Rod, Marian, and Holly Morris on tue 8 aug 00
I seem to be having more trouble than usual with crazing on my terra =
cotta work. I am using a very reliable ^06-2 commercially made terra =
from Runyan in Flint Michigan. I am using a variety of clear glazes =
(commercial and some home-brews) over slips and underglazes. The problem =
seems to occur no matter what is beneath the clear. I even tried =
commercial casting slip where I do slip over the terra, since I figured =
it would be compatible with the commercial glazes. I have tried bisque =
firing to 04 (and I know areas of my small kiln go higher), which nicely =
toasts the terra. Also, have fought the demon of impatience on opening =
the glaze fire in order to slow down the cooling.=20
Do I need to work on slowing the bisque firing down? How can I =
accomplish this with my non-automatic electric kiln? I usually fire 2-3 =
hours on lo, 2-4 hours on 4.5, and then shoot it up to 6, which usually =
lasts 1-2 hours, just as the manual says. How does one accomplish a soak =
when the cone shuts the kiln off?=20
I really want to get those craze lines out, since I am working on usable =
dishes.
Jonathan Kaplan on wed 9 aug 00
>I seem to be having more trouble than usual with crazing on my terra =
>cotta work. I am using a very reliable ^06-2 commercially made terra =
>from Runyan in Flint Michigan.
What is reliable? While for your work this may be the case and I am sure it
is, one needs to understand the nature of low temperature clay bodies at
the earthenware range. Using ball clay, perhaps Red Art for Terra Cottas,
some talc, it is possible to produce clay bodies that at that temperature,
work, mostly. The problem is in the fluxing of these bodies. Talc is ok but
rarely produces the glass needed for a vitrified body. Most terra cotta and
white earthenwares are poorly fluxed and prone to delayed moisture crazing.
If you make your own clay bodies, the addition of a small percentage of
frit 3124 will greatly help vitrify the body and greatly help with the
glaze fit.
I am using a variety of clear glazes =
>(commercial and some home-brews) over slips and underglazes. The problem =
>seems to occur no matter what is beneath the clear.
The problem can be attributed to either the clay body (above) or in the
glaze. Most clear galzes at this temperatre are frit and EPK. There are
many variations of this. It is dificult to match the expansions of both the
clay and the glaze to avoid both crazing and shivering. The trick is using
a combination of low expansion frit with other low expansion materials,
which at this temperature is dificult.
>Do I need to work on slowing the bisque firing down?
WIll prbably not make any difference what so ever.
How does one accomplish a soak =
Turn the switches down for about 15-30 minutes. A pyrometer will greatly
help maintain your temperature. But even so, the fault is in the clay and
glaze and altering the temperature will not help.
>I really want to get those craze lines out, since I am working on usable =
>dishes.
The earthenware range is indemic with problems. Yes, they can be solved. We
work in this range and over the years have also struggled with grazing and
shivering, and solved the problems by working with the glaze and modifying
over the years to have a wide firing range and incorporating other
materials into the glaze.. But to summarize, clay bodies at this temperatur
are weak and not vitrified. The glaze also does not help produce stronger
ware, as it should being under a slight compression.
I would suggest that you move into the cone 3-5 range. We press, cast, and
throw both terra cotta and white clays in this range with great success.
If you wish, contact me off the list if you have any other questions. I
would be happy to help.
Jonathan
Jonathan Kaplan
Ceramic Design Group LTd/Production Services
PO Box 775112
Steamboat Springs, CO 80477
(970) 879-9139 voice and fax
http://www.sni.net/ceramicdesign
UPS: 1280 13th St. Unit13
Steamboat Springs, CO 80487
ferenc jakab on thu 10 aug 00
I really want to get those craze lines out, since I am working on usable
dishes.
____________________________________________________________________________
__
Rod et al,
The crazing is because the glaze and the clay or slip under it have
different co-efficients of expansion. The only way to cure this is to ensure
that clay and glaze have similar expansion. There are people on the list who
could help with this if they had an analysis of both the clay and the glazes
and the slip. Using commercial glazes and slips is really a hit and miss way
of doing it.
If you can forward these analyses I can run them through my Matrix Glaze
calc program but I don't have the expertise that Ron Roy has for instance. I
think every potter should familiarise them selves with Seger formula and
learn to apply those principle to their own glaze formulation practises.
Potters who can't formulate glazes to match their clay body are like doctors
who can't use a stethoscope.. Sorry! but that's how I feel about the
degeneration of rigorous knowledge in all the crafts and trades.
Feri.
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