Mike Bailey on fri 16 jul 99
Hi to all, especially the glaze/clay specialists,
The recent discussion about Carol Baker's crazing problem revolved
around the problem of glaze fit (I thought the replies to this were very
good) and it's prompted me to raise this question crazing and heatwork
as something of a side issue.
The usual advice about crazing is that one of the ways to cure it
(crazing) is to increase the heatwork. (Altering the glaze or body are
other remedies) The idea is that if you increase the heatwork above
1100 deg C.- the free silica forms cristobalite, which contracts with an
inversion at 220 deg C., which causes the body to shrink (aptly called
the cristobalite squeeze).
HOWEVER, in some case of earthenwares and probably all porcelains the
opposite is true. Increased heatwork actually increases crazing!
The explanation, I think, lies in the fact that these particular bodies
have fluxes that absorb the free silica as a glaze/glass within the clay
matrix. Hence the cristobalite form of silica is not present to assist
with glaze fit on cooling.
I did some test 18 months ago which now show this really well - it's
taken that long for the crazing to develop. Various earthenware clays
were bisqued from at 1075 deg C. and at 25 deg C increments up to 1200
deg. C. The glazing was at 1080 deg. c. Two out of four white
earthenware bodies showed a progressive increase in crazing from the
extra heatwork. I asked the clay manufacturers about these bodies and it
seems that these white earthenware are made from ball clays to which
lime has been added. This presumably is the flux that is helping to
vitrify the clay at a lower temperature. In doing so, any free silica is
absorbed.
In the case of porcelains it is the alkalis from feldspar that are the
principle fluxes that vitrify the body (and absorb any free silica)
Does anyone agree/disagree or got any observations !?
Cheers,
Mike Bailey.
--
Mike Bailey
Michael Banks on sat 17 jul 99
Mike Bailey's post on this subject dovetails nicely with Greg Lamont's query
concerning calculating body expansion.
I've found that the higher a ceramic body is fired (or heatworked), the
closer it's expansion approaches the figure predicted from glaze calc
programs. This is to be expected because these programs incorporate CTE's
(expansion coefficients) measured from glasses and bodies eventually become
completely glassy if fired high enough. This expansion figure is normally
much lower than that achieved before the onset of total vitrification
(excepting some special purpose bodies, such as flameproof ware). So Mike I
do agree :)
Therefore, a glaze may fit a particular body perfectly when fired to
maturity (for stoneware usually exhibiting a small remnant porosity, but if
fired higher -to a glassy zero porosity state, the glaze may no longer fit
at all and craze. Porcelain and bone china present particular difficulties
to the ceramist, because they are routinely fired to a near glassy state.
And for these bodies Greg, their expansions come close to that predicted by
glaze programs. Porcelain glazes therefore must by formulated to lower
expansions in order to fit these glassy bodies, compared to stoneware
glazes. The contraction of these bodies on cooling is insufficient to
compress a high expansion stoneware-type glaze enough to prevent crazing.
The main reason why mature stoneware has expansions higher than that
predicted by glaze programs is because they largely consist of high
expansion crystalline minerals. Quartz and cristobalite are well known
examples and the body content of these two are manipulated by ceramists to
raise body expansion (and therefore contraction) by virtue of their
spectacular crystallographic inversions. But there are other important high
expansion minerals present too which also contract via inversions or
smoothly. These include the feldspars and pyroxenes. The expansions of these
are almost impossible for glaze programs to predict because their CTE's are
anisotropic (different in 3 different axes). This in the case of the
pyroxenes is quite extreme as they form long needle-shaped crystals.
Solid state reactions in the ceramic body often result in an increased
content of high expansion minerals at or around the maturation point. It is
the job of the formulating ceramist to ensure that a good balanced choice of
fluxes, alumino-silicates and silica be utilised to prolong the firing range
in the high-expansion maturation zone and reduce the tendency for sudden
vitrification. This ensures that the potter (whose kiln often exhibits a
wide range in temperature) does not receive underfired crazed pots, mature
uncrazed pots and overfired crazed pots from the same load!
Michael Banks,
Nelson,
New Zealand
> ----------------------------Original message----------------------------
>
> Hi to all, especially the glaze/clay specialists,
>
> The recent discussion about Carol Baker's crazing problem revolved
> around the problem of glaze fit (I thought the replies to this were very
> good) and it's prompted me to raise this question crazing and heatwork
> as something of a side issue.
>
> The usual advice about crazing is that one of the ways to cure it
> (crazing) is to increase the heatwork. (Altering the glaze or body are
> other remedies) The idea is that if you increase the heatwork above
> 1100 deg C.- the free silica forms cristobalite, which contracts with an
> inversion at 220 deg C., which causes the body to shrink (aptly called
> the cristobalite squeeze).
>
> HOWEVER, in some case of earthenwares and probably all porcelains the
> opposite is true. Increased heatwork actually increases crazing!
>
> The explanation, I think, lies in the fact that these particular bodies
> have fluxes that absorb the free silica as a glaze/glass within the clay
> matrix. Hence the cristobalite form of silica is not present to assist
> with glaze fit on cooling.
>
> I did some test 18 months ago which now show this really well - it's
> taken that long for the crazing to develop. Various earthenware clays
> were bisqued from at 1075 deg C. and at 25 deg C increments up to 1200
> deg. C. The glazing was at 1080 deg. c. Two out of four white
> earthenware bodies showed a progressive increase in crazing from the
> extra heatwork. I asked the clay manufacturers about these bodies and it
> seems that these white earthenware are made from ball clays to which
> lime has been added. This presumably is the flux that is helping to
> vitrify the clay at a lower temperature. In doing so, any free silica is
> absorbed.
>
> In the case of porcelains it is the alkalis from feldspar that are the
> principle fluxes that vitrify the body (and absorb any free silica)
>
> Does anyone agree/disagree or got any observations !?
>
> Cheers,
>
> Mike Bailey.
>
> --
> Mike Bailey
>
Craig Martell on sun 18 jul 99
Hello Mike:
In regard to the heatwork/crazing idea I would agree that increasing
cristobalite development would decrease the tendency for a glaze to craze.
I wouldn't agree that this is the best avenue of approach for glaze fit.
Especially in regard to ware that will be subject to temperature stresses
such as teapots, casseroles, mugs, and so forth. I'm mostly thinking of
high fire pots too such as stoneware and porcelain. I'm not really well
versed in earthenware stuff so perhaps someone else has or will respond to
this part of your query.
My experience with blending clays for high fire is to assemble a good blend
of clays based on color, texture, working properties, and then add the
feldspar and silica additions based on vitrification point and glaze fit. I
wouldn't agree that increased heatwork plays a role in glaze fit though.
Clays are designed to fire to a target temp range where they are mature and
strong anything beyond this may be detrimental and most folks don't overfire
for lots of good reasons. Soaking is another thing though. You can hold a
high temp and allow a more complete melt to take place. Would this affect
the expansion properties of the body and or glaze? Maybe, but I don't think
it would be a major factor. I think that the percentage of added spar and
silica have more bearing. They are pretty much melted and fully involved in
the silicate chain stuff at above cone 8 so they are going to do what they
do to free silica. My prefered method of promoting glaze fit is to add
ground quartz or crystalline silica to the body that will affect volume
change at the quartz inversion point of about 1032 F. The problem with
cristobalite is that it will invert quickly and radically at about 435 F.
Which is in the range of ovens and ovenware. It will also affect expansion
below this point in teapots, mugs etc. Not good stuff.
regards, Craig Martell in Oregon
Ron Roy on thu 22 jul 99
Hi Mike, well I started answering Mike but slipped into something else I
think -
This is fairly complicated - and my rant will probably raise more questions
that it answers - There is a great deal of confusion about fit.
I ask that you keep this in mind when reading about this subject - when
anyone says expansion what they really mean is expansion when heated and
contraction when cooled. When these terms are used we are talking about
reversible expansion and contraction. If a clay is cooled it gets smaller -
when heated it gets bigger - only very small amount of movement - but
movement that can crack glazes and clay - particularly when bonded
together. If you heat a piece of clay to 500C and cool it to 200 that piece
of clay will always be the same size at each temperature no mater how many
time you do it. If there is melting on heating the expansion rate will
change - what we are talking about here is - for the better or worse for
our particular fit problem.
Earthenware - unless there is significant Talc present - no cristobalite is
there unless added as a body ingredient.
I am unclear as to whether any cristobalite is produced with CaO present in
bodies fired below 1100C
Above 1100C there can be some cristobalite produced at cone 6 - but I have
only seen it in a body with talc present.
Above cone 6 - particularly bodies fire at cone 10 with some fluxing by
iron in reduction - I have seen lots - especially in bodies with micro fine
silica added and not enough KNaO present. All our silica (200 to 400m) has
lots of this super fine silica now. Mike is right about the porcelains -
25% silica and 25% spar - never seen any cristobalite and it's because all
that KNaO melts the cristobalite crystals as they form denying a seed for
the further build up of more crystals.
So - heat work certainly can increases cristobalite - especially in the
higher ranges - and this can prevent crazing - and it can also produce
glaze dunting with lower expansion glazes - best to have some idea of which
are high and which are low before you decide to fire longer to prevent
crazing.
In bodies that are not producing significant cristobalite - extra heat work
will result in less quartz - simply because more of it will be drawn into
the melt - not be crystalline and not go through the inversion on cooling
and help to keep glazes in compression. This is a bit tricky though - if
the glaze has not set (solidified, frozen) when the body gets smaller
because of the beta to alpha inversion - then it will not result in as much
added compression of the glaze because the glaze - still being somewhat
thermoplastic - will be able to adjust to the decrease in size. It is the
case that some boron glazes are still soft at 573C.
The old advice - add silica to the glaze or the body to cure crazing -
illustrates the role of free quartz and/or amorphous silica in influencing
overall glaze fit. The crystalline silica (quartz) because it makes the
body get smaller at 573C and the amorphous (non crystalline) silica because
it has a low coefficient of expansion/contraction when compared to the
other oxides we find in glazes.
This all comes back to how to cure crazing - or the opposite problem which
results in dunting and shivering. Some say change bodies - or add silica to
the offending body - well you better have a handle on the expansion of your
other glazes before you go skipping down that road. My advice is to find
out - through calculation or testing - more about the expansion of all your
glazes before changing the clay.
Or - fix the glaze. It is true that some glazes are so out of whack that
they will be impossible to fit to some bodies - take shino's for example.
On the other hand - I handle hundreds of crazing problems every year and
can fix the great majority of them - easily. It is not difficult to do -
and I am dismayed when I see potters trying to cure a fit problem by
altering firing to fix what is really an oxide based problem.
There is one case where firing can make fitting all glazes difficult. If
you fire too fast the glaze does not get a chance to bond well with the
clay - this means that crazing and shivering show up over a very small
difference - drives some potters nuts - and it is very difficult to deal
with - perhaps this is why firing higher gets rid of some crazing - more
time for the two materials to bond properly. Yet another reason not to fast
fire.
Sorry folks - got carried away again - trouble with having a dilatometer -
makes me want to tell everyone to have a look - watch for a great article
in Studio Potter - not the current issue - the next one - Peter Songhen has
got the handle on the silica/cristobalite thing nailed. I know cause I did
the dilatometry for him.
RR
>----------------------------Original message----------------------------
>Hi to all, especially the glaze/clay specialists,
>
>The recent discussion about Carol Baker's crazing problem revolved
>around the problem of glaze fit (I thought the replies to this were very
>good) and it's prompted me to raise this question crazing and heatwork
>as something of a side issue.
>
>The usual advice about crazing is that one of the ways to cure it
>(crazing) is to increase the heatwork. (Altering the glaze or body are
>other remedies) The idea is that if you increase the heatwork above
>1100 deg C.- the free silica forms cristobalite, which contracts with an
>inversion at 220 deg C., which causes the body to shrink (aptly called
>the cristobalite squeeze).
>
>HOWEVER, in some case of earthenwares and probably all porcelains the
>opposite is true. Increased heatwork actually increases crazing!
>
>The explanation, I think, lies in the fact that these particular bodies
>have fluxes that absorb the free silica as a glaze/glass within the clay
>matrix. Hence the cristobalite form of silica is not present to assist
>with glaze fit on cooling.
>
>I did some test 18 months ago which now show this really well - it's
>taken that long for the crazing to develop. Various earthenware clays
>were bisqued from at 1075 deg C. and at 25 deg C increments up to 1200
>deg. C. The glazing was at 1080 deg. c. Two out of four white
>earthenware bodies showed a progressive increase in crazing from the
>extra heatwork. I asked the clay manufacturers about these bodies and it
>seems that these white earthenware are made from ball clays to which
>lime has been added. This presumably is the flux that is helping to
>vitrify the clay at a lower temperature. In doing so, any free silica is
>absorbed.
>
>In the case of porcelains it is the alkalis from feldspar that are the
>principle fluxes that vitrify the body (and absorb any free silica)
>
>Does anyone agree/disagree or got any observations !?
>
>Cheers,
>
>Mike Bailey.
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
| |
|
