Bubble Bubble Toil And Trouble

[Tim T]

I'll try and remember to root out a test tile when I'm in the workshop tomorrow.

The approach does keep Si and Al constant - whether or not they are correct is a different matter, in the same way as with the Currie approach the flux is held constant, whether or not the flux is correct. So you are effectively making two orthogonal tests by combining these 2 approaches, and the correct glaze probably lies somewhere inbetween, e.g. increasing Si and/or Al as the flux mixture becomes more active.

Keeping Si and Al constant often means radical changes to the materials in the 4 corners, e.g. you may start off using Feldspar around the "normal" glaze but then have to switch to frits for low K or Na (as an aside, for the frits I have, why is there a preponderance of Na over K?). Then a combination of Insight and Excel to juggle the formulae.

The normal balanced flux can be wherever you want. For example, if you think it is low in K and want to try increasing it, you would put the starting point somewhere along the low K side, whereas if you had a glaze that worked OK but you wanted to see what the effect of playing with the fluxes would be, you may put the starting point in the middle, or if you wanted a lower firing glaze through adding more active fluxes the start point would be where the active fluxes are low.

[curt]

Tim, the currie method keeps the proportions of individual flux ingredients constant relative to each other buy it does not hold the flux constant.  In a standard currie tile, the flux package goes from 100% of the glaze in corner C all the way down to 35% of the glaze up in corner B.  Perhaps this is what you meant anyway.

 

I believe the reason most of our frits have more Na than K is because they are made primarily for the glass industry and not for potters.  Mainstream glass recipes almost always have high sodium as the main flux.    Compared to potassium, sodium melts at a higher temperature and rather suddenly late in the firing process which is helpful for fast fire glazes in industry.

 

Going to have to think more about the feasibility of switching glaze ingredients (eg, feldspar to frit) in the middle of a currie tile.  My first thought is that would have a major impact on the glaze and make it difficult to compare cells across the tile, or? 

[Tim T]

Curt, you said what I meant to say!

Thanks for explaining the preponderance of Na in fluxes.

 

The image shows what I mean by varying the fluxes (in this case Ca and KNaO) - though when making it I got distracted so the bottom rows (below the white line) are flipped left to right compared to the top. Also, I was playing around with 2ndary fluxes in the glaze, so some of the effects are minimal. KNaO is used because I couldn't get low enough K with what I had in stock, so the K:Na ratio changes in the glaze.

 

In this case, the starting glaze was high in Ca and KNaO as I wanted to see if reducing either or both of these would reduce the bubbling.

 

As I see it, there are a couple of ways that the compositions of the 4 corners could be worked out, given one corner as a starting point:

- Use the Seger formula and vary the proportions of the fluxes, but keep Si and Al constant

- or use the molar formula, and keep the proportions of everything except the fluxes you are varying constant, so this is akin to having a base glaze and adding increasing amounts of the 2 fluxes to it

Because of the presence of Boron, and its ambivalence in the Seger approach, I decided to take the 2nd approach for this test.

So, from the starting glaze there is 87% excluding the Ca and KNaO. I then set the targets for Ca and KNaO for the other 3 corners and rescale the proportions for these so they come to 100%. In this case the low targets were set by what was achievable with the materials to hand.

 

Anyway, the 4 corners are:

 

Top left - High Ca, high KNa, low rest of flux

Potash feldspar 29, Soda feldspar 7, Lead bisilicate 27, Kaolin 4, Silica 19, Whiting 3, Colemanite 13

CaO 0.34, MgO 0.02, KNaO 0.25, PbO 0.39, Al2O3 0.30, B2O3 0.30, SiO2 3.0

 

Top right - High Ca, low KNa

Lead bisilicate 28, Kaolin 21, Silica 36, Colemanite 13, Whiting 3

CaO 0.46, MgO 0.02, PbO 0.52, Al2O3 0.40, B2O3 0.40, SiO2 3.97

 

Bottom left - Low Ca, low KNa, high rest of flux

Lead bisilicate 29, Kaolin 14, Frit 3195 27, Silica 30

CaO 0.27, KNaO 0.13, PbO 0.59, Al2O3 0.45, B2O3 0.45, SiO2 4.53

 

Bottom right - Low Ca, high KNa

Potash feldspar 25, Lead bisilicate 29, Kaolin 2, Frit 3195 26, Silica 18

CaO 0.23, KNaO 0.30, PbO 0.48, Al2O3 0.36, B2O3 0.37, SiO2 3.6

 

This was fired to 1120 with a 15 minute soak in oxidation.

 

Although incidental to discussing the approach I'm using, the effects on the glaze are:

As Ca increases, the bubbles increase in number and get smaller, also making the glaze more opaque. Top right is also noticeably more matt and there is more shrinkage in the glaze.

Changing KNa had very little effect at the low Ca end, though at the high Ca end it helped offset the effect of too much Ca.

 

Tim

 

[High Bridge Pottery]

Not sure I can completely understand what you have tried to do. Looks to me like the silica and alumina are varying a lot across the tile.

[Tim T]

Joel,

To clarify a bit more:

From the starting glaze, using the molar formula, there is 13% CaO + KNaO and 87% of Si, Al and the rest.

To get the low Ca, low KNaO glaze, what I ideally want is 0% of both CaO and KNaO, so the target content of the other components is multiplied by 100/87.

In fact, I can't achieve this. the closest I can get to zeroing both of these out gives 4% CaO and 2% KNaO, totalling 6%, and 94% of the other components (the MgO isn't intended and isn't a flux at this temperature, so I'm happy for it to go to 0).

Thus Si, Al and the other components not being tested will have increased by 94/87 in the molar formula due to the reduction of Ca and KNaO, but their proportions will be the same.

In the Seger formula, there are 2 effects: to make the fluxes add up to 1, the fluxes other than KNaO and CaO will increase. Also, because the amount of flux in the glaze has reduced, the amounts of Si and Al will appear to increase.

As I'm working with molar proportions in making these changes, it may have been clearer if I'd given the molar formulae here:

Base glaze: 7.5% CaO, 0.4% MgO, 5.3% KNaO, 8.5% PbO, 6.5% Al2O3, 6.5% B2O3, 65.2% SiO2

Low Ca and KNa glaze: 4.2% CaO, 0% MgO, 2.1% KNaO, 9.2% PbO, 7% Al2O3, 7% B2O3, 70.4% SiO2

 

Hope this is clearer,

Tim

[curt]

Hi Tim,

 

I may be missing something, but I agree with Joel that the silica and alumina seem to moving around a lot based on the numbers you have provided. 

 

I think than no matter wether you use a seger formula approach or mole %, if you want to hold silica and alumina constant, then  the numbers for silica and alumina should be the same for all four corners and everywhere in between, ie, all 35 cells on a currie tile.  That means that any changes in CaO or KNaO across the grid are compensated only by adjustments in mole % of the remaining fluxes, rather than any changes in silica and alumina.

 

Does that make sense?

 

I thought your comments on the tile you put up were interesting, and seem to confirm that KNaO help the Ca get digested in the melt.  I am thinking the small bubbles are coming from a too-viscous melt as KNaO gets too low?

[curt]

Here are some micro bubble shots using a digital microscope at 200x and 800x magnification.  The large measure marks are one millimetre, and the small ones are one tenth of a millimetre. 

 

One of the advantages of testing glazes on a high quality translucent porcelain is that you can see your surfaces with x-ray vision due to the ability to backlight the test tile from behind!  This tile is from a series of cone 10 clear glaze tests on Southern Ice porcelain to which I added some basic coloring oxides, in this case copper carbonate.

 

These bubbles are small but still there, although not visible to the naked eye, I think mainly because there are small and few.  I attribute the limited number of bubbles in this sample partly to the clean porcelain and partly to the copper carbonate, which a powerful flux as well as being one of the least stable glaze ingredients we use. 

 

First 200x

 

 

 

and then 800x.  Sorry this is a little blurry but this is the best of several tries, pretty hard to get focus right at this level of magnification!

 

 

[High Bridge Pottery]

I think after a few more reads I am starting to see a bit more what you are doing Tim. The Si/Al ratio stays the same but flux to everything else varies. If I look at the low Ca Low Kna that has much more SiAl to flux compared to the high Ca High KNa. I do remember a few tests back I was trying something similar and the ratio of flux to everything else seemed a lot less important than Si to Al ratio but I think it is still worth considering.

 

Weirdly it looks like the most silica alumina to flux is the most melted. Is this because of the high lead in the glaze? It seems to me like the lead is influencing the melt much more than the other fluxes. Along the bottom you are taking out kaolin and silica and trading for feldspar, which seems to make the melt worse. Is that what I am seeing? Strange that increasing flux to SiAl and adding in KNa would make the melt worse.

 

Keeping on the Low KNa side and up to high Ca you are taking out frit and trading for more silica, more kaolin, colemanite and whiting and it has gone quite unmelted which makes a bit of sense. The frit will melt much easier and you have taken a chunk of KNa with it. Adding back in the potash and soda feldspars brings the melt back to something better.

 

Ignoring all the silica, alumina and boron it seems like CaO is really a high temp flux but it is still hard to say as all the values are moving around. Both the high KNa ones look very similar even with the varying CaO and it's not until KNa goes and you have too much CaO that it becomes too refractory. Seems like there is a certain amount of work lead can do but it only goes so far.

 

In terms of bubbles I think you want a well melted glaze that has had time to be molten for a while and melt all silica and get rid of bubbles generated. I think high CaO at that temperature will hamper your efforts in getting that kind of melt.

[Tim T]

Joel, that's right. As Boron and lead are both glass formers and fluxes, it didn't make sense to arbitrarily classify them as one or the other, and then just vary the flux content - which would probably make more sense with some of my stoneware glazes which lack these complications!

Yes, lead is a very strong flux, and at earthenware temps the flux is much more critical than with stoneware - and it is too cold for some like MgO to even start behaving as a flux.. The other fluxes are there primarily because they help prevent leaching of the lead, rather than for their fluxing power, and in this test I was more concerned about whether KNaO or CaO affected the bubbles, than their powers as a flux - though as you say, high Ca clearly has a detrimental effect.

Neither a long soak, nor increasing firing to 1190C helped, so I think the next tests are to play with more Pb and/or B as the primary fluxes, or perhaps to add some Cryolite as the Flourine will definitely get things rocking! I don't want to fire much above 1120 as the clay goes darker than I want and also starts to bloat, plus the lead starts vaporising and spreading to unglazed areas.

 

Curt, that's a good idea using backlit porcelain - I just have a hand lens and light at present. As a matter of interest, how are you firing it? I've done very little porcelain work, but played around a bit and using some of the porcelains available here in the UK I couldn't get any transparency when fired to 1290C in an electric kiln - they were rolled out to about 2mm thickness.

[curt]

Hi Tim, the porcelain I am using is fired to normal stoneware temperatures (Cone 10) in a standard stoneware firing cycle. 

 

I think transparency per-se is probably too much to ask for.  My porcelain is vitreous and glassy, but it is certainly not transparent, but it is translucent.  Translucency to me just means that some significant amount of light can pass through. 

 

2mm is plenty thin, so if you hold the piece up to the light and cannot see your fingers moving behind it then it is not a translucent porcelain in my view.  If it seems to have plenty of glassy phase and low (close to zero) porosity, then there is probably too much iron in it, or titanium, or both.       

[Tim T]

Curt, looks like I tried the wrong porcelains! I didn't get any noticeable translucency, and all was fully vitrified, but it was interesting to see how fairly small changes in firing temperature had a noticeable effect on colour.

Anyway, here's the results of my latest tests. If you imagine the original Currie tile above this one with normal IDs of A-D in the corners, then this one is below it, with IDs of C, D at the top and E, F below. I've intentionally pushed the limits with E and F here.

The area to the bottom left of the green line was bubble free, but as you can see not craze free! There's not enough Boron to cause a problem, so I reckon the next step is to go for the top left of the bubble free area but try less feldspar. There's only one crack in the top left, so I'll probably get away with it most of the time with thinner, more normal glaze thicknesses, but if I try less feldspar then it should make the glaze more bullet proof.

FYI, formulae for all the corners and the original glaze formulae are below. It's interesting how far the original recipe is in the "bubble zone", even though it came from a reliable source.

Glaze A B C D E F Original

CaO 0.34 0.34 0.34 0.34 0.34 0.35 0.34

MgO 0.02 0.02 0.02 0.02 0.02 0.02 0.02

K2O 0.20 0.20 0.20 0.20 0.02 0.02 0.20

Na2O 0.05 0.05 0.05 0.05 0.22 0.22 0.05

PbO 0.39 0.39 0.39 0.39 0.39 0.39 0.39

Al2O3 1.02 1.08 0.25 0.25 0.03 0.04 0.30

B2O3 0.30 0.30 0.30 0.30 0.30 0.30 0.30

SiO2 3.26 8.97 1.71 6.61 1.08 4.50 3.00

 

 

Tim

[High Bridge Pottery]

Seems you missed out this picture

 

Add in a smidge of lithium feldspar, I was surprised how much it lowers the expansion and relieves crazing at 0.02 unity, trace amounts really.

[Tim T]

Whoops, here's the pic.

Yes, I'll have a play with Li as well - but it'll be a few weeks as I have to get some stuff through the kiln for a show at the beginning of December

 

[curt]

Hi Tim,

 

Staring hard at those numbers for a few minutes, as you move toward the bottom, seems like amongst the fluxes you have switched out some potassium for sodium.  However, the big change seems to be radical reductions in both alumina and silica as you go down. 

 

I guess the right two columns show that silica is much too high over there - and alumina is much too low - to provide the foundations for any kind of balance between structure (alumina) and glass formers (silica, boron).

 

Not sure how much help you are getting from the Ca and Mg??  These are higher melting and maybe too high for this glaze?

 

I think you are right about the sweet spot being somewhere around the green line.   However, absolute levels of both alumina and silica seem dangerously low in this region.  

 

Have you thought about zooming and investigating more closely around the 12, 13, 17, 18 area?  This region seems to be non-crazing, but still relatively free of bubbles.    

 

You may have already said so, but I am assuming this glaze is decorative rather than functional?

[Tim T]

Switching potassium to sodium was just necessitated by the available frits I had - the top row is the bottom row of a Currie tile that was based on the original glaze recipe using soda and potash feldspar, whereas as you go down the rows these were swapped out for frit 3110.

The Ca is just about OK, and is here to reduce lead release as this comes down with a more complex mix of fluxes; the Mg is just a trace from the Colemanite and will just be a filler at this temperature.

This test was done as a wide ranging, low resolution test- bottom left has the lowest  Al and Si I could manage, with Si increasing to the right. The rightmost 2 columns are just there to match up to the test tile above it.

Yes, I'll be zooming in with a higher resolution test next, now I've got a better target area.

No need to call out the lead stasi, my work is purely decorative, though I'm giving myself the challenge of making it a safe lead glaze.

[ivolucien]

Has anyone considered using molybdenum trioxide for its surface tension reduction / wetting action?

 

I read a thesis paper on its use as a glass "fining" additive.  It's expensive, but you only add 0.2 - 0.5% dry weight. Parmelee mentions it in passing, but apparently it's pretty effective at helping bubbles to break surface and heal over.  I just received 2 oz of the dry powder and will report back on my tests over the next month. I'm trying the disulfide because it's cheaper -- not sure if I'm shooting myself in the foot with that choice though.

[curt]

Yep, have considered moly, just hadn't got around to procuring or testing it, not least of all because surface tension is not at the top of my list of "critical problems that need working on today". Very interested in your tests though, because I do think it is nonetheless important.

[High Bridge Pottery]

Is that thesis paper available anywhere? I have never even heard of the stuff.

[glazenerd]

Joel:

 

I often go to my Dictionary of Glass by Charles Bray- to find alternate solutions. Glaze is at its simplest form- a glassy matrix. Glass makers have long used one product to get rid of bubbles- Antimony Oxide.. When I was doing cone 10 crystalline, common to get bubbles with MGO and CU additions- so just 0.10 - 0.25% does the job right nice. I have been using MoSo4 for awhile trying to grow Moly crystals. You think crystalline glaze is tough- try to grow moly's. Point being, at cone 6, Moly is nearly impossible to get a complete melt- this might not hold true for MolyTriOx. I have used up to 12% lithium, and 8% lithium hydroxide and still have not gotten a complete melt of Mo. It tends to separate and pool in the glaze. However, again I am only speaking about MoS, have never tried the triox version. 

 

Warning: Antimony Oxide is considered a toxic dust- full safety measures should be observed.

Edit: do any of the materials used in your recipe need to be calcined to drive off moisture before firing?

 

Nerd

[High Bridge Pottery]

I ended up spending my day reading glass papers and came across this book, reading back through the thread I noticed you already posted it! There was one good section I wanted to post about CO2 being easily dissolved in alkali rich glass but when silica comes along it gets kicked out of the melt. Trying to get my head around fining agents but it's confusing. So many different reactions going on.

 

 

 

 

Happy Melbourne Cup Day.  Like a dog with a bone, cant let the bubble thing go, not yet.  The usual dogged internet surfing has turned up some interesting explanations and causes of bubble formation, which I want to set out here, lest they all leave my brain as I sleep tonight...  I am hopelessly out of my depth scientifically speaking, but as my wife will attest that has never stopped me before.... 

 

Warning: If you don't care about how glaze melts and why bubbles form stop reading here!

 

The most interesting and useful information by far comes from our colleagues in the field of glass, who deal with many of the same materials and issues that we do in ceramics.  In particular I found Introduction to Glass Science and Technology by Shelby to be very helpful (turned up on Google books).

 

Bubbles come from lots of sources, including the ones we know well like the decomposition of different glaze ingredients, but the ones we should be worried about are carbon dioxide and sulphur products (sulphide and sulphate).  During glaze melting, bubbles form around grains of silica sand as they react with things like Lithium, Sodium and Potassium Oxides that we introduce through our glaze ingredients. 

 

As temperatures rise and the melt proceeds more and more silica melts and creates little localities in the glaze where the prevsiously happy bits of feldspar now start giving off carbon dioxide and other gases..  However, at the same time, more and more silica is making the melt more and more viscous!  For a while, many of these gases simply get absorbed (saturated) into the surrounding liquid and disappear.  However, when the concentration of a gas is too high to be any longer absorbed into the melt, it is said to be supersaturated, and, voila, bubbles of that gas start to form.  Carbon dioxide is the principle one that does this in silicate melts like our glazes.

 

Before you eyes slowly shut and you drift off before banging your head on the desk (oops did that already happen?), why does this matter?????  It matters because carbon dioxide bubbles will keep emerging out of the now supersaturated glaze liquid LONG AFTER all the initial glaze ingredients have dissolved.  There was me thinking oh, well, when LOI is over I am safe.  LOL, not by a long shot.  This strongly suggests we can stop worrying about the breakdown of glaze materials in the early stages of the firing, and start worrying much more about how much silica we have in the glaze and whether or not it is all melted yet....  And, it means that key time for bubble management is definitely at the end of the firing.

 

Oh, and don't forget "reboil."  That is where a previously-formed good mature bubble-free glass or glaze starts to generate bubbles where there were none, before due to higher and higher heat.  This is a problem particularly with sulphur contaminants in our glazes and clays, which undergo a 3-fold increase in their supersaturation potential between 1100 C and 1400 C.    Kind of makes you wonder how clean those glaze ingredients really are....  However, more to the point, lowering your temp for a hold wont just allow more time for big bubbles to pass out of the glaze.  It ALSO may cause some of the sulfter to be reabsorbed into the melt, ie, bubbles disappearing by themselves rather than having to be passed out of the glaze! 

 

As a related issue, particle size of your glaze ingredients also seems to matter.  And there is a "right size," because if particles are too large then decomposition persists into the latter stages of the firing creating glaze defects and bubbles,   But, if particles are too small, this increases the likelihood that glaze particles group together early and trap gasses.  However, my take is that most of us are erring on the size of glaze particles that are too large if anything.  Hence, ball milling seems to hold some promise....

 

Little bubbles (less than 0.4mm) are called "seed" in glass-land, and anything bigger is called a bubble.   This matters because seed is much more difficult to get rid of than bubbles, but mercifully much less visible as well.  The process of getting rid of seed and bubbles is called "fining".  It can take 50% longer to fine out small bubbles than large ones...  The bigger the bubbles are, the faster they pass.  Sometimes in glass-land they use specific chemical additives called fining agents which makes lots of big bubbles which collect the seed on their way to the surface of the melt. 

 

Whether or not bubbles pass out of a liquid depends on their size (bigger is better!), how viscous the melt is (this changes over time during a firing of course), how dense the melt liquid around the bubbles is (higher temps give less density, which is good for bubble passing but too high may also cause reboil, which is bad - what temp you hold at starting to come into focus now - maybe not the highest?) 

 

Want to melt all the bubbles out of your silica-based glaze by force??  Better get a better kiln because you will need to go to 2200 C to get commercial-grade bubble-free silica glass.  Silica melts very slowly. 

 

We tend to think of lithium having super-powered melting properties in a glaze, but research shows that whether you use potassium, sodium or lithium matters far less than the amount of whichever one you use.  Decreasing effect after 10% in glass-land and virtually no additional effect after 20% R2O.  This would seem to match up with what I have seen on currie tile experiments.

 

The glass forming temperature when you use calcium is about 200 C higher than if you used sodium, potassium or lithium due directly to the viscosity of the melt.   Of course eutectics probably mean a mix is even better.  All you whiting users whose glazes lean heavily on calcium be advised.

 

OK, will stop there and take a breather.

 

Conclusions: 

 

1.  The end of your firing is where you will have the biggest impact in getting rid of bubbles.

 

2.  Use the cleanest glaze ingredients you can get to reduce the chance of bubbles

 

3.  Use glaze ingredients with small particle sizes

 

4.  Holds:  definitely, because

 

a) The more viscous the melt, the longer the hold, the better.  And since the smaller bubbles are, the longer it takes them to pass (so if you have to have bubbles, hope for big ones, cause at least you can do something about them!)

 

and

 

if you lower the temp for your hold you have a good chance for bubbles (particularly sulphur ones) to be re-absorbed into the melt).

 

I found in interesting after hoovering this kind of stuff to go back and look at the bubble photos again.   Is it just me or is there lots of unmelted silica floating around in some of them....?

 

As ever, very interested in the reflections and practical experiences of forum readers on this stuff....

[jrgpots]

This is coming from far left field...so, take my free advice for what is it worth. Remember it is "out of the box."

 

Gasses come out of a supersaturated solution if the atmospheric pressure drops. I was curious if you are firing in reduction. Placing a kiln in reduction increases the atmospheric pressure. This decreases the outgassing of CO2. But, if you open up the damper at the end of reduction thus reducing the atmospheric pressure, bubbles will form.

 

So, if you are firing in reduction, try changing how quickly you adjust the damper. A slower rate of change in the atmospheric pressure reduces the bubble production.

 

If you are firing in oxidation consider pressurizing the kiln during the end of the firing cycle. This would reduce bubble formation.

 

Jed

[High Bridge Pottery]

Interesting thoughts, they do go along with a bit of what I have been reading.

 

There seems to be two solutions to bubbles, either you fine them out so they bubble up to the surface or you get them to go back into the melt. A mixture of both seems to be the option. Interestingly when you lower the temperature the saturation level increases so a lot of the seed bubbles can be removed by dropping 50c~ from top temp and holding. Going to have to test that.

 

For dissolution back into the melt you can increase the pressure but that seems to be difficult to do. Fining you can lower the pressure to increase the bubble size but again seems complicated.

Fining by adding something to the glaze that produces a lot of gas at the right time seems the best way to go. To me it's counter intuitive that to have less bubbles you must make more.

 

In regards to oxidation/reduction it seems they change how gasses are released, with reduction being at a much lower temperature. I am not sure from what I have been reading if they are firing in reduction, they are adding a source of carbon to the glaze to 'reduce' the glaze. Also talk about increasing the water content in the kiln to produce off gassing of sulphates.

 

When I can I am going to try sourcing some graphite and see if I can get any results without adding sodium sulphate. There must be some in the raw materials anyway? At least I hope so.

 

Added a few interesting graphs. They are all related to glass making but they must have some impact in ceramics, right? Interesting that all the CO2 seems to have gone by 1000c and SO2 later on.

 

 

 

Also a little graph on particle size and how it affect their melt times.

 

[curt]

Joel thanks for the charts, good stuff.  It seems to pretty much line up with my own research conclusions from glass-land, which you kindly included in your post.   

 

I am not surprised that gas release takes place at lower temps in reduction (as compared to oxidation), given how a reducing atmosphere makes so many fluxes that much more potent (eg, iron).  If the melt gets going earlier and is that much more aggressive in reduction, one would think that gasses must be produced - and evolved, or gotten rid of - that much earlier.  

 

I had to stare at (and even think about!) your second chart a while to figure out why they omitted the line for Oxygen (O2) which was included in the first chart  .    More interesting yet was the line for CO, which was mostly irrelevant in the oxidation chart but pretty important early in the story in the reduction chart.  Probably explains those occasional small explosions we would sometimes get around 950 C in our LPG firings, hehe.

 

I think the glass story is very compelling for us in the ceramics biz, particularly in analysing glaze reactions.

 

However, the main difference in my mind is that the glass people are basically heating up all their materials in a big cauldron until they are soup, and letting that soup bubble away until most (all?) reactions amongst those materials are pretty much complete. 

 

AND, they are not trying to get this soup to homogenize and refine itself while simultaneously melting it on to some kind of partially baked ceramic surface underneath, like we are.  I think many of our difficulties in ceramics land come from the clay underneath, and how that interacts with the glaze, rather than the glaze itself.   

 

It has made me wonder about industry and how they address these issues in a practical production sense.  I am starting to think that industry's approach is simply to calcine EVERY ONE of the materials in the clay COMPLETELY before even trying to apply the next layer (the glaze).   Bisqueing to Cone 3 and then applying earthenware glaze would be one specific example of this.  Whereas in stoneware production we only bisque to around 1000 C, still leaving the part of the calcining between 1000 C and 1280C to happen while we are also trying to apply the glaze (!).

 

Can anyone confirm or debunk this idea? 

[Magnolia Mud Research]

Joel,

 

Can you post the citations (what authors, books, papers, websites, etc. they came from) for the charts?  It would help in answering Curt's confirm/debunking question.
 
LT

[Callie Beller Diesel]

Also by this logic...

Would the higher bisque that is recommended for clay bodies higher in organics and sulphur be then counterproductive if you're considering adding some kind of seeding agent (like gypsum) to your glaze bucket in order to clarify a glaze?