Shoe and tire impressions in dirt, sand or snow can be recovered by a combination of photographic techniques and taking a three-dimensional cast of the impression. Dental stone, also called casting powder, a simple, inexpensive casting material, can record sufficient detail for comparison purposes. It is always recommended that photographs using oblique lighting from a number of sides be done before casting material is placed into the impression. When dealing with impressions in snow or sand, it is recommended to use Snow Print Wax™ to "fix" the impression so it will not be destroyed or damaged when the casting material is poured in.
Before mixing the casting material, it is recommended to photograph the impression. 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".
Casting powder may be mixed in a bowl with a spatula or other stirring device. The mixing ratio is approximately 12 oz. of water to two pounds of casting powder. This amount 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 casting powder into an 9" x 12" zip-top plastic baggie. Measure water into a 12 oz. 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 casting powder, which may not be available at the scene.
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 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.
The camera used should be placed on a tripod, which will allow the film plane to be parallel with the surface containing the evidence impression. Some impression evidence 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 than color films. 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 of the print and take a picture. Take additional exposures with the strobe held at an oblique angle to the right, to the top and to the bottom of the impressions. If added details can be seen by holding the light in other directions, take additional 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 evidence impression. A larger, 3-part, folding scale which is handy for tire track impressions is also available. The advantage of an "L" shaped scale is that if the scale divisions on each leg of the scale are not equal in the photograph or if the circles with cross hairs 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.
- Catalog No. 4-1101....... Casting Powder, 25 pounds
- Catalog No. 4-1105....... Footprint Casting Frame
- Catalog No. 4-1106....... Tire Track Casting Frame