Paul Lewing on tue 20 jan 98
A friend of mine has asked me to help him fix a glaze so he can get UL
approval on his handthrown sinks. We ran the numbers on it today, and
I think I may have solved it, but I wonder what others with more
experience with this sort of thing may think.
He doesn't want me to publish the recipe, but I'll tell you what I
know. He uses both a porcelain body and the same porcelain body with
20% Ione grog added, and fires to ^10 in reduction. He uses the same
glaze on both bodies, and even under a field microscope after the
application of India Ink, there are no crazes visible on either body.
However, when the test pieces are boiled and frozen, the porcelain
body passes the test, but the one with the grog does not.
His glaze has a coefficient of expansion of 6.07. It has an amount of
magnesium very slightly above the limit formulas that I use, but
everything else is within the limits, although the alumina and silica
are in the upper part of the limits. Based on my very limited
experience at fitting glazes to clay bodies (something I usually don't
care about) and the results of a series of tests he'd run, we decided
the COE was too low. By taking out 5% of the EPK and 5% silica, we
kept the Si:Al ratio about the same, and upped the COE to 6.28.
So here are my questions:
1. Does 6.07 seem like too low an expansion number, and is an increase
to 6.28 enough of a change to work here, or too much?
2. Why does adding Ione grog, which is basically Al and Si, raising
the COE? Or is it? Wouldn't adding Al and Si lower the COE of the
clay body, therefore lowering the COE of a glaze that would fit it?
3. He read in Hamer that soaking in water can change the expansion of
a porous body by swelling up the body and loosening the fit of a glaze
under compression. Is this true, and could that be why the glaze fits
both bodies dry, but does not fit the more porous grog body after
soaking?
4. Should he just take my suggestion, which is to only send them
samples of the body that passes the test, and assume they'll never see
or recognize the fact that he uses two bodies. After all, it's not
like it's a porcelain and Rod's Bod were're talking about here. The
two bodies do look very much alike.
Thanks for all the good advice I know we'll get.
Paul Lewing, Seattle
DONPREY on wed 21 jan 98
In a message dated 1/20/98 10:45:54 AM, you wrote:
<<3. He read in Hamer that soaking in water can change the expansion of
a porous body by swelling up the body and loosening the fit of a glaze
under compression. Is this true, and could that be why the glaze fits
both bodies dry, but does not fit the more porous grog body after
soaking?>>
Paul, the boiling/freezing procedure is essentially an accelerated aging test.
I suspect that given enough expansion/contraction cycles the glaze would craze
on both bodies. Opening the body with grog just lowers the threashold. That
doesn't mean, of course, that the glaze can't be fixed to pass a one-cycle
test.
Don Prey In Oregon
Kris Baum on thu 22 jan 98
Paul Lewing sent a question about his friend with the sinks ...
I wonder why a sink would get UL (Underwriters Laboratories?)
approval? I thought they just tested appliances for electrical
safety. Please explain! Maybe we should be getting UL approval and
don't know it!
Also, I have to question the ethics of getting the porcelain sink
"approved" and then slipping in the grogged sinks under the same
approval, even though they wouldn't pass, because no one could tell
them apart? If the sinks are virtually identical in appearance
anyway, why not just limit production to the porcelain and not use
the grogged porcelain?
Kris
mailto:shubunki@erols.com
David Hewitt on fri 23 jan 98
Paul,
If you wish to reduce / eliminate crazing by adjusting a glaze, I think
you need to reduce the coefficient of expansion so as to place the glaze
under less tension and desirably under slight compression.
I would have thought that Hamer's comments about soaking in water can
change the coefficient of expansion of a body was really referring to
earthenware bodies.
My guess - I stress *guess* - is that the grog has changed the expansion
of the body slightly.
David
In message , Paul Lewing writes
>----------------------------Original message----------------------------
>A friend of mine has asked me to help him fix a glaze so he can get UL
>approval on his handthrown sinks. We ran the numbers on it today, and
>I think I may have solved it, but I wonder what others with more
>experience with this sort of thing may think.
>
>He doesn't want me to publish the recipe, but I'll tell you what I
>know. He uses both a porcelain body and the same porcelain body with
>20% Ione grog added, and fires to ^10 in reduction. He uses the same
>glaze on both bodies, and even under a field microscope after the
>application of India Ink, there are no crazes visible on either body.
>However, when the test pieces are boiled and frozen, the porcelain
>body passes the test, but the one with the grog does not.
>
>His glaze has a coefficient of expansion of 6.07. It has an amount of
>magnesium very slightly above the limit formulas that I use, but
>everything else is within the limits, although the alumina and silica
>are in the upper part of the limits. Based on my very limited
>experience at fitting glazes to clay bodies (something I usually don't
>care about) and the results of a series of tests he'd run, we decided
>the COE was too low. By taking out 5% of the EPK and 5% silica, we
>kept the Si:Al ratio about the same, and upped the COE to 6.28.
>
>So here are my questions:
>1. Does 6.07 seem like too low an expansion number, and is an increase
>to 6.28 enough of a change to work here, or too much?
>
>2. Why does adding Ione grog, which is basically Al and Si, raising
>the COE? Or is it? Wouldn't adding Al and Si lower the COE of the
>clay body, therefore lowering the COE of a glaze that would fit it?
>
>3. He read in Hamer that soaking in water can change the expansion of
>a porous body by swelling up the body and loosening the fit of a glaze
>under compression. Is this true, and could that be why the glaze fits
>both bodies dry, but does not fit the more porous grog body after
>soaking?
>
>4. Should he just take my suggestion, which is to only send them
>samples of the body that passes the test, and assume they'll never see
>or recognize the fact that he uses two bodies. After all, it's not
>like it's a porcelain and Rod's Bod were're talking about here. The
>two bodies do look very much alike.
>
>Thanks for all the good advice I know we'll get.
>
>Paul Lewing, Seattle
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
URL http://digitalfire.com/education/people/hewitt.htm
Tony Hansen on fri 23 jan 98
>So here are my questions:
>1. Does 6.07 seem like too low an expansion number, and is an increase
>to 6.28 enough of a change to work here, or too much?
I wouldn't worry so much about the actual figure as testing the ware
to see if the glaze fits.
>3. He read in Hamer that soaking in water can change the expansion of
>a porous body by swelling up the body and loosening the fit of a glaze
>under compression. Is this true, and could that be why the glaze fits
>both bodies dry, but does not fit the more porous grog body after
>soaking?
That's it. Sanitary ware has to be very close to zero porosity.
The insulator and sanitary ware industries use a fushine dye test
to check porosity. I have the procedure written up somewhere. Small
chips of the porcelain are put in this dye and then under high
pressure for many hours. Breaking a piece allows you to see how
far the dye penetrates into the surface. I work for a company that
that does this with a simple piston assembly under a hydraulic press.
--
-------
T o n y H a n s e n thansen@digitalfire.com
Get INSIGHT, Magic of Fire at http://digitalfire.com
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