Jennifer Buckner on wed 23 jun 10
Ron Roy wrote:
Hi Jennifer,
The glaze is somewhat short of Boron so it is probably an underfired
matte - in other words - melted enough to feel like a melted glaze but
not melted enough to be a stable glaze.
If those pots on the top shelf were short pots they probably did not
get high enough to be "almost melted."
Yes, they are short - about 4" to 41/2" high.
Call it an underfired matte glaze and make sure it gets a full cone 6
I thought it had gotten a full ^6. I use large but not
self-supporting cones. ^6 was completely over, touching the cone
holder, ^7 was tipped to about 10:00. I'm puzzled, doesn't this tell
me I had a full ^6 firing? I was also watching the temps very
carefully during the firing because I'd had a glitch during the
previous firing and Rob Batty of L&L was giving me some help over the
phone. The temps got to 2165o as I'd programmed.
- not the best kind of glaze to use as a liner - looks like it will be
crazing as well.
John Hesselbert wrote:
A couple thoughts. You said the cones confirmed cone 6 but were they
on the top shelf with your pots?
Yes, I put cones on each of the 3 shelves.
Top shelves of electric kilns are often cooler than the middle.
This glaze is a bit marginal for good melting at cone 6 but should be
OK if it really got there. But at cone 5 or 5 1/2, with all that
zircopax and rutile, if you don't get really good melting you would
feel a little sandpapery. Try it in the hottest part of your kiln
next time and, if you haven't already done it, put a cone pack on
that top shelf and see what is really going on.
Should I re-glaze the pots before re-firing?
Another possibility is that the zircopax and/or the rutile didn't get
well dispersed. Did you sieve through an 80 mesh sieve when you mixed
the batch. If not, try that and see if it helps.
Yes, I sieved it 3 times just a day or so before glazing, and I
thought I'd stirred it up well before glazing
Third, if you still are having problems after trying the above,
adding another 5% GB would add enough boron to get you more
comfortably in the cone 6 melting range. Try that on a small fraction
of the batch first to see if you like the result. The unity formula
of the original is below. You can see the boron level is only 0.195.
I like to see closer to 0.25 at cone 6.
I'll certainly try it.
Eric Hanson wrote:
possibly the silica was not 365 mesh or finer and possibly the rutile was
granular instead of glaze grade (?)
The silica is 365 mesh and the rutile (light) is very powdery.
Mike Gordon wrote:
Other than it might be the change in G.B. since the recipe was
posted, I immediately thought it might be brick dust from the kiln
arch or lid if you fire electric. Try it on a lower shelf,
There's no dust scattered on the parts of the kiln shelf that were
not covered by pots. Also, the surface of the pots is so uniformly
gritty that I'd have to say it didn't come from random "shedding"of
the lid. But I will try firing them on the middle shelf.
Gents: Thanks for the helpful hints. I'll try the suggestions and
see where they take me.
Jennifer
Jennifer G. Buckner jennifer@buckner.cc
Jennifer Buckner on wed 23 jun 10
Hello Clayarters,
I wonder if anyone can help me with a glaze problem. I glazed some
stoneware and porcelain pots with a white ^6 glaze called Krakowski
Cream which I found in the Clayart archives. I believe it's a
revision of a recipe from Jane Deboos. (see below). Apparently Alisa
Claussen tested it and reported a "semi-matte, smooth surface". My
test tiles, dipped in a 500 gm batch, came out with a semi-matte and
smooth surface. However, the pots, bisqued to ^04, dipped in a
10,000 gm batch for a count of 5 and fired on the top shelf of the
kiln, came out with the same nice off-white colour but with a surface
that feels like very fine sandpaper. Can anyone give me a clue as to
why this happened? Could it have been my glazing technique? Is it
the glaze and, if so, can I fix it?
Krakowski Cream
Gerstley Borate.............12.4
Potash Feldspar.............41.8
Silica..............................17.0
Kaolin (EPK).................2.3
Whiting...........................7.4
Zircopax.........................5.4
Dolomite.........................5.4
Bentonite.........................1.0
Rutile...............................7.3
The firing schedule was:
100o/hr to 200o
400o/hr to 1950o
108o/hr to 2165, 10 min. hold
AFAP to 1800o
off
There's a thermocouple offset of +50 on each of the 3 thermocouples
on the kiln so the temps were really higher and the cones confirmed that.
Any help would be appreciated.
Jennifer (in northern VT)
Jennifer G. Buckner jennifer@buckner.cc
Eric Hansen on wed 23 jun 10
possibly the silica was not 365 mesh or finer and possibly the rutile was
granular instead of glaze grade (?)
-h-
On Wed, Jun 23, 2010 at 9:45 AM, Jennifer Buckner wrot=
e:
> Hello Clayarters,
>
> I wonder if anyone can help me with a glaze problem. I glazed some
> stoneware and porcelain pots with a white ^6 glaze called Krakowski
> Cream which I found in the Clayart archives. I believe it's a
> revision of a recipe from Jane Deboos. (see below). Apparently Alisa
> Claussen tested it and reported a "semi-matte, smooth surface". My
> test tiles, dipped in a 500 gm batch, came out with a semi-matte and
> smooth surface. However, the pots, bisqued to ^04, dipped in a
> 10,000 gm batch for a count of 5 and fired on the top shelf of the
> kiln, came out with the same nice off-white colour but with a surface
> that feels like very fine sandpaper. Can anyone give me a clue as to
> why this happened? Could it have been my glazing technique? Is it
> the glaze and, if so, can I fix it?
>
> Krakowski Cream
> Gerstley Borate.............12.4
> Potash Feldspar.............41.8
> Silica..............................17.0
> Kaolin (EPK).................2.3
> Whiting...........................7.4
> Zircopax.........................5.4
> Dolomite.........................5.4
> Bentonite.........................1.0
> Rutile...............................7.3
>
> The firing schedule was:
> 100o/hr to 200o
> 400o/hr to 1950o
> 108o/hr to 2165, 10 min. hold
> AFAP to 1800o
> off
>
> There's a thermocouple offset of +50 on each of the 3 thermocouples
> on the kiln so the temps were really higher and the cones confirmed that.
>
> Any help would be appreciated.
>
> Jennifer (in northern VT)
>
> Jennifer G. Buckner jennifer@buckner.cc
>
John Hesselberth on wed 23 jun 10
On Jun 23, 2010, at 9:45 AM, Jennifer Buckner wrote:
> However, the pots, bisqued to ^04, dipped in a
> 10,000 gm batch for a count of 5 and fired on the top shelf of the
> kiln, came out with the same nice off-white colour but with a surface
> that feels like very fine sandpaper. Can anyone give me a clue as to
> why this happened? Could it have been my glazing technique? Is it
> the glaze and, if so, can I fix it?
Hi Jennifer,
A couple thoughts. You said the cones confirmed cone 6 but were they on =3D
the top shelf with your pots? Top shelves of electric kilns are often =3D
cooler than the middle. This glaze is a bit marginal for good melting at =
=3D
cone 6 but should be OK if it really got there. But at cone 5 or 5 1/2, =3D
with all that zircopax and rutile, if you don't get really good melting =3D
you would feel a little sandpapery. Try it in the hottest part of your =3D
kiln next time and, if you haven't already done it, put a cone pack on =3D
that top shelf and see what is really going on.
Another possibility is that the zircopax and/or the rutile didn't get =3D
well dispersed. Did you sieve through an 80 mesh sieve when you mixed =3D
the batch. If not, try that and see if it helps.
Third, if you still are having problems after trying the above, adding =3D
another 5% GB would add enough boron to get you more comfortably in the =3D
cone 6 melting range. Try that on a small fraction of the batch first to =
=3D
see if you like the result. The unity formula of the original is below. =3D
You can see the boron level is only 0.195. I like to see closer to 0.25 =3D
at cone 6.
Good luck,
John
Recipe Name: Krakowski Cream
Unity Oxide
.115 Na2O
.174 K2O
.151 MgO
.56 CaO
1.000 Total
.312 Al2O3
.195 B2O3
.004 Fe2O3
3.098 SiO2
0 TiO2
0 P2O5
9.9 Ratio
75.1 Exp
Comments: =3D20
-----------------------------------
Calculations by GlazeMaster=3D99
www.masteringglazes.com
------------------------------------
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ron Roy on wed 23 jun 10
Hi Jennifer,
The glaze is somewhat short of Boron so it is probably an underfired
matte - in other words - melted enough to feel like a melted glaze but
not melted enough to be a stable glaze.
If those pots on the top shelf were short pots they probably did not
get high enough to be "almost melted."
Call it an underfired matte glaze and make sure it gets a full cone 6
- not the best kind of glaze to use as a liner - looks like it will be
crazing as well.
RR
Quoting Jennifer Buckner :
> Hello Clayarters,
>
> I wonder if anyone can help me with a glaze problem. I glazed some
> stoneware and porcelain pots with a white ^6 glaze called Krakowski
> Cream which I found in the Clayart archives. I believe it's a
> revision of a recipe from Jane Deboos. (see below). Apparently Alisa
> Claussen tested it and reported a "semi-matte, smooth surface". My
> test tiles, dipped in a 500 gm batch, came out with a semi-matte and
> smooth surface. However, the pots, bisqued to ^04, dipped in a
> 10,000 gm batch for a count of 5 and fired on the top shelf of the
> kiln, came out with the same nice off-white colour but with a surface
> that feels like very fine sandpaper. Can anyone give me a clue as to
> why this happened? Could it have been my glazing technique? Is it
> the glaze and, if so, can I fix it?
>
> Krakowski Cream
> Gerstley Borate.............12.4
> Potash Feldspar.............41.8
> Silica..............................17.0
> Kaolin (EPK).................2.3
> Whiting...........................7.4
> Zircopax.........................5.4
> Dolomite.........................5.4
> Bentonite.........................1.0
> Rutile...............................7.3
>
> The firing schedule was:
> 100o/hr to 200o
> 400o/hr to 1950o
> 108o/hr to 2165, 10 min. hold
> AFAP to 1800o
> off
>
> There's a thermocouple offset of +50 on each of the 3 thermocouples
> on the kiln so the temps were really higher and the cones confirmed that.
>
> Any help would be appreciated.
>
> Jennifer (in northern VT)
>
> Jennifer G. Buckner jennifer@buckner.cc
>
Stephani Stephenson on thu 24 jun 10
I claim no knowledge of the glaze or the clay.
but i do know that the clay you use can affect the glaze response in a BI=
=3D
G way.
same glaze can be completely different, one clay to the next.
so, i find glaze formulas pretty useless ,unless I know which claybody th=
=3D
ey
are fired on.
one possible explanation of the variation.
cheers
Stephani
John Hesselberth on thu 24 jun 10
On Jun 24, 2010, at 9:52 AM, Jennifer Buckner wrote:
> They both came out exactly the same. When I tested the
> glaze on smaller items, I also used Tucker's Mid Cal 5 and all 3 of
> them came out with a smooth semi-matte finish.
Jennifer, were your smaller items all test tiles that were sitting flat =3D
on a shelf? If so it could well be a heat work situation. Tiles flat on =3D
a shelf cool much more slowly because of the heat content of the shelf =3D
itself. You are soaking for 10 minutes and then crash cooling to 1800 if =
=3D
I remember correctly. But flat tiles sitting on a shelf will have a lot =3D
more time for the glaze to smooth out, particularly if it is barely =3D
melted.
John
John Hesselberth
http://www.masteringglazes.com
http://www.frogpondpottery.com
"I love everything that's old: old friends, old times, old manners, old =3D
books, old wines." Oliver Goldsmith, "She Stoops to Conquer" (1773)
Jennifer Buckner on thu 24 jun 10
At 01:23 AM 6/24/2010, Stephani Stephenson wrote:
>I claim no knowledge of the glaze or the clay.
>but i do know that the clay you use can affect the glaze response in
>a BIG way.
>same glaze can be completely different, one clay to the next.
>so, i find glaze formulas pretty useless ,unless I know which claybody the=
y
>are fired on.
>
>one possible explanation of the variation.
Thanks Stephani,
I know that the clay will affect the glaze, but I was using 2
different clays - Tucker's Mid Smooth Stone and Sheffield's 20266 ^6
porcelain. They both came out exactly the same. When I tested the
glaze on smaller items, I also used Tucker's Mid Cal 5 and all 3 of
them came out with a smooth semi-matte finish.
> cheers
>Stephani
Jennifer G. Buckner jennifer@buckner.cc
| |
|
