Dan Goodsell on thu 2 mar 00
I use a nice fat tenmoku with 10% Alberta Slip and 9% Red Iron Oxide. When I
first changed from Albany to Alberta I had no problems with glaze fit (though
the color was a bit different). Lately I have been having a serious
shivering problem on about 25% of the pieces with this glaze, particularly on
pots in the front-top area of my 46 cu. ft. downdraft kiln. My other glazes
seem fine. I reduce at cone 5 for 20 minutes and again at cone 11 for 20
minutes. My clay body is IMCO 8-11 Red 412. The other change I made was in
floor shelf configuration to accommodate a different size shelf. Since this
change I have had trouble reaching temperature with the final part of the
firing, from cone 7 to cone 11, taking up to 14 hours (ouch). I am changing
back to the old floor set up in hopes of speeding up the firing. My question
is has anyone had shivering problems they could attribute to Alberta Slip or
to a very long firing cycle? Any suggestions would be appreciated.
Dan Goodsell
Hank Murrow on fri 3 mar 00
>----------------------------Original message----------------------------
>I use a nice fat tenmoku with 10% Alberta Slip and 9% Red Iron Oxide. When I
>first changed from Albany to Alberta I had no problems with glaze fit (though
>the color was a bit different). Lately I have been having a serious
>shivering problem on about 25% of the pieces with this glaze, particularly on
>pots in the front-top area of my 46 cu. ft. downdraft kiln. My other glazes
>seem fine. I reduce at cone 5 for 20 minutes and again at cone 11 for 20
>minutes. My clay body is IMCO 8-11 Red 412. The other change I made was in
>floor shelf configuration to accommodate a different size shelf. Since this
>change I have had trouble reaching temperature with the final part of the
>firing, from cone 7 to cone 11, taking up to 14 hours (ouch). I am changing
>back to the old floor set up in hopes of speeding up the firing. My question
>is has anyone had shivering problems they could attribute to Alberta Slip or
>to a very long firing cycle? Any suggestions would be appreciated.
> Dan Goodsell
Dear Dan; I wonder if the longer firing schedule is leading to the
development of crystobalite in the body, and subsequent shivering. Hank in
Eugene
Craig Martell on fri 3 mar 00
Dan asked:
> My question
>is has anyone had shivering problems they could attribute to Alberta Slip or
>to a very long firing cycle?
Hi:
I'd wager a pint or two of Bishop's Finger Ale that your long firing to
cone 11 with a red claybody is causing cristobalite formation in the clay
which will cause some glazes to shiver. You may also see some thermal
shock to mugs and ovenware pots if you make that kinda stuff. Alberta
doesn't have a real radical coefficient of expansion so it's probably not
the culprit. It really depends on the total glaze composition too and you
don't say what that is.
Solutions may be to use another claybody or shorten the firing a bit and
fast cool. This will get rid of some of the cristobalite but maybe not
enough. If a claybody contains sufficient amounts of feldspar, the
cristobalite is normally neutralized by the body glass. If you really want
to know about the clay you can prepare fired samples to be tested with a
dilatometer. This will yield the absolute picture on cristobalite if
that's the problem.
regards, Craig Martell in Oregon
Ron Roy on fri 3 mar 00
The problem is probably the extended firing - lots of cristobalite is
produced over the long slow firing above 1100C - particularly with that
kind of high iron body.
I predict you will get back to normal with a faster firing from 1100C to
cone 11. Cristobalite is produced in reduction and in oxidation.
RR
>----------------------------Original message----------------------------
>I use a nice fat tenmoku with 10% Alberta Slip and 9% Red Iron Oxide. When I
>first changed from Albany to Alberta I had no problems with glaze fit (though
>the color was a bit different). Lately I have been having a serious
>shivering problem on about 25% of the pieces with this glaze, particularly on
>pots in the front-top area of my 46 cu. ft. downdraft kiln. My other glazes
>seem fine. I reduce at cone 5 for 20 minutes and again at cone 11 for 20
>minutes. My clay body is IMCO 8-11 Red 412. The other change I made was in
>floor shelf configuration to accommodate a different size shelf. Since this
>change I have had trouble reaching temperature with the final part of the
>firing, from cone 7 to cone 11, taking up to 14 hours (ouch). I am changing
>back to the old floor set up in hopes of speeding up the firing. My question
>is has anyone had shivering problems they could attribute to Alberta Slip or
>to a very long firing cycle? Any suggestions would be appreciated.
> Dan Goodsell
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Dan Goodsell on sat 4 mar 00
Thanks for the input on my question re: shivering. The formula for my cone 11
tenmoku is:
Custer Feldspar 43
Whiting 13
EPK 15
Silica 19
Red Iron Oxide 9
Alberta Slip 10
As both Craig and Hank suggest I will try to get my firing cycle back to a
shorter length to remedy the shivering problem. I am not familiar with a
"dilatometer" or with Bishop's Finger Ale, though I am near Chico, Ca. home
of Sierra Nevada Brewery, so maybe a Pale Ale or Clebration Ale will do.
Dan Goodsell
Craig Martell on mon 6 mar 00
Hello again Dan:
OK, I'll knock of the comments about esoteric beers and oddball equipment
for evaluating claybody performance. Sierra Nevada ales are pretty good
though!
Your glaze is posted below in receipe and seger formula. It's a good,
balanced glaze. The alumina is just a tad bit high but if your firing to
cone 11 I think it's fine. Probably would be at cone 10 as well. The
expansion is not in the range that would make me think that shivering is
the fault of the glaze itself. I would think for sure that the clay is
developing a lot of cristobalite and this is causing the
shivering..........regards, Craig Martell in Oregon
custer feldspar..... 43.00 39.09%
whiting............. 13.00 11.82%
alberta slip........ 10.00 9.09%
epk kaolin.......... 15.00 13.64%
silica.............. 19.00 17.27%
iron oxide red...... 10.00 9.09%
========
110.00
BaO 0.00* 0.02%
CaO 0.63* 7.92%
MgO 0.04* 0.34%
K2O 0.22* 4.71%
Na2O 0.10* 1.43%
TiO2 0.01 0.10%
Al2O3 0.63 14.30%
P2O5 0.00 0.04%
SiO2 4.54 60.88%
Fe2O3 0.29 10.27%
Cost/kg 0.48
Si:Al 7.23
SiB:Al 7.23
Expan 7.63
Ron Roy on mon 6 mar 00
Hi dan,
A dilatometer is a machine for measuring how much a material expands and
contracts when heated and cooled - in our case clay and glazes.
I have just compared your tenmoku with mine and see the expansion is qiute
a bit lower than mine - In fact I have tested my celadon - which has an
expansion close to yours - on a stoneware body that I know has cristobalite
in it - Cause I have a dilatometer - and the glaze cracked the clay - so I
am as certain as I can be that the problem is your extended firing.
If you try some porcelain with that glaze - you will probably find it fits
well - and there is never any cristobalite in feldspar fluxed porcelaine.
RR
>----------------------------Original message----------------------------
>Thanks for the input on my question re: shivering. The formula for my cone 11
>tenmoku is:
> Custer Feldspar 43
> Whiting 13
> EPK 15
> Silica 19
> Red Iron Oxide 9
> Alberta Slip 10
>
>As both Craig and Hank suggest I will try to get my firing cycle back to a
>shorter length to remedy the shivering problem. I am not familiar with a
>"dilatometer" or with Bishop's Finger Ale, though I am near Chico, Ca. home
>of Sierra Nevada Brewery, so maybe a Pale Ale or Clebration Ale will do.
>Dan Goodsell
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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