Fluorine In Glazes On Earthenware - A Technical Challenge

[Tim T]

Hi,

I've been experimenting with glazes containing  fluorine  on some earthenware pots, inspired by some of John Chalke's work (see https://www.pinterest.com/larzgaydos/john-chalke/).

The experiments have been interesting, but I am trying to understand the chemistry of what is going on, and so learn how to manage these better.

The base glaze is 50/50 Nepheliny syenite and cryolite by weight (from the book Dry glazes), with a standard red earthenware clay, and sometimes with a white engobe .Firing is to 1050C with a 15min soak.

I've created a gallery folder called Fluorine where you can see some of the results.

 

The first things to note are that the glaze is hihly reactive and highly fluid - even when only a very thin layer of glaze is applied to the pot. Two examples of this: there are the remains of a thin walled hand built vase that the glaze effectively melted so it folded in half (this doesn't happen with other glazes on the same style of work); the inside of this piece was unglazed, but the glaze has worked through to the inside. Another is the underside shot of the bowl, which wasn't glazed on the underside. You can see a rough base where it broke off from the part that had glued to the kiln shelf; some glaze in the middle of the base where it had worked its way through the pot (about 8mm wall thickness) - the bottom of the bowl was unglazed; and some grinder marks where the glaze run was cut through to free it from the kiln shelf. The glaze ate about 6mm into the kiln shelf!

Furthermore, the pots themselves are generally very weak - as weak as when dried prior to biscuit firing, for example, and some have developed cracks in them as well.

 

The assumption about fluorine in glazes is that it simply gasses off, but this obviously isn't happening, and as fluorine is a much more powerful oxidiser than oxygen I would also be surprised if this happened.

 

On the small tumbler, and to a lesser extent the faceted bowls where there is no engobe under the glaze, there is a smooth glassy glaze on the outside, but then this is easily knocked off by hand to come off in flakes, leaving a very finely pitted surface underneath. So I guess that a gas is being given off at the glaze/body level, but the glaze above it has already sealed over and the gas cannot escape, so a thin foam forms that is very friable when solidified. There are some bubbles in the surface visible in the side shot of the bowl, so some gas is escaping.

 

In general, where the glaze has any thickness it forms  an attractive amber with either crystals or bubbles in it, about 1 - 2mm dia. But there is often a greenish tinge, and also the colour of the clay in contact with the glaze is significantly darker than would be expected, more in line with what would be expected by over firing the pot to about 1250C.

 

I think part of what is happening  is that the Fl in the Cryolite becomes a gas, and then reacts with the iron in the clay to form FeF2 and/or FeF3. FeF3 can be formed by putting iron wool in fluorine gas (see YouTube for some fun videos!) and forms greenish crystals, so this is definitely there. I've found less info on FeF2, but it is reddish-purple, and this may account for the darker colour of the clay?

 

If you look at the view of the bowl from above, a large crystal has formed in the bottom, about 25mm across, and then some more elongated crystals reaching out from there. What this is, I have no idea!

 

I think the fluorine will also be displacing some of the oxygen in the silica in both the glaze, which will weaken it, but I don't think this is the only reason why the clay hasn't vitrified to any significant extent at all. Anyone got any ideas here?

 

This should be enough of a challenge for the more technically minded readers here!

 

 

 

 

 

[bciskepottery]

This guy might be helpful: Jon Singer. Brilliant.

 

http://www.jonsinger.org/

 

http://jossresearch.org/

[Marcia Selsor]

Jon is indeed brilliant and a master glaze wizard. I love running into him at NCECA. he always whips out is most recent glaze tests.

Marcia

[Tim T]

Oh, I remember reading about fluorescent glazes on his web site. Yes, that's a good idea.

[Tyler Miller]

I think you're getting the melted pots because alumina is cryolite soluble when the cryolite is liquid.  I'd hazard a guess that silica is also soluble.  That's likely no kind of revelation, but maybe it's a cue to look at the chem a little differently than normal?

[Callie Beller Diesel]

You might try gently reaching out to his widow, Barbara Tipton. (He passed in March of 2014.)

She is a generous woman, and may point you towards some resources. (Although she isn't likely to give you specific formulas)

Their work tended to cross polinate, as it will when you live with someone.

[Pieter Mostert]

Tim, I think the corrosion may be due to the formation of silicon tetrafluoride. The following is a quote from Out of the Earth, Into the Fire by Mimi Obstler, in the section on fluorspar, and I'm guessing something similar happens with cryolite.

"At temperatures somewher between 1652^oF and 2190^oF (provided silica is present, the disassociating temperature of fluorspar in the absence of silica rises to 2300^oF - 2500^oF), fluorspar breaks apart and releases volatile fluorine gas. The liberated gas attracts silica from the glaze and/or claybody to form silicon tetrafluoride (SiF₄), which escapes from the kiln, leaving behind the newly separated calcium. ...

"The consequences of using fluorspar can sometimes be injurious to the ceramic surface, the kiln interior, and even the potter. The formation of silicon tetrafluoride robs the glaze and/or claybody of needed silica. In addition, the forcible exit of silicon tetrafluoride is often marked by blisters and pinholes. ... Although there is a difference of opinion as to whether the liberation of this volatile gas injures kiln interiors (see Parmalee 1973), Ceramic Industry reports definitively that silicon tetraflouride corrodes furnace linings. Our experience confirmed the corrosive nature of silicon fluoride. A test series that added 20% - 40% of fluorspar to Sanders Celadon glaze not only corroded the kiln shelves, but also attacked the surface of adjoining pots. Even more important is the fact that tetraflouride gas is a known carcinogenic material"

[glazenerd]

Tim:

 

Remember the octet rule from chemistry: "every element will lose or gain an electron to become like the nearest noble gas."  Flourine is one away from the nearest noble gas - Neon. Fluorine can be produced from fluospar only in a reduction environment: carbon monoxide had to be present for it to form. What you are dealing with is reactivity:  Ionization energy measures peak energy released:

 

Lithium 520.2 kj/mol   Fluorine: 1681.0 kj/mol  ....so yes you are setting a bomb off in your glaze

 

The other issue is electron affinity: the power to repel (negative charge) or attract ( positive charge) atoms from other elements:

 

Lithium: 59.6 Kj/mol     Fluorine:  328.0 kj / mol   you dropped a giant magnet in your glaze: does not surprise me a bit that it is pulling atoms from the clay.

 

Lithium has always been considered the strongest flux; Fluorine makes it look like childs play.

Edit: and due to its low molecular weight.... will do so in a hurry.

Nerd

[Callie Beller Diesel]

Two things to this discussion:

1)John Chalke did indeed fire reduction, both high and low fire

2) I saw the man about a month before he passed. He was very sick indeed with cancer. I will politely say that his approach to studio hygiene was definitely "old school."

Edit: as were those of many of the instructors I have been exposed to. ("I'm supposed to show you this way, but I don't do it myself at home.")

[bciskepottery]

Hamer and Hamer (The Potter's Dictionary of Materials and Techniques) has a strong caution against using fluorite -- basically, it is not recommended for direct use in glaze recipes.  They also state fluorine gas is a health hazard in the pottery workshop, is noticeable in the workshop atmosphere, and, over time, will etch the workshop windows until they are opaque.  Their discussion also notes fluorine gas can cause blistering in glazes.  A short entry in the book, but rather informative. 

 

Can fluorine only be produced in a reduction atmosphere? Or can it also be produced in oxidation or neutral atmospheres?  I would think fluorspar could also pick up an electron through eutectics and is not dependent on atmosphere. 

[glazenerd]

There are three grades of fluorspar; the only grade that is suppose to be sold in the pottery world is the acid grade. If that holds true, then you are dealing with 2-3% fluorine. I asked the same question about the release of fluorine gas in an oxidizing environment, with no specific answers. I ended up calling the chemist at the lab supply store, one of the benefits of being a customer. He could not give a definitive answer either, but indicated the temperatures would have to be higher than what I was firing to. So no seems to have an exact answer about the temperature required to release fluorine gas. I have several roof top ventilators in my studio to keep the heat down, gases out, and particulates from floating around.

 

Nerd

[glazenerd]

Tim:

 

Spent a little time going through mineralogical data about cryolite. I do not depend solely on glaze book data: can hear the stones coming at me now. Some info for you: cryolite can come in several grades- no big surprise there. However, read up on halide minerals- might find some info to be useful. Cryolite has a melt temp of 1012C, and you ran to 1050C- so you had it broke down to its cations and anions. In reading a journal from some Swedish chemists- they were testing samples and getting high levels of hydroflouric cations. They cited the presence of a yellow florescence: which I saw on several of your pieces. Some other interesting points;

 

"Molten cryolite is used as a solvent for aluminium oxide"  

- all clays have a fair amount of Al2O3

 

"A halide ion is a halogen atom bearing a negative charge. The halide anions are fluoride (F^−), chloride (Cl^−), bromide (Br^−), iodide (I^−) and astatide (At^−). Such ions are present in all ionic halide salts. Halide minerals contain halides."

 

Given the yellow film I saw, and the melt temp of cryolite vs your peak temp: think you produced a hydroflouric cations- IE solvent = ,melted clay

 

Nerd

[Tim T]

Sorry for the slow response to all your useful comments, the day job is pretty full on at the moment.

 

One thing I may not have made sufficiently clear originally is that all the "interesting" behaviour only happens when the glaze goes directly onto the clay. If I put an engobe on before bisque firing, and then glaze with the cryolite/neph sy glaze, the clay seems OK, all I get is some attractive black speckles breaking through onto the engobe (where I guess some of the fluorine is breaking through the engobe to the clay?). This is why I think the iron in the earthenware is critical. FYI the engobe recipe is 10 calc. kaolin, 16 ball clay, 16 wollastonite, 32 pot spar, 20 frit 3124, 10 zirc.

 

Also, note that I'm using Cryolite (Na3AlF6), not flourite.flourspar (CaF2). Yes, there are several grades of cryolite. I;'m using the purest, with lower grades having more Fe and Si impurities.

 

Tyler, yes the Al dissolves into the cryolite. As people probably know, this is used in the manufacture of aluminium. The aluminium seems to form a stable solution (see http://www.diva-portal.org/smash/get/diva2:125541/FULLTEXT01.pdf).

With a  bit more poking around, a similar process is used to get high purity Si for solar panels, and there are a couple of papers on melting Si in cryolite: https://www.researchgate.net/publication/256696848_Charge_transport_properties_of_cryolite-silica_meltsand http://link.springer.com/article/10.1007%2Fs12633-014-9214-2#/page-1

Possibly most interesting is this older paper on Si and Al in cryolite, resulting in SiF4 being produced as Pieter suggested: http://www.chempap.org/file_access.php?file=276a737.pdf

 

SiF4 has a BP of about -90C, so will be gassed off unless it reacts with something else en route. It then reacts with water (probably outside of the kiln) to form either hydroflouric acid or H2SiF6 (see http://nzic.org.nz/ChemProcesses/production/1C.pdf),in both cases losing Si. So, if the gas leaves the piece, Si will be depleted from the glaze/body. As the cryolite/neph sy glaze has approx. equal numbers of F and Si atoms, a significant amount of the Si will come from the clay.

 

Looking at references from glass makers, they say oxygen is necessary for production of F - my guess is that with its high reactivity, it doesn't care whether it is oxidation or reduction.

 

Glazenerd, do you have the refernce to the Swedish docs for the efflorescence - that may be a useful clue.

 

Lots more reading to do when I get some free time to concentrate on this.

 

I know there are health and safety concerns with this stuff, and hydroflouric acid is pretty nasty stuff, but I've looked into these properly and am happy that my setup meets H&S standards I am happy with, so I'd like to keep the discussion on the glaze chemistry.

 

Work's busy till the end of the month, so I probably won't be able to respond to any comments before then (an advantage is that I can skip half a day from a meeting in Delft this week to look at local Delftware - at a total tangent to this thread, there are still a couple of local makers, but they have dropped using local clay with a tin slip, using imported white clays from the UK and Germany, and painting directly onto the bisqued item.

 

Tim

[glazenerd]

Tim:

 

Start about pg 17- hope you like boring stuff.

http://2dgf.dk/xpdf/bull29-01-02-1-45.pdf

 

Can save you the read: they tested cryolite samples from around the world; made chemical analysis. Noting that ore samples with yellow florescence had much higher levels of F. As a result produced much higher % of hydrofluoric acid  54% F was recorded in high purity samples. 

 

Nerd

[Tim T]

Thanks, I'll read it when back from my travels. Don#'t worry, I plough through papers in my day job, though not in this field