iandol on sun 12 nov 00
I had always believed that pinholes, porosity and bubble inclusions in a =
glaze were caused by the late evolution of carbon dioxide from glazes =
which were compounded from recipes which contained carbonate derived =
fluxes, though I do know there are other possibilities.
However, a recent glaze test series where carbonates were deliberately =
excluded had several examples which contained widespread subsurface =
porosity and examination with a X-10 hand lens confirmed this =
observation. The majority of the remaining samples were free from the =
defect but a few which were opaque had a textured surface which =
suggested that there had been gas evolution. More than three quarters of =
the 36 samples were smooth with no suspicion of pinholing.
An immediate thought was that these gas inclusions might be due to the =
evolution of water vapour from the Talc or Kaolin. But since F. Hamer =
suggests dehydroxylisation of clay will be complete by 600 deg Celsius =
and 900 deg Celsius for Talc, well below the cone 8 to which the =
samples were fired, this seemed an improbable explanation. Also, since =
the tiles had been bisqued to cone 08 it seems unlikely that the gas =
should emanate from the clay.
The only significant points of note:
There had been a healthy reduction, evidence for which were copper red =
and a celadon blue green test tiles which were testing a previous a =
recipe from a previous tile for colour response as well as delicate =
transparent greens from mixtures afflicted with the bubbles.
The basic components were Soda felspar, Wollastonite, Talc and Kaolin.
Porosity was only seen in the high felspar, low kaolin, high talc, low =
Wollastonite corner of the tile and was most pronounced in the recipe =
which had maximum felspar and talc and minimum kaolin and Wollastonite.
The mixtures were fired on two clays and both gave similar results which =
seems also to rule out the clay as a factor.
Now I do have ideas about this which may not be popular. But I would =
appreciate any other valid opinion or logical explanation as to why =
there should be such widespread gas inclusions in an otherwise =
uncoloured transparent glaze when there is no apparent cause, according =
to the usual rules.
Looking forward your ideas,
Ivor Lewis. Redhill, South Australia.
Craig Martell on sun 12 nov 00
Hello:
Ivor asked for a "valid opinion" about the bubbles in his tests. I'll
venture an opinion and you guys can judge whether or not it's "valid"!! :>)
I was thinking that cone 08 may be a bit low to remove all the carbonaceous
material from the clay and if the glaze has significant viscosity under
temperature, it may take too much time for the gasses to liberate and reach
the glaze surface.
It is also stated in Hamer & Hamer, plus other texts that soda feldspar is
unstable above cone 8, I think, and will start to outgas. One approach you
may want to try is changing the soda spar to potash spar which is stable
throughout all of the high temp range. If this cures the bubbles, there is
a high probability that the soda spar is the culprit. Again, I do think
that viscosity of the glaze will have a bearing too and you would see
problems in areas of temp difference in the kiln.
regards, Craig Martell in Oregon
John Hesselberth on sun 12 nov 00
iandol wrote:
>Now I do have ideas about this which may not be popular. But I would
>appreciate any other valid opinion or logical explanation as to why there
>should be such widespread gas inclusions in an otherwise uncoloured
>transparent glaze when there is no apparent cause, according to the usual
>rules.
>
>Looking forward your ideas,
A glaze powder on the surface of a pot contains lots of included air. I
haven't ever tried to measure the bulk density of unfired glaze applied
to a pot surface but I bet it is pretty low compared to a pure gas-free
melted glaze. As the glaze starts to melt there is very little driving
force to "eject" that included air. I expect much of it just stays put.
I expect a truly bubble-free glaze is a pretty elusive beast. Nearly all
glazes will have tiny bubbles if examined carefully enough.
By the way it is also my opinion that most carbonate generated gas does
leave. I expect most of the oxidation occurs below the "seal-over"
temperature. And there would be a driving force to expel that gas (or an
equal volume of included air).
Regards, John
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is, perhaps, still necessary to say that the very best glazes cannot
conceal badly shaped pots..." David Green, Pottery Glazes
Gavin Stairs on mon 13 nov 00
At 10:19 PM 11/11/00, Ivor wrote:
>...
>Porosity was only seen in the high felspar, low kaolin, high talc, low
>Wollastonite corner of the tile and was most pronounced in the recipe
>which had maximum felspar and talc and minimum kaolin and Wollastonite.
Ivor, I appreciate your relatively complete protocol report. Makes the
guessing a bit less random.
Perhaps the bubbles are inclusion bubbles of air from the initial
porosity. With almost complete spar and low clay, the melt might come on
rather abruptly overall, resulting in trapped air after the glaze had
sealed over. This would not occur where there was enough clay to provide
channels for air to escape during consolidation of the sinter mass, or at
least not as much.
Gavin
Gavin Stairs
Stairs Small Systems
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
phone: (416)530-0419 stairs@stairs.on.ca
Cameron Harman on mon 13 nov 00
As many of you know, I do not comment very often as mostly the
comments by others are well stated, plus I don't have as much time
as I would like any more. However, I thought that I could add
factual anecdotal information for your interest.
The explanations by Craig Martell and John Hesselberth are right
on. In the early 50s my father was head of the Ceramic Department
at Battelle Memorial Institute in Columbus, OH. They had a project
from the US makers of dinnerware to find out what they could do to
get brighter glazes on their ware.
Their detailed investigation showed that there was a large bubble
population in the less bright ware and that Syracuse China (at
that time) had the brightest ware ( so bright that dad could spot
their ware whenever he went into a restaurant). Interestingly
enough, the Syracuse ware had almost zero bubbles in the glaze.
Dad postulated that the degassing of the ware that took place
after the glaze crusted over on the surface caused the bubbles. As
a result he suggest a slower firing rate below 1700 degrees (for
their ware) to allow the degassing of the body to take place while
the gasses could still escape through the glaze coating.
As expected, the glazes came out bright and bubble free. On a
factory tour of Syracuse China, they found that the Syracuse
firing curve matched the recommendations.
Different ware and different glazes will demand adjustments to the
temperature, but I have seen this approach work many times over
the years. In this case it doesn't matter what causes the bubbles
if you give them time to get out before the glaze surface is too
hard.
It should be interesting to note that years later when I visited
this same Syracuse factory, they had lost all knowledge of this
work and were having troubles with a high bubble population in
their glazes. That just shows how the little changes made over the
years can sometime accumulate in a negative way.
Cameron
--
**********************************************************
Cameron G. Harman, Jr. 800-556-0766 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
get your free ezine: http://www.kilnman.com/ezine/ezine.html
THE place to go for solutions to your drying and firing problems
**********************************************************
ferenc jakab on mon 13 nov 00
Porosity was only seen in the high felspar, low kaolin, high talc, low
Wollastonite corner of the tile and was most pronounced in the recipe which
had maximum felspar and talc and minimum kaolin and Wollastonite.
Ivor,
Wollastonite is Calcium Silicate, You probably know this. On another
tangent, any gas could cause this effect but my inclination is that your two
clays contain enough Calcium Carbonate to liberate CO2 at those
temperatures. A further thought is that reducing the Wollastonite has
reduced the flux in the glaze not permitting it to mature sufficiently. I
had both these problems recently with Bennett's Fine Terracotta.
Feri
bea pix on mon 13 nov 00
is there anyone on Long Island in NY who has a (propane gas) kiln at
home and could tell me what the county or town (mineola or
thereabouts) regulations might be?
thanks
Tom Buck on mon 13 nov 00
Ivor:
as you say, the bubbles/pinholes are not caused by carbon dixoide,
an unlikely case at C9/10 R even if carbonates were used in the glaze mix
(assuming you fire at a reasonable rate, ie, not too fast).
What could be outgassing at 1300 C? A possible candidate is
a compound of silicon and fluorine. Fluorine is often encountered in clays
and feldspars, and it is possible that the bound fluorine would not
dissociate until high temperatures. And then the free F would link with
silicon to form a volatile compound that would find a path out of the body
and the glaze.
A second possible source of outgassing would be certain metallic
oxides that are affected by reducing agents and the high temperature. For
example, Zinc Oxide itself doesn't melt (and therefore form vapour) until
1975 C. But it is easily reduced to Zinc metal by CO/H2 at high
temperature, and the Zinc immediately vapourizes. Since this is an ongoing
process, and slow, outgassing will continue for some time.
Ponder on these ideas, friend Ivor. BFN. Peace. Tom.
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
Dave Murphy on mon 13 nov 00
Tom & Ivor:
I was extremely interested to hear from Tom Buck that a culprit could be =
zinc oxide. The rutile blue glaze that I use and experience these nasty =
tricks sometimes contains a small percentage of zinc. As this usually =
volitizes could I just leave it out. The main feldspar is Neph Sye.
Barbara Murphy
Waterloo County Pottery
Waterloo Ontario
Canada
Jeff Lawrence on tue 14 nov 00
Ivor was meditating on bubble formation in the absence of carbonates...
Hello Ivor!
Back before I learned not to use a high-talc body for cone 6, I noticed a
related phenomenon.
As you probably knew, the talc outgasses and makes glazes look ugly. My body
(1:1 talc:clay) looked good under cone 6 glazes after a single good firing
(1 hour soak at 2210, two hours soak at 1900 or so). I had a temmoku that
looked great and Floating Blue was beautiful too. Until I refired them, that
is.
After the second firing, the temmoku turned algae green and bubbly opaque
while the Floating blue turned greenish brown with blue highlights.
Another firing of the temmoku greened it up even more.
My conclusion is that talc in a clay body oozes its noxins bit by bit, and
would bet an ice-cold beer that your outgassing comes in part from talc
inclusions, if they are present.
Any theories why the temmoku brown turned green?
Jeff Lawrence ph. 505-753-5913
Sun Dagger Design fx. 505-753-8074
18496 US HWY 285/84 jml@sundagger.com
Espanola, NM 87532 www.sundagger.com
Michael Banks on tue 14 nov 00
Ivor,
Feldspathic melts are naturally bubbly. Try a simple experiment: make a
fusion button of any pure potash feldspar by firing to cone 10. Examine the
glossy whitish globule with a magnifying lens. The globule is whitish
because of innumerable tiny bubbles. These bubbles are the reason why
feldspathic glazes such as pale celedons and other pale Sung dynasty type
glazes have such an exquisite milkyness.
The bubbles derive not from volitile alkalis in the feldspar, but from the
inevitable tiny mineral, fluid and gaseous inclusions present in all natural
feldspar. Examining feldspar crystals microscopically, reveals many of these
are sericite (white mica) plates aligned parallel to the feldspar cleavage
planes. Sercicite forms inside feldspar during subsolidus (below melting
point) chemical adjustment of the feldspar lattice as it cools from high
temperature; a little potash, water (sometimes plus fluorine) and alumina is
exsolved by the cooling feldspar. Firing the feldspar in a glaze reliberates
the gases from the decomposing sericite. The tiny gas bubbles are trapped in
the high viscosity melt and are extremely difficult to clear.
Other carbonate-free, fluoride-free, anhydrous minerals also produce bubbly
glazes. Silica flour is often manufactured by grinding vein quartz lumps, or
sand derived from vein quartz. But even quartz derived from eroded granite
or sandstone or gneiss contains many inclusions which survive fine grinding.
Vein quartz forms in nature from hydrothermal solution precipitation (hot
water). Many fluid inclusions are normally entrapped in vein quartz and are
easily visible under a light microscope at medium magnification. These fluid
inclusions usually consist of water plus gas, but liquid carbon dioxide ones
are quite common too. The immense pressure contained inside the glassy solid
quartz walls is sufficient to maintain the carbon dioxide as a liquid.
Fluid inclusion study is a entire sub-discipline of optical microscopy in
the Earth Sciences and has yielded large amounts of data about conditions
prevailing deep in the earth at elevated temps and pressures.
Most other natural mineral products used by potters contain potential gas
producing inclusions too. Only by formulating your glazes from pure frits,
might you be able to escape the bubbles. But then, examining powdered frit
by microscope also reveals inclusions...
Michael Banks,
Nelson,
New Zealand
----- Original Message -----
Ivor Lewis wrote:
> Porosity was only seen in the high felspar, low kaolin, high talc, low
> Wollastonite corner of the tile and was most pronounced in the recipe
which
> had maximum felspar and talc and minimum kaolin and Wollastonite.
Michael Banks on fri 17 nov 00
Hi Jeff,
You are right to be wary of "talc". The water content of pure talc is 4.8%
(hydroxyl radicals yielded on decomposition) and this constitutes the main
volitile component of this mineral, but some commercial talcs contain
significant carbonate in the form of magnesite (MgCO3) -and will therefore
de-gas carbon dioxide. Talc is often commercially recovered from
talc-magnesite rock. Recently I fielded a query from a clayart correspondent
in the UK who had trouble from a "talc" from his ceramic materials supplier,
which actually contained a large proportion of magnesite. So some talcs at
least, cannot be strictly regarded as being a carbonate-free material.
I've found that talc (even the pure stuff) can be the source of quite severe
problems if not dispersed well in glazes and bodies. I'm involved in
manufacturing and marketing five white bodies and two glazes containing
talc. Where customers are supplied these in powder form, complaints arise
from time to time concerning bubbling, pinholing and cratering of glaze
coats. Inevitably this is sourced to incomplete mixing/screening on their
part. Talc is a greasy, hydrophobic material and requires very thorough
mixing. All our wet clay sales are mixed in big, high energy blungers and
the talc is added first to the water, ensuring the longest residence time in
a turbulent, low viscosity mix. The care required in mixing talc bodies
argues against using dry-powder/wet pugging methods to produce these clays.
Your floating blue glaze could be turning green on the second firing, due to
the affinity titanium dioxide has for iron. After the first firing,
floating rutile inclusions have not had long enough to dissolve in the
glass. Second time around, more titanium goes into solution and becomes an
extremely powerful scavenger of ambient iron, even from the underlying
ceramic body. Iron plus titanium have a pronounced yellowing effect, along
with cobalt giving greens.
Michael Banks,
Nelson,
New Zealand
----- Original Message -----
Jeff Lawrence wrote;
> Ivor was meditating on bubble formation in the absence of carbonates...
> Back before I learned not to use a high-talc body for cone 6, I noticed a
> related phenomenon.
>
> As you probably knew, the talc outgasses and makes glazes look ugly. My
body
> (1:1 talc:clay) looked good under cone 6 glazes after a single good firing
> (1 hour soak at 2210, two hours soak at 1900 or so). I had a temmoku that
> looked great and Floating Blue was beautiful too. Until I refired them,
that
> is.
>
> After the second firing, the temmoku turned algae green and bubbly opaque
> while the Floating blue turned greenish brown with blue highlights.
>
> Another firing of the temmoku greened it up even more.
>
> My conclusion is that talc in a clay body oozes its noxins bit by bit, and
> would bet an ice-cold beer that your outgassing comes in part from talc
> inclusions, if they are present.
>
> Any theories why the temmoku brown turned green?
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