Jonathan Kaplan on thu 22 jun 00
snip....
>> fired at cone 06, 05, & 04. I then glaze fire at cone 04. I am using a
>> standard terra cotta and the Arbuckle white glaze. I have read mention of
>> something called dunting, but it only states that dunting or cracking is a
>> possible problem. I don't know if that is what is happening. I would
>> appreciate some help from anyone who knows about this. Thanks, Tonya
You may want to consider bisque firing at cone 04, and then glazing at a
lower temperature such as 06 or so. The 04 bisque will insure that the
offending materials in your terra cotta red clay, and there are alot, also
known as the LOI or loss on ignition, are completely burned out. If not,
the results show up in your glaze such as pinholing.
Most of the majolica galzes that we come across can be successfully used in
the cone 06 to 04 range because they are frit based.
If you bisque higher than 04 you may have difficulty with glaze application
as the body may be too tight. Most low temperature clays run a fairly tight
range for bisque: too low and the LOI doesn't happen completely. Too high
and the body tightens up and it may be difficult to get a good glaze
deposit. The other downside is that clay bodies in this range are really
quite structurally weak and very susceptible to delayed moisture crazing.
The best solution I have used is to add a percentage of frit 3124 to the
body to tighten it up and produce some glass phase. Works fine.
Dunting cracks are indeed cooling cracks and are easy to identify: the
crack is well defined and the cracked edges are sharp with no glaze roll
over into the crack. Curb that over anxious zeal and let the kiln cool by
itself until you can safely unload it with your bare hands.
Good luck!
Jonathan
Jonathan Kaplan, president
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
voice and fax 970 879-9139
jonathan@csn,net
http://www.sni.net/ceramicdesigin
Plant Location:
1280 13th Street Unit 13
Steamboat Springs CO 80487
(please use this address for all deliveries via UPS, comman carrier, FEd
Ex, etc.)
Tjo62@AOL.COM on thu 22 jun 00
I have been doing some majolica with small to med. sized bowls and mugs
without any problem. But when I fired some (several) large wheel thrown and
slab formed platters, they cracked completely into pieces, either in the
glaze firing or within one or two days after the glaze firing. I have bisque
fired at cone 06, 05, & 04. I then glaze fire at cone 04. I am using a
standard terra cotta and the Arbuckle white glaze. I have read mention of
something called dunting, but it only states that dunting or cracking is a
possible problem. I don't know if that is what is happening. I would
appreciate some help from anyone who knows about this. Thanks, Tonya
Bill Weaver on thu 22 jun 00
Tonya,
I have been using Linda's glaze for several years without problem. I did Have some
cracking similar to what you described when I fired to quickly, and did't soak at
1650 degrees for about 45min. I also experienced it once when I did't fire down but
simply turned everything off when cone 05 was tipping slightly. Are you using the
03 glaze or the the 06?
Tjo62@AOL.COM wrote:
> I have been doing some majolica with small to med. sized bowls and mugs
> without any problem. But when I fired some (several) large wheel thrown and
> slab formed platters, they cracked completely into pieces, either in the
> glaze firing or within one or two days after the glaze firing. I have bisque
> fired at cone 06, 05, & 04. I then glaze fire at cone 04. I am using a
> standard terra cotta and the Arbuckle white glaze. I have read mention of
> something called dunting, but it only states that dunting or cracking is a
> possible problem. I don't know if that is what is happening. I would
> appreciate some help from anyone who knows about this. Thanks, Tonya
>
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Ron Roy on mon 26 jun 00
This is a fit problem - the glaze is not contracting on cooling as much as
the clay and the difference is enough to break the pot.
The problem with low fire clays - they are not vitreous enough to keep
water out - water enters the pot over time - the clay rehydrates and gets
bigger - and delayed crazing is the result. To counteract this (delay the
crazing for as long as possible) we make the glaze have a low expansion. On
some bodies the result is glaze dunting.
It is possible to increase the expansion of that glaze so it will no longer
be a problem on that body.
There is a down side to this - your next batch of clay will be different -
that's the way it is at earthenware temperatures - and there is no way to
forecast clay/glaze fit so you might have to adjust again. It is the
problem with low fire clays and glazes.
My recommendation is to go to cone 6. You can use the same techniques
decorating and control the clay and glaze much better.
RR
>I have been doing some majolica with small to med. sized bowls and mugs
>without any problem. But when I fired some (several) large wheel thrown and
>slab formed platters, they cracked completely into pieces, either in the
>glaze firing or within one or two days after the glaze firing. I have bisque
>fired at cone 06, 05, & 04. I then glaze fire at cone 04. I am using a
>standard terra cotta and the Arbuckle white glaze. I have read mention of
>something called dunting, but it only states that dunting or cracking is a
>possible problem. I don't know if that is what is happening. I would
>appreciate some help from anyone who knows about this. Thanks, Tonya
>
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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