Special Situations

Special techniques must be used in casting under water or in snow. Let's take a look at these.

Casting under water. It maybe necessary to cast a footprint or tire print that is entirely underwater, where the location and amount of water present precludes draining or removal of the water. A section of stovepipe may be used to direct dry plaster to the print location and prevent waste of the plaster. The dry plaster should be sifted into the stovepipe and directed into the impression. This method can be used even in fairly deep water. Three to 4 parts of salt maybe added to 10 to 12 parts of plaster of Paris to hasten the setting time.

Casting in snow. The temperature, depth, and adhesive quality of the snow, and the condition of the ground surface, are all important factors. Plaster of Paris emits some heat as it hardens and has a tendency to melt the snow, destroy the print and impair details of the cast. You should make trial casts in the snow away from the impression to determine which of the several techniques are best in the situation with which you are faced.

The imprint may be strengthened by spraying with plastic spray. Then a thin layer of talcum powder should be dusted into the imprint with a syringe to act as insulation against the heat. The impression should again be sprayed with the plastic. Several coats of spray may be necessary to suitably fix the impression.

A retaining wall is especially necessary here to prevent spread of the plaster to areas not insulated. In uninsulated areas, the heat of the setting plaster might melt the snow and water might run under the prepared imprint, further contributing to its possible destruction.

Other Casting Materials and Possible Uses

In addition to plaster, other casting materials may be used to obtain prints. Let's look at some of these and their possible uses.

Dental stone. Use of dental stone allows for a cast of only one layer approximately one-quarter inch thick. It develops into a very durable and a lighter cast than plaster of Paris, and reinforcing materials are not necessary. It is highly recommended for use and is much more economical and convenient.

Silicone rubber. Silicone rubber, although more expensive than plaster of Paris, offers some distinct advantages. Silicone rubber's advantages are that casts can be made quickly, water or heat is not needed, and castings are rubbery, eliminating the problem of breakage during handling or shipping. The rubber freezes at extremely low temperatures, making it extremely useful in casting tracks or prints in the snow.

Silicone rubber, a fast cure, room temperature vulcanizing rubber, requires no heat and sets up into a solid rubbery mass. The catalyst (a substance that causes or accelerates a chemical reaction) is supplied in a small tube with the silicone rubber package. The catalyst should be thoroughly mixed with the liquid rubber just before using. About one-half teaspoon of catalyst to a pound of the rubber is normal.

Curing time of the rubber can be varied by altering the amount of catalyst added. A table is supplied with the material. A curing time of 5 to 10 minutes for prints in dust is recommended.

Care must be taken when mixing to preclude the formation of air bubbles as they may obscure details in the cast. When catalyzed, the rubber will remain workable for about 5 minutes at 77F, after which it will set. Lower temperatures will lengthen the setting time.

Suggested uses of silicone rubber include foot and tire prints, dust prints, tool and jimmy marks, casting of parts of the human body, and fingerprints. Silicone rubber is not recommended for surfaces bearing natural patterns such as leather or fabrics as the detail of the print is obscured by the detail of the natural surface.

Epoxy casting resin maybe used to make a positive from the silicone rubber impression.

Silicone rubber can be used to obtain fingerprint impressions found in putty and in caulking around window panes when they cannot be removed and forwarded to a laboratory.

There are other casting materials and uses available that are not covered here but are found in Army field manual, FM-19-20, Law Enforcement Investigations.

Special Techniques

Obtaining footprints, tool marks, and tire impressions require that special technique be used.

Footprints. When a search indoors is to be made for footprints, it is advisable first to darken the room. Use a flashlight, with the beam aimed low, to search floors, windowsills, and furniture. Oblique lighting will often show up traces not visible with ordinary or direct light.

Footprints on carpets should not be overlooked, as appropriate photographic techniques may produce valuable evidence photographs. Excellent results have been obtained by using a high-contrast film and a high-contrast paper for the print.

Footprints on solid surfaces made by dust particles adhering to a shoe can be lifted by the use of large sheets of ordinary fingerprint-lifting tape.

Tool marks. Tool marks on metal surfaces, such as hammer blows on a safe, are usually extremely faint and the details microscopic. The wax portion of the "Posmoulage" material is excellent for use in these cases. Other marks having deeper indentations may be cast using any of the materials previously mentioned These types of impressions, including "jimmy" marks on doors and windowsills, are not easily destroyed so other substances may also be used.

Tool marks may be destroyed or changed if tampered with by an untrained or unskilled investigator. Original evidence is more useful for scientific examination and evaluation and is less subject to attack in court than reproductions. You may often be unable to make photographs and casts that represent the evidence sufficiently for identification purposes at a laboratory.

Tire impressions. The circumference of a tire is 5 to 8 feet. The probability of identifying or matching a tire track with a particular tire increases with the length of the cast or casts obtained. Ideally, consecutive tire casts should be made equaling the circumference of the tire involved.

Casts should be made of each individual tire track if more than one track is found. Testimony that the combination of the designs taken from a set of four tire impressions found at the crime scene corresponds to the designs and the wheel positions of the four tires on the suspect's automobile is of obvious value.

Each cast and tire submitted should be fully identified as to the wheel position. (See fig. 16-7.) Sketches, photographs, and other recorded entries should likewise identify wheel position.