Lili Krakowski on tue 10 jun 08
Come now, Ivor! ClayArt has been horsing around re: how to ask nicely,
pretty please, but this is going too far!
"I wish to take advantage of your vast historical knowledge of the
processes and materials used by potters. Perhaps you have an answer to
my inquiry."
For one: we are of an age, and have, probably, been in clay equally long.
And you have a far better clay library.
But beyond that. I know infinitely less than the actual scientists among
us.
All I know is that alumina always is considered the "sticker" --a term I
first encountered in Andrew Holden's "The Self-Reliant Potter"--Without
alumina the glaze would run off the pot...or why problems with crystal
glazes of necessity low in Alumina.
Now what exactly alumina DOES, how it blends into the glass is a mystery to
me. I do know it makes a glaze more refractory: but also know
refractoriness could be achieved with other materials.
Sorry. Good luck. Stick to it!
Lili Krakowski
Be of good courage
John Hesselberth on tue 10 jun 08
Hi Ivor and Lili,
Ivor, don't you just love Lili's spunk. You are not going to
successfully pull her chain without having yours pulled in return.
But assuming your question was genuine and not just baiting some of
us who can't resist swallowing the hook--for which you do have a
reputation--, I would offer the following:
Hamer and Hamer say, in the 3rd edition, page 6, that "the role of
alumina is to act as an intermediate oxide linking the alkaline and
acidic oxides in stable glazes". They show a nice illustration of
this on page 7. Now in the world of polymers we would call this a
cross-linking agent and it seems to me that term would also be
appropriate in the world of ceramics. Cross linking will increase the
viscosity of a material and, hence, alumina's reputation that it
keeps the glaze on the pot. That is also probably why it is called a
"stabilizer". But if alumina from the body cross linked with silica
in the glaze it would also serve as an "adhesive" chemically bonding
the two together. I suspect this is the way Obstler is using the term.
Regard,
John
On Jun 10, 2008, at 11:07 AM, Lili Krakowski wrote:
> Come now, Ivor! ClayArt has been horsing around re: how to ask
> nicely,
> pretty please, but this is going too far!
>
> "I wish to take advantage of your vast historical knowledge of the
> processes and materials used by potters. Perhaps you have an answer to
> my inquiry."
>
> For one: we are of an age, and have, probably, been in clay
> equally long.
> And you have a far better clay library.
> But beyond that. I know infinitely less than the actual scientists
> among
> us.
>
> All I know is that alumina always is considered the "sticker" --a
> term I
> first encountered in Andrew Holden's "The Self-Reliant Potter"--
> Without
> alumina the glaze would run off the pot...or why problems with crystal
> glazes of necessity low in Alumina.
>
> Now what exactly alumina DOES, how it blends into the glass is a
> mystery to
> me. I do know it makes a glaze more refractory: but also know
> refractoriness could be achieved with other materials.
>
> Sorry. Good luck. Stick to it!
>
>
>
> Lili Krakowski
>
> Be of good courage
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ivor and Olive Lewis on wed 11 jun 08
Dear Lili Krakowski,
Thanks, a reply I appreciate.
Yes, I was counting back time to be amazed that is now fifty years
since I constructed my first potter's wheel.
Back then there was a notion that a clay body had similarities to its
human counterpart. Alumina provided the "Bones", Silica the "Flesh"
and the alkalies were the "Blood". And we all know what a sticky mess
blood can make.
Best regards,
Ivor.
Ivor and Olive Lewis on wed 11 jun 08
Dear John Hesselberth,
Thanks for your summary.
You may be correct in your speculation that was what Mimi Obstler
intended when she wrote that Aluminium Oxide is the "Glue". But the
photomicrographs of Glaze/Clay interfaces I have seen say more about
the ability of a molten glaze, which is intended to be homogeneous, to
dissolve the surface of a pot and the subsequent regrowth of
crystalline compounds such as Mullite to lock clay and glaze into a
single unit. If this is the case, Alumina is not acting independently
but in concert with Silicon, Oxygen and Alkali elements.
Well, my first edition of Hamer shows the same illustrations, which
exclude the alkali elements. Note that he tells us " When a Glaze
finally melts the Alumina is dissolved into the molten glass". He also
makes a remark about its effects on Viscosity but nothing about
adhesion.
The Zinc Crystalline pot Avril Farley gave me indicates that Alumina
deficient glazes adhere securely to the clay beneath them and the
glaze flows down the pot because it has a very low viscosity.
Viscosity and Adhesion are hardly synonymous.
My interest in is genuine. Lili was kind enough to provide a
reference which I will follow through if it is in the SA library
system. And I like Mimi Obstler's "Core" concept for the principle
ingredients of a glaze. I see the value of her book for people who
have access to the ingredients she used and other's who are looking
for a pathway to follow.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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