Scott Nelson on tue 9 nov 04
Hi all, (I posted this message a couple days ago on rec.crafts.pottery,
but have since found this forum so I thought I'd post it here as well...
some nice folk there suggested underfiring as the culprit.)
I'm having a hard time with a store bought clear glaze (cone 6). I bought
this glaze in powder form and mixed it myself. Since then I'm having random
problems with it, usually on big bowls with lots and lots of work invested
in them.
Problem is: milky white translucent spots develop in place where I guess the
glaze
is to thick, except we've been extremely carefull about it not being to
thick. What
would cause a glaze to be so extremely sensitive to thickness? Or is it a
combination
of the firing conditions and this specific glaze? I spoke to a local potter
about it and
he said he makes his own clear glaze and never sees this problem but has
seen it with
store bought glazes before. I posted some pics for anyone crazy (or nice!)
enought to
have a look I would sincerely love any advice or comments...
www.dramaworks.ca/badpots/badpots.htm
Thanks,
Scottn
Ivor and Olive Lewis on wed 10 nov 04
Dear Scott Nelson,
Without knowing the composition of a glaze it is always difficult to
give definitive answer but three ideas come to mind.
If the translucent areas have a blue cast this may be due to the Boron
Oxide content of the glaze, assuming that boron frit is an ingredient.
this seems to be indicated by your images.
Or, where the coating is thick, cooling is being retarded allowing
some micro crystallisation of either Calcium silicate or Magnesium
silicate, again depending on the composition of the glaze.
Finally, if temperature rise was curtailed before all of the
ingredients had dissolved there might be some residual cloudiness that
could be cured by increasing the dwell time at your maturity
temperature, allowing more heat to soak into the thicker glaze areas.
BLISTERS
I think you are trailing a high Iron slip. If so the Iron oxide is
beginning to decompose and evolve Oxygen. Since the glaze is molten it
is tending to prevent this gas escaping, hence, the blisters.
Suggestion to remedy this is to cool slowly , keeping the glaze in the
fluid state until the reaction ceases and the bubbles smooth out.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "Scott Nelson"
To:
Sent: Wednesday, 10 November 2004 4:36
Subject: Commercial clear ^6 glaze turns milky in spots??
> Hi all, (I posted this message a couple days ago on
rec.crafts.pottery,
> but have since found this forum so I thought I'd post it here as
well...
> some nice folk there suggested underfiring as the culprit.)
>
> I'm having a hard time with a store bought clear glaze (cone 6). I
bought
> this glaze in powder form and mixed it myself. Since then I'm having
random
> problems with it, usually on big bowls with lots and lots of work
invested
> in them.
> Problem is: milky white translucent spots develop in place where I
guess the
> glaze
> is to thick, except we've been extremely carefull about it not being
to
> thick. What
> would cause a glaze to be so extremely sensitive to thickness? Or is
it a
> combination
> of the firing conditions and this specific glaze? I spoke to a local
potter
> about it and
> he said he makes his own clear glaze and never sees this problem but
has
> seen it with
> store bought glazes before. I posted some pics for anyone crazy (or
nice!)
> enought to
> have a look I would sincerely love any advice or comments...
>
> www.dramaworks.ca/badpots/badpots.htm
>
> Thanks,
>
> Scottn
>
>
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