A Closer Look at Casting Materials
Written by David A. Thornton   

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A closer look at casting materials

MODERN DENTISTRY has helped make mouths healthier, brighter and more attractive—but it has also added a few techniques to the crime-scene investigator’s tool kit as well. Polyvinylsiloxane, dental stone, and alginate casting materials have been adapted from their traditional uses in dentistry (such as making dentures) to the casting of impressions at crime scenes.

Many of the requirements for these products are the same: sterility, fast setting times, ability to render small details, and good releasing characteristics. To provide better contrast for microscopic examination, many of the products are tinted. To get these products to meet the needs of both dentistry and crime-scene investigation, a little chemistry is used to create the needed properties.

Dental stone, made from gypsum (or, technically, CaSO4.2H2O) can be a colorless, yellowish, or white mineral. Manufacturers frequently add dye to tint the product.

The use of gypsum as plaster or cement dates to the ancient Egyptians, Greeks, and Romans. Commonly used by artists, plaster of Paris was named for the ancient quarries found just outside of Paris, France. Gypsum plaster and cement differs from plaster of Paris by its hardness and strength. A process called calcination causes the gypsum to become denser than the materials used in plaster of Paris. The gypsum is heated, under pressure, to about 300°F. Calcium chloride may also be added to make the gypsum crystals more regular and to increase the calcination. The heating draws off about 3/4 of the water molecules in the gypsum.

Anyone who has poured dental stone directly into untreated snow to cast a shoe print has learned that the hardening process is an exothermic reaction that produces heat as the dental stone cures. As the gypsum recrystallizes, the formation of chemical bonds generates heat. The amount of heat created by the reaction is proportional to the amount of dental stone mixed. Spray waxes and other preparations are used to prevent the snow from melting.

Compression strength—one of the reasons that dental stone is the preferred casting material—is directly related to the effort of mixing the powder and water. The manufacturer obtains the material’s hardness information (printed on the side of the bag) under ideal, industrial conditions. Simple hand mixing in the field will not meet the published specifications, so casts can still be fragile.

Getting the consistency correct is also important. The amount of water used to mix the dental stone can affect the results. Too little water will cause the loss of necessary details and too much water affects the overall strength of the cast. The consistency rating, or the ratio of dental stone to water, is associated with compressive strength. A rating of 30 translates to 30 cm3 of water to 100 g of casting material. For tire and shoe tracks, a rating of 30 to 35 is recommended. A rating higher than 35 may lose critical detail when the print is cleaned.

Dimensional stability, tear strength, and quick curing have made polyvinylsiloxane (PVS) popular for casting impression evidence. The material has very good detail capture and easily releases from the surface. At room temperatures, it generally takes only two to three minutes to make the cast. After curing 24 hours, dental PVS casting materials shrink only 1/4 to 1/10 compared to other silicone casting materials, creating reliable casts of impressions. The dental PVS is also less smelly and easier to work with in the field.

PVS was developed in the 1970s as a silicone elastomer, which is a large molecule with the property of viscoelasticity. Silicone and silicon are two distinctly different materials and should not be confused. Silicon is a crystalline element used in semi-conductors and microchips. Silicone, on the other hand, is an inert synthetic compound.

Dental PVS consists of two parts: a silicone-polymer base component, a vinyl polymethylsiloxane crosslinking agent that forms bonds between polymers, and an elemental metal catalyst. Combining the parts causes the mixture to cure and retain surface features. The long bond lengths between atoms allow the material to conform easily to the surface, creating accurate reproductions of impressions. Once the two parts are mixed, kneading the material keeps it pliable until it is ready to be used.

Alginate casting materials—traditionally used to create negative impressions of teeth—can also be used in forensic science to create molds of bite marks in food. It is also sensitive enough to record the ridgelines on fingertips. Alginate is used extensively in the food industry as a food additive in everything from indigestion tablets to processed cheese.

Alginate comes from the sea as an anionic polysaccharide, a form of polymeric carbohydrate found extensively in the cell walls of brown algae. The alginate powder, when combined with two or three parts water to one part powder, forms a rubbery, irreversible hydrocolloid gel. The amount of water used in the mixture affects the handling times, setting, accuracy, and fragility of the mold.

Using alginate casting materials requires pre-planning and quick hands, as the material sets quickly. The item that will be casted should be prepared before the alginate is mixed. It also shrinks as it dries; consequently, it should never be used to make a positive cast of something such as a shoe print. At least one agency discovered that the shrinkage can be significant, as a shoeprint casted with alginate shrunk from about a size ten to a size eight. Only through the diligent efforts of a footprint expert to calculate the rate of shrinkage was the print finally admitted into evidence.

The dental field has provided some excellent materials for crime-scene investigators to use in their search and recovery of evidence. Like all tools in the box, understanding how and why the products are used ensures the best results. Reading the product labels is a good starting point.

About the Author

David A. Thornton is a crime-scene investigation training consultant with Thornton Consulting & Investigation in Thornton, Colorado. He has 17 years of law-enforcement experience, is a professional educator and law-enforcement trainer, and is a Certified Senior Crime Scene Analyst with the International Association for Identification.

 
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