Lili Krakowski on mon 8 jan 07
I do not have a digital kiln. Nor a pyrometer. I do not know how cones and
pyrometers relate.
Having said which.
I do not know why you had pinholing, Gay, and so cannot opine! (ah, yes,
there are times like that.) Pinholing can be caused by overfiring as well
as underfiring, and it can be caused by the body. If you fire to 2185 F and
Orton c.6 is 2246 the appearance is that you are underfiring. (So I
wonder: is your pyrometer correct? Could it be off?)
Question is: underfiring WHAT? When you go to the store and you try on
clothes, you may find that in one brand the size 6 jeans fit, in another it
is a 8, in another a 10. (These are fictional sizes I am told some actual
people wear, but I do not believe it!) When you buy clothes the one given
is your body. What is best for it. When you fire a glaze the criterion is
what "fits" that glaze.
Back to glaze. If you are firing by your cones then it is what the glaze is
doing when the cone has bent properly that matters.
If you are firing by your pyrometer then you must go to the temp on the
pyrometer where your glaze is "done".
I believe in cones. I have found them very reliable, trustworthy and like
that.
Now as to the pinholes. Ok. They are caused as a rule by escaping gases.
Fine. Gas can escape from the body --either because it has not been bisqued
high enough, or because something in it has not finished gasing out when you
turn the glaze kiln off. This is a special problem where glaze is applied
thickly. If you have ever been in a crowded bus or subway and tried to
elbow your way to the door from the back, but not quite made it, you only
can get off at the next stop. If a material wants to get out and the glaze
is too thick to let the gases out at the correct stop....
Again I repeat my endless nag: test all glazes on at least three bodies.
Why would overfiring cause pinholing? Because a material that does not
decompose down here starts to decompose at higher temps.
Soaking at max temp, or cooling slooooowly can correct pinholing. I believe
very strongly in heating slowly. Soaking and slow cooling allow pinholes to
heal over. Slow heating allows the gases to escape at a more relaxed pace.
And last--something I know nothing about. Slip cast bodies can create
pinholing. But someone who knows about slipcasting will have to explain.
Lili Krakowski
Be of good courage
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