CASTING

If an impression is found indoors in dust, or if it is made only from dirt adhering to the object, no preparation may be required.

An impression found outdoors should be examined to determine if any loose particles have been blown or otherwise introduced into it. Any such particles should be carefully removed. A syringe maybe used to gently blow dust that may have gathered-a pair of tweezers to lift out small stones-or a pocket knife to flick out such debris. Care should be taken to avoid destroying portions of the impression.

The syringe can also be used to good advantage to withdraw any water accumulated in the impression, or it can be drawn off by cutting a small channel at one side of the impression to allow drainage.

Plaster of Paris Casts

Plaster of Paris is especially suitable for taking impressions of foot and tire prints or other marks (in dirt mud, sand, or snow) that do not require microscopic detail accuracy. Where such microscopic detail is required, a casting medium other than plaster should be selected.

Preliminary steps. You should make a practice cast of your own shoe print before attempting to make the cast that may eventually be used as evidence. In sandy and loamy soils the particles frequently lack cohesion, and the print is therefore fragile.

Strengthen the impression to support the weight of the casting material and to prevent the destruction of fine detail. This is best accomplished by spraying the impression with a plastic spray or shellac. Hair net spray makes a good substitute if the others are not immediately available. Application of a direct spray may cause destruction of details. Direct the spray against a baffle made of cardboard or some other material to cause a fine mist to settle gently onto the print.

The number of coats to be given can best be determined by an examination of the test print. Usually from 3 to 10 coats in sandy soils are sufficient.

Shellacs, varnishes, and plastics are used only to solidify the print and are not always essential to the casting process.

When the plastic spray has completely dried, a thin mist of light oil allowed to settle on the print will make it easier to remove the completed cast from its surroundings.

If the print is in a good solid earthy material, no further preparation is necessary.

A retaining wall should be placed around the print to confine the plaster to a convenient area and to allow the cast to be built up to the desired thickness. The retaining wall may be constructed of any available material, including earth, to build a damlike structure.

If strips of material are assembled to be used as a kit, and it is intended that they be used over again, give them a light coat of oil before they are used. That will permit them to be easily removed from the cast.

Mixing the plaster. The plaster of Paris mixture may be prepared by one of two methods, both of which require the sifting of the plaster into water. Never add the water to the plaster; add the plaster to the water.

One method is to sift the dry plaster slowly into the water while constantly stirring the mixture. In the second method, the plaster is added to the water by sifting the plaster around the edges of the container. Plaster is added until it begins to rise to the surface of the water. When cracks, similar to those seen in dried mud, appear, no more plaster should be added

Mix by stirring beneath the surface to a thick creamy consistency. If lumps appear, they should be removed

It is advantageous first to mix a thin mixture to place in the impression to record the finest of details. The thin mixture is able to flow freely into the impression. Follow this with a thicker mixture. Do not make the thin mixture too thin, as it may wash away details. Experimentation will best indicate just the right proportions.

If a mixture is made too thick, it must be discarded and a new batch mixed. The mixture is ready to pour when it has reached the consistency of thick cream.

Pouring and strengthening the cast. When pouring the plaster into the impression, it should be poured from a low level and the force of the plaster falling into the impression should be broken by letting it drop first onto a spatula or the hand.

Move the ladle or stick in a circular motion to cover the entire area of depression. Put the plaster into the impression at a place where very little, if any, detail exists. These precautions should be taken to prevent destruction of important details.

After a layer of plaster about one-half to three-fourths inch has been poured, the reinforcing material should be added. If sticks or wire are used, take care to prevent the ends from protruding through the bottom of the cast. If dried twigs or wood are used, they should first be soaked in water, as they will absorb water from the cast, making it more fragile.

Twigs, sticks, and pieces of wire should be laid at random in the cast and not laid parallel. If the reinforcing material is laid parallel in a single direction, the cast may fracture in the same place in which the pieces of reinforcing material have been laid. Wire mesh does not present this problem.

After the reinforcing material has been introduced into the cast, the rest of the plaster can be added until the desired thickness of the finished cast has been reached.

Additives. While a thin mixture of plaster of Paris will record more fine detail than a thick mixture, it will require a much longer setting time. One-half teaspoon of salt added to each pint of water used in fixing the plaster will hasten setting. The more salt, the faster the setting. Sugar added to the water will retard the setting. A saturated solution of borax, 1 part of borax to 10 parts water used will retard the setting from 15 to 30 minutes. It also makes the impression harder and sharper. Substances used to hasten or retard the setting should be added to the water before the plaster is added.

Completing the Cast. Before the cast is completely set, it should be properly marked for identification. The identifying data can be scratched into the surface of the cast with any suitable instrument.

As a minimum, the data should contain the case number, date, your initials, and any such other information deemed necessary. An arrow indicating the direction of north will enable you to locate the exact position of the cast in relationship to other evidence found at the crime scene.

When several casts are made at the same location number them consecutively. The number of each cast should be entered in your notebook, along with data describing the original location. To assist the laboratory examiners with their examinations, a number of casts may be taken of different shoe and tire impressions found at the crime scene. Often, a print made by a particular shoe will clearly indicate details and characteristics that are not found in a second print made by the same shoe.

Removing the cast. After the cast has completely hardened, it may be removed for further processing. Plaster usually hardens within 20 to 30 minutes after preparation. During the process of setting, the plaster becomes warm. When it starts to cool, the plaster has hardened sufficiently for removal.

Care should be exercised in its removal. Even though it has been reinforced, plaster is still fragile.

After the plaster has been removed, any excess dirt may be dislodged by washing the cast gently in a pan of water, with a hose, or under a faucet. If a hose is used to wash it off, remove the nozzle since water under pressure may round off edges or obliterate small details. The use of a brush or water under pressure should be avoided to prevent possible damage to details in the face of the cast.

If portions of the surrounding soil are to be collected for submission to the laboratory for petrographic comparison with the soil sticking to the shoes or clothing of a suspect, it maybe appropriate to send the cast to the laboratory without washing. This will provide the laboratory with samples of the soil directly under the shoes of the suspect at the time of the imprint, and may be of aid to the examiners in making a comparison of the soils.