Gary W. Wagoner on mon 20 may 96
Marvin Bartell mentions having formulated a body which resists thermal
shock well, but which has a tendency to crack in drying. The formula given
does look like one which would have considerable drying shrinkage, which he
is reluctant to remedy with grog or sand, since he worries that might
adversely affect the thermal shock resistance. I use a similar body which
also had a tendency to crack in large forms (platters in particular); I
solved the problem by calcining some of the clay which goes into the mix.
In my case, I am using about 8-10 per cent of the total clay weight as
calcined Hawthorn fireclay. This reduced the drying shrinkage enough to
eliminate the cracking without making the clay unpleasantly coarse for
throwing.In his body calcined clay might have the additional advantage of
having a coefficient of expansion identical to the remainder of clay in the
body formula.
Gary Wagoner
Auburn University
SANTERRE ROBERT on tue 21 may 96
Gary,
Great comments on a practical approach to solving the "pesky" cracking problem.
Two questions that could shed further light for some of us somewhere on the
learning curve:
1.) What in the formula given suggested to you there would be a drying
shrinkage problem?
2.) What is the actual process (start to finish) you use to do "calcining" of
the Hawthorne fire clay? I've seen the term "calcined" used in several posts
recently, but am unfamiliar with the actual process.
Bob
santerre_robert@lilly.com
>----------------------------Original message----------------------------
>Marvin Bartell mentions having formulated a body which resists thermal
>shock well, but which has a tendency to crack in drying. The formula given
>does look like one which would have considerable drying shrinkage, which he
>is reluctant to remedy with grog or sand, since he worries that might
>adversely affect the thermal shock resistance. I use a similar body which
>also had a tendency to crack in large forms (platters in particular); I
>solved the problem by calcining some of the clay which goes into the mix.
>In my case, I am using about 8-10 per cent of the total clay weight as
>calcined Hawthorn fireclay. This reduced the drying shrinkage enough to
>eliminate the cracking without making the clay unpleasantly coarse for
>throwing.In his body calcined clay might have the additional advantage of
>having a coefficient of expansion identical to the remainder of clay in the
>body formula.
>Gary Wagoner
>Auburn University
Gary W. Wagoner on tue 21 may 96
>----------------------------Original message----------------------------
>Gary,
>
>Great comments on a practical approach to solving the "pesky" cracking problem.
>Two questions that could shed further light for some of us somewhere on the
>learning curve:
>
>1.) What in the formula given suggested to you there would be a drying
>shrinkage problem?
>
>2.) What is the actual process (start to finish) you use to do "calcining" of
>the Hawthorne fire clay? I've seen the term "calcined" used in several posts
>recently, but am unfamiliar with the actual process.
>
Robert; those are some questions I should have anticipated.
1.)The formula was comprised of Goldart and ball clay primarily, both of
which are quite plastic and have considerable drying shrinkage. Commonly
some fire clay (less plastic and lower shrinkage) would be used to lessen
the tendency to crack.The calcined clay I use has no plasticity and reduces
shrinkage even more.
2.)Calcining is merely firing of the clay in its raw, powder form. I have a
large bisque-fired jar I fill with the Hawthorn clay and run it through an
ordinary bisque. At that point, the chamical water is broken off of the
clay molecule, making it non-plastic but still a fine powder easily mixed
into a clay formula .Many slip recipies call for calcined Kaolin, which is
produced in the same fashion and for the same reason- to reduce drying
shrinkage (in that case of a slip which is to be applied to a dry or
bisqued piece where normal shrinkage would make the slip peel.)
Gary Wagoner
Auburn University
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