Carole Fox on tue 1 aug 00
Dear Clayart Friends,
This has really been bugging me...Can anyone explain to me why terra =
sigillatta loses its burnish at high temperatures? It doesnt make sense =
to me because everything else just gets shinier as the temp. goes up.=20
Thanks for your time!
Carole - in Elkton, Maryland, feeling very indecisive about where to =
start my digging for a pit-fire
R12396@AOL.COM on tue 1 aug 00
Carole,
My guess would be this. polishing or burnishing compacts fine clay
particles, smoother equals reflective...heat expands..moving clay particles
further apart...rougher...less reflective equals duller...?What do you think?
Rich in hot Sacto.
Tom Buck on tue 1 aug 00
Carole Fox:
You ask: why does a terra sigillata not go glossy if overfired?
Take a moment or two to consider what TS is... extremely small particles,
and most often they are clay-only particles, ie, SiO2+Al2O3. TS particles
are certainly smaller than one-millionth of a metre (yard), more likely 10
to 100 times smaller. These ultrafine particles arise by the method of
preparation, namely, by sedimentation (letting a thin slurry of clay
settle for many hours), then taking only the middle layer as TS.
The upper layer is decanted and discarded, then the middle layer
is decanted and set aside as TS, and the bottom layer of coarse particles
is then discarded (or otheriwse used in potworks). One doesn't fuss too
much about getting a TS layer with sharp boundaries.
Others on Clayart have said they use a thin slurry of TS (density
1.12 -1.15 grams/millilitre) applied to biscuit (bisqued in the range of
C010 to 07). They are some potters who apply TS to airdried ware and then
single-fire; but most prefer the less risky method applying TS to biscuit.
TS people vary in how many coats they apply to the pot, from 3 to 5 seems
common.
Let's assume you apply TS to soft-biscuit (C010), using 3 coats.
The fine TS particles fill in the hollows between the larger somewhat
fused claybody peaks and valleys. Now, fire to C06 perhaps C04 for hard
biscuit. The pot undergoes more shrinkage, another 4-6%, and the fine TS
particles tend to coalesce into very thin, well-fused areas inside the
valleys now closer together.
Ok, now consider what is most likely to happen if one goes beyond
C04, say to C1: the claybody gets closer to its slump point, shrinkage
increases, and the TS particles get incorporated into the body itself. So,
when cooled the pot will exhibit a dry matt surface, with the TS effect
long gone.
For TS to exhibit maximum effect, the claybody should be an open
one (ie, lots of coarse particles), and the firing of the biscuit should
be within a narrow range, the actual top Cone being dependent of the
claybody's properties.
Ok, you TSers, give this idea a good going over.
til later. Peace. Tom B.
Tom Buck )
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
John Guerin on tue 1 aug 00
In a message dated 8/1/00 1:18:25 PM Central Daylight Time, cfox@DCA.NET
writes:
<< Dear Clayart Friends,
This has really been bugging me...Can anyone explain to me why terra
sigillatta loses its burnish at high temperatures? It doesnt make sense to me
because everything else just gets shinier as the temp. goes up.
Thanks for your time! >>
The explanation that was conveyed to me is that since clay is composed of
microscopic platelets of silica, the burnishing action compresses the clay
surface and aligns the platelets in parallel so that they present a solid
surface that reflects the light. The problem is that when the clay vitrifies
at high temperature (~900 degrees F) the platelets tend to warp. The
'cupping' action of the platelets decreases the reflective factor
significantly. However when I was studying with Mark Tabho, a Hopi-Tewa
Potter, he showed me an old Indian trick to restore the shine. They apply a
coating of liquid floor wax (acrylic) after the pot is fired. It dries almost
immediately and can be buffed with a soft cloth. This only works on a
burnished surface where the pores of the clay surface have been sealed.
JASCO TILE AND GROUT SEALER (available at Home Depot) works also and is more
durable.
John Guerin
Tucson, AZ
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