David Gallagher on mon 13 mar 06
Ok so its probably not that amazing.
But it has got me very curious.
I started doing some testing of celadons at ^8-9 Red.
After doing a line blend, picked my favorite and made
up a 2.5 gallon batch.
I wanted a darker green, on the iron bearing stoneware
body.
And that is what the glaze test showed. Yippie for me.
The recipie is pretty basic.
Neph Sy, clay, silica and for the green 2% Black Fe2.
(I don't have it infront of me so I may have left
something out)
Loaded up the communal kiln and some fired it over the
weekend.
My greens came out. Some variations from light olive
to a nice dark green.
Chalked the variation up to uneven reduction and all
is ok.
Next kiln load went in and came out...
Same glaze batch, same application, and now its a nice
powder blue?
So I'm curious if anyone has some clues as to why this
happened? I didn't have a chance to check the kiln log
to see if the temp was different and that is where I
will start.
Did I accidentally make a heavier application?
Does a stronger, or a consistant reduction encourage
the green?
Any insight would be appreciated.
Thanks
David
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Hank Murrow on mon 13 mar 06
On Mar 13, 2006, at 8:33 AM, David Gallagher wrote:
> Next kiln load went in and came out...
> Same glaze batch, same application, and now its a nice
> powder blue? So I'm curious if anyone has some clues as to why this
> happened? Any insight would be appreciated.
Dear David;
I imagine that your firing altered somewhat. The blue color is the
color of iron in the FeO form rather than the Fe2O3 form, so you cooled
slowly enough to re-oxidize the first load, AND/OR reduced less heavy
on that load. If you wish to encourage the greenish tones, use American
kaolins (in both body and glaze), which are contaminated with titanium
which takes the color away from blue towards green. Blue celadons are
best achieved in glazes with little to no Sodium, little to no Titania,
perhaps small amounts of BaO, small amounts (.5 to 1%) of Iron oxide,
and placed on bodies which share these qualities with the glaze. The
original Song porcelains from the twelfth century around Jingdezhen
were made from Petuntse. This is a hydrothermally altered (means that
the alkaline content was reduced by weathering action of hot gases and
waters from the cooling volcanic magmas below) volcanic ash..... very
low in both Titania and iron and containing around 3% potassium in the
clay lattice itself. Their glaze was made by adding 15% to 20% of
limestone to the body, which originally was entirely Petuntse. As they
worked the veins of Petuntse farther from the weathered surface
deposits, they had to add Kaolin to provide enough plasticity to throw
the stuff. At the present time in Jingdezhen, their bodies contain only
around 30% Petuntse......and are very different from the original
composition.
Interestingly, one of my teachers(visiting this week after NCECA) is
David Stannard, who prospected and developed porcelains from weathered
volcanic ashes from Ca to AK along the pacific Rim volcano belt. Close
inspection with electron microscopy reveal these western petuntse rocks
to be largely identical to the Chinese samples given similar treatment
by him.
I mine and process several of these deposits to make translucent
porcelains which ring like bells and are bluish in color, with a very
long firing range(Cones 8_12!). I aslo mine a Petuntse that has been
contaminated/blessed with iron and titania along with the weathering
gases, which is not translucent(due to the Ti and Fe), but which
slightly self-glazes due to some residual solubles, and is gorgeous.
The one in the middle is Alaskan petuntse, the one on the right is the
Oregon Petuntse, and the one on the left is an altered Andesite (very
dark volcanic rock) from the Santiam River in Oregon. Each is glazed
with the body itself with 20% added Limestone. the dark one is fired to
cone 6 electric(due to the iron content) and all ring like
bells......loud and persistently. The one in the middle is translucent
(count your fingers through it when held to the light).
Hope this long reply helps you a little. I showed samples of these at
NCECA in the Clayart room. I sent you pics privately, as Clayart cannot
post them yet.
Cheers, Hank
www.murrow.biz/hank
John Britt on mon 13 mar 06
David,
Don't know what happened but heaver more consistent reduction can create
blue celadons. Lots of factors to consider though.
Hope it helps,
John Britt
www.johnbrittpottery.com
Craig Martell on tue 14 mar 06
John was saying:
>Don't know what happened but heaver more consistent reduction can create
>blue celadons. Lots of factors to consider though.
Hello John:
What is this based on? I've never seen this happen if the chemistry isn't
within the parameters for a blue celadon. I use three celadons, two green
and one blue. The firing has less to do with the outcome than application
and chemistry.
regards, Craig Martell Hopewell, Oregon
John Britt on wed 15 mar 06
Hey Craig,
It has been my experience that the firing matters just as much as the
chemistry, application and clay body. I don't know which one trumps the
other but they all matter.
I have applied a blue celadon glaze very thin on dark stoneware and had a
gray celadon. A thin coat of blue celadon on porcelain may be transparent
but with a thick coat it is a nice blue. I have even had an otherwise
transparent glaze turn blue celadon on a stoneware body, in heavy
reduction. (Otherwise it will be gray)
A blue celadon that is fired in uneven reduction can create a pot with
half blue and half greenish snotty color. (I have photos in my book on
page 66, tile 14)
I have also had blue celadons from soda feldspars, (Page 66, tile 12 and
13) and nepheline syenite. This proves that firing can trump chemistry.
Because most people say that you can't get blue celadon with soda feldspar.
So, I can only say, from my experience, thickness matters as well as
firing, recipe and clay body.
Hope that is clear, as it is 6:00 a.m.,
John Britt
www.johnbrittpottery.com
John Britt on wed 15 mar 06
Craig,
I should have specified that by =93chemistry=94 I mean =93know composition=94=
that
produces blue celadons. Because I am reading =93correct chemistry=94 to mean=
,
the limits that are set through observation. So if we construct a glaze
and fire it, then adjust it, fire it, etc., we then, though this
experience, construct a set of =93limits=94 or parameters for a blue celadon=
.
But this does not always include what is not in our experience.
I am not being condescending, merely trying to be clear about the terms I
was using.
Hope it helps,
John Britt
www.johnbrittpottery.com
Marcia Selsor on wed 15 mar 06
I worked with developing a nice light blue celedon for Southern Ice =20
when I was in Hawaii and firing to ^10 reduction. I used burnt umber.
It was really nice. Now I want to work on it for ^6 reduction for =20
Frost. When I do, I'll let you know.
Marcia
On Mar 15, 2006, at 4:32 AM, John Britt wrote:
> Craig,
>
> I should have specified that by =93chemistry=94 I mean =93know =20
> composition=94 that
> produces blue celadons. Because I am reading =93correct chemistry=94 =
to =20
> mean,
> the limits that are set through observation. So if we construct a =20
> glaze
> and fire it, then adjust it, fire it, etc., we then, though this
> experience, construct a set of =93limits=94 or parameters for a blue =20=
> celadon.
> But this does not always include what is not in our experience.
>
> I am not being condescending, merely trying to be clear about the =20
> terms I
> was using.
>
> Hope it helps,
>
> John Britt
> www.johnbrittpottery.com
>
> ______________________________________________________________________=20=
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