mel jacobson on fri 10 mar 00
according to older data from japan, a single re/fire does
not do any damage to the clay body. mr. uchida re/fired
about 30% of his pots. (of course one pin hole in a pure
white pot looked like hell.)
he often told me that a second or third re/fire would break down the
integrity of the clay body.
perhaps one of our chemist/engineers could clarify this point.
mel/mn
http://www.pclink.com/melpots (website)
from minnetonka, minnesota, u.s.a.
Tom Buck on sat 11 mar 00
mel....many claybodies contain too much of minerals/chemicals that lead to
bloating/dunting if overfired. i recall one body i threw years ago that
weeped at C3, was tight at C5/6 and bloated at C7+ Why? Because it
contained a significant amount (10%???) of nepheline syenite, a mineral
with high sodium oxide and, compared to other feldspathic minerals, less
silica. Common feldspars (G200, Custer, F-4) have a crystalline structure
that can be expressed in simple chemical terms: 1 mole flux oxides to 1
mole alumina to 6 moles silica. Neph sy is 1:1:4 approx.
Another factor is the possibility of formation of cristobalite, a
crystalline (structured) form of silica. Perhaps Ron Roy will say once
again how overfiring or very slow cooling will lead to cristabolite and it
in turn making the claybody more discontinuous and easily broken.
There are also bodies with unusual high amounts of iron compounds,
or titanium compounds, and perhaps fluorine compounds. Under certain
firing regimens these compounds could lead to failed pots, and refiring
would certainly enhance this effect.
There, that's all I want to say for now. Let Michael, Ron,
Evan, Mary et al come forth and add to the discussion.
til later. Peace. Tom B.
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
On Fri, 10 Mar 2000, mel jacobson wrote:
> ----------------------------Original message----------------------------
> according to older data from japan, a single re/fire does
> not do any damage to the clay body. mr. uchida re/fired
> about 30% of his pots. (of course one pin hole in a pure
> white pot looked like hell.)
>
> he often told me that a second or third re/fire would break down the
> integrity of the clay body.
>
> perhaps one of our chemist/engineers could clarify this point.
>
> mel/mn
> http://www.pclink.com/melpots (website)
> from minnetonka, minnesota, u.s.a.
>
Ron Roy on sat 11 mar 00
Hi Mel,
Any refire to the same cone does more heat work - everything gets melted
more - no question about this. Some clays and glazes have enough latitude
built into them to pass this extra melting OK - others will go over the
edge.
The more ominous aspect of refiring - at higher temperatures - is the
formation of more cristobalite in certain kinds of bodies. The longer you
stay between 1100C and top temperature the more cristobalite will be
generated in those bodies that are prone to it. Cristobalite is produced on
the way up and on the way down.
If you are increasing the expansion (on heating) and contraction (on
cooling) as cristobalite does - you are affecting the fit of glaze on that
clay - in some cases it will result in cracked pots.
The latest issue of Studio Potter has a great article on cristobalite
complete with dilatometer charts. If you are curious about why some bodies
have cristobalite and others do not - check it out.
RR
>----------------------------Original message----------------------------
>according to older data from japan, a single re/fire does
>not do any damage to the clay body. mr. uchida re/fired
>about 30% of his pots. (of course one pin hole in a pure
>white pot looked like hell.)
>
>he often told me that a second or third re/fire would break down the
>integrity of the clay body.
>
>perhaps one of our chemist/engineers could clarify this point.
>
>mel/mn
>http://www.pclink.com/melpots (website)
>from minnetonka, minnesota, u.s.a.
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Michael Banks on sun 12 mar 00
A deceptively simple (but complex) question this one. I would think it
would depend on the composition of the clay body.
For example, low-alkali, quartz free bodies which develop interlocking,
flexible acicular minerals during firing, such as mullite or pyroxenes -will
remain tough after multiple firings. Also, low alkali, high-fire vitreous
porcelains are remarkably stable to repeated firings, due to their low
thermal expansions. An extreme example of this is spark-plug porcelain
which has a lot to put up with!
The main problem with ordinary potters stoneware clays, is that they
generally have quite a lot of free quartz in them. This mineral imparts a
high coefficient of thermal expansion (CTE), so that glazes fit without
undue crazing. This high CTE is applied fairly abruptly around the 573oC
alpha/beta inversion point. To complicate matters, the behaviour of this
quartz during repeated firings depends on it's grainsize.
The finest fraction (generally regarded as less than 50 micron)
progressively recrystallizes as cristobalite if there is insufficient
alkalis around to melt them into low CTE silica glass. The more time the
fine quartz is held above the conversion temperature, the more quartz
recrystallizes as cristobalite. Repeated firings produce more cristobalite,
as the period above the conversion temperature is cumulative. Even quartz
grains coarser than 50 microns are progressively converted with prolonged
(or multiple) spells above yellow-heat. Bodies with significant
cristobalite contents are prone to dunting, shivering and general loss of
integrity caused by the severe alpha cristobalite/beta cristobalite
inversion between 200-250 oC. The CTE of cristobalite is so high that the
contrast in CTE between it and the surrounding low-CTE minerals and glass in
the body matrix is intolerable and impossible to contain by the limited
flexibility of the structure.
Ideally a strong stoneware body should have a minimum of fine-grained
quartz, enough alkali (e.g. feldspar) present to dispose of the fine-grained
quartz -inevitably present, but enough coarse quartz sand to allow
reasonable glaze fit. Mr. Uchida was probably referring to the effects of
multiple firings on bodies of this type (as bodies with a lot of sub 50
micron quartz would not likely survive two firings). It is probable that
the loss of integrity he was referring to, resulted from coarse quartz sand
and not cristobalite.
It is my belief that coarse grains of quartz (which do not melt or invert to
cristobalite during repeated refirings) become repeated cracked-up by being
subjected to repeated journeys through the 573 oC inversion point. More
specifically, by multiple cooling through this point. A strong
mullite-bearing body matrix can probably expand elastically to absorb the
573 degree expansion of the quartz grains. But on cooling, the brittle
quartz is shattered under tension as it becomes abruptly smaller than it's
cavity in the body. I first became aware of this in examining quartz
crystals in igneous rocks which are commonly intensively cracked, compared
to surrounding feldspars (which do not have an inversion) etc, which are
not. I can provide a photograph of this phenomenon (in jpg format) via
e-mail to anyone interested.
So repeated firings of quartz crystals (which do not heal their cracks at
stoneware temps) progressively weaken the body by degrading the internal
quartz sand. The cracks in the quartz will propagate into those parts of the
surrounding matrix that are vitreous, weakening the body by meandering
networks of microcracks. A runaway chain reaction ensues. Repeated firings
will increase the degree of vitification of the matrix, increasing the
propensity of cracking to propagate. The progressive shattering of the
quartz sand removes the grog effect of the sand, which normally interferes
with crack propagation. Every repeated firing further diminishes the size
of the quartz, until at some stage it will be fine enough to convert to
cristobalite. Then the body will then fail spectacularly -if not long
before.
Michael Banks,
Nelson,
New Zealand
mel/mn wrote:
> ----------------------------Original message----------------------------
> according to older data from japan, a single re/fire does
> not do any damage to the clay body. mr. uchida re/fired
> about 30% of his pots. (of course one pin hole in a pure
> white pot looked like hell.)
>
> he often told me that a second or third re/fire would break down the
> integrity of the clay body.
>
> perhaps one of our chemist/engineers could clarify this point.
Phyllis E. Tilton on sun 12 mar 00
Mel: In many of ceramic magazines and books, pieces are shown that have some
stunning effects that are explained by multiple firings. The underglazes,
engobes,etc have been layered on. Apparently the clay body holds up under all
this or they just show the ones that came thru it. I have wondered if they
fire at lower cones that would be kinder to the pots. The refirings that you
mention-were they in wood or gas?
Phyllis Tilton
Daisypet@aol.com
Wishing that I were coming to NCECA but circumstances keep me home this
year. I even had a place to stay! I will NOT miss Charlotte next year.
Ron Roy on tue 14 mar 00
Michael - There are some micro photographs in "Ceramic Masterpieces" by
Kingerly and Vandiver of quartz crystals in porcelain. The fissures
produced by the quartz inversion are around the outside of the crystal.
On the next page there is the same type of photo (1000x) of some long fired
soft paste porcelain and you can see the quartz, cristobalite, glass and
wolastonite.
On page 82 there is another of a Sung dynasty body with the same fissures
around the quartz grains.
It is one of the most interesting books in my library and I believe it has
been reprinted.
RR
>----------------------------Original message----------------------------
Michael wrote......
>It is my belief that coarse grains of quartz (which do not melt or invert to
>cristobalite during repeated refirings) become repeated cracked-up by being
>subjected to repeated journeys through the 573 oC inversion point. More
>specifically, by multiple cooling through this point. A strong
>mullite-bearing body matrix can probably expand elastically to absorb the
>573 degree expansion of the quartz grains. But on cooling, the brittle
>quartz is shattered under tension as it becomes abruptly smaller than it's
>cavity in the body. I first became aware of this in examining quartz
>crystals in igneous rocks which are commonly intensively cracked, compared
>to surrounding feldspars (which do not have an inversion) etc, which are
>not. I can provide a photograph of this phenomenon (in jpg format) via
>e-mail to anyone interested.
>
>So repeated firings of quartz crystals (which do not heal their cracks at
>stoneware temps) progressively weaken the body by degrading the internal
>quartz sand. The cracks in the quartz will propagate into those parts of the
>surrounding matrix that are vitreous, weakening the body by meandering
>networks of microcracks. A runaway chain reaction ensues. Repeated firings
>will increase the degree of vitification of the matrix, increasing the
>propensity of cracking to propagate. The progressive shattering of the
>quartz sand removes the grog effect of the sand, which normally interferes
>with crack propagation. Every repeated firing further diminishes the size
>of the quartz, until at some stage it will be fine enough to convert to
>cristobalite. Then the body will then fail spectacularly -if not long
>before.
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Michael Banks on wed 15 mar 00
Thank you Ron, I'll try to access a copy here. It's great to actually see
these processes in action.
A couple of points.
1. Bear in mind that the photomicrographs you describe are in fine-grained
(i.e: -200 mesh or finer) vitreous porcelain, bodies which are acknowledged
not to usually degrade by refiring. Sandy stonewares with 70 to 50 mesh
quartz content, do.
2. At 1000X, the quartz grains must be very small indeed to fit inside the
field of view, i.e: only a few tens of microns in diameter. The contraction
forces these small grains can generate are too puny to effect a well-knitted
porcelain matrix, but are sufficient to spring them loose from it.
3. Many stoneware bodies have quartz sand of up to 70 mesh (200 micron), or
even 50 mesh (300 micron). These large grains are much more likely to
shatter internally than the -200 mesh (-75 micron) quartz in porcelain,
because of the many more inclusion trails present on coarse quartz. These
are planes of weakness acting like perforated dotted lines. Interestingly,
inclusion-rich quartz grains melt preferentially in contact with feldspar.
Thus residual grains that have escaped fusion are tough little,
weakness-free grains. (This is analogous to the remnant ice blocks which
are left over after the melting of big icebergs, very hard, inclusion-free,
blue ice -called growlers).
Refiring stoneware bodies causes the semi-molten body matrix to re-glue
itself to the outside of quartz grains, putting them under great tensional
stress when cooling through the 573 degree inversion contraction. At this
point the matrix is no longer pyroplastic and is rigid. Successive
refirings do seem to progressively weaken stoneware and I put this down to
quartz sand comminution.
Michael
Ron Roy wrote:
> ----------------------------Original message----------------------------
> Michael - There are some micro photographs in "Ceramic Masterpieces" by
> Kingerly and Vandiver of quartz crystals in porcelain. The fissures
> produced by the quartz inversion are around the outside of the crystal.
>
> On the next page there is the same type of photo (1000x) of some long
fired
> soft paste porcelain and you can see the quartz, cristobalite, glass and
> wolastonite.
>
> On page 82 there is another of a Sung dynasty body with the same fissures
> around the quartz grains.
>
> It is one of the most interesting books in my library and I believe it has
> been reprinted.
>
> RR
>
> >----------------------------Original message----------------------------
> Michael wrote......
>
> >It is my belief that coarse grains of quartz (which do not melt or invert
to
> >cristobalite during repeated refirings) become repeated cracked-up by
being
> >subjected to repeated journeys through the 573 oC inversion point. More
> >specifically, by multiple cooling through this point. A strong
> >mullite-bearing body matrix can probably expand elastically to absorb the
> >573 degree expansion of the quartz grains. But on cooling, the brittle
> >quartz is shattered under tension as it becomes abruptly smaller than
it's
> >cavity in the body. I first became aware of this in examining quartz
> >crystals in igneous rocks which are commonly intensively cracked,
compared
> >to surrounding feldspars (which do not have an inversion) etc, which are
> >not. I can provide a photograph of this phenomenon (in jpg format) via
> >e-mail to anyone interested.
> >
> >So repeated firings of quartz crystals (which do not heal their cracks at
> >stoneware temps) progressively weaken the body by degrading the internal
> >quartz sand. The cracks in the quartz will propagate into those parts of
the
> >surrounding matrix that are vitreous, weakening the body by meandering
> >networks of microcracks. A runaway chain reaction ensues. Repeated
firings
> >will increase the degree of vitification of the matrix, increasing the
> >propensity of cracking to propagate. The progressive shattering of the
> >quartz sand removes the grog effect of the sand, which normally
interferes
> >with crack propagation. Every repeated firing further diminishes the
size
> >of the quartz, until at some stage it will be fine enough to convert to
> >cristobalite. Then the body will then fail spectacularly -if not long
> >before.
>
> Ron Roy
> 93 Pegasus Trail
> Scarborough
> Ontario, Canada
> M1G 3N8
> Evenings 416-439-2621
> Fax 416-438-7849
>
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