Ben Shelton on fri 23 jun 06
I am interested in what causes opacity or translucency, or transparency in
glazes that don't contain opacifying agents such as tin, or zirconium
opacifiers.
I have seen pictures of glazes such as historical celedons and white sglazes
on porcelain that are translucent without being too shiny and yet the use of
opacifiers in ancient times was very unlikely. So..
What chemistries contribute to opacity and translucency?
In the case of old celedons could it be titanium in the iron source? Whicj
leads to the question "What was the historical source of iron in celedon wares?"
Thanks all,
Ben
Dave Finkelnburg on fri 23 jun 06
Ben,
Unfortunately, it's probably easier to make an
opaque glaze than a truly clear one! It takes a
balance of glass former, stabilizer and flux in a
glaze to get a really clear glaze.
Bubbles, unmelted ingredients, tiny crystals and
separate phases in the glaze are four ways to get
opacity in a glaze. High alumina, high silica, and
high flux glazes all can be opaque.
Clay was the source of iron in many early high-iron
glazes. A reference such as Nigel Wood, "Oriental
Glazes," discusses this in detail. John Britt's great
book on high fire glazes also may answer some of your
questions.
Hope this is at least a little helpful.
Dave Finkelnburg
--- Ben Shelton wrote:
> I am interested in what causes opacity or
> translucency, or transparency in
> glazes that don't contain opacifying agents such as
> tin, or zirconium
> opacifiers.
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Tom at Hutchtel.net on sat 24 jun 06
Snail...
I think he said "historical celadons" and if you look at the @1600 and
before Korean Celadons, they were all quite opaque, often flecked, and very
beautiful IMHO. The modern highly melted celadon has nothing on them. If
you ever get a change to see the pots in the Ataka collection do not miss
it. Secondary opportunity would be to find a copy of "the radiance of Jade
and the clarity of water" which is the catalog/book from the display that
went around about 15 years ago.
Tom Wirt
John and Judy Hesselberth on sat 24 jun 06
On Jun 23, 2006, at 6:21 PM, Ben Shelton wrote:
> I am interested in what causes opacity or translucency, or
> transparency in
> glazes that don't contain opacifying agents such as tin, or zirconium
> opacifiers.
Hi Ben,
The simple answer is trapped bubbles of gas and crystals that form on
cooling. If you add to that materials that never go into the melt in
the first place you have lots of opportunities for opacification
without adding the standard ones of tin, zirconium, and titanium. In
fact making a perfectly transparent, bubble and crystal free glaze is
a real challenge. Look at any of your glazes under 10x or so
magnification and you will see what I mean.
Regards,
John
Snail Scott on sat 24 jun 06
At 06:21 PM 6/23/2006 -0400, you wrote:
I have seen pictures of glazes such as historical celedons and white sglazes
on porcelain that are translucent without being too shiny and yet the use of
opacifiers in ancient times was very unlikely...
>In the case of old celedons could it be titanium in the iron source?
Titanium changes the color of celadon. That's why
even the titanium content of the clay is important
to the color outcome.
And I wouldn't call celadon opaque to any degree
at all, but rather very transparent. Otherwise the
variation in the glaze thickness couldn't bring
out the depth of carving, as it does so well.
Whicj
>leads to the question "What was the historical source of iron in celedon
wares?"
Why not natural trace 'contamination' of a local
clay source? It doesn't take much iron to make a
celadon.
-Snail
Ivor and Olive Lewis on sat 24 jun 06
Dear Ben Shelton ,
Controlling the transmission and reflection of light in glazes and =
porcelains without introducing pacifying agents depends to creating a =
layer that has two substances, intimately mixed at the sub microscopic =
to microscopic level, that have indices of refraction that are very =
different from each other. In the case of some Celadon glazes this is =
due to widespread distribution of minute bubbles of gas (My suggestion =
that the gas is Oxygen from the decomposition of Red Iron Oxide) in a =
clear silicate glass. Refractive index of the gas will be of the order =
of 1.000. Refractive index of the glass is of the order of 1.550. When =
light passes from one to the other or vice verse it changes its =
direction of travel. Multiple internal reflections eventually reverse =
the path of the incident light so you see a white surface that appears =
opaque or semi opaque. In the case of Porcelain that becomes translucent =
it is air that has not escaped from the clay during fusion.
Titanium oxide has a very high refractive index. Ilmenite can be a =
source of iron. Suppliers quote analyses in terms of Elemental oxides. =
Mineral Assay gives us better knowledge.
Iron is one of the most common elements in the crust of the Earth. Most =
white clays contain less than one percent but it is sufficient to create =
blues, blue greens and even yellow tinges in glaze of white clay. I =
described a set of glaze samples that illustrated this some years ago. =
My concern was the widespread distribution of bubbles in my mature =
samples when there was no obvious source of gas in my ingredients.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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