Chris Schafale on tue 20 mar 01
Cindy inspired me to replicate and extend a little her tests with
Opal Blue, something I've been meaning to do, and I just got
through unloading the kiln (too hot as usual) and wanted to share
the results
Background:
The version of this base that I use is:
Frit 3134 50
Silica 25
EPK raw 15
EPK calcined 8
Total 98 (the other 2% is the LOI from the calcined EPK)
I fire to cone 5-ish, thusly:
50C/hr to 120C
150C/hr to 525C
75C/hr to 600C
150C/hr to 1050
60C/hr to 1172, hold 15 minutes
100C/hr to 1000, hold 30 minutes
shut down and cool naturally
My usual clay is a medium brown stoneware (Highwater Speckled
Brownstone).
First, I tried Cindy's experiment with adding alumina (I used
alumina oxide) to the version of the base that is colored with 5
rutile and 2.5 cobalt carb. On my clay this version ranges from a
fairly flat light blue (boring) where thick, to a nice green on edges. I
want to promote the green. Thinning the glaze helps, but you don't
get all green until it's too thin for functional coverage. So, I added
2% alumina oxide to one 500 gram batch, and 10% alumina oxide
to another. Then I mixed the two, by volume, to get the
intermediate grades of 4, 6, and 8% added alumina. My results
duplicate Cindy's to a large degree.
2%: rich, textured, deep, bluish green with blue places where thick
4%: darker green-blue, thicker spots blackish instead of blue
6%: looks black from a distance, but has blue-green speckles
under direct light
8%: brownish black, no blue at all
10%: more of a gray-ish black with slightly frosty looking spots,
slightly less shiny surface
The above were test tiles. On a small bowl made of brown
stoneware, the 6% version looked blackish-brown with some blue-
green streaks. Interestingly, on a porcelain test bowl, it still looks
very dark from a distance, but under light is a deep olive with subtle
hints of blue on edges.
The other test I did was to take the version that uses 10% RIO and
add cobalt, to see if I could get a nice black out of it. I could. 2%
did the trick nicely, 4% was very black indeed. The nifty thing was
that when I dipped the top of the 10 RIO/4 cobalt tile in the 5%
rutile-only version of the base base that Cindy described, I got a
lovely runny deep blue flowing over the black. Very striking -- can't
wait to see it on a pot. I just love that 5% rutile variation as a top
glaze -- I've never seen it look bad, and it usually looks terrific. Can
be a bit runny, though, so one has to control oneself and use it
only on the top half of pieces.
So, that's my story. I'd love to hear any ideas about why the
blue/green version gets black with too much added alumina.
Actually, it's amazing that it melts at all at cone 5 with that much
alumina, but I guess the overload of boron takes care of that.
Chris Schafale
where the rain is slapping the window panes, the wind is howling,
and it's 40 degrees. Did I mention that North Carolina spring
weather can be fickle? But it's supposed to be sunny and mid-60's
on Thursday....
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
Ababi on wed 21 mar 01
O.K. I will save your letter and see what I can do with your testing. Now
try this: I posted it before. Apply this glaze above low alumina
clear glaze, I give you here my favorite, The original as well as my
variation. The Opal blue you can apply either all over or on some parts,
preferred in away that it might move The upper glaze, Opal Blue, in this
time.
Koren 27
========
QUARTZ.............. 26.00 26.53%
WHITING............. 13.00 13.27%
*ENGLISH KAOLIN(Puraf 5.00 5.10%
ZINC OXIDE.......... 10.00 10.20%
*FELDSPAR soda....... 35.00 35.71%
BARIUM CARBONATE.... 9.00 9.18%
========
98.00
Koren Strontium
===============
QUARTZ.............. 27.15 27.15%
*FELDSPAR soda....... 36.55 36.55%
WHITING............. 13.58 13.58%
*ENGLISH KAOLIN(Puraf 5.22 5.22%
ZINC OXIDE.......... 10.44 10.44%
STRONTIUM CARBONATE. 7.05 7.05%
========
100.00
Good glaze!
Ababi Sharon
ababisha@shoval.ardom.co.il
http://members4.clubphoto.com/ababi306910/
----- Original Message -----
From: "Chris Schafale"
To:
Sent: Wednesday, March 21, 2001 2:49 AM
Subject: Opal Blue, more tests for cobalt green
> Cindy inspired me to replicate and extend a little her tests with
> Opal Blue, something I've been meaning to do, and I just got
> through unloading the kiln (too hot as usual) and wanted to share
> the results
>
> Background:
> The version of this base that I use is:
> Frit 3134 50
> Silica 25
> EPK raw 15
> EPK calcined 8
> Total 98 (the other 2% is the LOI from the calcined EPK)
>
> I fire to cone 5-ish, thusly:
> 50C/hr to 120C
> 150C/hr to 525C
> 75C/hr to 600C
> 150C/hr to 1050
> 60C/hr to 1172, hold 15 minutes
> 100C/hr to 1000, hold 30 minutes
> shut down and cool naturally
>
> My usual clay is a medium brown stoneware (Highwater Speckled
> Brownstone).
>
> First, I tried Cindy's experiment with adding alumina (I used
> alumina oxide) to the version of the base that is colored with 5
> rutile and 2.5 cobalt carb. On my clay this version ranges from a
> fairly flat light blue (boring) where thick, to a nice green on edges. I
> want to promote the green. Thinning the glaze helps, but you don't
> get all green until it's too thin for functional coverage. So, I added
> 2% alumina oxide to one 500 gram batch, and 10% alumina oxide
> to another. Then I mixed the two, by volume, to get the
> intermediate grades of 4, 6, and 8% added alumina. My results
> duplicate Cindy's to a large degree.
> 2%: rich, textured, deep, bluish green with blue places where thick
> 4%: darker green-blue, thicker spots blackish instead of blue
> 6%: looks black from a distance, but has blue-green speckles
> under direct light
> 8%: brownish black, no blue at all
> 10%: more of a gray-ish black with slightly frosty looking spots,
> slightly less shiny surface
>
> The above were test tiles. On a small bowl made of brown
> stoneware, the 6% version looked blackish-brown with some blue-
> green streaks. Interestingly, on a porcelain test bowl, it still looks
> very dark from a distance, but under light is a deep olive with subtle
> hints of blue on edges.
>
> The other test I did was to take the version that uses 10% RIO and
> add cobalt, to see if I could get a nice black out of it. I could. 2%
> did the trick nicely, 4% was very black indeed. The nifty thing was
> that when I dipped the top of the 10 RIO/4 cobalt tile in the 5%
> rutile-only version of the base base that Cindy described, I got a
> lovely runny deep blue flowing over the black. Very striking -- can't
> wait to see it on a pot. I just love that 5% rutile variation as a top
> glaze -- I've never seen it look bad, and it usually looks terrific. Can
> be a bit runny, though, so one has to control oneself and use it
> only on the top half of pieces.
>
> So, that's my story. I'd love to hear any ideas about why the
> blue/green version gets black with too much added alumina.
> Actually, it's amazing that it melts at all at cone 5 with that much
> alumina, but I guess the overload of boron takes care of that.
>
> Chris Schafale
> where the rain is slapping the window panes, the wind is howling,
> and it's 40 degrees. Did I mention that North Carolina spring
> weather can be fickle? But it's supposed to be sunny and mid-60's
> on Thursday....
> Light One Candle Pottery
> Fuquay-Varina, North Carolina, USA
> (south of Raleigh)
> candle@intrex.net
> http://www.lightonecandle.com
>
>
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