John Sankey on wed 21 nov 07
It is generally believed that the expansion for a stable glaze
must be 1-10% less than that of the clay. Glaze expansions much
less than this can result in shivering - the breaking off of
sharp pieces of glaze under thermal stress. Glaze expansions
higher than that of the clay can cause crazing - fine cracks in
the glaze surface that will trap contaminants, thus making the
item unsuitable for containing food.
However, analysis of successful glazes from the Clayart archives
calls this into question - glazes can have expansions well
outside this range and still be successful. The mean COE of these
glazes is 6.9x10-6/K as expected, but the range is far wider than
10%: +-1.9
It should be noted, however, that almost none of the testers
performed any functional tests, such as a freezer to boiling
water cycle.
See the graph at
http://sankey.ws/glazecoe.gif
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John Hesselberth on thu 22 nov 07
Hi John,
How did you get the expansion data for the hundreds of clay bodies
they were used on to compare with the glaze? Clay bodies vary quite
widely in thermal expansion characteristics.
Regards,
John
On Nov 21, 2007, at 4:42 AM, John Sankey wrote:
> However, analysis of successful glazes from the Clayart archives
> calls this into question - glazes can have expansions well
> outside this range and still be successful. The mean COE of these
> glazes is 6.9x10-6/K as expected, but the range is far wider than
> 10%: +-1.9
John Hesselberth
www.frogpondpottery.com
"Man is a tool-using animal....without tools he is nothing, with
tools he is all" .... Thomas Carlyle
John Sankey on sat 24 nov 07
"How did you get the expansion data for the hundreds of clay bodies
they were used on to compare with the glaze? Clay bodies vary quite
widely in thermal expansion characteristics."
Almost all the tests were done by one person, Alisa Clausen, on
what she describes as a "white, mid-range stoneware". I'm not
aware of any commercial stoneware that has expansion outside
6.5-7.7; the data includes glazes as low as 3.5 that didn't
shiver, and as high as 13 that didn't craze. Again, I stress that
Alisa did not do freezer to boiling water tests, and that my
calculated expansions use Appen coefficients normalised to Ron's
data. I submit however that the range observed is well beyond
uncertainties of calculation or body.
If anyone is willing to do more thorough survival testing of
extreme glazes, I could make up a list of the extreme expansion
glaze names with calculated expansions for them.
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John Hesselberth on sat 24 nov 07
On Nov 24, 2007, at 8:07 AM, John Sankey wrote:
> the data includes glazes as low as 3.5 that didn't
> shiver, and as high as 13 that didn't craze.
And one more question. Did you screen out all of the non glossy
glazes since we know COE calculations are only reasonable
approximations for glossy glazes?
John
John Hesselberth
www.frogpondpottery.com
"Man is a tool-using animal....without tools he is nothing, with
tools he is all" .... Thomas Carlyle
Ron Roy on sat 24 nov 07
Hi John,
How ware is glazed has a bearing in this. If a low expansion glaze is
applied inside and out the stress tends to equalize and the shivering does
not show up till the difference is too great.
Use a low expansion glaze as a liner and a crazed glaze on the outside and
the problem will show up much sooner.
RR
>It is generally believed that the expansion for a stable glaze
>must be 1-10% less than that of the clay. Glaze expansions much
>less than this can result in shivering - the breaking off of
>sharp pieces of glaze under thermal stress. Glaze expansions
>higher than that of the clay can cause crazing - fine cracks in
>the glaze surface that will trap contaminants, thus making the
>item unsuitable for containing food.
>
>However, analysis of successful glazes from the Clayart archives
>calls this into question - glazes can have expansions well
>outside this range and still be successful. The mean COE of these
>glazes is 6.9x10-6/K as expected, but the range is far wider than
>10%: +-1.9
>
>It should be noted, however, that almost none of the testers
>performed any functional tests, such as a freezer to boiling
>water cycle.
>
>See the graph at
>http://sankey.ws/glazecoe.gif
>
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>Include 'Byrd' in the subject line of your reply
>to get through my spam filter.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
John Sankey on sun 25 nov 07
"Did you screen out all of the non glossy glazes since we know
COE calculations are only reasonable approximations for glossy
glazes?"
I tried that - the mean is lower (statistically nonsignificant
amount), the variance was the same. Besides, I understand that
linear superposition is reasonably valid for all glazes that are
melted overall; glazes that are matte because of underfiring are
noted in the database and excluded from calculations.
"If a low expansion glaze is applied inside and out the stress
tends to equalize and the shivering does not show up till the
difference is too great."
Alisa responds: "All of my tests are done on an extruded tube.
Only the outside of the tile has glaze on it. So, it would be
safe to say that if my glazes were going to shiver, they would
have by now, I think. There is no equalizing of the glazes
because they are glazed only on one side, the outside."
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