John Hesselberth on wed 11 aug 04
On Wednesday, August 11, 2004, at 05:48 PM, Wood Jeanne wrote:
> So my questions for interested glaze experts are: What
> reaction between the soil (or sand?) and the glaze
> cause the chemical change?
> And: in the cases of glaze disintegration did the
> glaze not fit the clay?
> And finally: If the glazes were stable would they stay
> fused to the pot and the same color condition after
> 2000+ years?
Hi Jeanne,
It is probably just acidic or alkaline attack on the glazes, i.e. they
are just being gradually dissolved or eaten away. This would happen
very, very slowly in dry climates--more rapidly in humid or moist
climates. And, of course, it will also happen more or less rapidly
depending on the inherent stability of the glaze, i.e. did it have an
appropriate level of silica and alumina to be stable, was it thoroughly
melting in firing, and was it not overloaded with colorants.
The answer to your last question is a definite maybe. I think it would
also depend on the environment. I doubt any glaze would stand 2000
years of exposure to acid rain in a warm humid climate, but it probably
would last OK buried in the desert.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Wood Jeanne on wed 11 aug 04
Hi Everyone,
The past week I had the opportunity to visit several
museums & look at ancient pottery, one of my great
loves. These pots, plus others I've seen raise a
question. I hope glaze experts on this list might also
be interested in.
In some cases, the Chinese stoneware pots glaze has
begun to flake off or disintegrate. In other cases
glazed Ancient Near Eastern pottery that has been
buried in for many years has gone through a chemical
change causing the glazes to change color and look
lustrous, very pretty, like a thinly applied pearl
luster.
In all cases the fired clay itself is as sturdy as the
day it was fired.
So my questions for interested glaze experts are: What
reaction between the soil (or sand?) and the glaze
cause the chemical change?
And: in the cases of glaze disintegration did the
glaze not fit the clay?
And finally: If the glazes were stable would they stay
fused to the pot and the same color condition after
2000+ years?
Thanks,
Jeanne W.
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Ivor and Olive Lewis on thu 12 aug 04
Dear Jeanne Wood,
Archaeologists would give a better explanation but this is my simple
take on the matter.
In all soils and burial sites over the centuries water has percolated
into the soil and around buried pots. Most waters are Acidic, due to
the solubility of Carbon dioxide in water. The water also collects
Humic Acid, a product of organic decomposition. Most glazes are based
on Alkali metal or Alkali Earth compounds with the elements NA, K and
Ca predominating. All of these are soluble in acid solutions. So the
surface of the glaze etches. Eventually this process reaches the
interface of the glaze with the fired clay. Many old wares were low
fired and contained Lime. Now this begins to dissolve in the acid
solutions. So fragments of glaze can flake from the pot.
Poor glaze fit will exacerbate the problem because crazing gives added
access points for the acid water to penetrate.
Iridescence is due to the nature of the leached glaze surface. It
becomes micro porous but still remains transparent. This structure can
interfere with the way light behaves, like precious opal, causing both
diffraction and refraction, separating white light into spectral hues.
A common colour in glass which has suffered this in arid climates is a
pale natural Magenta.
If glazes were not subject to forces in corrosive environments they
would stay pristine for an indefinite period of time.
Hope that covers the field.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
wayne on thu 12 aug 04
Jeanne:
I think to answer your question we would
need to speak with a 2000 year old potter.
Anyone on the list? LOL
Since we're on the subject of stability....
How long should one "reasonably" expect
pottery to last? A decade? A century?
A millenium? or just until the customer gets it home ?
Personally, I have no hope that any of my
work would survive to the year 3000
(though some of the chunkier pieces might
having been buried already :>). This of course
assumes that our species itself survives.
Do we succumb to the "throwaway" society we
are(have) become and expect it to last only a year?
Or do we shoot for millenia and if so....
how the heck do you test for that?
Let's see...
freeze thaw tests...assuming maybe four freeze/thaws
per year, times a thousand years, means 4000
cycles, or almost 11 years at once per day (10.96)
Does anyone want to run glaze tests that long?
Wayne Seidl
Key West, Florida, USA
North America, Terra
Latitude 81.45W, Longitude 24.33N
Elevation 3.1 feet (1m)
----- Original Message -----
From: "Wood Jeanne"
So my questions for interested glaze experts are: What
reaction between the soil (or sand?) and the glaze
cause the chemical change?
And: in the cases of glaze disintegration did the
glaze not fit the clay?
> And finally: If the glazes were stable would they stay
> fused to the pot and the same color condition after
> 2000+ years?
>
Earl Krueger on fri 13 aug 04
On Aug 12, 2004, at 3:34 AM, wayne wrote:
> How long should one "reasonably" expect
> pottery to last? A decade? A century?
Slightly longer than Christmas tree lights.
About 3 years.
That's longer than the majority of John or
Jane Q Public will keep functional ware
before they tire of it and it goes to the
yard sale, thrift shop or dump.
If this offends you, get a grip. It's GREAT!
Potters don't have to build in planned
obsolescence; the buyer takes care of that
for them. All you have to do is keep creating
new designs and glazes so they will buy more.
What great fun.
Earl K...
Bothell, WA, USA
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