Talbott on wed 5 mar 97
Tony...
WOOOO!!! ... How many glazed bars broke at 250 lbs? Could there be
some variable other than glaze fit that caused perhaps only that one bar to
break? How many bars were in each test group? Seems out of line, as in
the laws of physics. Do not remember the stress and strain formulas but
the glazed surface can exert only so much force. I think there must be
some variable that you are overlooking that causes these results to vary so
much. What were the dimensions of these bars and what was the average
glaze (vitreous) thickness after firing? Something does not seem to add
up... think about it.... Is there a physicist in the house? ... Marshall
>----------------------------Original message----------------------------
>I did some fired strength testing bars today that were incredible.
>I had about 6 different sets of cone 06 glazed bars, all the glazes were
>beautiful, no crazing or shivering. The bar groups broke very consistently
>indicating reliable results.
>
>The glazeless bars broke at 1200 lbs.
>The strongest glazed bars were 1600 lbs.
>The weakest were 250!
>
>Again it appears that the single most profound effect on ware strength
>is glaze fit. And you simply cannot determine if your glazed ware is
>strong or weak by simply doing a visual inspection. In a worst case
>scenario here a potter could increase his ware strength 6-fold by a
>simple glaze recipe adjustment!
>
>--
>=================================================================
>Tony Hansen, IMC - Get INSIGHT 5 beta or The Magic of Fire II at
>http://digitalfire.com or http://www.ceramicsoftware.com
1ST ANNUAL CLAYARTERS' GALLERY - NAPLES, MAINE (Summmer 1997)
{contact me directly for more information}
Celia & Marshall Talbott
Pottery By Celia
Route 114
P.O. Box 4116
Naples, Maine 04055-4116
(207)693-6100 voice and fax
clupus@ime.net
Clayarters Live Chat-Room 10:00 pm Eastern Standard Time on Saturday Nites
SHORT CUT if the room has already been created then go to:
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------------------------------------------------------------------------------
Gavin Stairs on wed 5 mar 97
At 08:49 AM 05/03/97 EST, you wrote:
>----------------------------Original message----------------------------
>Tony...
> WOOOO!!! ... How many glazed bars broke at 250 lbs? Could there be
>some variable other than glaze fit that caused perhaps only that one bar to
>break? How many bars were in each test group? Seems out of line, as in
>the laws of physics. Do not remember the stress and strain formulas but
>the glazed surface can exert only so much force. I think there must be
>some variable that you are overlooking that causes these results to vary so
>much. What were the dimensions of these bars and what was the average
>glaze (vitreous) thickness after firing? Something does not seem to add
>up... think about it.... Is there a physicist in the house? ... Marshall
>
Hi Marshall,
I'm not a physicist, but an engineer who works in high energy physics. Will
that do?
The reason for the dependence of strength on glaze fit is not very
mysterious if you know how brittle materials like ceramic break. The
characteristic of a brittle material is that it is elastic right up to the
point that it breaks. When it breaks, all of the energy that has been
stored in elastic deformation is then released. Brittle fractures tend to
start in some little flaw, and then propagate. The elastic energy that is
released does the propagating, so once the little flaw begins to crack, the
rest happens very fast. Like breaking glass. If you get a little local
failure in a car windshield, it will act just like this, but in sort of stop
action, since the amount of elastic energy available is slight. Just where
the stone hits there will be a few radiating cracks. If you press against
the glass, they will propagate before your very eyes. It takes very little
extra energy. That's why, if you get a stone ding on your windshield, you
have to get it repaired right away, or it will soon spread too far to fix.
The premature failure of poorly fitting glazes arises because the little
flaws I was talking about are usually on the surface of the pot or bar.
Where the glaze is. So the cracks originate in the glaze, at the surface.
If the glaze expands (contracts) more than the clay, it will be in tension
when it cools down. This preload means that part of the capacity of the
glaze to resist breakage is already used up. If it is extreme, the glaze
may already be broken (crazed) when you take it out of the kiln. Or, it may
craze at some future date. Every craze line is a flaw which can initiate a
crack. This is a case very like the windshield. It takes a very small
pressure to propagate the crack. That is, the test bar (or teapot) is weak,
and will show an anomalously low modulus of rupture. If the glaze is a good
fit, it will be under no preload stress, and the bar will break at
approximately the same stress as the unglazed bar. Actually, since the
glaze surface may be smoother, with fewer flaws, it may not break as easily
as the bare bar. A glaze which contracts less than the clay will also be
under pre-stress when it is removed from the kiln, but this time in
compression. Clays and glasses are less susceptible to failure under
compression that under tension, so this may tend to strengthen the material,
like the prestressed glass in you car windows. (The windshield is not
prestressed: it is laminated. The side and rear windows are prestressed,
which means that they have been cooled rapidly by means of air jets during
manufacture, so that the outer layer passed the solid point before the
inside, resulting in the outside layer being compressed when the liquid
glass inside cools and shrinks.) Like these windows, the ware is stronger,
because the Outside layer, where the breaks occur, doesn't come into tension
until the prestress is overcome. But, when it does break, there is more
elastic energy to propagate the cracks, so they tend to be more destructive.
The pot may shatter.
I have been careful to say "usually" when locating the flaws at the surface.
In the case of the crazing glaze, this is almost always the case. In the
case of the compressed glaze, there is another common failure mode which
doesn't involve the surface. While the glaze is under compression, the clay
right next to it is under tension. If these forces are great enough, the
clay may fail in tension before the glaze fails in compression. Or the
glaze may fail in compression. The pot may crack, the glaze may spall off
flakes at the surface, or it may crack at the interface, leaving hollows
under the glaze.
There is an impact test which can tell you which condition your glaze/ware
is in. If you do what the stone did to your windshield, that is hit the
surface with a sharp object, you will see a characteristic breakage pattern
in the glaze. If the glaze is under compression, the cracks will be
circular, with the impact point at the center. If the glaze is under
tension, they will radiate. If the glaze is a good fit, there will be both
radiating and circular cracks. In the extreme cases, these patterns will be
very extensive, or the whole piece may shatter. The optimal fit is the one
where the pattern is confined to the immediate vicinity of the impact, and a
heavier blow is needed to start the fracture. This is a moderate
precompression, or a fit in which the glaze contracts slightly less than the
clay beneath it. Ideally, such a test should be done on the pot itself, so
that all of the geometric complexities are included.
The standard bar rupture test that Tony Hansen was doing measures one
particular aspect of the problem: the apparent tensile failure stress of the
glazed (or unglazed) standard specimen. This modulus of rupture permits
different cases to be compared in a standard way. Glaze fit can be
estimated by means of the difference between the modulus of rupture of
glazed and unglazed specimens. Note, however, the weasel words above: for
a practicing studio potter, it is perhaps better to do an impact test on
samples of the actual ware. If you can bear it.
Gavin
Tony Hansen on thu 6 mar 97
> WOOOO!!! ... How many glazed bars broke at 250 lbs? Could there be
> some variable other than glaze fit that caused perhaps only that one bar to
> break? How many bars were in each test group?
Very perceptive. There were 5 bars in each group and they broke producing
consistent results. The 250 was an average of the low group.
However I did overlook the fact that there were two bodies involved. Both
were talc/ball clay mixes and were visually iddentical. Yet the one body was
almost twice as strong as the other. So you are right,
this did throw out the results a bit. However the fact remains that you or
I could just as easily be using the 250lb combination as the 1600lb one and
never know the difference until we strength tested. This case proved that
by switching clay bodies you could double the strength of your ware, by optimizi
your glaze on the low strength body you could double it, by switching bodies and
optimizing the glaze you could quintuple it.
--
=================================================================
Tony Hansen, IMC - Get INSIGHT 5 beta or The Magic of Fire II at
http://digitalfire.com or http://www.ceramicsoftware.com
Denis Whitfield on thu 6 mar 97
Marshal is right to be a bit sceptical about the results if they are based
on single test bars. It is normal to use a MINIMUM of ten bars of unglazed
and ten of glazed and then to compare the AVERAGE of the breaking strengths.
If there is a difference between the averages, statistical analysis can be
applied to test the degree of certainty that the difference is a function of
the glaze/no glaze rather than chance.
Denis
>----------------------------Original message----------------------------
>Tony...
> WOOOO!!! ... How many glazed bars broke at 250 lbs? Could there be
>some variable other than glaze fit that caused perhaps only that one bar to
>break? How many bars were in each test group? Seems out of line, as in
>the laws of physics. Do not remember the stress and strain formulas but
>the glazed surface can exert only so much force. I think there must be
>some variable that you are overlooking that causes these results to vary so
>much. What were the dimensions of these bars and what was the average
>glaze (vitreous) thickness after firing? Something does not seem to add
>up... think about it.... Is there a physicist in the house? ... Marshall
>
>>----------------------------Original message----------------------------
>>I did some fired strength testing bars today that were incredible.
>>I had about 6 different sets of cone 06 glazed bars, all the glazes were
>>beautiful, no crazing or shivering. The bar groups broke very consistently
>>indicating reliable results.
>>
>>The glazeless bars broke at 1200 lbs.
>>The strongest glazed bars were 1600 lbs.
>>The weakest were 250!
>>
>>Again it appears that the single most profound effect on ware strength
>>is glaze fit. And you simply cannot determine if your glazed ware is
>>strong or weak by simply doing a visual inspection. In a worst case
>>scenario here a potter could increase his ware strength 6-fold by a
>>simple glaze recipe adjustment!
>>
>>--
>>=================================================================
>>Tony Hansen, IMC - Get INSIGHT 5 beta or The Magic of Fire II at
>>http://digitalfire.com or http://www.ceramicsoftware.com
>
> 1ST ANNUAL CLAYARTERS' GALLERY - NAPLES, MAINE (Summmer 1997)
> {contact me directly for more information}
> Celia & Marshall Talbott
> Pottery By Celia
> Route 114
> P.O. Box 4116
> Naples, Maine 04055-4116
> (207)693-6100 voice and fax
> clupus@ime.net
>
> Clayarters Live Chat-Room 10:00 pm Eastern Standard Time on Saturday Nites
>
> SHORT CUT if the room has already been created then go to:
>
> http://webchat9.wbs.net/webchat3.so?Room=PRIVATE_Clayarters
>
>
> or if room has not been created go to WBS at:
> http://webchat5.wbs.net/webchat3.so
>
> and set up a "Private Room"... just use the name: Clayarters
>------------------------------------------------------------------------------
>
>
****************************************************************************
DR DENIS WHITFIELD
Senior Lecturer
Co Director, Centre for Ceramic Research, Design and Production
Department of Visual and Peforming Arts
University of Western Sydney, Macarthur
PO Box 555
CAMPBELLTOWN NSW 2560
AUSTRALIA
email: d.whitfield@uws.edu.au
phone: 02 97729345
fax: 02 97723244
****************************************************************************
Cindy on fri 7 mar 97
Okay, Gavin.
I'm impressed (and I mean it), but a little confused. Does this mean that
if I see circular cracks, I need to increase or decrease my glaze
expansion? My mind just doesn't work the same way yours does, apparently.
>
This is too much convolution for me to follow without sitting down and
making a chart. Help, please!
Cindy
> There is an impact test which can tell you which condition your
glaze/ware
> is in. If you do what the stone did to your windshield, that is hit the
> surface with a sharp object, you will see a characteristic breakage
pattern
> in the glaze. If the glaze is under compression, the cracks will be
> circular, with the impact point at the center. If the glaze is under
> tension, they will radiate. If the glaze is a good fit, there will be
both
> radiating and circular cracks. In the extreme cases, these patterns will
be
> very extensive, or the whole piece may shatter. The optimal fit is the
one
> where the pattern is confined to the immediate vicinity of the impact,
and a
> heavier blow is needed to start the fracture. This is a moderate
> precompression, or a fit in which the glaze contracts slightly less than
the
> clay beneath it. Ideally, such a test should be done on the pot itself,
so
> that all of the geometric complexities are included.
>
Ron Roy on fri 7 mar 97
>Hi Marshall,
>
>I'm not a physicist, but an engineer who works in high energy physics. Will
>that do?
>
>The reason for the dependence of strength on glaze fit is not very
>mysterious if you know how brittle materials like ceramic break. The
>characteristic of a brittle material is that it is elastic right up to the
>point that it breaks. When it breaks, all of the energy that has been
>stored in elastic deformation is then released. Brittle fractures tend to
>start in some little flaw, and then propagate. The elastic energy that is
>released does the propagating, so once the little flaw begins to crack, the
>rest happens very fast. Like breaking glass. If you get a little local
>failure in a car windshield, it will act just like this, but in sort of stop
>action, since the amount of elastic energy available is slight. Just where
>the stone hits there will be a few radiating cracks. If you press against
>the glass, they will propagate before your very eyes. It takes very little
>extra energy. That's why, if you get a stone ding on your windshield, you
>have to get it repaired right away, or it will soon spread too far to fix.
>
>The premature failure of poorly fitting glazes arises because the little
>flaws I was talking about are usually on the surface of the pot or bar.
>Where the glaze is. So the cracks originate in the glaze, at the surface.
and there was lots more after this.
Thanks Gavin - I understand it better now. One little addition - if you cut
clay glazed with a crazing glaze and look at it with enough magnification
you will see: As the glaze fractures to relieve the tension it also pulls
the body apart (micro crack) along the craze line. No doubt these "body
faults" contribute to the phenomena that Gavin is explaining.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Talbott on fri 7 mar 97
Tony... For the life of me I still can not understand how proper glaze fit
can so dramatically effect the ware strength...I can understand a 10%-20%
increase from worst to best...but beyond that I am quite skeptical.
A couple of questions: What is the thickness of the clay bars? What is
the approximate thickness of the glaze being applied before firing? What
are the firing temps.. both bisque and glaze? What is the type of claybody
being used for test? What is the glaze material being used. Finally how
is the breaking strength actually measured? Sorry to be such a busy
body...just my nature..
Best Wishes... Marshall
>----------------------------Original message----------------------------
>> WOOOO!!! ... How many glazed bars broke at 250 lbs? Could there be
>> some variable other than glaze fit that caused perhaps only that one bar to
>> break? How many bars were in each test group?
>
>Very perceptive. There were 5 bars in each group and they broke producing
>consistent results. The 250 was an average of the low group.
>However I did overlook the fact that there were two bodies involved. Both
>were talc/ball clay mixes and were visually iddentical. Yet the one body was
>almost twice as strong as the other. So you are right,
>this did throw out the results a bit. However the fact remains that you or
>I could just as easily be using the 250lb combination as the 1600lb one and
>never know the difference until we strength tested. This case proved that
>by switching clay bodies you could double the strength of your ware, by
>optimizi
>your glaze on the low strength body you could double it, by switching
>bodies and
>optimizing the glaze you could quintuple it.
>
>--
>=================================================================
>Tony Hansen, IMC - Get INSIGHT 5 beta or The Magic of Fire II at
>http://digitalfire.com or http://www.ceramicsoftware.com
1ST ANNUAL CLAYARTERS' GALLERY - NAPLES, MAINE (Summmer 1997)
{contact me directly for more information}
Celia & Marshall Talbott
Pottery By Celia
Route 114
P.O. Box 4116
Naples, Maine 04055-4116
(207)693-6100 voice and fax
clupus@ime.net
Clayarters Live Chat-Room 10:00 pm Eastern Standard Time on Saturday Nites
SHORT CUT if the room has already been created then go to:
http://webchat9.wbs.net/webchat3.so?Room=PRIVATE_Clayarters
or if room has not been created go to WBS at:
http://webchat5.wbs.net/webchat3.so
and set up a "Private Room"... just use the name: Clayarters
------------------------------------------------------------------------------
Gavin Stairs on sat 8 mar 97
At 08:42 AM 07/03/97 EST, you wrote:
>----------------------------Original message----------------------------
>Okay, Gavin.
>
>I'm impressed (and I mean it), but a little confused. Does this mean that
>if I see circular cracks, I need to increase or decrease my glaze
>expansion? My mind just doesn't work the same way yours does, apparently.
>>
>This is too much convolution for me to follow without sitting down and
>making a chart. Help, please!
>
>Cindy
>
Hi Cindy,
Thanks for the opportunity to expand on this a bit.
Actually, it's pretty subjective. You'll have to develop an idea of what to
expect by doing tests. I'd suggest a line blend on a series of tiles, with
your chosen glaze and body, between a version that always crazes and one
that shivers. Make up a few tiles of each type, well labelled. They don't
need to be very big, but big enough to see the patterns. Maybe a couple of
inches square or round. Then fire to your normal maturation, and then test
them: Wear goggles, and beware of flying slivers of glaze. Clean up well
afterwards.
Find some nice, moderately heavy, somewhat pointed object, like a ball-peen
hammer, or a steel ball. Lay the tiles out on a flat table top, with a
stiffish pad underneath, like a layer or two of cardboard. Test your
strength on some of the crazed tiles. When you know how hard you have to
hit, work your way up a series of tiles, and see what happens.
Case A) The glaze has too great expansion for the body.
A glaze that wants to craze (but hasn't yet) will crack easily, and the
radiating cracks will predominate. You must hit hard enough that there is
enough energy in the blow to radiate the cracks. If you're not sure, press
on the tile and see if the cracks grow easily.
Case B) The glaze has too slight expansion for the body.
A glaze that wants to shiver will do so easily, and the glaze, or the tile,
may even shatter with a light blow. If it remains intact, there will be a
predominantly circular pattern, and the radial cracks will be short and
resist growth. A light pressure may cause the tile to fall apart, after it
has been hit.
Case C) The glaze is just right!
The glaze will take a heavier blow to crack. When it cracks, there will be
just a slight network of radiating and circular cracks. The glaze may not
break at all: the whole tile may break first. This is similar to case B,
but it will happen with a heavier blow.
Here's the why of the crack patterns:
If the glaze is under tension (Case A), it has some local energy stored up
which will tend to lengthen any crack. The cracks you initiate with your
hammer all start at one point, so the cracks will start there and go
straight ahead (more or less). They don't care where they came from, but
they will appear to come from the same point of impact. Ergo, a radiating
pattern.
If the glaze is under compression (Case B), it has excess energy stored, but
it tends to want to hold the glaze together, and break the tile. So the
whole tile may come apart just like case A, but more likely the glaze will
break in tension right where you hit it because the blow will form a
depression at the point of impact (like if you punch a pillow) which will
cause a strong ring of tension around the impact region. The cracks will
follow this depression, so they will be circular, and there won't be much in
the way of radiating cracks. It may also flake or shatter, especially under
the impact.
If the glaze is just right (Case C), it is under moderate compression. This
makes it hard to start a crack, but there isn't enough energy stored to
cause a catastrophic breakage (shattering) when it does crack. So small
cracks stay small, and the piece is hard to break. When it does break, it
may come apart into a few pieces.
If I were setting this up to do for quality control, I would do it by
dropping a heavy ball onto the samples, so I could see how high the ball had
to be to break the sample. This measures how hard it is to break the
sample. The "just right" would be close to the maximum, but you might want
to err a little on the safe side. That is a slightly higher expansion
glaze. This will result is less danger of shattering on breakage.
Hope this helps. If you want more, why not ask me directly.
Gavin
Tony Hansen on sat 8 mar 97
> Tony... For the life of me I still can not understand how proper glaze fit
> can so dramatically effect the ware strength...I can understand a 10%-20%
> increase from worst to best...but beyond that I am quite skeptical.
I've done hundreds of these. The worst offenders are cone 10R iron ware.
I've seen ware I can easily tear apart with my bare hands. I've measured
lows
of 1500 lbs to highs of 8000 lots of times with the only variable being
the
glaze. I know there are probably other factors related to
the glazes so that all the difference cannot be totally attributed to
thermal
expansion differences. But a crazed glaze has started a crack and in
ceramics
a crack is a very severe weakeness looking for an opportunity to induce
failure.
> A couple of questions: What is the thickness of the clay bars?
There were about 1 inch.
> What is the approximate thickness of the glaze being applied before firing?
Typical thickness, i.e. credit card.
> What are the firing temps.. both bisque and glaze?
Cone 06 for both.
> What is the type of claybody being used for test?
Talc:Ball clay classic casting mix.
> What is the glaze material being used.
Boron:kaolin mixed with opacifier.
> Finally how is the breaking strength actually measured?
On a dedicated $5000 device intended to apply a load to the
center point of a
bar supported at both ends. It applies increasing force
at a constant rate
and records the load at fracture.
--
=================================================================
Tony Hansen, IMC - Get INSIGHT 5 beta or The Magic of Fire II at
http://digitalfire.com or http://www.ceramicsoftware.com
Corinne P. Null on sun 9 mar 97
>
>Find some nice, moderately heavy, somewhat pointed object, like a ball-peen
>hammer, or a steel ball. Lay the tiles out on a flat table top, with a
>stiffish pad underneath, like a layer or two of cardboard. Test your
>strength on some of the crazed tiles. When you know how hard you have to
>hit, work your way up a series of tiles, and see what happens.
>
I would love to perform this test on my clay and claybody, but haven't a
clue how heavy an object to drop on the tiles. Anybody have any
suggestions of useful objects for such a task? "ball-peen hammer" isn't in
my vocabulary, and I don't have any steel balls laying around!
TIA
Corinne
Tony Hansen on mon 10 mar 97
> >Find some nice, moderately heavy, somewhat pointed object, like a ball-peen
> >hammer, or a steel ball.
We used a variation of this test on electrical insulators where I
worked.
They had welded up a frame and make a pendulum arrangement with a steel
ball.
This allowed for perfect repeatability in testing glaze fit and impact
resistance.
--
=================================================================
Tony Hansen, IMC - Get INSIGHT 5 beta or The Magic of Fire II at
http://digitalfire.com or http://www.ceramicsoftware.com
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