Lili Krakowski on wed 26 jan 05
I am puzzled as to why I never read about replacing kaolin or ball clay =
with pyrophilite in a glaze to reduce crazing. Now that lithium and =
lithium spars are considered no-nos why is no one talking about using =
potassium carb, calcined? Rarely if ever do I see a recommend of "just =
calcine some of your clay." Wot am I missing?
(I have used pyrophilite and calcined clay in glazes. Never yet =
calcined pot.carb)
Lili Krakowski
Be of good courage
Ivor and Olive Lewis on thu 27 jan 05
.<< Now that lithium and lithium spars are considered no-nos why is no
one talking about using potassium carb, calcined? >>
Dear Lili,
An interesting question, but one which raises others.
When were we told, and who told us, that we were not to employ the
Lithium Felspars?
Let's take the Potassium Carbonate part first. How much do potters
know about the properties of this material ? Very little I would
suspect but you might be interested to learn that it is deliquescent,
that is, it reacts with atmospheric water to form a highly alkaline
solution. So lightly calcining it below its melting point would dry
it out. Contrary to the belief that all carbonates decompose to
release their metallic oxides when heated Potassium carbonate does
not, at least not a the working temperatures of most potter's kilns.
The anhydrous material melts just under 900=BA C and in that condition
will attack free silica and hot pottery, as does Sodium Carbonate.
As Mel remarked on today's posting, we should discard the myths. There
are others we might talk about.
<clay with pyrophyllite in a glaze to reduce crazing>>
Pyrophyllite. Interesting material. Another Hydrated Alumino Silicate
but with twice the volume of Silicon dioxide when compared to Kaolin.
Does it have problems. I would not know. It could be that amounts
needed to eliminate crazing may be high enough to significantly change
the character of the glaze. Did you think about that?
It is classed as "infusible" by Dana who says it "Exfoliates like
vermiculite" when heated . This might cause a coating that contained
it to crack away from the surface of a pot. So if it were
pre-heat-treated to de-hydroxylate it why not use it as a glaze
ingredient. Whether this would be an advantage over adding free
silica, which is one of the ways recommended to reduce the incidence
of crazing I would not know. A lot will depend on the decomposition
reactions and the solubility in whatever is causing a glaze to melt.
I would anticipate that the best way to find out is to prepare some
samples which incorporate Pyrophyllite as an ingredient and send them
to Ron Roy for testing.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Daniel Semler on thu 27 jan 05
Hi Lili,
Got me again :) From such a short email, so many questions arise. I pulled a
bunch of references on these but to no real avail.
> I am puzzled as to why I never read about replacing kaolin or ball clay with
> pyrophilite in a glaze to reduce crazing.
I looked up pyrophyllite and it seems that these are low/non-alkaline clays. So,
while using them to reduce expansion would work, a straight sub would alter the
flux balance, wouldn't it ? Have you ever substituted a very large proportion
in a glaze ? If so, did the glaze change noticeably ? It seems it is more often
mentioned in connection with adjusting expansion in bodies.
> Now that lithium and lithium
> spars are considered no-nos
Are lithium spars a no-no ? I realise that lithium carb. has some toxicity, but
the MSDS on spodumene seems no worse than other spars. Is there another reason
?
> why is no one talking about using potassium carb,
> calcined? Rarely if ever do I see a recommend of "just calcine some of your
> clay." Wot am I missing?
I don't know how calcining some of the clay works, though I have a vague
recollection of hearing it somewhere. Can I find a reference ? No. So when you
use calcined clay in place of some of the raw clay how much do you sub ? Is it
1 gram of calcined replacing 1 gram of raw ? If so then I can understand why
this might have a benefit. If not ....
>
> (I have used pyrophilite and calcined clay in glazes. Never yet calcined
> pot.carb)
On the potassium carb, it seems that calcined or not it would only reduce
crazing if it were replacing soda ?
Apologies for the dissection, but I'm studying crazing at present and I'm
curious as always.
Thanx
D
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