William Lucius on wed 16 mar 11
From a technological viewpoint the addition of quartz sand to a clay body r=
=3D
esults in significant decrease in vessel wall strength. Imagine a whole lo=
=3D
t of round quartz grains that all of a sudden expand when passing through q=
=3D
uartz inversion during bisque firing. The quartz sand expands enough to pu=
=3D
sh the surrounding clay away. On cooling back the quartz sand shrinks back=
=3D
=3D2C leaving not only a void between the quartz sand grain and the clay=3D=
2C b=3D
ut also a swarm of microfractures that were created by the quartz expansion=
=3D
. Those fractures grow=3D2C especially with use of pots for cooking=3D2C w=
hich=3D
eventually leads to vessel wall failure. As noted in the paragraph below=
=3D
=3D2C prehistoric Pueblo potters crushed igneous rocks (or ground up potter=
y =3D
sherds) because their angularity and lack of quartz bypassed the problem of=
=3D
weak pots.
=3D20
Kathy Hensler and her collogues refer to the cultural preference for crushe=
=3D
d trachyte (a dark volcanic rock) in the Chuska Region of the American Sout=
=3D
hwest. =3D20
"Various technological studies indicate Chuskan ceramics are harder than th=
=3D
eir sand tempered
analogs (see for example Neupert 1993)=3D2C yet strength is less likely to =
re=3D
flect temper
than clay source and firing=3D2C and in itself does not answer the question=
o=3D
f why trachyte was
adopted if clay sources remained the same=3D2C as discussed above. One expl=
an=3D
ation entails a
property of the mineral quartz. Quartz grains when heated at typical cookin=
=3D
g fire temperatures
swell slightly causing detrimental long-term effects on cooking pot fabrics=
=3D
(Rice 1987). This
quartz effect holds for all sand-tempered and crushed sandstone- and quartz=
=3D
ite-tempered sherds=3D2C
but not for trachyte-tempered sherds as this rock type lacks quartz. The po=
=3D
or thermal
performance of quartz sand-tempered ceramics was also demonstrated experime=
=3D
ntally with
sherds from Cove-Red Valley. Multiple lines of evidence in this experiment =
=3D
demonstrated the=3D20
superior heat-resistance of trachyte-tempered as opposed to quartz sand-tem=
=3D
pered sherds
(Hensler 1999). Thus=3D2C we contend that the net effect of trachyte temper=
i=3D
s to increase ceramic
heat-resistance---leading to longer cooking pot use life and probably lesse=
=3D
r breakage rates in
firing."
http://www.nmacweb.org/My_Homepage_Files/Download/chuska_paper%5B1%5D.pdf
=3D20
However=3D2C even serving ware pots usually not used for cooking also exhib=
it=3D
the use of crushed rock or sherd=3D3B so it is not quite as simple as a cu=
lt=3D
ural response to a technological problem. =3D20
=3D20
Of course most studio pottery will never be used for cooking over a campfir=
=3D
e. And most of us include a second glaze firing that I would assume might =
=3D
serve to heal the microfractures by melting them shut. Overfired sandy cla=
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ys develop an unsightly pebbly surface that results from excessive clay shr=
=3D
inkage. Early Pueblo pots with quartz sand temper often exhibit the same p=
=3D
ebbly surface=3D2C suggesting that the potters intentionally overfired thei=
r =3D
pots to decrease the incidence of microfractures. I have found that moloc=
=3D
hite is an appropriate proxy for crushed sherds or ground up trachyte for m=
=3D
y replications as well as my studio pottery.=3D20
=3D20
William A. Lucius=3D2C Ph.D.
Board President and Director Institute for Archaeological Ceramic Research =
=3D
(IACR)
iacr@msn.com
www.instituteforceramicarchaeology.org=3D20
http://www.ourlkcpage.multiply.com
http://www.leuppkilnconferenceorg.web.officelive.com
=3D
C. Tullis on thu 17 mar 11
This is of course is dependent on the flux content, the granular size of =
=3D
the sand=3D20
and the percentage of sand added to the clay body
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