Lili Krakowski on wed 13 sep 06
Pinholing is a misery, with a number of possible causes.
You write that the glazes have done fine on another body, and are ok on the
test tiles. You fired The Other Clay to the same cone
in glazing, but it was NOT mature. The present clay is vitrified. So there
are perceptible differences in the bodies. Fine.
Clay bodies are mixtures of materials. The two bodies in question may
contain
different materials.
A possible pinhole cause comes from organic materials in the body
components, and the fineness or coarseness of the clay.
You might start by
slowing your bisquing way way down, to allow organic materials to
burn out before the surface tightens up too much. We worry, rightly,
about steam from the residual water in the pot plus the chemical
water in the clay. But forget that organic matter
in the clay needs plenty of time and opportunity to burn out.
Otherwise it burrows it's gassy way through the glazes, leaving pinholes.
(Cooper points out that organic matter burning out puts the body into
reduction.)
You say some of the pinholes are like craters. That suggests serious
gas formation, not enough time and temp for the glaze to heal well. Also
some colorants will make a glaze more refractory (needing a higher
glaze firing temp) and some produce quite a bit of gas.
So a change in firing cycle might help.
There is a possibility --well beyond my expertise, however--that an
ingredient in
your clay body--which you tell you were warned not to fire over co.5,
but are taking to co.6--is beginning to decompose at co.6. I would find
that strange, because bodies generally are formulated to do ok on either
side of the cone for which they are designed, but it is possible.
Then the two bodies may accept glaze differently. The pinholes just might
come from the glaze application. The test tiles may do ok because the way
you apply glaze to them is more compatible with the body. Your might want
to experiment with applying the glaze in different ways, and with the glaze
itself, the liquid slop, thicker and thinner. I have found that the
dampness/dryness
of the bisque plays a part in application success.
And then...and let no one tell you this never happens, because every old
potter has war stories--there may be something amiss with the batch of that
clay body that you got. The boxes may have code numbers, check, call, ask
for a box from another batch.
A true story: A potter whose work was extremely prim, proper, pristine used
a just-off-white clay. One day the supplier (now gone, no one
you would know, and certainly not the blessed, wonderful Jack Wolfe)sent
a mislabeled batch of speckled clay. By the time the disaster was noticed,
the speckles had found their way into his recycling bin! Neither apologies,
nor refunds could make up for the time lost, orders deliveries postponed
etc.
Test each new batch before integrating it into the studio....
A hint: Think of your clay body as you would of bread. Think of the kiln
as your toaster. Raisin bread needs different toaster treatment/time than
rye; White Italian bread needs a different time in the toaster than a
bagel...and so
on....
THROWING STICKS are fun, and our very own Ivor wrote a lovely piece in PMI
a while ago--Ivor, please tell us date--about making one's own. Meanwhile
get a few old wooden kitchen spoons at the thrift shop. You can whittle
them to your taste, or use them as is....
The throwing suggestions you have been given all are perfect. When people
say slow the wheel down--they mean SLOW. Mine runs at about 40 revolutions
per minute when I am throwing...a bit more when trimming. Throw as dry as
you can...and leave plenty of clay at the top before you start collaring.
TALENT: A quote from Frans Wildenhain: "Everyone has talent. Nobody wants
to work!" Talent or no talent, what makes the potter is work, work, work,
and work. (Or why I must get going).
STIRRING STORY: The reaction as you have been told would not be to
the manganese...but what are the other ingredients? Could you have a
reaction to
one of them?
Another war story. When I was a young student I mixed a whole bucket of a
wood ash
glaze with my bare arm. I was not attentive to the fact that the ash was
unwashed. My
arm felt a bit funny while I was stirring. It emerged from the bucket with
a bright red
opera-length glove. I had that peeling glove for easily a week--till I
grew new skin.
Ain't used no unwashed ash since!
Lili Krakowski
Be of good courage
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