mary simmons on tue 9 nov 99
Well, I HATE being wrong, but Michael Banks has shown me the error of my
ways....SO petalite is mined by the ton in various parts of the world, and
synthesized only by geologists in experimental petrology labs. Mea culpa.....
HOWEVER,
Petalite IS a feldspathoid--it's structure is VERY different than other
feldspathoids (it's really wierd). Looking at this mineral the way Michael
banks did, it does seem to be a "silica-saturated feldspathoid". But,
there is no mention of this term in any of the mineralogy or petrology
books that I have--not a one of them says anything about a high silica
content in this mineral. How can this be, we might well ask, when Klein &
Hurlbutt's _Manual of Mineralogy_ reports a theoretical composition for
petalite as:
Li2O 4.9%
Al2O3 16.7%
SiO2 78.4%
78% silica is higher than K-feldspar silica content! How can I still say
that this is a low silica mineral????? (and I am, geologically speaking)
It is REAL important to remember that we geologists and potters look at
analysed formula in terms of oxides, even though that is not what they
are--we do not have molecules of lithium oxides and alumina oxides and
silica oxides floating around in a random sort of way and call it petalite.
Looking at minerals in this way is just an easier way to cope with the
complexities of the chemistry. What we DO have are STRUCTURES of these
oxides which compose a larger structure of a mineral, such as petalite.
And in these structures, all Si is not silica.
In the mineral petalite, SOME of the Si and all of the Al and all of the Li
are each bonded to 4 oxygen atoms in tetrahedrons. The Si-O structures,
when attached to each other compose silica. The rest of the Si is involved
in a sheet-like structure with the formula Si4O10--which is similar to the
micas, and probably is what gives petalite it's wierd platy habit--these
sheets and the Si, Al and Li tetrahedra are all bound together into the
mineral petalite.
Now, Si4O10 is a silicate but is NOT silica any more than is a mica or a
clay mineral. So, in this mineral, the Si-O in tetrahedral coordination is
considered silica--hence we can call petalite a feldspathoid and get away
with it, though there is a lot of Si elsewhere in the mineral. We know
that all the Si in a silicate does not end up in a glaze as silica
glass--and chalking up the analyzed % of SiO2 in any mineral as all silica
seems illogical to me as well.
Perhaps this is all just splitting atoms and is not useful to glaze
chemistry....
Be that as it may, this has caused me to think a lot about glazes and how
things that start out with crystal structures end up as glass..........And,
how silicates crytallize FROM a melt. WHEN does the silica in the mineral
break down to glass, and does it happen at different times, according to
what structure the silica is in (tetrahedra vs sheets)?
Maybe no one cares about all this, but it seems VERY important to me.....
It seems really important to know how and when these oxides are released
into the glaze, and that we should NOT just tally up the oxides as if
they're all equally available at the same time.
Now remember that I am new to potteryville, and so do not have the
experience with glazes that a lot of you have, so whatever I say is as a
geologist trying to understand glaze chemistry. And I have a BURNING
question for those of you who've made it this far.......
Let's say we are using Custer feldspar to get some fluxing oxides into a
glaze. Fluxes lower melting temperatures by breaking bonds in the
mineral's structure, and by doing so, turn the silicate minerals to glass.
It is not reasonable at all to think that the silica that is contained in
the feldspar has it's bonds broken by the K2O and/or CaO WITHIN the
feldspar structure. It seems more logical to me that another source of a
flux that is NOT bound into a silicate is required to flux that
feldspar--and once THAT happens, the K2O and CaO are released into the melt
to flux away at something else.
I read somewhere (probably on Tony Hansen's fabulous web site) that CaO
fluxes at 2 temps: once at ~ 800C and again at ~1200C...yes? So, CaO
derived from calcite (for instance) could break bonds in a feldspar, which
would liberate other oxides, such as more CaO, and/or Na and/or K2O IN the
feldspar.
Yes, anyone? Do silicate minerals, when melted, decompose immediately and
simultaneously into glass + oxides? Or WHEN do the bonds break?
It also seems logical that a Li-bearing silicate would behave very
differently from Li2CO3 for instance--because the bonds are not the
same.....is it more difficult to get the Li out of a silicate than out of a
carbonate?--if you are using an Li silicate, wouldn't you need something to
flux the Li out of the silicate structure? I presume that carbonates
naturally decompose with heat and water--???
One more very fine, yet interesting point: "petalite commonly alters to
montmorillonite, which may occur as a fibrous pale to deep pink coating on
the petalite"--Deer, Howie and Zussman _The Rock-Forming Minerals_
I look forward to responses and hope that I have not made anyone glaze over
(no pun intended........well, maybe ever so slightly intended)
Mary
PS Michael--some K-spars are pink because of a little Fe stuffed in.
Sometimes K-spars are white, and sometimes they are turquoise......
Mary Simmons
Dept of Earth and Planetary Sciences
Northrop Hall
University of New Mexico
Albuquerque, NM 87131-1116
(505)277-9259
piedra@unm.edu
Craig Martell on thu 11 nov 99
Mary wrote:
>Be that as it may, this has caused me to think a lot about glazes and how
>things that start out with crystal structures end up as glass..........And,
>how silicates crytallize FROM a melt. WHEN does the silica in the mineral
>break down to glass, and does it happen at different times, according to
>what structure the silica is in (tetrahedra vs sheets)?
I would say that the point that silica forms glass in a glaze is persuant
to what else is there in terms of alkali. Glazes are interactions between
acids and alkali. I know that I'm telling you stuff that you know so just
take this as MY verbalization of what I think is happening. All this stuff
is expressed in temperature ranges instead of set melting points because of
the complexity of the glaze matrix and how all these different crystals
interact, and when they start to vibrate due to temp etc. But you asked
about silica in a mineral, and not in a glaze. Potash feldspar, orthoclase
and microcline will start to sinter and melt at around 2000 F. or a bit
above that. So here you have a mineral that really has it all in terms of
a glaze ingredient. Silca, an acidic glass former, alumina which is
amphoteric (halfway point between alkai and acid), and KNaO, the alkaline
flux. Things start to happen in a glaze at the point where the first
alkali starts to react with the other constituents. There is some soda in
potash spar and it will begin to fuse first and the potassium will follow
shortly. As the temperature advances, reactivity quickens. The end result
of full fusion is a function of heat, which is temperature and time.
I've done feldspar fusions at two temperatures. Cone 10 reduction and cone
6 oxidation. Usually, I pack dry feldspar powder into a one half inch
crucible so I have a known volume and shape to deal with. I can compare
rates of fusion by observing deformation once the sample has been fired. I
have done fusions with Kingman, Custer, and g-200 potash spars. Kona f-4,
minspar, and NC 4 soda spars. Petalite and Ceramic grade spodumene(mined
at Kings Mountain North Carolina) and Nepheline Syenite, which is a
feldspathoid as you already know. The most reactive material at both temps
is nepheline syenite. The soda spars are next and are pretty close in
terms of fusion rate. Then the potash spars with custer being the least
fusible due to the higher silica content. Spodumene and Petalite are last
and fuse at an undetermined and variable point. The higher the percentage
of silica in these minerals, the more heat work needed to fuse and deform
them or there is a need to introduce more alkaline radical oxides.
Potash spars don't fuse much at cone 6 ox. The soda spars do better, soda
being a more reactive alkali than potash. Line blends of potash spars and
lithium carb, zinc oxide, gerstley borate, calcium borate frit will produce
a more active melt. As little as 10% gerstley borate with potash spar will
produce a totally fused, flat melt of a packed half inch crucible. Lithium
is less than that but as most know you have to be careful with
lithium. Blends of potash spars with nepheline syenite will also produce
improved fusion at cone 6.
At cone 8 and above, there is a much wider selection of fluxes owing to
increased temp.
>Maybe no one cares about all this, but it seems VERY important to me.....
>It seems really important to know how and when these oxides are released
>into the glaze, and that we should NOT just tally up the oxides as if
>they're all equally available at the same time.
Good point. The end point of the fire and the time it takes to get there
plus the time spent lingering in the highest range of reactivity determines
how all these oxides will be used by the glaze. This is also determined by
the balance of alkai in the seger formula against the amounts of silica and
alumina in the glaze. The alumina-silica ratio is a big determiner here
too. Glazes can be engineered for full fusion at the target temp so that
all the oxides are available and enter into fusion. Then one is buffaloed
by the cooling thing and devitrification. If you want all or most of the
silica to be glass, you need to inhibit crystal formation with the alumina
content, or by cooling the glaze fast. You can get a completely different
glaze from the same receipe by slow or fast cooling if the right alkai are
present. This happens geologically with granite and rhyolite, which are
the same general composition but cool at different rates producing
different rocks. Granite cools slowly and has a more massive crystal habit
and rhyolite has cooled much quicker and is glassier with a finer
crystalline stucture.
>Yes, anyone? Do silicate minerals, when melted, decompose immediately and
>simultaneously into glass + oxides? Or WHEN do the bonds break?
I think that once they are melted and in an excited fluid state,
disassociate. I guess one thing that helps this is gasses being liberated
from the matrix. Depending on what is in the glaze this can be more or
less a factor. Soda spars gas as does fluorspar and cornish stone. CO2 is
released and C0 is ingested. All of this isn't immediate unless the temp
rise is really violent I would think. It's more of an ongoing process that
isn't fully finished until the glaze is fully cooled and set. What has
disassociated may or may not get back together with the same pals. I think
this depends on what valencies are present and how fast the glaze cools. I
think that a quick cool will form a more random association of ions whereas
a slower cool may allow more selective joining. Not REAL sure about this
but I have read in Hamer that this is a definite possibility.
There is a lot more that I'd like to speculate on but it's late and I'm
tired. Not complaining, just giving a reason for bailing out. I'm
hopefully not the only one interested in this discussion so I'll pass the
keyboard to someone else.
The above stuff isn't meant as "the answer" to Mary's points, just my
thoughts on the subjects.
regards, Craig Martell in Oregon
John Hesselberth on fri 12 nov 99
mary simmons wrote:
>And I have a BURNING
>question for those of you who've made it this far.......
>
>Let's say we are using Custer feldspar to get some fluxing oxides into a
>glaze. Fluxes lower melting temperatures by breaking bonds in the
>mineral's structure, and by doing so, turn the silicate minerals to glass.
>It is not reasonable at all to think that the silica that is contained in
>the feldspar has it's bonds broken by the K2O and/or CaO WITHIN the
>feldspar structure. It seems more logical to me that another source of a
>flux that is NOT bound into a silicate is required to flux that
>feldspar--and once THAT happens, the K2O and CaO are released into the melt
>to flux away at something else.
>
>I read somewhere (probably on Tony Hansen's fabulous web site) that CaO
>fluxes at 2 temps: once at ~ 800C and again at ~1200C...yes? So, CaO
>derived from calcite (for instance) could break bonds in a feldspar, which
>would liberate other oxides, such as more CaO, and/or Na and/or K2O IN the
>feldspar.
>
>Yes, anyone? Do silicate minerals, when melted, decompose immediately and
>simultaneously into glass + oxides? Or WHEN do the bonds break?
Hi Mary,
I think you have just stated very well why glaze chemistry is still one
part science and one part mystery (some would even say black magic). An
even simpler example of the dilemma you are raising is found in plain old
vanilla silica or flint. It is pretty well known that the particle size
to which it is ground is important with respect to how well it
incorporates itself into the "glass" of glazes. Use 200 mesh silica and
you very well may get a different glaze than if you use 325 mesh silica.
There are a lot of "time at temperature" and "parts per million chemistry
and physics" things going on as a glaze forms and the overall chemistry
and physics is only understood in the crudest sort of way. That is
exactly why I won't predict whether or not a glaze is stable to acids or
bases without testing it.
Anyhow, welcome to the mysterious world of glazes. Unfortunately most of
the world's ceramic experts stopped doing basic research on glazes
shortly after World War II and turned their attention to the part of the
ceramic spectrum that finds applications in space, electronics and
medicine. Maybe you can help us sort some of this out.
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 time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
mary simmons on sat 13 nov 99
Craig--
>I would say that the point that silica forms glass in a glaze is persuant
>to what else is there in terms of alkali. Glazes are interactions between
>acids and alkali.
So, where does Ca, NOT being an alkali, fit in? Are you lumping it in with
the alkalis and I am just being sniggly? I've read that CaO a pretty
powerful flux, and deduced (rightfully or wrongfully?) that CaO from
non-silicates (ie CaCO3) help to unzip the silicates and release other
fluxes that might be contained therein--e.g. NaO, K2O and more CaO. CaCO3
is easily dissolved in surface waters on Earth, so it seems logical that it
would dissociate in a glaze fairly quickly, and release CaO to go and wreck
crystal structures.
I am trying to understand the process by which fluxes lower melting
tempertuares.
>I know that I'm telling you stuff that you know so just
>take this as MY verbalization of what I think is happening.
likewise.....
>All this stuff
>is expressed in temperature ranges instead of set melting points because of
>the complexity of the glaze matrix and how all these different crystals
>interact, and when they start to vibrate due to temp etc.
OK, then there are TWO things going on? Heat makes bonds vibrate and
expand until they break. How do fluxes break bonds? Or do they, really?
I have this image of little revolutionary CaO's running around a glaze with
scissors, snipping bonds, and liberating imprisoned pals. :) I LIKE
giving thermodynamics a punch line!
>But you asked
>about silica in a mineral, and not in a glaze. Potash feldspar, orthoclase
>and microcline will start to sinter and melt at around 2000 F. or a bit
>above that.
Sinter=to cause to become a coherent mass without melting. ???? What does
sintering mean EXACTLY, or at least in this context???????
And, how do you know when something has started to sinter, or melt, that
it is melting equally in all parts of the mineral, no matter what the
composition or structure?
>So here you have a mineral that really has it all in terms of
>a glaze ingredient.
Yeah....but it doesn't have the stuff we need in the proper quantities.....
>Silca, an acidic glass former, alumina which is
>amphoteric (halfway point between alkai and acid), and KNaO, the alkaline
>flux. Things start to happen in a glaze at the point where the first
>alkali starts to react with the other constituents.
Again, doesn't Ca also get involved EARLY????
>I've done feldspar fusions at two temperatures.
I am not understanding what you mean.......are you fusing fine particles of
feldspar to each other? Or are you melting fine particles of feldspar into
a liquid? Why you are doing this? ......are you doing this to see WHEN you
get the alkalis and Ca-oxides and Si out of the feldspars?
>Potash spars don't fuse much at cone 6 ox. The soda spars do better, soda
>being a more reactive alkali than potash. Line blends of potash spars and
>lithium carb, zinc oxide, gerstley borate, calcium borate frit will produce
>a more active melt. As little as 10% gerstley borate with potash spar will
>produce a totally fused, flat melt of a packed half inch crucible.
GB has Ca in it-is THAT why it makes a more active melt, or because it is a
frit? or because of the boron?
Lithium
>is less than that but as most know you have to be careful with
>lithium. Blends of potash spars with nepheline syenite will also produce
>improved fusion at cone 6.
K + Na makes a more active flux than either of these alone?
>
>>Maybe no one cares about all this, but it seems VERY important to me.....
>>It seems really important to know how and when these oxides are released
>>into the glaze, and that we should NOT just tally up the oxides as if
>>they're all equally available at the same time.
>
>Good point. The end point of the fire and the time it takes to get there
>plus the time spent lingering in the highest range of reactivity determines
>how all these oxides will be used by the glaze. This is also determined by
>the balance of alkai in the seger formula against the amounts of silica and
>alumina in the glaze.
--which is why we don't just use feldspars alone as the total glaze....
>The alumina-silica ratio is a big determiner here
>too. Glazes can be engineered for full fusion at the target temp so that
>all the oxides are available and enter into fusion. Then one is buffaloed
>by the cooling thing and devitrification. If you want all or most of the
>silica to be glass, you need to inhibit crystal formation with the alumina
>content, or by cooling the glaze fast.
You might also want to be sure you haven't over-saturated with some cation
or other (Ca, Mg, etc)--I've read that you can get Mg and Ca-silicate
crystals in a glaze.
You can get a completely different
>glaze from the same receipe by slow or fast cooling if the right alkai are
>present. This happens geologically with granite and rhyolite, which are
>the same general composition but cool at different rates producing
>different rocks. Granite cools slowly and has a more massive crystal habit
>and rhyolite has cooled much quicker and is glassier with a finer
>crystalline stucture.
In rhyolite rocks you do get chunks (we call them phenocrysts--we do this
so that geology has lots of jargon, AND so we don't have to go take more
calculus)--of feldspar, quartz and accessory minerals like biotite, Fe/Ti
oxides and etc., within that glassy matrix. The phenocrysts crystallize in
the magma chamber prior to eruption--they don't happen during cooling. In
granites, things are hot for long periods of time (thousands-millions of
years), so things have a chance to swim around and find like-minded
individuals so to speak.
>
>>Yes, anyone? Do silicate minerals, when melted, decompose immediately and
>>simultaneously into glass + oxides? Or WHEN do the bonds break?
>
>I think that once they are melted and in an excited fluid state,
>disassociate.
But, when you have a complicated silicate, like a feldpsar, it seems
logical that things would not all dissociate at the same time.......I'm am
SO out on a limb here, thinking that with the different bonding situations
going on that some things would dissociate faster than others, depending on
bond strength. Where oh where do fluxes fit in here and what EXACTLY do
they do?
I guess one thing that helps this is gasses being liberated
>from the matrix. Depending on what is in the glaze this can be more or
>less a factor. Soda spars gas as does fluorspar and cornish stone. CO2 is
>released and C0 is ingested. All of this isn't immediate unless the temp
>rise is really violent I would think. It's more of an ongoing process that
>isn't fully finished until the glaze is fully cooled and set. What has
>disassociated may or may not get back together with the same pals.
I'd think it really unlikely that any of the original minerals would
re-equilibrate during cooling. WE have to think of thermodynamics
(eeeuuuww)--it's not just temperature that rules, there are other variables
as well--pressure, Gibb's Free Energy, available elements, and etc.
Contrary to how we like to make life easy by assuming that all minerals are
in a constant state of equilibrium with respect to temperatures (and
pressure) as things cool. If rocks really did act that way, ALL rocks on
surface of the earth would be the same. Consider diamonds, which
crystallize at the base of the crust ~30km or greater below the surface,
and would never crystallize at the surface of the earth (nor because
Superman squeezed a handful of coal!). Yet there they are all over the
place...jewelry stores, all over Liz Taylor. It takes a HUGE amount of
energy to break the bonds in a diamond--much more energy than is needed for
the diamond to endure it's out-of-equilibrium conditions during it's ascent
to the surface or AT the earth's surface. SO, in a glaze, we'd have to
have the proper temperature AND pressure AND composition, AND it would have
to be energetically more favorable for the mineral to re-crystallize as
what it was originally, than for it to remain as a glass (or something
else). THAT is what makes it unlikely.......
>this depends on what valencies are present and how fast the glaze cools. I
>think that a quick cool will form a more random association of ions whereas
>a slower cool may allow more selective joining.
Very true that TIME is a real important part of this--In volcanic rocks,
such as rhyolite, you get sandine, which is a high temperature K-spar--it
is completely disordered, with respect to Al and Si cations which are
randomly distributed between 2 crystallographically distinct tetrahedra,
whereas an orthoclase (also a K-spar, found in granites and pegmatites)
crystallizes at a lower temp, so the Al and Si cations can move around and
find each other, and occupy those tetrahedral sites in a more ordered
fashion. Microcline is the low temp K-spar and is completely ordered, with
respect to those tetrahedral sites.
Here we are at the core of my question (which I have posed to several
petrologists): in a melt are ions swimming around looking for things to
hook up with, and do they find an O or two to bond to, THEN jump into a
silicate structure or does the silicate structure get born full grown,
instantaneously?
It's all so non-simple. But, I think it is really important to look at the
structures these minerals possess, and not to look at the glaze as simply
moles of this and that.
Thanks for the discussion--I really am happy that you took the time to
write all this--there is so much I don't understand, and I want to KNOW!!!!!!
It's about Beer-thirty, and though I don't drink often, my brain hurts now,
and I am going home.......
Mary
Mary Simmons
Dept of Earth and Planetary Sciences
Northrop Hall
University of New Mexico
Albuquerque, NM 87131-1116
(505)277-9259
piedra@unm.edu
mary simmons on sat 13 nov 99
Hi John--thanks for jumping in!
you said:
>I think you have just stated very well why glaze chemistry is still one
>part science and one part mystery (some would even say black magic).
Wow! So I am not alone in these woods, er rocks?????
An
>even simpler example of the dilemma you are raising is found in plain old
>vanilla silica or flint. It is pretty well known that the particle size
>to which it is ground is important with respect to how well it
>incorporates itself into the "glass" of glazes. Use 200 mesh silica and
>you very well may get a different glaze than if you use 325 mesh silica.
This makes sense to me.....smaller particles = fewer bonds to break. Fewer
bonds to break means more glass, faster.......
>Anyhow, welcome to the mysterious world of glazes. Unfortunately most of
>the world's ceramic experts stopped doing basic research on glazes
>shortly after World War II and turned their attention to the part of the
>ceramic spectrum that finds applications in space, electronics and
>medicine. Maybe you can help us sort some of this out.
In order for me to sort out why glazes work, maybe I will! My first
project to that end is to make some thin sections of glazes, pertially
melted glazes and un-fired glazes. Thin sections are 30 micron thick
sections of rocks that are glued to glass slides and viewed under a
polarizing microscope. (I'd have to impregnate the unfired glaze with epoxy
so that a thin section could be made) Due to the 2 polarizers and the
different refractive indexes that each mineral has, the colors that you see
are diagnostic of most minerals. Quartz and feldspar however are
white-gray-black in thin section, but have other optical properties that
are diagnostic. I want to see what crystallizes, and when. Thin sections
are a first step, but in order to see if/when bonds break, I'd have to use
something more powerful than a microscope. Though I am not a graduate
student anymore, I am employed here in the geology department, known for
it's world class analytical equipment--SEM's TEM, xray diffractometer, XRF,
mass-spec.
I can take photos of these thin sections under the microscope and find some
way to post them. I found a site where you can look at regular minerals
that I know and love (mmmmmm, kyanite) in thin section. They are RALLY
Beautiful!!!!. It's at:
http://www.science.ubc.ca/~eoswr/cgi-bin/db_minerals/search.cgi
I appreciate any FRIENDLY information/discussion anyone has--you guys all
have the experience with glazes!!!
thanks!
Mary
Mary Simmons
Dept of Earth and Planetary Sciences
Northrop Hall
University of New Mexico
Albuquerque, NM 87131-1116
(505)277-9259
piedra@unm.edu
John Hesselberth on sun 14 nov 99
mary simmons wrote:
>This makes sense to me.....smaller particles = fewer bonds to break. Fewer
>bonds to break means more glass, faster.......
Probably not fewer bonds to break--at least not a meaningful difference.
The particle sizes for both 200 and 325 are still way above molecular
sizes. And you don't normally break chemical bonds when you physically
divide. It is probably more of a question of time to melt and diffuse
through the melt--plain old heat, mass transfer and, maybe, momentum
transfer considerations. Of course chemical kinetics may be important
too but ,personally, I'd be very surprised if this were limiting. This is
thick gooey stuff, even at its peak temperature. Those molecules and
ions don't move around and mix with each other very quickly. It would be
interesting to know how far they do move, but I would bet it's not more
than a few molecular diameters unless you soak at temperature for a long,
long time. So you don't get a homogeneous mix at the molecular level.
That's why I can put a very thin coat of a clear glaze on top of a copper
containing glaze that leaches badly and cut the leaching down by an order
of magnitude. The two separate glazes largely retain their integrity even
though the one starts out as a very thin coating of particles lying on
top of the other. If I still were a practicing chemical engineer I could
probably make some estimates of how fast these things intermingle in the
melt, but I'm afraid I'll have to leave that to someone who is more
up-to-date on things like the Navier-Stokes equations and other such
stuff.
Fun, huh? Does kind of make one thirsty for a beer.
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 time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
Hank Murrow on tue 16 nov 99
>----------------------------Original message----------------------------
>Mary wrote:
>
>>Be that as it may, this has caused me to think a lot about glazes and how
>>things that start out with crystal structures end up as glass
and Craig answered;
>I would say that the point that silica forms glass in a glaze is persuant
>to what else is there in terms of alkali. Glazes are interactions between
>acids and alkali. As the temperature advances, reactivity quickens. The
>end result of full fusion is a function of heat, which is temperature and
>time.
The end point of the fire and the time it takes to get there
>plus the time spent lingering in the highest range of reactivity determines
>how all these oxides will be used by the glaze. You can get a completely
>different glaze from the same receipe by slow or fast cooling if the right
>alkai are present. This happens geologically with granite and rhyolite,
>which are
>the same general composition but cool at different rates producing
>different rocks. Granite cools slowly and has a more massive crystal habit
>and rhyolite has cooled much quicker and is glassier with a finer
>crystalline stucture.
Not REAL sure about this
>but I have read in Hamer that this is a definite possibility.
>There is a lot more that I'd like to speculate on but it's late and I'm
>tired. Not complaining, just giving a reason for bailing out. I'm
>hopefully not the only one interested in this discussion so I'll pass the
>keyboard to someone else.
>
>The above stuff isn't meant as "the answer" to Mary's points, just my
>thoughts on the subjects.
>
>regards, Craig Martell in Oregon
Nor are my comments meant to be conclusive; But I thought i would pass on
some references from David Stannard (a potter/mentor) which I have found
very useful in building a mental image of what happens at various scales
during the melt. David suggests that Cardew's comments in the appendix to
"Pioneer Potter" (p.304,I think) concerning Ionic Potential are penetrating
indeed:
" Linus Pauling figured out these simple arithmetic ratios, & I bet his
book "The Nature of the Chemical Bond" is still the most direct & simple
treatment of Ionic Potential around.
Also, I got a *tremendous* lot of imagery out of the opening chapters of Brian
Mason's book "Principles of Geochemistry", 1st ed.(Wiley & Sons, N.Y.,
1952) I didn't like his revised edition-- better for geology students, maybe,
but not for me.
W.A.Weyl wrote a book, collected 1930s series of articles from Ceram. Soc.,
called "Colored Glasses" (Dawsons of Pall Mall, 1959) which I found *very*
useful for the static "network former/modifier theory" treatment. Good for
imagining competing color tendencies. Later he switched to Dynamic
modelling-- more satisfying for professionals, & for me in principle, but
too hard to follow & make simple-minded shop use of it!). May have to use
interlibrary loan to get these books." David Stannard
This notion of the Ionic Potential goes a long way to explain why certain
ions are 'more melty' than others. It also explains much of the mystery of
Boron, and why Phosphorus is a glass-former (actually, network former).
Hope these remarks will help, Hank in Eugene
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