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pinholes on slipped pots

updated wed 23 may 12

 

Lili Krakowski on tue 22 may 12


Tracey Duivestein writes (and I edit!):

"[I get]... glaze pinholes when I apply glaze over thick slip... I =3D
tried 3 types of clay with 15 different glazes with the
same results. Most test cylinders without the slip are pinhole free.

" The slip is ... same body as the thrown cylinders (using
borehole water & sieved through a 60 mesh sieve) & applied just after
turning. ....> I have used different firing schedules, increased the =3D
bisque temperature
(from cone 06), mixed various glazes (boron frits & zinc) & used =3D
various
glaze thickness without any success."

My guess is different from Ron's.

You have X clay body, and then you add a slip made of X. I assume with =
=3D
some additives.
But the slip is applied thickly. So your clay body gives off gases as =3D
it is bisqued, then slip plus
body give off more gasses as the pot is glaze fired.

BUT the slip is of finer particle size than the body, because you say =3D
you sieve it.
Which suggests the slip is denser, "tighter" than the body alone,
and makes passage of gases more difficult. And, as you say the slip is =3D
applied thickly,
and I assume the additives to the slip are fluxes of some sort, the slip =
=3D
is very dense
and gases cannot escape freely.

Actually I would look at the clay body. You say some of the =3D
unslipped pots
also pinhole...A reason to look at the clay body.

When you say "I tried 3 types of clay" do you mean for the pots or for =3D
the slip?

Maybe the time has come to try a different slip recipe? That actually =3D
may work..

PS: The general assumption is that pots give off gasses only in the =3D
early stages of firing...
At lower temps...But in my limited experience there are materials that
begin to "gas" only at higher temps...which, when we fire at c.6 have =3D
finished their
gassing out quite yet. Tell us, please: Is the clay body you are using =3D
a true c.6 body,
or is it one of those labeled from c.1 to c8 ? =3D20

A helpful test might be to check body vitrification...if the body has =3D
more absorbency
that 1.5% the problem might be the body has not matured enough
to guaranty no more materials giving off gas.

Please let us know...





Lili Krakowski
Be of good courage