Tony Hansen on fri 7 nov 03
of clay body cone6,10, 12 etc.
On average the higher temperatures have potential to
be stronger. However I have measured cone 02 bodies that
are stronger than the average cone 10 body.
Alumina can have a compressive strength of up to 500,000 psi
partly by virtue of being fired to a very high temperature.
I have been under the impression that a clay body that is formulated to
reach it\'s optimum point of vitrification at cone 6 is just as strong as one
that is formulated to vitrify at cone 10 or higher. Have I misunderstood
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
----- Original Message -----
From: \"John Hesselberth\"
Sent: Thursday, November 06, 2003 6:26 PM
Subject: Re: cone 6 as ovenware?
> On Thursday, November 6, 2003, at 03:32 PM, John Rodgers wrote:
> > But vitrified claybodies should not have this problem. Water is not
> > entrained. Vitrification can be accomplished at cone 6, but at cones
> > like 10,11, and 12, it\'s the best, and the ware is a lot stronger to
> > resist breakage of any kind.
> I could easily make the case that cone 6 is preferred over cone 10-12
> for ovenware. First it can be adequately vitrified with no problem--not
> by using a cone 10 body at cone 6, but by using a well formulated cone
> 6 body. Second, cone 6 does not normally have a cristobalite
> problem--cone 10 bodies can easily have this problem if they contain
> excess silica and that is bad news for oven ware. The cristoballite
> phase conversion occurs at 226 deg C (439 deg F)--within the range of
> normal oven temperatures. This results in a rapid 3% change in volume.
> That can easily crack a casserole dish. Of course, properly formulated,
> cone 10 bodies should be OK also, but it is a lot easier for a potter
> to tell if a body is properly vitrified than if it (at cone 10-12) has
> a cristobalite problem.
> So my answer to your question is that cone 6 stoneware is fine for
> ovenware if you make sure the glaze is durable and fits well, and the
> body is well vitrified (below 3% water absorption). It may well be
> preferred over cone 10-12.
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Ron Roy on fri 7 nov 03
of clay body cone6,10, 12 etc.
This has to do with other than maturing temperature - more to do with
mullite developement, the degree of vitrification, cristobalite, free
quartz and glaze fit.
In industry they are obsessed with low expansion bodies - that way - when
unevenly heated - because the amount of expansion is not much or none -
there is no breakage. Finding bodies with lower expansion is part of the
trick - when you find em - the probelm becomes - how to kake glazes that
Underfired clay has better ability to cope with heat shock because it is
more flexible but take in water and will be a problem.
Over vitrified clay are more brittle (read glassy) and can't take uneven
heating because they are less flexible.
I think most failures are due to improper glaze fit but that is not the
only reason ware fails in the oven.
I usually say - keep the clay on the thick side and the glaze on the thin
side - get the clay/glaze fit right, shoot for a clay absorption of about
2%, make shapes that transfer stress in an even way and find some lower
> I have been under the impression that a clay body that is formulated to
>reach it's optimum point of vitrification at cone 6 is just as strong as one
>that is formulated to vitrify at cone 10 or higher. Have I misunderstood
>Craig Dunn Clark
15084 Little Lake Road