The effects of thermal alteration on saw mark сharacteristics
Brouchoud, Jordan Elizabeth
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This project examined the effects of burning on saw mark characteristics of isolated semi-fleshed white-tailed deer (Odocoileus virginianus) long bones as a substitute for human remains. Different classes of saws were examined to determine which type of saw mark characteristics are obliterated when burned and which are not. The saw mark characteristics that were examined are superficial false start scratches, false start kerfs, and completely sectioned cuts with breakaway spurs/notches. The long bones were burned at the Boston University School of Medicine using a muffle furnace, at differing temperatures and at differing time increments. The author hypothesized that the higher the temperature and the longer the duration of bone burning, the greater will be the obliterative effect on saw mark characteristics. All samples were examined using a Motic® Digital Light Microscope 12 VDC with a Nikon® MKII Fiber Optic Light attached with accompanying Motic® imaging and measuring software. Distances were measured between striations on complete cuts, false start kerf widths, and false start scratch widths using the Motic® imaging and measuring software. Images were also taken of the cross sections of the kerf floors. The striations on the kerf walls, false start kerf widths, and false start scratch widths were compared to the control samples. Measurements taken from false start scratches, false start kerfs, and complete cuts were averaged and compared to the averages from each temperature and the control samples, to assess the degree of shrinkage from thermal alteration. The false start kerf profile shapes were blindly examined and classified into Class A, B, C, or D(following the system of Symes 1992) and compared to the control samples. Kerf flare and blade drift were examined to determine if thermal alteration obliterated those saw mark characteristics. The chainsaw false start kerfs and complete cuts were examined macroscopically to determine what effects thermal alteration had on those types of marks. All thermally altered samples were assessed for color change, heat-related fracturing, and whether or not the saw marks were still visible. The author found that all saw marks made with the mitre saw, crosscut saw, and bow saw were still visible and identifiable, even in a fractured state and, when burned up to 700°C for one hour. Most of the false start kerf samples were classified into the correct kerf profile shape as outlines in Symes (1992). False start kerfs and complete cuts made with the chainsaw were blindly examined and showed that these marks are distinct and easily identifiable when the bone is completely intact or has very minimal fracturing. The crosscut saw false start scratch and crosscut saw complete cut samples showed signs of shrinkage. The average width of the false start scratch samples burned at 700°C for one hour was about 50% smaller than the control sample's average width. The same was true for the complete cut striation widths. Shrinkage did not appear to alter the crosscut saw false start kerf widths or the bow saw false start scratch widths. For all cuts made with the mitre saw shrinkage did appear to alter the samples. Warping did occur where some of the burned averages were larger than the control sample averages. This suggests that some warping did take place by widening the kerf, thus changing the analysis of the saw mark characteristics. Blade drift and kerf flare were seen in the samples and thus were not affected by thermal alteration. The author's hypothesis was not rejected, because in some cases thermal alteration did modify the saw mark characteristic measurements and in some cases thermal alteration did not alter the measurements. Some of the saw marks were affected by shrinkage, while others were not. False start kerf profile shape classification was not affected by thermal alteration. The chainsaw samples were affected most by the thermal alteration, because of the obliterative effects of heat-related fracturing which progressed generally with the greater temperature.