Michael Banks on mon 8 feb 99
Fluorspar (fluorite), calcium fluoride, 51.3% calcium, 48.7% fluorine. The
primary ore of fluorine and a valuable industrial mineral with many
applications, including being a powerful ceramic flux. It is unlikely to
become unavailable in the near future, check out other suppliers Jeri.
The only possible substitute is cryolite (sodium aluminium fluoride) which
is 32.8% sodium, 12.8% aluminium and 54.4% fluorine. Any glaze where
cryolite is substituted for fluorspar will have to have substantial
adjustments to replace the lost lime and more anti-flux added to compensate
for cryolites greater fluxing power. But the short answer is that you
should be able to still get fluorspar.
Fluorides such as fluorspar and cryolite give unique colours with some
colouring oxides. But, potters using them face two problems. Both minerals
emit fluorine gas when overfired (which is not healthy and will frost the
pottery windows) and current glaze calculation software does not include
fluorine as a flux, which it undoubtedly is.
Does anyone know why fluorine is not accomodated in glaze calc programs?
Michael Banks
Nelson,
New Zealand
-----Original Message-----
I need info about this flurospar stuff. I can't find it any where and
therefore I don't know what to substitute for it. It is no longer available
at MN Clay or Continental and I have need of it or a substitute for it.
PLease help. Jeri
Alexander Solla on mon 8 feb 99
Micheal-
This idea of Flourine being a flux but not being included in glaze calc
programs has really bothered me. Apparently, it is presumed to have all been
reduced to its gaseous state. Wish that were wholly true. My experiments have
shown this to not be completely true.
I have been playing with flux saturates for the last year as part of my MFA
thesis. My favorite crystal forming glaze is made up of equal parts frit and
cryolite. I vary the frits according to what we are running out of and what
color range I want then. Usually 3185, 3819, 3195, 5301 are my first choices.
Most result in super fluid glazes which when slow cooled over three days,
result in glazes where crystals grow UP out of the glass, making a jagged
surface, like a forest of crystals growing, sometimes can reach 1/32 of a inch
higher than the fluid puddle. As the crysals agglomerate, they matte out
the surface gloss, only showing shine where the glaze thins to the point where
the crystals cant grow. Very wild and absolutely amazing colors!
Good luck with the search for Flourospar. Shouldnt be too hard to find.
Alex in Utah.
slpbm@cc.usu.edu
Gavin Stairs on mon 8 feb 99
At 01:37 PM 2/8/99 EST, you wrote:
....
>Fluorides such as fluorspar and cryolite give unique colours with some
>colouring oxides. But, potters using them face two problems. Both minerals
>emit fluorine gas when overfired (which is not healthy and will frost the
>pottery windows) and current glaze calculation software does not include
>fluorine as a flux, which it undoubtedly is.
>
>Does anyone know why fluorine is not accomodated in glaze calc programs?
....
The general assumption is that all metallic salts, including fluorides, are
"oxidized" in firing, and result in the oxides that are conventionally used
in calculation programs. In fact, fluorine and chlorine are more powerful
oxidizers than oxygen, so there is some good reason to suppose that they
might remain in the glaze under some circumstances.
The main dynamic which replaces halides (fluorine, chlorine, bromine,
iodine...) with oxygen is diffusion. The rule of diffusion is that a high
concentration tends to move to a region of lower concentration. So, if you
pour a layer of fresh water over a layer of salt water, the salt will
diffuse into the fresh water until the whole is slightly saline. The
counter tendency is for the denser salt water to remain below the lighter
(more buoyant) fresh water. Which one wins depends on the time and
temperature and lots of dynamic variables, all of which can be reduced to
the concept of diffusion pressure.
In a pottery glaze, there is a diffusion pressure which tends to bleed the
halide ions into the kiln atmosphere, which has a low concentration of
them. They are replaced by oxygen ions, which are in a relative abundance
in the kiln atmosphere. The longer you leave the system at high
temperature, the more complete this reaction will be.
The counter reaction occurs when you salt glaze. Then, the kiln atmosphere
is rich in chloride, which replaces oxygen in the glaze, and which results
in the characteristic salt glaze. The conventional wisdom is that this
glaze coat is just like any oxide coat, but I don't believe it. I am sure
that it has a relative abundance of chloride ions replacing oxygen. This
has a strong fluxing action.
The reason for the great fluxing strength of the halides is that they are
monovalent, and replace divalent oxygens (one bond replacing two bonds).
The double oxygen bonds are what cross-link silica glass. The halides
break glass bonds, making the matrix less cohesive, and easier to melt.
So why don't calculation programs accommodate this? Well, it is a subject
that has received relatively little attention, and it is quite difficult to
quantify, since the halides are fugitive. It is rather like the question
of the effect of zinc and other fugitive fluxes. They are there at the
outset, and may have a decisive effect on the glaze behavior during
melting, but relatively little may remain at the end of the process. Glaze
calculation programs of the current generation just aren't up to that
challenge.
Gavin
Gavin Stairs
Stairs Small Systems (S3)
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
(416)530-0419 stairs@stairs.on.ca
Edouard Bastarache on tue 9 feb 99
Hello all,
if you can live without fluorine here is a theoritical substitution for
fluorspar:
for each 1% use 0.03% Sio2,
0.97% Whiting
Later,
Edouard Bastarache
edouardb@sorel-tracy.qc.ca
http://www.sorel-tracy.qc.ca/~edouardb/
-----Message d'origine-----
De : Michael Banks
@ : CLAYART@LSV.UKY.EDU
Date : 8 fivrier, 1999 13:48
Objet : Fluorspar???
>----------------------------Original message----------------------------
>Fluorspar (fluorite), calcium fluoride, 51.3% calcium, 48.7% fluorine. The
>primary ore of fluorine and a valuable industrial mineral with many
>applications, including being a powerful ceramic flux. It is unlikely to
>become unavailable in the near future, check out other suppliers Jeri.
>
>The only possible substitute is cryolite (sodium aluminium fluoride) which
>is 32.8% sodium, 12.8% aluminium and 54.4% fluorine. Any glaze where
>cryolite is substituted for fluorspar will have to have substantial
>adjustments to replace the lost lime and more anti-flux added to compensate
>for cryolites greater fluxing power. But the short answer is that you
>should be able to still get fluorspar.
>
>Fluorides such as fluorspar and cryolite give unique colours with some
>colouring oxides. But, potters using them face two problems. Both
minerals
>emit fluorine gas when overfired (which is not healthy and will frost the
>pottery windows) and current glaze calculation software does not include
>fluorine as a flux, which it undoubtedly is.
>
>Does anyone know why fluorine is not accomodated in glaze calc programs?
>
>
>Michael Banks
>Nelson,
>New Zealand
>
>-----Original Message-----
>
>I need info about this flurospar stuff. I can't find it any where and
>therefore I don't know what to substitute for it. It is no longer
available
>at MN Clay or Continental and I have need of it or a substitute for it.
>PLease help. Jeri
Michael Banks on wed 10 feb 99
I've enjoyed the posts from Alex, Edouard and Gavin on this subject.
Alex's information is instructive about the stability of fluorine in glazes.
I believe this element is much less fugitive than coventional wisdom
suggests. I believe fluorine is quite different to the other halogens in
its great affinity for silicates, it eats them like a rottweiler chomping a
rabbit. Chlorine, bromine and the others can't touch quartz. Fluorine
vaporizes it!
The fact that fluorine has a reputation for boiling out and blistering, may
be due to its phenomenal fluxing power and reactivity, rather than being
fugitive. If it is a more powerful flux than any other, the glaze will be
more easily over-fluxed and boil unexpectedly when fired too high. Also,
being more electronegative than oxygen, fluorine has the ability to supplant
oxygen in silicate chains, possibly liberating free-oxygen gas which then
could exit the glaze causing blisters (analogous to manganese dioxide). So
it is possible that fluorines reputation for volatility may be somewhat
undeserved and misunderstood.
My own experience of low-firing fluoride-bearing glazes is, that if you
adjust for the phenomenal fluxing power, a fluoride-fluxed glaze can be
absoutely stable and unblemished. It will however be runnier than most and
will mature several cones lower than its seger formula would suggest. The
crucial fact here is that when the unity formula does not include fluorine,
the predicted maturation pint is not accurate because a crucial flux is not
accounted for.
I believe fluorine is able to occupy sites in silicate liquids (molten
glazes) without diffusing out (sorry Gavin, I don't agree that it wafts out
like chlorine), is capable of staying-put in solid-solution in clear solid
glazes and forms stable mineral inclusions in variagated glazes.
Evidence for these assertions are as follows;
- Frit & enamel makers have long exploited fluorines unrivalled fluxing
power and I beleive the final product still has its fluorine content
unchanged after melting and wet grinding. For example ferro frit 5301
(4101). Manufacturers would probably not waste fluorspar, if half of it
wafted-off up the chimney.
- In nature many magmas (glazes?) contain dissolved fluorine. These are
generally alkali and alumina rich in composition (mostly alkali granites,
nepheline syenites etc). Volcanic glasses from solidification of these when
they erupt at the surface, are also fluorine-rich. Fluorine-rich fumeroles
are rare and when they occur, are due to the hot-water alteration
(destruction) of volcanic glass (ash beds) below. Fluorine does occur in
primary volcanic gas, mostly as hydrogen fluoride, but this is miniscule
compared to the amount staying underground, stabily bonded to silicates.
- Also in nature, fluorine-rich minerals are often the first to crystallize
out as solid grains, the best example being fluor-apatite (fluor-calcium
phosphate) which occurs in a great range of igneous rocks, especially
basalt - the most common rock on the earths surface.
- Finally, the earths mantle is significantly loaded with fluorine, despite
having being kept at a white-hot temperature for the last 4.5 billion years.
Very little has diffused out over all this time for a good reason, it can
form a stable bond with silicates even under these extreme conditions. Much
of the original water there however, has long diffused to the surface.
Fluorine is the most electronegative and reactive element in the known
universe. I would assert that it is also the most powerful flux that
ceramicists can get their hands on. The information from nature would
suggest that glazes high in alkalies, alumina and calcium phosphate (bone
ash) are best able to contain it.
Michael Banks,
Nelson,
New Zealand
Gavin Stairs on thu 11 feb 99
At 10:32 AM 10/02/99 EST, Michael Banks wrote:
....
>more easily over-fluxed and boil unexpectedly when fired too high. Also,
>being more electronegative than oxygen, fluorine has the ability to supplant
>oxygen in silicate chains, possibly liberating free-oxygen gas which then
>could exit the glaze causing blisters (analogous to manganese dioxide). So
>it is possible that fluorines reputation for volatility may be somewhat
>undeserved and misunderstood.
....
>I believe fluorine is able to occupy sites in silicate liquids (molten
>glazes) without diffusing out (sorry Gavin, I don't agree that it wafts out
>like chlorine), is capable of staying-put in solid-solution in clear solid
>glazes and forms stable mineral inclusions in variagated glazes.
....
Hi Michael,
Just a (not so) short rebuttal. I, too, believe that fluorine hangs about
in glazes! However, I'm also quite sure that it diffuses out over time at
high temperature. This is normal behaviour, even for such highly reactive
species as fluorine. Fluorine is the most electronegative, then chlorine,
then oxygen. The rest of the halides are not so reactive. Chlorine and
oxygen are much closer together than either to fluorine, so chlorine can be
more readily displaced by oxygen. All of these will move about in a highly
fluxed, hot glaze. When sufficiently liquid, the diffusion pressures I
mentioned before will indicate which directions the species will tend to
prefer. This stuff only pertains to behaviour at high temperature, in
fairly liquid melts. In lower temperature glazes, where the fluorine is
fluxing, but the glaze remains thixotropic, the mobility will be much less.
This is also true of chlorine, to a lesser extent, as in a glaze including
salt (sodium, potassium, calcium chloride) as a constituent (rather
difficult to control, like Egyptian paste).
The real question is, what happens when the glaze cools? Where does the
fluorine end up? Wherever it is, it will be highly polar (ionic). It will
probably prefer bonding to other fluxes (Na, K, Ca, etc.), as these are
also highly ionic. The result will be isolated ionic pairs in a glass
matrix. This could have very interesting effects on the glaze properties,
not all of them desirable. For one thing, they will considerably weaken
the glass. But they might also impart unusual textures and colours. I
believe this is what is happening in salt glazes, and I would guess than
fluorine glazes will have similar properties, but will follow a different
firing history, with less fluorine mobility, and therefore higher residual
halide content in the end.
The salt glaze, which conventionally should be a very poor fit to the body
because of the high sodium, probably ends up fitting only because of the
fugitive chloride fluxing, resulting in a glaze coat much lower in sodium
than expected otherwise.
There is a relatively easy way to check the fugitive nature of fluorine and
chlorine. That is to make a glaze or frit, and then to attempt to remelt
it. One of these days I will have a go at it. Unfortunately, I seem to be
rather tied up in the problems of my day job these days.
The remaining problem of how to include these effects in a glaze
calculation program requires a new model, I guess. Certainly one could
simply reduce the maturity of a glaze, but by how much, and with what
limits?
This would seem to be a whole new area for experimentation. I'd like to
get the ACerS phase diagrams on CD-ROM, but the cost is a bit steep. Maybe
I could buy it, and then sell research services? Tell me your mixture, and
I'll look it up for you?
Gavin
Edouard Bastarache on thu 11 feb 99
Hello Michael,
a few years ago i tested a few chromium green glazes for high fire as
suggested by Richard Behrens in Glaze Projects.Not having fluorspar in my
lab i substituted for it using my proposition i send to the list
recently, the results were excellent, all the glazes were well fluxed, the
recipies and pictures were then sent to the GlazeBase project.
What do you think?
Later,
Edouard Bastarache
edouardb@sorel-tracy.qc.ca
http://www.sorel-tracy.qc.ca/~edouardb/
-----Message d'origine-----
De : Michael Banks
@ : CLAYART@LSV.UKY.EDU
Date : 10 fivrier, 1999 10:32
Objet : Fluorspar???
>----------------------------Original message----------------------------
>I've enjoyed the posts from Alex, Edouard and Gavin on this subject.
>
>Alex's information is instructive about the stability of fluorine in
glazes.
>I believe this element is much less fugitive than coventional wisdom
>suggests. I believe fluorine is quite different to the other halogens in
>its great affinity for silicates, it eats them like a rottweiler chomping a
>rabbit. Chlorine, bromine and the others can't touch quartz. Fluorine
>vaporizes it!
>
>The fact that fluorine has a reputation for boiling out and blistering, may
>be due to its phenomenal fluxing power and reactivity, rather than being
>fugitive. If it is a more powerful flux than any other, the glaze will be
>more easily over-fluxed and boil unexpectedly when fired too high. Also,
>being more electronegative than oxygen, fluorine has the ability to
supplant
>oxygen in silicate chains, possibly liberating free-oxygen gas which then
>could exit the glaze causing blisters (analogous to manganese dioxide). So
>it is possible that fluorines reputation for volatility may be somewhat
>undeserved and misunderstood.
>
>My own experience of low-firing fluoride-bearing glazes is, that if you
>adjust for the phenomenal fluxing power, a fluoride-fluxed glaze can be
>absoutely stable and unblemished. It will however be runnier than most and
>will mature several cones lower than its seger formula would suggest. The
>crucial fact here is that when the unity formula does not include fluorine,
>the predicted maturation pint is not accurate because a crucial flux is not
>accounted for.
>
>I believe fluorine is able to occupy sites in silicate liquids (molten
>glazes) without diffusing out (sorry Gavin, I don't agree that it wafts out
>like chlorine), is capable of staying-put in solid-solution in clear solid
>glazes and forms stable mineral inclusions in variagated glazes.
>
>Evidence for these assertions are as follows;
>
>- Frit & enamel makers have long exploited fluorines unrivalled fluxing
>power and I beleive the final product still has its fluorine content
>unchanged after melting and wet grinding. For example ferro frit 5301
>(4101). Manufacturers would probably not waste fluorspar, if half of it
>wafted-off up the chimney.
>
>- In nature many magmas (glazes?) contain dissolved fluorine. These are
>generally alkali and alumina rich in composition (mostly alkali granites,
>nepheline syenites etc). Volcanic glasses from solidification of these
when
>they erupt at the surface, are also fluorine-rich. Fluorine-rich fumeroles
>are rare and when they occur, are due to the hot-water alteration
>(destruction) of volcanic glass (ash beds) below. Fluorine does occur in
>primary volcanic gas, mostly as hydrogen fluoride, but this is miniscule
>compared to the amount staying underground, stabily bonded to silicates.
>
>- Also in nature, fluorine-rich minerals are often the first to crystallize
>out as solid grains, the best example being fluor-apatite (fluor-calcium
>phosphate) which occurs in a great range of igneous rocks, especially
>basalt - the most common rock on the earths surface.
>
>- Finally, the earths mantle is significantly loaded with fluorine, despite
>having being kept at a white-hot temperature for the last 4.5 billion
years.
>Very little has diffused out over all this time for a good reason, it can
>form a stable bond with silicates even under these extreme conditions.
Much
>of the original water there however, has long diffused to the surface.
>
>Fluorine is the most electronegative and reactive element in the known
>universe. I would assert that it is also the most powerful flux that
>ceramicists can get their hands on. The information from nature would
>suggest that glazes high in alkalies, alumina and calcium phosphate (bone
>ash) are best able to contain it.
>
>Michael Banks,
>Nelson,
>New Zealand
Phyllis E. Tilton on fri 12 feb 99
Another vein of thought but still regarding fluorine only in the form of
fluoride. We have fluoridated water. In using it for making glazes, even in
it's very diluted amount, can it ultimately have an effect on the chemical
balance? My other thought is in the recycling of clay--from the slurry bucket,
evaporating, etc. If the fluoride does not volatilize but concentrates with
the evaporation of water, can this have an effect on the clay body? I get
these icky,picky, whatever thoughts about minuscule things but was once told
that you can't learn much if you don't ask questions. I hope this isn't a dumb
question but I'm sure I'll learn something.
Thanks
Phyllis Tilton
Daisypet@aol.com
Michael Banks on fri 12 feb 99
To Edouards question (appended below), I would say that he obviously added
enough whiting to supply the Behrens glaze flux needs, in leu of fluorspar.
This being a plain chromium green glaze, the main irreplacable benefit
afforded by the fluoride component in cobalt and copper glazes, being the
special colour effects, is not an issue. In other words, it doesn't sound
like the fluorspar was needed in this glaze to start with and as it has no
ability to improve chrome green, - it was entirely redundant.
To Gavins post (also appended below, abridged): I would say that unlike
chlorine, fluorine can form a number of ultra-stable mineral inclusions in
cooled glazes, just like it does in nature. These include crystals of
fluorapatite, phlogopite and cryolite to mention a few. The presence of the
these may decrease glaze durability to a marginal degree, but no more than
most other opacifiers and fluorapatite may actually increase the durability
of alkaline glazes. Fluorapatite is the mineral formed in teeth enamel in
people exposed to fluoridated water and is notably chemical and abrasion
resistant. People worried about glaze durability should add bone ash to the
recipe to encourage the formation of fluorapatite.
I agree that in purely-amorphous transparent glazes, fluorine may make the
glaze softer, but this is often the price paid for special colour effects,
and such glazes are usually not intended for non-ornamental surfaces. I
can't say in what form the fluorine takes in these clear glasses (as opposed
to the semi-opaque glazes discussed above), as I'm currently in Indonesia -
a long way from my text books back in NZ, but I doubt that it occurs as
monomolecular alkali halides. These are soluble and would leach from frit
when it was wet ground, which is not observed. Calcium fluoride, cryolite
etc, are certainly possibilties and while soft, are quite insoluble.
I would say that your suggested fluorine-containing frit experiment Gavin,
is easily done, but why the fluorine in a frit such as ferro 5301 (aka 4101)
should survive one firing and not two escapes me. If the fluorine is
contained in interstitial monomolecular alkali fluorides (as you suggested)
or monomolecular fluorspar or cryolite (as I've postulated) it still is not
going to have significan vapour pressure at frit melting temps ~900 degrees
C.
Michael Banks
Jakarta
-----Original Message--1
> a few years ago i tested a few chromium green glazes for high fire as
>suggested by Edouardin Glaze Projects.Not having fluorspar in my
>lab i substituted for it using my proposition i send to the list
>recently, the results were excellent, all the glazes were well fluxed, the
>recipies and pictures were then sent to the GlazeBase project.
>What do you think? Snip
>Edouard Bastarache
-----Original Message--2
>The real question is, what happens when the glaze cools? Where does the
>fluorine end up? Wherever it is, it will be highly polar (ionic). It will
>probably prefer bonding to other fluxes (Na, K, Ca, etc.), as these are
>also highly ionic. The result will be isolated ionic pairs in a glass
>matrix. This could have very interesting effects on the glaze properties,
>not all of them desirable. For one thing, they will considerably weaken
>the glass...
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