ferenc jakab on sun 5 nov 00
A friend of mine is doing his masters on glaze chemistry and has access to
XRD and XRF equipment. He has been to an archaeological dig in Thailand in
an area where there was a thriving potting industry which suddenly
disappeared in the C18th. He has bought back a lot of shards to separate the
glazes and then analyse them. The glazes are celadons and chuns. He also
brought back some shards from ware that had been sourced from these
potteries which had been found in shipwrecks off Indonesia. The interesting
thing is that some of these glazes were badly leached out, probably by sea
water. One sample he showed me was badly leached on one side and still had a
beautiful green celadon on the other where the shard had been protected by
the shell of a sea snail which had become attached to it. I was very
surprised as I expected the glaze to be too stable for this sort of
leaching. The affected glaze looked a little like a pumice stone. Very
porous.
Feri.
Paul Taylor on mon 6 nov 00
Dear Fenenc .
For some time I suspected that my celadon glazes were changing color
with time becoming richer. If the sea can bleach them then the atmosphere
can re oxidize them over time - especially the more matt glazes.
But I will not give up getting a good color - without the benefit of
millennia.
I have Just found out that Whiting and wolistonite can have titanium
content as high as 0.6 It is so difficult when dealing with even the best
suppliers to get accurate info . When you need to be fussy and do not want
to buy a hundred tons.
How do I persuade a multi national that my obsession is worth while.
Regards from Paul Taylor
http://www.anu.ie/westportpottery
> From: ferenc jakab
> Reply-To: Ceramic Arts Discussion List
> Date: Sun, 5 Nov 2000 23:09:41 +1100
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Leaching Celadons
>
> A friend of mine is doing his masters on glaze chemistry and has access to
> XRD and XRF equipment. He has been to an archaeological dig in Thailand in
> an area where there was a thriving potting industry which suddenly
> disappeared in the C18th. He has bought back a lot of shards to separate the
> glazes and then analyse them. The glazes are celadons and chuns. He also
> brought back some shards from ware that had been sourced from these
> potteries which had been found in shipwrecks off Indonesia. The interesting
> thing is that some of these glazes were badly leached out, probably by sea
> water. One sample he showed me was badly leached on one side and still had a
> beautiful green celadon on the other where the shard had been protected by
> the shell of a sea snail which had become attached to it. I was very
> surprised as I expected the glaze to be too stable for this sort of
> leaching. The affected glaze looked a little like a pumice stone. Very
> porous.
>
> Feri.
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
Michael Banks on tue 7 nov 00
Hi Feri,
I believe I know why feldspathic celedon glazes will not be stable immersed
in seawater: These glazes probably were formulated from a high proportion
of potash feldspar in the recipe. The resulting glaze (glass) has a high
potash content. Submersion of such a glaze in a sodic brine, or seawater
with its' high sodium content, results in the potash ion being replaced by
the concentrated aqueous sodium.
This effectively destroys the glaze. Seawater has significantly less potash
relative to the sodium content. Therefore immersion of a rich source of
potash ions (glaze) in it results in a strong ionic gradient pushing the
potash into solution. Sodium ions replace the potash via ionic transfer,
however in the process the structural integrity of the glass is disrupted.
Glazes formulated with sodium (e.g: soda feldspar) as the primary flux would
be significantly more stable -as the glass would be less likely to be in
disequilibrium with the surrounding brine.
The fate of the potassic celedons is shared by glassy volcanic rocks,
feldspars, clays and micas with potassium content that end up in seawater,
or buried in sediment with a high brine content. The strongly bonded
crystalline minerals such as feldspar survive the process (unlike glasses),
but are converted to the sodic version, i.e: orthoclase (potash feldspar) is
converted to albite (sodium feldspar). Rocks like basalt lose their potash
content and become more sodic in chemistry. Geologists call these rocks
spilites and the process spilitisation. Spilites are a sure indicator that
the rock has been soaked in sodic brine for a period.
Michael Banks,
Nelson,
New Zealand
----- Original Message -----
Ferenc Jakab wrote (abridged):
> ...brought back some shards from ware that had been sourced from these
> potteries which had been found in shipwrecks off Indonesia. The
interesting
> thing is that some of these glazes were badly leached out, probably by sea
> water. One sample he showed me was badly leached on one side and still had
a
> beautiful green celadon on the other where the shard had been protected by
> the shell of a sea snail which had become attached to it. I was very
> surprised as I expected the glaze to be too stable for this sort of
> leaching.
ferenc jakab on tue 7 nov 00
Michael,
Thanks for the thoughts. Youre knowledge of geochemistry (mineralogy) is
always an eye opener to me. It makes sense to me. I'll pass the message on
to my friend at Uni. Knowing him, he may have reached similar conclusions.
Do you have objections to his communicating with you if he should choose to
do so? He is working with Neville Rosengren at La Trobe, Bendigo.
Feri.
| |
|
