean it. The cast should be allowed to air dry for between 24 to 48 hours.

The cast should never be placed in an airtight container or wrapped in plastic. Paper bags can be used but they do not afford the best protection. The bag can easily tear and the cast can fall out or otherwise be damaged. Dried casts should be stored in cardboard boxes. Cast boxes are available with the dimensions of 17 1/4" x 11" x 2 1/4". They are sold in quantities of 25.

Snow Print Wax™

For impressions in the snow, it is recommended that prior to casting, Snow Print Wax™ be sprayed into the impression. Because Snow Print Wax™ is waxy, it insulates the snow and prevents loss of footwear or tire track detail from the exothermic (heat producing) reaction of the hardening of the dental stone.

The process is easy. Note the photographs in the next column. Spray a few layers of Snow Print Wax™ in the impression, allowing each layer to dry for a moment before adding another one.

Gently pour the prepared dental stone impression material onto the Snow Print Wax™ and allow it to harden. Perform the other procedures as you normally would, at this point. If you add sticks to improve structural integrity, do so. If you scratch in the date, case number and your initials, do so.

When the dental stone is cool and hard, carefully remove it from the impression in the snow. The Snow Print Wax™ will adhere to the dental stone casting material. The red color makes photography of the details in the dental stone easier than a totally white cast. Also, it is a good warning sign to other persons at the crime scene, so they won't step on the impressions or drive over the suspect tire tracks. One can of Snow Print Wax™ should do 7-10 prints, depending on the size of the impression.

Photographic Scales

The camera should be placed on a tripod allowing the film plane to be parallel with the surface of the evidence impression. Some examiners prefer black and white photographs over color photographs as they feel the range of grays with black and white film shows the details better. Illumination of the impression should be done at an oblique angle from a number of directions. In other words, hold the strobe at an oblique angle to the left, right, top and bottom of the print, respectively to take your pictures.

With photography of this nature, scales should be included in the photograph. The Bureau photomacrographic scale set was designed by the Federal Bureau of Investigation for this particular purpose. The "L" shaped scale and straight scale are white with black marks on one side and black with white marks on the other. Choose the side that will show the best contrast with the surface and place it close to, but not overlapping, the impression. A larger, folding scale which is handy for tire track impressions is also available. The advantage of an "L" shaped scale is that if the divisions on each leg of the scale are not equal in the photograph or if the circles with crosshairs are not absolutely round, you will know that the camera was not parallel to the impression. If the angle of error is not severe, it can be corrected in the darkroom. Tilt the paper carrier to bring the "L" shaped scale back to parallel perspective.

	Spray a few layers in the impression
	Gently pour prepared dental stone onto the Snow Print Wax™ and allow it to harden.
	When the dental stone is cool and hard, carefully remove it from the impression.

Additional Reading

Footwear Impression Evidence by William J. Bodziak
Tire Imprint Evidence by Peter McDonald
Techniques of Crime Scene Investigation by Barry A. J. Fisher

Ordering Information

Casting Powder, one gallon
Catalog No. 4-1100 $ 16.95

Casting Powder, 25 pounds
Catalog No. 4-1101 34.95

Shoeprint Casting Frame
Catalog No. 4-1105 14.95

Tire Track Casting Frame
Catalog No. 4-1106 22.95

Footprint/Dustprint Boxes, 25 boxes
Catalog No. 3-3015 44.95

Bureau Scale, set of 2
Catalog No. 6-3880 7.95

Folding Evidence Scale, inches
Catalog No. 6-3835 7.95

Folding Evidence Scale, metric
Catalog No. 6-3830 7.95

BODZIAK/Footwear Impression
Catalog No. 8-5038 89.95

McDONALD/Tire Imprint Evidence
Catalog No. 8-5029 79.95

FISHER/Tech. of Crime Scene Invest.
Catalog No. 8-5016 75.00