jklay on mon 4 oct 04
Anyone, =20
After a cursory search in the archives I came across someone who was =
also perplexed as to the lack of information on "dunting". Perhaps =
dunting is not the appropriate term for the problems I have had with =
cracking after the fire. I am well aware that a thick application of =
glaze on the inside and a thin one on the outside can cause cracking =
(dunting ?) some times years after the pot was fired.
I was reminded of that recently when I set a small, hard object down not =
too gently, with a thump, near one of my pieces. Simultaneously I heard =
that dreaded "ping:" Sure enough, a piece that had been fired at least =
ten years ago cracked. =20
I am about to send a stoneware recipe to my custom mixer and am =
wondering what body composition would best minimize this problem as I =
would like to use a thicker glaze on the inside and no glaze or a =
thinner glaze on the outside. The body I had been using contained =
feldspar but no flint. Flint was eliminated in prior formulations =
because of shivering. =20
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Ron Roy on mon 4 oct 04
Hi jklay,
I need to know - what temperature you are firing to - if the dunting only
happens with certain glazes - I am assuming that any glazes that craze will
not do this for instance.
The problem is probably a low expansion glaze on the inside of the ware -
and if there is any cristobalite in the body - that will make it worse. You
are right to get the silica out - and also make sure there is at least 10%
spar in the body - if you are firing above cone 9.
See Peter Sohngens article on the subject - Studio Potter vol 28 #1
If you care to send me your proposed body recipe I can give you an opinion
- if you care to send some glaze recipies I can probably tell you which
ones would most likely to cause dunting.
RR
>Anyone,
> After a cursory search in the archives I came across someone who was
>also perplexed as to the lack of information on "dunting". Perhaps
>dunting is not the appropriate term for the problems I have had with
>cracking after the fire. I am well aware that a thick application of
>glaze on the inside and a thin one on the outside can cause cracking
>(dunting ?) some times years after the pot was fired.
>I was reminded of that recently when I set a small, hard object down not
>too gently, with a thump, near one of my pieces. Simultaneously I heard
>that dreaded "ping:" Sure enough, a piece that had been fired at least
>ten years ago cracked.
> I am about to send a stoneware recipe to my custom mixer and am wondering
>what body composition would best minimize this problem as I would like to
>use a thicker glaze on the inside and no glaze or a thinner glaze on the
>outside. The body I had been using contained feldspar but no flint.
>Flint was eliminated in prior formulations because of shivering.
>
>
>
>---
>Outgoing mail is certified Virus Free.
>Checked by AVG anti-virus system (http://www.grisoft.com).
>Version: 6.0.752 / Virus Database: 503 - Release Date: 9/3/2004
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Carolynn Palmer on mon 4 oct 04
Having experienced dunting and also mixing my own clay body for over 30
years, I did what you did, removed the flint from my recipe.
It did not solve the problem completely, especially on thrown pieces over 8
inches in diameter.
I fire at cone 6 in an electric kiln. (Used to fire to 9 and 10 in the same
kilns, but have backed it down to 6 in an effort at economizing.)
Through a bit of research, I learned about Mullite and Kyanite. (Mullite is
calcined Kyanite.)
After adding 5% to 10 % of Mullite to my clay body, which contains a good
bit of Custer Feldspar, I believe I have this dunting under control.
(I read that Mullite is supposed to increase the pots resistance to thermal
shock during use - don't know about that - had no problems before adding
Mullite.)
And my cone 6 glazes still fit.
In my opinion there is not nearly enough information available to potters on
the recipes for clay bodies using currently available clays and additions.
-Carolynn Palmer, Somerset Center, Michigan
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