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• alteration experimants by Barbara Purdy (1974) on Florida charts have <br />produced some insights into the effects of theanal treatment which are <br />relevant to this discussion. <br />Purdy confirmed her initial expectations that the heating of microcrys- <br />tallix~e rocks does alter their chemical properties so as to make than <br />easier to flake. She also specified a rnanbPS of attributes (color change, <br />vitreousness, or "potlid" fractures) which were good clues that thermal <br />alteration had occurred but was unable to isolate any ci~aracteristics <br />whidz indicated unequi.vocably that alteration had occ~uzed intentionally. <br />She indicated that "a forest fire or hearth situation" was as likely to <br />alter the material as was an intentional exposure to heat (Purdy, 1974, <br />p. 52). However, one of the major contributions of her study was, as <br />she relates, accidental. <br />In one of the experiments, some chart materials literally "exploded" <br />• while being heated. In subsequent experiments to determine the cause of <br />this rock failure, Purdy (1974, pp. 41-42) discovered that <br />"Explosion occurred on all occasions at 400° C when the <br />material was taken to 400° C without allowing the tempera- <br />ture to be raised slowly or at least leaving it at 350o C <br />for an extended period." <br />And further, <br />"When the temperature was raised rapidly a crackling <br />noise (decrepitation) was often heard at 350° C and <br />always heard at 400o C when the material was removed <br />fran the oven without allaying it to first cool." <br />The debris produced fran these "explosions" evidences "...potlid fracturing <br />and blocky, angular flakes with no bulbs of percussion," and "resembles <br />exactly what one would expect to result from too rapid expansion and con- <br />traction as might occur in a forest or if a rock had been placed in or <br />neaz a hearth" (Purdy, 1974, p. 41). <br />These findings are important because of the general similarities between <br />the resultant debris from intentionally rapid heating of rocks and much <br />• of the debitage recovered at 5ILC139. This latter miaterial is characteris- <br />tically blocky or angular with no discernible bulbs of percussion and many <br />47 <br />