MOLDING

It may be desirable to produce a mold from a cast that had been made previously. It might be advantageous to provide several investigators with

Figure 16-7.-Identifying wheel positions in curved tire tracks.

copies of the cast so they can search independently for the shoe that made the original impression. Once a mold is made of the cast, as many copies as needed can be reproduced. Casts made with agar moulage compositions require that a mold be made shortly after the cast to meet evidentiary requirements.

Plaster of Paris

To make a plaster of Paris mold from a plaster of Paris cast, the cast should first be given a thin coating of light oil, otherwise it will be most difficult, if not impossible, to separate the cast from the mold. With this precaution having been taken, the procedure for producing the mold is almost identical to that used when making the cast.

Plaster of Paris is mixed and poured into a container or onto a flat surface with the area contained by framing material. A rubber photochemical tray of appropriate size makes a good container. The cast, after being coated with oil, is placed into the plaster mixture to an appropriate depth. When the plaster mixture has set, the cast may be removed, leaving a mold that maybe used for making other casts.

If duplicate casts are to be made of plaster of Paris, the mold must be coated with oil or the same problem of separating the cast from the mold will again present itself.

Positive Moulage-Agar Compositions

Positive moulage material, a wax, may be melted over a slow fire in any pan from which it can be poured.

If the cast is hollow, the melted moulage should be poured in, allowed to stand 2 or 3 minutes, and then poured out. This procedure is then repeated as necessary, allowing a slow buildup of wax to the required thickness.

The moulage may be brushed or poured into other-than-hollow negative casts. Since the moulage will not adhere to anything, it may be used for making molds from plaster casts. No oiling of the cast is necessary. The cast can be appropriately framed and the material brushed onto the cast, or it may be applied with a spatula.

The mold should be reinforced with wire mesh or cloth. Bandages make excellent reinforcing material.

LABORATORY EXAMINATION

A plaster of Paris cast, even with reinforcing, is fragile evidence and must be handled carefully. The cast should be wrapped in soft paper or cotton to avoid abrasion and possible destruction of fine identification points. It should then be well wrapped in strong wrapping paper and secured. The cast may then be placed in a box, cushioned on all sides by excelsior or other shock-absorbing material.

Wax and modeling clay casts, while not as fragile as plaster, present other problems, such as being easily deformed by pressure. These types of casts must be protected from pressure as well as abrasive action. Photographs of the impressions from which the casts were taken should also be forwarded with the casts to a laboratory to aid the examiner in the analysis.

Articles such as shoes or tools, to be shipped to the laboratory for comparison purposes, should be boxed and shipped separately. They should be protected from the accidental addition of marks which might invalidate them as evidence. The chain of custody requirements are equally applicable to this type of evidentiary material as to any other.

The examination of cast and mold evidence usually is done in the fingerprint division of a laboratory. There are two types of characteristic-class and individual. Class characteristics are those marks and lettering that result from the manufacturing process. Individual characteristics are those marks peculiar to certain objects, such as cuts, tears and uneven wear which result from its daily use.

These characteristics allow the examiner to establish positive or negative identifications with a particular object. Remember that this type of evidence might also be used to eliminate suspects. The importance of this very valuable evidence should not be minimized, and every effort should be made to secure casts of impressions found at the crime scene.