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Depth Of Calcination Measurement In Fire Origin Analysis
By: Patrick Kennedy, Kathryn Kennedy, & Ronald Hopkins Originally Presented at the Fire and Materials 2003, 8th International Conference and Exhibition, San Francisco, CA January 28, 2003 Tel: (941) 351-6409Email Mr. Kennedy Website: www.kennedy-fire.com |
This research project was performed in conjunction with the 2002 National Advanced Fire, Explosion, and Arson Investigation Training Program cosponsored by the National Association of Fire Investigators (NAFI), the Eastern Kentucky University Fire and Safety Engineering Technology Program (EKU), and the National Fire Protection Association (NFPA).
The research was designed to illustrate and, if appropriate, support the system for measuring depth of calcination on fire exposed vertical gypsum wallboard and used in fire patterns analysis as recommended in NFPA 921.
The tests and data collection were conducted in March 2002 on full-scale room fire evolutions, using ten fire investigators of widely varying experience, from novices to full time professionals, to make and record depth of calcination measurements with no previous knowledge of the actual origins of the test fires.
Additional supplemental laboratory bench testing was conducted as background research into the loss of density of gypsum wallboard from heat exposure at the Forensic Fire Science and Technology Laboratories of John A. Kennedy and Associates.
Research test results were good, providing accurate and reproducible fire movement analysis and supporting the calcination measurement techniques, systems, and tools advocated by NFPA 921. Collected data was comparable among participants with widely varied fire investigation experience and after only minimal instruction and practice in the calcination depth measurement techniques and tools.
Calcination is a chemical and physical change in the nature of common GWB produced by heating to temperatures in excess of 80°C (176°F.). This calcination can be defined as: driving out of volatiles (almost exclusively water) from the gypsum component of GWB, in essence, dehydration.3 When heated above 80°C, approximately 75% of the chemically bound water is driven off.4 A chemical change in the nature of the gypsum also occurs during the heating process. The calcium sulfate dihydrate (CaSO4 • 2H2O ) becomes calcium sulfate hemihydrate (CaSO4 • 1/ 2H2O + 1.5H2O) at about 100°C (212°F.) and then anhydrous calcium sulfate (CaSO4 + 1.5H2O) at about 180°C (356°F.).5
Significant mass loss and a corresponding decrease in density occur within the calcined portion of the GWB during the calcination process.6 (This loss of mass and density was re-confirmed in the laboratory bench testing conducted in connection with this research project.)
In NFPA 921-2001 the suggested methods for determining depth of calcination are by "...use a visual observation of cross-sections or a probe survey."9
The research project reported here was designed to investigate whether practical and scientific validation for the traditionally used "probe survey" method could be found and to compare that method to the less often utilized and more cumbersome, visual cross-section method.
"Probe survey" methods have been in use successfully by professional fire investigators for both calcination and "depth of char" analyses since the 1950's.10 Similar to its use in depth of char analysis, it is this difference in density between the calcined andoriginal portions of the GWB that is measured using the "probe survey" method. In general, the "probe" method consists of using a calibrated probe caliper-like device to determine the depth of heat treatment to structural lining surfaces, particularly GWB and charred wood. The instrument is inserted perpendicularly into the surface of the heat-treated material, and by feeling the difference in resistance (density) between the pyrolized and non-pyrolized cross-sections, the relative amount of heat treatment is noted.11 Comparing the relative depths of a series of measurements tells the investigator which points of measurement were more heat treated than others, with the deeper measurements being closer to a single source of heating.
Visual observations of subtle color changes in individually cut cross-sections of the GWB are an alternate method of discerning the relative changes (calcination).12 But the cross-sectioning method has inherent procedural drawbacks to the practical fire investigation. It is considerably more labor intensive, time consuming, and involves perception of sometimes indistinct color changes frequently made more indistinct by the actual process of cutting the crosssection samples under field conditions, or made more confusing by the presence of impregnated smoke staining in the gypsum from the burning paper backing.
Practical experience and the research results of this study have concluded that the "probe survey" method, while encompassing some of the same subjective interpretations as the "cross-sectioning" method, is faster, more practical, and considerably less time consuming and labor intensive. In addition, the "probe" method does less destruction to the evidentiary values of the heat-treated surface. This leaves the evidence in place to be available for subsequent investigations and lessens the possibility of complaints of spoliation of evidence. The analytical results of the two methods are statistically comparable.
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Patrick Kennedy, is the Principal Expert Fire and Explosion Analyst for John A. Kennedy and Associates, the world's oldest established fire investigation firm. With over forty-five years of professional experience, Patrick Kennedy is also the senior-most active fire analyst, with more years of experience than any other active fire investigation professional.
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