Building Materials

Sometimes a burglar or assailant enters or leaves premises through a window, via a roof or ceiling, or by breaking down or forcing a door. This can produce a range of wide range of trace evidence derived from the building materials used in that particular dwelling. Trace evidence is often invisible and will adhere to the clothing, hair, skin, and footwear of a suspect without the person being aware of it. Forensic examination of the suspect may produce evidence that can link the person to the scene of the crime through the presence of tiny amounts of building materials.

Forensic analysis of building materials covers a wide range of substances, such as brick, plaster, slate, loft insulation, glass, and wood. The broad principles for collecting and examining such materials are the same. The evidence has to be collected from around the site of entry or escape from the scene of the crime, by brushing, taping, picking with tweezers, or vacuuming. The samples need to be stored in a separate unused container and transferred to the forensic laboratory through a careful chain of custody. Examination of the suspect and his or her clothes for matching trace evidence of building materials has to be done with great care and preferably not by the same investigator who was at the scene of the crime. Otherwise, fragments of brick dust or glass, for instance, could be unknowingly transferred to the suspect.

Most building material trace evidence is in the form of fibers or dust. For instance, loft insulation is composed of glass fibers. The first step is to examine the material by eye, in good lighting, and then under a microscope. Various microscopic techniques are used to establish the nature of the material. In comparison microscopy, the sample is compared to known reference samples of various types of brick or plaster. The exact color of the sample can be established by microspectrophotometry.

There are various analytical techniques that can determine the chemical composition of a building material. Forensic scientists may use infrared spectroscopy, neutron activation analysis, or x-ray diffraction as appropriate. The analysis of building material evidence may tell the investigators a great deal about how an entry or exit to a building was made by a suspect. This provides a vital link in reconstructing the events before and after the crime took place.