Ivor and Olive Lewis on wed 12 oct 05
Dear Friends,
Some twenty five years or so ago Olive was given a Christmas present by =
one of her young students. It was a cream jug in the form of a Cow, =
Dutch in origin and very much in the Delft Ware tradition. Manufactured =
as a slip cast piece in bone china or porcelain it is typical of the =
export genre with cobalt blue underglaze decoration and a clear glaze. =
It is a glaze which is fascinating because it is water clear over the =
majority of the surface but transparent strong Robin's Egg Blue where it =
has collected in the hollows of the form. I have often wondered how it =
was that the thin areas were colourless and transparent whereas the =
thicker accents were such a delightful colour in contrast to the harsher =
Cobalt hue.
I was reading one of the articles in CM Sept written by Jayne Shatz =
about her adventures with the development of her cone 6 glazes for =
electric firings. Of her Clear Glaze she said "It was clear on the =
surface and pooled in the crevices, where it transformed into a lovely =
robin's egg blue"
The recipe given is
Clear Base Glaze=20
(Cone 6 Oxidation)
Barium Carbonate 8.7%
Gerstley Borate 25.0%
Whiting 8.7%
Custer Felspar 38.0%
Silica (Flint) 19.6%=20
If there is no colourant in this style of glaze, how would you explain =
the colour where the glaze pools.
Is it due to some form of impurity in one or more of the ingredients? =
Is it an optical effect induced by one or more of the ingredients? Is it =
due to interaction with the porcelain Clay or one of its ingredients?
Shatz Porcelain =20
Nepheline Syenite 26%
EPK 38%
Jackson Ball Clay 10%
Silica (Flint) 26%=20
Or is it a function of this Artists firing schedules, which are not =
described.
Just a little thing but such a puzzle!
Best regards
Ivor Lewis.
Redhill,
S. Australia.
Tony Ferguson on wed 12 oct 05
Ivor,
My only thoughts are that it might be a "chun" like effect. The same thing has happened to me in the wood kiln with a temmoku-blue in the thicker areas. Maybe trace minerals are the culprit as well as firing atmosphere.
Tony Ferguson
Tony Ferguson
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Duluth, Minnesota
Artist, Educator, Web Meister
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Donna Kat on thu 13 oct 05
On Wed, 12 Oct 2005 10:04:19 +0930, Ivor and Olive Lewis
wrote:
>Dear Friends,
>It is a glaze which is fascinating because it is water clear over the
> majority of the surface but transparent strong Robin's Egg Blue where it
> has collected in the hollows of the form. >The recipe given is
>
>If there is no colourant in this style of glaze, how would you explain the
> colour where the glaze pools.
If I were to guess, I would think that the cobalt bled into the glaze ever
so slightly and pooled into the depths....
Ivor and Olive Lewis on fri 14 oct 05
Dear Donna Kat ,
The two blues have totally different characters. The Cobalt underglaze =
on the Cow Creamer (Which is a commercial product) shows no signs of =
movement, even when observed with the aid of a hand lens. The =
transparent blue where the clear transparent colourless glaze is pooling =
has the attributes of a Blue Topaz gem and shows nuances of aquamarine.
Thanks for your suggestion.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ivor and Olive Lewis on sat 15 oct 05
Dear Donna,
You may well be correct relating to the item that I have been looking at =
but what about the glaze recipe in CM about Stoneware electric firing? =
Did you read the article?
Since Iron is one of the most prolific elements in the crust of the =
earth it is almost impossible to avoid some degree of contamination in =
the minerals and chemicals we use.
My own opinion is that it is due to the reduction of one of the iron =
oxides when firing at porcelain temperatures, possibly over cone 10 even =
in an electric kiln.
Thanks for your contribution.=20
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Jim Murphy on sat 15 oct 05
Hi ya Ivor,
"Blue" may very well be indicative of increased Fe2+ levels.
I recall reading a research article about "float glass" ... different
glass-chemistry I know ... whereby "blue" color was attained as the
Fe2+:Fe3+ "redox ratio" was increased to 0.50.
"Yellows" & "greens" could be made with lower redox ratios, while higher
ratios around 0.60 yielded "amber".
Best wishes,
Jim Murphy
on 10/15/05 1:49 AM, Ivor and Olive Lewis at iandol@WESTNET.COM.AU wrote:
> My own opinion is that it is due to the reduction of one of the iron oxides
> when firing at porcelain temperatures, possibly over cone 10 even in an
> electric kiln.
Ivor and Olive Lewis on sun 16 oct 05
Dear Jim Murphy,=20
I think you are on the mark with the idea of Fe3+=3D>Fe2+. The =
controlling factors are temperature and Oxygen Partial Pressure. Values =
of interest to potters can be estimated from the Ellingham Diagram (P =
308, Chemistry of the Elements: Earnshaw and Greenwood)
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
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