Bruce Girrell on wed 21 jan 04
Recently I posted that I had obtained some opalescence in a glaze test, but
provided no further details. Here is some more information that may be of
interest to those following the thread.
I performed a set of glaze tests on celadons following the methodology of
Tichane. There are three major ingredients to the celadon: feldspar, silica,
and calcium carbonate. Iron, in small quantity, provides the color when
fired in reduction. I made a recipe grid (a la Currie) in which I varied the
feldspar and silica quantities but held the calcium and iron quantities
constant. I used Kona F4 feldspar, 200 mesh flint, whiting, and ochre as my
ingredients. Each dry ingredient was passed through an 80 mesh sieve to
ensure that there were no major lumps.
Following Tichane's notation, where a mix of 50 parts F4 feldspar, 30 parts
silica, 20 parts whiting and 1 part ochre would be denoted 532.1, I made the
following combinations:
712.1 722.1 732.1 742.1 752.1
612.1 622.1 632.1 642.1 652.1
512.1 522.1 532.1 542.1 552.1
412.1 422.1 432.1 442.1 452.1
312.1 322.1 332.1 342.1 352.1
212.1 222.1 232.1 242.1 252.1
The 30 combinations were mixed to a specific gravity of approximately 1.5
for application (enter Brongniart's formula). The glazes were applied to
three different clay bodies, a porcelain, an ovenware, and a clay body that
had been formulated for raku use. Each tile had a single dip all over, a
double dip over half its length and a triple dip at the top. All were fired
as closely together as possible on a single shelf. The kiln was kept in
reduction throughout the entire firing except for a short time of about 1/2
hour at a temperature near 1000 degrees C when it slipped into a neutral
atmosphere. The kiln stalled when forced back into reduction and it took
over an hour to get it climbing again. Once the OxyProbe woke up, the
reduction was held between 0.5 and 0.65 except as just noted.
In my attempts to overcome the stall I boosted the gas pressure to from
below 5 PSI to 10 PSI. This resulted in blazing past cone 10 in a remarkably
short period of time. Final temperature was cone 12 and was consistent
throughout the kiln except for one cone pack placed near the flame trough
that had cone 12 flattened and another pack near the flue where cone 12 was
not fully down. Cooling was done by simply closing the damper and bricking
up the burner ports.
On all clay bodies the mixture 222.1 showed signs of opalescence. It looks
like an electric blue glow within the glaze. The glaze had fully melted and
had run down to the bottom edge of the tile. The opalescence is primarily in
the pooled glaze. This combination also had numerous crystals of what appear
to be calcite. Calcite should not exist post-firing unless the reduction
atmosphere prevented the calcium carbonate from giving up its CO2.
Nevertheless, these crystals certainly remind me of calcite.
Tichane suggests that addition of phosphorous via bone ash will promote
opalescence. My best celadon was 522.1. Extending Tichane's notation I will
note the addition of 1 part of bone ash as 522.1B.1 and the addition of 20
parts bone ash as 522.1B2. I mixed the following glazes:
522.1B.1 522.1B.2 522.1B.5 522.1B1 522.1B2
Each glaze has approximately twice as much bone ash as its predecessor. The
glazes were once again mixed to about 1.5 specific gravity and applied as
before. Only porcelain was used this time. The kiln was again held in
reduction throughout the firing. Once again, the kiln wanted to slip into a
more oxidizing atmosphere near 1000 degrees, but this time I was ready for
it. Although the temperature plot becomes more ragged as the kiln and I
danced on the edge of oxidation/reduction, the kiln never really went into a
stall. I also kept the gas pressure down to 5 PSI throughout the whole
firing. Final temperature was cone 11 on the shelf with the test tiles, cone
12 down near the flame trough, cone 10 down every else except cone 10 at
3:00 near the flue.
The glazes all showed substantial bubbling. The one with the largest amount
of bone ash, 522.1B2, is a complete froth the never totally melted. None of
the glazes were melted to the extent of the first run. The bubbling caused
the glaze to appear much more opaque and the color is almost gone. There is
no sign of opalescence anywhere.
Future work:
Since the opalescence appeared on my bottom row in my initial test, I don't
yet know what would happen if I dropped the feldspar even further. It would
also be interesting to know what would happen with variation in the calcite
quantity, i.e., what glazes 221.1 and 223.1 would look like.
Bruce "Teacher, my lab report is done" Girrell
Hank Murrow on thu 22 jan 04
Dear Bruce;
I notice that in your bone ash tests the silica was 20%. Since the
Phosphorus in the bone ash is a glass former, I wonder what would
happen if you left out some or all of the silica as you add more and
more Bone ash. Since the P2O5 is acting as a glass former, perhaps the
silica is keeping it from melting. I have several glazes with high bone
ash (25% t0 50%!) and all of them have reduced silica. They melt fine.
Cheers, Hank in Eugene
On Jan 21, 2004, at 9:24 AM, Bruce Girrell wrote in part:
> Tichane suggests that addition of phosphorous via bone ash will promote
> opalescence. My best celadon was 522.1. Extending Tichane's notation I
> will
> note the addition of 1 part of bone ash as 522.1B.1 and the addition
> of 20
> parts bone ash as 522.1B2. I mixed the following glazes:
>
> 522.1B.1 522.1B.2 522.1B.5 522.1B1 522.1B2
>
> Each glaze has approximately twice as much bone ash as its
> predecessor. The
> glazes were once again mixed to about 1.5 specific gravity and applied
> as
> before. Only porcelain was used this time. The kiln was again held in
> reduction throughout the firing. Once again, the kiln wanted to slip
> into a
> more oxidizing atmosphere near 1000 degrees, but this time I was ready
> for
> it. Although the temperature plot becomes more ragged as the kiln and I
> danced on the edge of oxidation/reduction, the kiln never really went
> into a
> stall. I also kept the gas pressure down to 5 PSI throughout the whole
> firing. Final temperature was cone 11 on the shelf with the test
> tiles, cone
> 12 down near the flame trough, cone 10 down every else except cone 10
> at
> 3:00 near the flue.
>
> The glazes all showed substantial bubbling. The one with the largest
> amount
> of bone ash, 522.1B2, is a complete froth the never totally melted.
> None of
> the glazes were melted to the extent of the first run. The bubbling
> caused
> the glaze to appear much more opaque and the color is almost gone.
> There is
> no sign of opalescence anywhere.
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