konstantin on tue 7 dec 04
We use Laguna WSO clay for Raku firing in our school.This is a cone 10
clay but it works very well for that function.
But if I fire it @cone 6, with cone 6 glazes,all glazes (highest and
lowest expansion, Mastering cone 6glazes)) craze.
My guess is that "all the rules" do not apply because the temperature
at which I am firing the clay at is not "mature" or vitrified. But could
someone explain why, exactly.
Thank you,,
Konstantin
Ron Roy on thu 9 dec 04
Hi Konstantin,
Probably full of Kyanite or some other expansion/contraction reducing material.
If clay does not contract enough during cooling glazes are going to graze.
If you care to send me a small bar of that clay (3" long x 3/8") I will
measure it in my dilatometer and we can compare it to the clays in our
book.
Just let me know if you want to make the bars and fire them in a regular
glaze firing and I will send instructions on how to make em.
RR
>We use Laguna WSO clay for Raku firing in our school.This is a cone 10
>clay but it works very well for that function.
> But if I fire it @cone 6, with cone 6 glazes,all glazes (highest and
>lowest expansion, Mastering cone 6glazes)) craze.
> My guess is that "all the rules" do not apply because the temperature
>at which I am firing the clay at is not "mature" or vitrified. But could
>someone explain why, exactly.
>Thank you,,
> Konstantin
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Bob Masta on fri 10 dec 04
Konstantin and Ron:
I have noticed this phenomenon before as well. I first
noticed it with a commercial "wide firing range" body
that was recommended for low-fire or high-fire use.
But at cone 04, every glaze I tried crazed. So I kept
working my way up in temperature and finally at
cone 2 found a decent non-crazing glaze fit (high
boron glaze). The body was still nowhere near
mature as determined by absorption, but closer.
I think this same thing also happens on regular
stoneware bodies, though I haven't been as
methodical about testing it. My thought is that
the expansion rate is not a fixed value depending
only on chemical composition, but also on the
structure, so vitrification may permanently change
the expansion rate. Does this make sense?
Best regards,
Bob Masta
potsATdaqartaDOTcom
Ron Roy on sat 11 dec 04
Hi Bob,
I have a dilatometer - easy to do some testing if you really want to know
about that - you make the clay and fire it and I'll measure.
From my experience - and I have measured a lot of clay to date - over 400
samples of clay bodies - raw clay and glazes - I would think that firing
higher would increase the chances of crazing.
As more heat is applied - more of the quartz in the clay goes into
solution. Free quartz in a clay is an anti craze because of the inversion
at 573C. Melt more of that quartz and you lose some of the contraction
during cooling.
On the other hand - how much the glazes gets bonded to the clay has an
influence on both crazing and shivering - perhaps that is the effect you
are seeing.
As you increased the temperature to cone 2 - did you also change the glaze?
RR
>I have noticed this phenomenon before as well. I first
>noticed it with a commercial "wide firing range" body
>that was recommended for low-fire or high-fire use.
>But at cone 04, every glaze I tried crazed. So I kept
>working my way up in temperature and finally at
>cone 2 found a decent non-crazing glaze fit (high
>boron glaze). The body was still nowhere near
>mature as determined by absorption, but closer.
>
>I think this same thing also happens on regular
>stoneware bodies, though I haven't been as
>methodical about testing it. My thought is that
>the expansion rate is not a fixed value depending
>only on chemical composition, but also on the
>structure, so vitrification may permanently change
>the expansion rate. Does this make sense?
>
>Best regards,
>Bob Masta
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Daniel Semler on sat 11 dec 04
Hi Ron, Bob,
I almost sent this yesterday but thought it might just add confusion, but I
think it may be of interest.
I once fired two samples of a high fire white stoneware. I bisqued both as
normal ( cone 06). One sample I high fired - cone 10. I then applied a clear
crackle raku glaze to both and raku fired them both, around 1850 F - cone ??
earthenware - not sure it was done as usual by sight, but I think I recall both
in the same firing. Anyhow, to get to the point, the high fired sample barely
crazed at all. The lower fired one was crazed as per usual for that glaze. So
this could be due to either factor, lack of glaze body interface or better fit.
It seems that to do this sort of test it is tricky to isolate the contributing
factors.
Hope I have not merely add confusion.
Thanx
D
konstantin on sun 12 dec 04
Hi Ron, Bob and Daniel:
Thank you for your comments and your offer Ron, but for now I don't
want to trouble you yet. I want to do some more tests and if that does
not clear up the problem I will contact you then. I think that the
Kyanite idea is very plausible because when I add a locally dug
earthenware (^04 ) in1-1, 1-2, and 1-3 proportions to the WSO clay and
fire to ^6 the body is mature at this temp. but I am still getting the
crazing for both high and low expansion glazes.
I am new to clay and pottery but have been making "things" mostly in
metal for 40 years. I am self thought and most of my work is born out
of my deep interest in the properties of materials. In exploring these
properties I find my place.
I find this list very helpful and inspiring.
Konstantin
Ron Roy on sun 12 dec 04
Hi Daniel,
Thanks for this - the dilatometry will show us exactly what is happening
with the clay body - I am interested to see what is happening.
RR
> I almost sent this yesterday but thought it might just add confusion, but I
>think it may be of interest.
>
> I once fired two samples of a high fire white stoneware. I bisqued both as
>normal ( cone 06). One sample I high fired - cone 10. I then applied a clear
>crackle raku glaze to both and raku fired them both, around 1850 F - cone ??
>earthenware - not sure it was done as usual by sight, but I think I recall both
>in the same firing. Anyhow, to get to the point, the high fired sample barely
>crazed at all. The lower fired one was crazed as per usual for that glaze. So
>this could be due to either factor, lack of glaze body interface or better fit.
>
> It seems that to do this sort of test it is tricky to isolate the
>contributing
>factors.
>
>Hope I have not merely add confusion.
>
>Thanx
>D
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Daniel Semler on sun 12 dec 04
Hi Ron,
Actually, I've thought more about this. Have you ever taken the same claybody
fired at a range of temps and then done dilatometry on the set of samples ? I
assume that the curves would vary indicating the change in the ceramic
composition as a function of heat work experienced.
Of course, that leads to a whole series of new questions, like how porous is
bisque really ? Anyhow, now really, getting off thread, sorry.
Thanx
D
Ron Roy on mon 13 dec 04
Hi Konstantin,
Did I mention there is a series of glazes in our book - designed to find
the expansion of clay bodies.
I would still like to dilatometer that clay - finding the outer limits for
clay bodies is a real interest for me.
RR
> Thank you for your comments and your offer Ron, but for now I don't
>want to trouble you yet. I want to do some more tests and if that does
>not clear up the problem I will contact you then. I think that the
>Kyanite idea is very plausible because when I add a locally dug
>earthenware (^04 ) in1-1, 1-2, and 1-3 proportions to the WSO clay and
>fire to ^6 the body is mature at this temp. but I am still getting the
>crazing for both high and low expansion glazes.
> I am new to clay and pottery but have been making "things" mostly in
>metal for 40 years. I am self thought and most of my work is born out
>of my deep interest in the properties of materials. In exploring these
>properties I find my place.
> I find this list very helpful and inspiring.
> Konstantin
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on mon 13 dec 04
Hi Daniel,
No but I would like to do it.
Free quartz has a profound effect on contraction during cooling - take a
look at the dilatometer charts on our book - you can see - even the
porcelain has lots of free quartz.
I am assuming that as melting progresses - more quartz in taken into
solution and becomes amorphous - which has a low COE - I just don't see how
it could be any other way.
It's way adding silica to a clay body is a cure for crazing - the quartz
crystal goes from B to A during the inversion at 573C - gets smaller and
affects the whole clay body. Cristobalite - another form of crystalline
silica - but goes through it's inversion about 200C - it also helps cure
crazing but - it happens at ovenware temperatures and is a danger because
of that.
RR
> Actually, I've thought more about this. Have you ever taken the same claybody
>fired at a range of temps and then done dilatometry on the set of samples ? I
>assume that the curves would vary indicating the change in the ceramic
>composition as a function of heat work experienced.
>
> Of course, that leads to a whole series of new questions, like how porous is
>bisque really ? Anyhow, now really, getting off thread, sorry.
>
>Thanx
>D
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Bob Masta on wed 15 dec 04
Hi Ron,
> I still would like to know what increased temperature does
> to expansion of clay bodies - I have not done any
> experiments on this so what I said is theory - if you do
> want to so the firing my offer is still good - it would make
> a good article if you would like to do that - help our
> collective understanding.
> If you care to take the time I would like to calculate the
> expansions of your glaze series - it will shed light on what
> happened in your tests.
Here are the glazes I used, adapted from "Glaze Projects"
by Richard Behrens, pg 40, "Simple Colemanite Glazes":
C/04 C/2
Gillespie Borate 60 55.0
EPK 15 17.5
Flint 25 27.5
The body was Rovin RO-45, purchased in 1989 and now long
obsolete. Rovin no longer has an analysis for this body.
Their representative says that according to the only
information she has, it was "kaolin based, no grog".
The original info from their old catalog says:
"White, low fire, no talc. A very white, wide firing
range low temperature body with excellent hand
building properties."
C/06 C/1 C/6 C/10
Shrinkage: 3.9 4.7 4.7 7.8 %
Absorption: 14.7 10.2 7.5 2.2 %
I used this body because it has essentially zero
shrinkage between bone-dry and C/2. (According to the
above, that should hold to C/6 as well, but I haven't
tested that.) I use this for projects where dimensions
are critical; it allows me to adjust things easily in the
bone-dry state, including drilling holes with accurate
spacing and even threading them for standard machine
screws.
The C/04 glaze crazed badly, the C/2 version didn't
show any crazing under 20x magnification... at least
initially. One of the applications was for shower knob
escutcheons that are always wet. Since the body isn't
really vitrified at C/2, they eventually developed fine
crazing due to the absorption.
Now that I'm taking another look at the stated absorption
numbers, I realize this body was far from maturity at C/2.
My conjecture about expansion ratio changing seems more
unlikely than ever. If you still think this merits more work, I
think it would be better to use a readily-available body,
maybe one formulated just for the purpose.
Best regards,
Bob Masta
potsATdaqartaDOTcom
Bob Masta on thu 16 dec 04
Hi, Ron:
I've been reviewing the dilatometer curves in MC6G to
try to understand how my conjectured change in the
expansion/contraction rate of a body with maturation
might affect the curves. If I understand correctly, it
would have to steepen the slope of the body curve to
better fit a relatively higher expansion/contraction glaze.
To test this, I assume you could fire samples of the same
body to different temperatures and compare the curves
for the cooling portion of each firing. I'm wondering if
you may have already done this experiment, perhaps
not with this goal in mind, by running curves for a bisque
firing and full-maturity firing on the same body. Since
there would be a maximal difference in maturity here, if
they don't show slope differences then we can probably
reject my conjecture without further ado.
Best regards,
Bob Masta
potsATdaqartaDOTcom
Ron Roy on thu 16 dec 04
Hi Bob,
The 04 glaze calculated expansion is 342.34 on the scale I use.
The other glaze has an expansion of 317.09.
There is a complication - the boron is over 12% in both cases - more so in
the 1st glaze - boron loses it's anti craze property when over 12% so both
those calculated expansions need to be adjusted up but I don't know by how
much.
When I first saw the 317.09 number I thought - shivering is a possibility -
that low and I start twitching - but the high boron makes that higher - but
be careful in that range - test for shivering.
I don't know if more experiments are worth it for you - working with
underfired clay is going to be a problem when moisture gets under the glaze
- body expands - glazes craze. If you lower the expansion enough to counter
act that - you run the risk of shivering.
The way around that is to use a high talc body - this encourages the
formation of enstatite which is an anti craze - and the MgO stops water
absorption.
MgO turns into a strong melter about cone 4/5 so bodies mature real fast -
watch out for that part.
Be glad to do the dilatometery - we should work together on the clay body I
guess. More for understanding than a solution.
RR
>> I still would like to know what increased temperature does
>> to expansion of clay bodies - I have not done any
>> experiments on this so what I said is theory - if you do
>> want to so the firing my offer is still good - it would make
>> a good article if you would like to do that - help our
>> collective understanding.
>
>> If you care to take the time I would like to calculate the
>> expansions of your glaze series - it will shed light on what
>> happened in your tests.
>
>
>Here are the glazes I used, adapted from "Glaze Projects"
>by Richard Behrens, pg 40, "Simple Colemanite Glazes":
>
> C/04 C/2
>Gillespie Borate 60 55.0
>EPK 15 17.5
>Flint 25 27.5
>
>The body was Rovin RO-45, purchased in 1989 and now long
>obsolete. Rovin no longer has an analysis for this body.
>Their representative says that according to the only
>information she has, it was "kaolin based, no grog".
>The original info from their old catalog says:
> "White, low fire, no talc. A very white, wide firing
> range low temperature body with excellent hand
> building properties."
> C/06 C/1 C/6 C/10
>Shrinkage: 3.9 4.7 4.7 7.8 %
>Absorption: 14.7 10.2 7.5 2.2 %
>
>I used this body because it has essentially zero
>shrinkage between bone-dry and C/2. (According to the
>above, that should hold to C/6 as well, but I haven't
>tested that.) I use this for projects where dimensions
>are critical; it allows me to adjust things easily in the
>bone-dry state, including drilling holes with accurate
>spacing and even threading them for standard machine
>screws.
>
>The C/04 glaze crazed badly, the C/2 version didn't
>show any crazing under 20x magnification... at least
>initially. One of the applications was for shower knob
>escutcheons that are always wet. Since the body isn't
>really vitrified at C/2, they eventually developed fine
>crazing due to the absorption.
>
>Now that I'm taking another look at the stated absorption
>numbers, I realize this body was far from maturity at C/2.
>My conjecture about expansion ratio changing seems more
>unlikely than ever. If you still think this merits more work, I
>think it would be better to use a readily-available body,
>maybe one formulated just for the purpose.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on sat 18 dec 04
Hi Bob,
The higher the curve - the higher the expansion on heating and the higher
the contraction on cooling.
Notice the sharp increase in expansion/contraction at 573C - that is the
quartz inversion. A cure for crazing is to add more quartz to a body -
thereby increasing the contraction during cooling. This helps to keep the
glaze under compression - thus helping to prevent crazing - as long as the
glaze is frozen at 573C. If the glaze is still pyro plastic - it simply
contracts along with the clay and no strain builds between body and glaze.
An interesting point - so how does adding silica (quartz) to a glaze also
help to cure crazing? Because quartz is no longer quartz in a melted glaze
- it's amorphous silica (not a crystal) and has a very low
expansion/contraction rate compared to all of the other oxides we use. The
one exception is Boron and then - only when below 12% in the glaze.
No - I have not diled any clay raw or body fired at different temperatures.
It only came up because there was the supposition that firing higher cured
crazing.
If the subject had been about high fired bodies - and cristobalite was
involved - then I would think it possible - longer firings can produce more
cristobalite - and cristobalite (another crystalline form of SiO2) can help
put glazes into compression - but it does it at a lower temperature (200C).
In the case of quartz - longer firing would tend to decrease the amount
because more of it would go into solution - becoming amorphous - and having
a low expansion (compared to quartz - lower the expansion - so the body
would not contract as much and crazing would get worse.
Hope this is understandable - if not - just ask for more.
RR
>I've been reviewing the dilatometer curves in MC6G to
>try to understand how my conjectured change in the
>expansion/contraction rate of a body with maturation
>might affect the curves. If I understand correctly, it
>would have to steepen the slope of the body curve to
>better fit a relatively higher expansion/contraction glaze.
>
>To test this, I assume you could fire samples of the same
>body to different temperatures and compare the curves
>for the cooling portion of each firing. I'm wondering if
>you may have already done this experiment, perhaps
>not with this goal in mind, by running curves for a bisque
>firing and full-maturity firing on the same body. Since
>there would be a maximal difference in maturity here, if
>they don't show slope differences then we can probably
>reject my conjecture without further ado.
>
>Best regards,
>
>Bob Masta
>
>potsATdaqartaDOTcom
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Bob Masta on sun 19 dec 04
Hi, Ron:
> No - I have not diled any clay raw or body fired at
> different temperatures.
> It only came up because there was the supposition that
> firing higher cured crazing.
Does your dilatometer record going in both temperature
directions? (You show only one curve in MC6G, which
I assume is the "down" curve since you are dealing with
glazes.) If so, then a normal run of a bisqued body in a maturity firing
will give all the info needed. If the coefficient of expansion
changed at the top temperatures as the sample matured,
then it wouldn't come back down the same curve as when
is went up as bisqueware. If it *does* come back on the
same path, then the question is resolved and maturity
doesn't change COE.
Best regards,
Bob Masta
potsATdaqartaDOTcom
Earl Krueger on mon 20 dec 04
On Monday 20 December 2004 01:55 am, Ron Roy wrote:
> I only dil on the way up - no control of temperature on
> the way down - if I could meter the temperature on the
> way down the lines would coin side.
Ron,
Are you saying that if you plot the expansion of a clay
body, previously fired to cone X, as it is heated and if
you could plot the contraction as it cooled you would
expect the two plots to exactly overlay each other?
If so I guess I could see that hapenning _unless_ during the
measurement you heated the sample to a high enough
temperature, above cone X, so that further chemical or
physical changes occured, such as the formation of
cristobalite. Then I would expect that the contraction
plot might not exactly overlay the expansion plot.
Have you ever plotted the same sample multiple times to see
if the heat work (whatever that is) accomplished during the
previous measurements had any effect on subsequent
readings?
Just curious.
--
Earl K...
Bothell WA, USA
"You may be disappointed if you fail,
but you are doomed if you don't try."
Beverly Sills (1929 - )
Ron Roy on mon 20 dec 04
Hi Bob,
I only dil on the way up - no control of temperature on the way down - if I
could meter the temperature on the way down the lines would coin side.
It is especially important when doing glazes - otherwise you would not find
the melting points except by guessing - go over the melting point a bit too
far and your sample welds to the apparatus and expensive repairs are the
result - been there done that - but only once.
I think you are right - that measuring a body fired to bisque and to
maturity would give useful info - but there are many changes that happen
depending on the type of clay and fluxes and I would like to see more - if
I can.
I have worked on a high talc body for a tile maker and it was interesting
to see cristobalite at cone 6 - or was it ensatite? or both? Also - if
cristobalite were formed - would it make up for the quartz that was lost
due to melting?
I have come to the conclusion that there is very little that is resolved
when it comes to combining the kind of materials we use - and beware the
generalizations we see constantly - they come from the hope that we can say
certain things are predictable. I don't understand why we are like that -
and I'm not saying I don't want to be able to say this is so each time -
but every time I say that I have to admit - it's only based on what I think
I know - I can't even say what I know because so much of it was told to me
by those who did not know. So it turns out I don't even know what I think I
know - never mind what I don't know.
So - perhaps you will understand - I have this need to know - and seeing
the stages all the way up will help - the horrible part is - will it be
true for the next body.
What ever you decide to do is fine with me - I am not being demanding - we
will learn something no mater what we do.
RR
>> No - I have not diled any clay raw or body fired at
>> different temperatures.
>
>> It only came up because there was the supposition that
>> firing higher cured crazing.
>
>Does your dilatometer record going in both temperature
>directions? (You show only one curve in MC6G, which
>I assume is the "down" curve since you are dealing with
>glazes.) If so, then a normal run of a bisqued body in a maturity firing
>will give all the info needed. If the coefficient of expansion
>changed at the top temperatures as the sample matured,
>then it wouldn't come back down the same curve as when
>is went up as bisqueware. If it *does* come back on the
>same path, then the question is resolved and maturity
>doesn't change COE.
>
>Best regards,
>
>Bob Masta
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ivor and Olive Lewis on tue 21 dec 04
Dear Ron,
You say << I have worked on a high talc body for a tile maker and it
was interesting to see Cristobalite at cone 6 - or was it enstatite?
or both? Also - if Cristobalite were formed - would it make up for the
quartz that was lost due to melting?>>
There are two things that can be done to find out more. The first is
to prepare and observe thin sections through a microscope to identify
the the mineral phases. The second is to use your percentage elemental
oxide analysis to locate the position of your recipe in the phase
diagram.
With 20% MgO, 25%Al2O3 and the remainder SiO2 you might have
Cordierite. Add in K2O and things start to get a bit complex, but fun
to speculate with.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "Ron Roy"
To:
Sent: Monday, 20 December 2004 8:25
Subject: Re: Glaze Crazing on Underfired Clay
> Hi Bob,
>
> I only dil on the way up - no control of temperature on the way
down - if I
> could meter the temperature on the way down the lines would coin
side.
>
> It is especially important when doing glazes - otherwise you would
not find
> the melting points except by guessing - go over the melting point a
bit too
> far and your sample welds to the apparatus and expensive repairs are
the
> result - been there done that - but only once.
>
> I think you are right - that measuring a body fired to bisque and to
> maturity would give useful info - but there are many changes that
happen
> depending on the type of clay and fluxes and I would like to see
more - if
> I can.
>
> I have worked on a high talc body for a tile maker and it was
interesting
> to see cristobalite at cone 6 - or was it ensatite? or both? Also -
if
> cristobalite were formed - would it make up for the quartz that was
lost
> due to melting?
>
> I have come to the conclusion that there is very little that is
resolved
> when it comes to combining the kind of materials we use - and beware
the
> generalizations we see constantly - they come from the hope that we
can say
> certain things are predictable. I don't understand why we are like
that -
> and I'm not saying I don't want to be able to say this is so each
time -
> but every time I say that I have to admit - it's only based on what
I think
> I know - I can't even say what I know because so much of it was told
to me
> by those who did not know. So it turns out I don't even know what I
think I
> know - never mind what I don't know.
>
> So - perhaps you will understand - I have this need to know - and
seeing
> the stages all the way up will help - the horrible part is - will it
be
> true for the next body.
>
> What ever you decide to do is fine with me - I am not being
demanding - we
> will learn something no mater what we do.
>
> RR
>
>
>
> >> No - I have not diled any clay raw or body fired at
> >> different temperatures.
> >
> >> It only came up because there was the supposition that
> >> firing higher cured crazing.
> >
> >Does your dilatometer record going in both temperature
> >directions? (You show only one curve in MC6G, which
> >I assume is the "down" curve since you are dealing with
> >glazes.) If so, then a normal run of a bisqued body in a maturity
firing
> >will give all the info needed. If the coefficient of expansion
> >changed at the top temperatures as the sample matured,
> >then it wouldn't come back down the same curve as when
> >is went up as bisqueware. If it *does* come back on the
> >same path, then the question is resolved and maturity
> >doesn't change COE.
> >
> >Best regards,
> >
> >Bob Masta
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
>
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Ron Roy on wed 22 dec 04
Hi Earl,
I have trouble understanding why any potter would not be curious Earl -
when I think of how much I understood before I started up with my
dilatometer - about clays and glazes - and how much I understand now - I
would never consider going back.
Expansion/contraction is exactly reversible - as long as the material has
not been altered - in this case by heat - so yes - if I could control the
cooling the and keep it steady - at the same rate as when heated - the
lines would match.
Remember - the graph describes expansion at a steady rate of heat increase
- and expansion rates are always - or should be - quoted at a certain
temperature.
Calculated rates are not because they are theoretical.
The samples are held - in a holder with a push rod - as the sample is
heated - any movement is transferred by the push rod to the exciter - then
to the built in calculator. The push rod and holder are made of fused
silica. Instructions say - don't go over 800C or you will have to replace
the holder & pushrod sooner. So I don't go over 800C with clays. I have to
with some glazes to find their melting point - but not unless I have to.
If I don't go over 800C with clay I will have practically no change in
clays - unless they have some boron in them - nothing I could measure in my
machine.
I have remeasured some samples - I calibrate using a fused alumina sample
and have done it many times - same graph each time.
When I do remeasure - I cannot tell if there has been any change Earl - I
have to position the graph paper by hand each time - the error in that
would mask any other differences from heat work. What I can say is - it
would be so small it would make no difference to any of us - variation of
base materials would be much more significant.
Thanks for asking Earl -
RR
>Ron,
>Are you saying that if you plot the expansion of a clay
>body, previously fired to cone X, as it is heated and if
>you could plot the contraction as it cooled you would
>expect the two plots to exactly overlay each other?
>
>If so I guess I could see that hapenning _unless_ during the
>measurement you heated the sample to a high enough
>temperature, above cone X, so that further chemical or
>physical changes occured, such as the formation of
>cristobalite. Then I would expect that the contraction
>plot might not exactly overlay the expansion plot.
>
>Have you ever plotted the same sample multiple times to see
>if the heat work (whatever that is) accomplished during the
>previous measurements had any effect on subsequent
>readings?
>--
>Earl K...
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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