Heat Treatment of microcrystalline quartz
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Photo of conchoidal fracture in quartz from:
http://dn.redwoods.edu/coursenotes/renner/geo_images/rocks_minerals/minerals/conchoidal-fracture.jpg
Heat treatment of flint very often makes it much easier to work. Longer, thinner flakes can often be struck from the core, and the stone becomes more glassy, with a greasy look to the conchoidal surfaces left after a flake has been struck.
Text below is an excerpt from the paper:
Heat-Treating Experiments With Onondaga Chert:Preliminary Results
by Frank L. Cowan
which may be viewed at:
http://wings.buffalo.edu/anthropology/Lithics/Files/thermal.pdf
Various investigators have used scanning electron microscopy to compare
the fracture surfaces of heated and unheated cherts (Draper and Flenniken 1984; Flenniken and
White 1983; Johnson 1985; Olausson and Larson 1982; Purdy 1974; Purdy and Brooks 1971;
Robins, et al 1978). In comparing fracture surfaces produced before and after heat-treatment, they
find that heat-treated cherts fracture to form much smoother surfaces. In unheat-treated cherts,
fracture fronts appear to pass between the quartz microcrystals, breaking the relatively weak bonds
holding the crystals together. At the high magnification used in SEM, these surfaces appear
topographically rough. In heat-treated cherts, however, the fracture appears to pass through many
of the microcrystals, leaving a much smoother fracture surface. The smoother surface reflects light
more evenly and produces an increased surface luster or "greasy" texture that is macroscopically
visible and has served as an indication of thermal alteration to many investigators (Bordes 1969;
Crabtree and Butler 1964; Rick 1978).
The physical/chemical changes that take place during heat-treatment are poorly understood.
Purdy and Brooks (1971) have suggested that impurities (particularly iron compounds) within the
microcrystalline quartz latticework may reach a eutectic melting point. They suggest that "minute
amounts of impurities (or compounds of the elements making up the impurities) in the
intercrystalline spaces of the chert are probably acting as fluxes (substances promoting fusion) to
fuse a thin surface film on the cryptocrystals. This fusion occurs when the melting point (eutectic
development) of the impurities is reached... Binding of the microcrystals results in a more
homogeneous material with the ability to fracture like glass rather than like a rock aggregate"
(Purdy and Brooks 1971).
3
Flenniken and Garrison (1975) have countered with the observation that the Purdy-Brooks
hypothesis depends upon particular mineral impurities which are not universally present in chert or
other materials which respond positively to heat-treatment. They suggested that interstitial moisture
present in the crystalline structure would expand forcibly as a gas with heating, and water vapor
would escape by micro-fracturing the crystalline latticework. Several observers have noted that
heat-treating results in small but measurable weight loss probably due to the loss of water
(Mandeville 1973; Purdy and Brooks 1971).
More recently, Schindler, et al. (1983) have offered another mechanism whereby heat-treatment
reduces the tensile strength of Bald Mountain jasper, although they do not claim that it as a
mechanism generalizable to a larger class of materials. In heat-treating the central Pennsylvanian
jasper to approximately 275 degrees C., Schindler, et al. noted that the jasper undergoes a
distinctive color change from yellow to deep red. Concurrent with the color shift, they observed
that there is a 50% reduction in point-tensile strength. In determining that the color transformation
was the result of a recrystallization of goethite to hematite, a denser material, they noted that the
volume reduction of the iron compounds left small "channels" permeating the reddened stone.
They reasoned that these small voids were the cause of the reduction in tensile strength, and
hence the thermal alteration mechanism for Bald Mountain jasper.
The last hypothesis, aside from positing a unique mechanism for a unique material, suffers from
a flawed research design. All of the jasper samples heat-treated and tested were very small (1 cm)
cubes (Schindler, et al. 1983:528). It is typical for heat-treated jaspers to undergo a dramatic color change only near the heated surfaces; reddening seldom penetrates more than a few millimeters
into the mass of the rock. Although unreddened, heat-treated jasper within larger stone masses is
nonetheless thermally altered as clearly demonstrated by reduced fracture toughness and
increased luster of fracture surfaces. The direct equation of color change with the alteration of
fracture characteristics forced Schindler, et al. (1983:537) to advance ludicrous technological
interpretations, such that large cores or bifaces would have to be heat-treated more than once
during the reduction process.
In most cherts (and other varieties of microcrystalline quartz and some quartzites), successful
heat-treatment usually involves raising the temperature of the stone to approximately 275 degrees
C., and maintaining that temperature for a period of several hours (Flenniken and White 1983;
Mandeville and Flenniken 1974). The critical temperature at which thermal alteration occurs and
the length of time the material must remain at that temperature is apparently quite variable with
differing kinds of materials. Flenniken and Garrison (1975) have found that Arkansas novaculites
require very high temperature curves, with thermal alteration only successfully occurring with
sustained temperatures of 450-500 degrees C. Purdy and Brooks (1971) reported that color changes
consistently took place in Florida chert heated to between 240-260 degrees C. when iron was
present in concentrations of at least 1,100 parts per million. Critical temperatures for effecting
textural changes were 350-400 degrees C. Rick (1978:60) reported that Burlington chert underwent
color changes (apparent iron oxidation) at approximately 230-290 degrees C., but that textural
changes were not apparent at less than 290-370 degrees C. Personal experimentation has
indicated that Burlington cherts may be successfully heat-treated at much lower temperatures.
Sustained temperatures of 275 degrees C. appears to be sufficient, although higher temperatures
will produce higher, more vitreous lusters. My experiences with heat-treatment suggest that
generally more vitreous materials, and darker colored cherts and flints require less heat than lighter
colored cherts, and, in fact, many high quality materials can not survive the high temperature
curves suggested in some of the literature. This observation is concordant with the experiences of
other knappers experimenting with heat-treatment (Crabtree and Butler 1964; Flenniken, personal
communication, 1984; Rick 1978:61).
The upper temperature limit of successful heat-treating is definitely fixed at 573 degrees C. (at
one atmosphere of pressure), the low quartz/high quartz inversion point (Hurlbut and Klein
1977:411-412). At 573 degrees C., at one atmosphere of pressure, quartz undergoes an
instantaneous and reversible transformation of crystal structure. "This displacive transformation
from low to high quartz involves only minor atomic adjustments without breaking of Si-O bonds.
Upon cooling high quartz, through the inversion point at 573 [degrees]C, Dauphine twinning may
be produced" (Hurlbut and Klein 1977:415). Although Hurlbut and Klein (1977:411) report that the
inversion may be repeated "over and over again without physical disintegration of the crystal", this
temperature does represent the apparent threshold beyond which microcrystalline quartz
aggregates can not cross without disintegrative effects. Whether because of the reversible
alteration of individual crystal symmetry or the morphological transformation to Dauphine twinned
low quartz, the effect is that a rock unit of chert no longer has any structural integrity, and tends to
become friable and sometimes chalky and will crumble rather than break conchoidally with any
applied force. Several investigators have noted this decrepitation (Mandeville 1973; Mandeville
and Flenniken 1974; Purdy 1974, 1975; Schindler, et al 1982), although apparently some have not
been aware of the crystalligraphic basis for it.
The controlled nature of successful heat-treatment becomes apparent with the consideration of
temperature change rates. Rapid heating results in explosive fractures as excess moisture trapped
in voids, fissures, pockets, and in the interstices between microcrystals vaporizes. Differential
expansion of the brittle material results in characteristic "potlid" fractures. Overly rapid cooling
results in reticulated fractures or "crazing". Rapid cooling and uneven mass shrinkage causes
distinctive "reticulated" fractures which occur abruptly across the thickness of a chert blank and
often leave a rippling surface across the fracture oriented to the broader dimensions of stone (Purdy
1975).
Conceptually and observationally, the distinction between heat-treatment and fortuitous burning
is essential. Poorly controlled heat-treatment attempts can produce all of the fractures discussed
above. Nonetheless, the abundant burnt and pot-lidded chert debitage and artifacts recovered
from most archaeological sites pertains to fortuitous spatial congruences of debitage and fires
burning for other purposes, congruences which are not necessarily the result of synchronous
activities or deposition. Post-depositional fires, whether humanly caused or the result of natural
processes, may alter cherts, although the results are most likely to result in characteristc fractures of
thermal shock, 'smoked' surfaces or structural deterioration to blue-black color and chalky textures.
The distinctive textures of controlled thermal alteration for technological purposes occur on
surfaces flaked after heat-treatment. This is not to say that heat-treating attempts will always be
successful, as the following experiments with Onondaga chert will show. Nonetheless, once the
appropriate heat-treating conditions and variables for particular materials are learned and
understood, it would be likely that the frequency of unsuccessful heat-treating attempts would be
low. If the heat-treatment of chert were a highly risky undertaking, with the likely loss of time, labor
and scarce raw materials, it would be unlikely that it would be pursued as a maximizing strategy.
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Photo of conchoidal fracture in quartz from: