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OK, now we are trucking.  I feel like the stories are converging so I just want pin down a couple of things.  I wouldn't call these theories so much as proposed facts (thereby automatically attracting ire and inviting disagreement and argument! :lol: )

 

1.  EVERY ceramic object fired to stoneware temperatures - and lets be clear that means BOTH stoneware and porcelain - have BOTH mullite AND glassy phase in them when the firing is over. 

 

2.  The amourphous (glassy) phase is mostly melted silica, while the mullite phase is mostly alumina, but in the finished, properly fired object, these two solid phases exist side by side simultaneously in the fired object, intermingled.

 

3.  Mullite forms after (as) fluxes suck the silica out of kaolin (metakaolin) particles, leaving a high alumina spinel phase.

 

4.  Any combination of alkali and alkali earth fluxes can be used to do the fluxing, and will produce (irreversibly) both mullite AND glassy phase when fired to stoneware temperatures. 

 

Do you agree or disagree with statements 1, 2, 3 and 4?

 

These statements might (or might not) seem innocuous, but (like Nerd) I believe it is important to establish a core understanding of what is going on in a clay body firing in order to provide a platform to characterize how the different fluxes work, either on their own or together in different combinations. 

 

Similarly, I also think it is important (at least for purposes of this discussion) that we let go of preconceived (and sometimes somewhat ill-founded) notions about the differences between porcelain and stoneware, and start to embrace the similarities, so that we can increase understanding.  Glassy phase is no more or less desirable than mullite is, in either stoneware or porcelain.  They both play important - but different - roles in every high-fire body.   How much of each is a variable that we can and should control, and it will affect how the fired body looks and works.  More glassy phase looks more like porcelain, less glassy phase looks more like stoneware.  Just choices.

 

I don't think we need to wonder whether or not fluxes like Magnesium and Calcium work as body fluxes- that is well known and has been for hundreds of years.  That was one of the messages from the Limoge clay-body chemistry I posted above, which uses significant amount of both Mg and Ca in addition to the K and Na.  And better acknowledge Ba in there as well.  And others.   The fact that the "mainstream" (not ready to call them "industry" or "trade" yet) clay body recipes you have gathered do not employ these fluxes is simply a matter of choice - or oversight - depending on how you might want to see it.

 

Nerd, you are spot on about particle size being very important in this story. My clay body formulation testing using primarily locally collected native materials has made me painfully aware of how important particle size is to vitrification.  I will try to add some of that evidence to this discussion in due course.  In any case, I heartily agree that if reducing porosity and absorption is the objective, smaller particle sizes make a significant difference.

 

Re molochite (which is just ground up mullite), I don't disagree that it might aid body development, but I think this may be "gilding the lily".  One should be able to produce well-matured, low-porosity stoneware without having to resort to this.  I don't think there is anything magic about 72% to 75% silica.  I am already working with stoneware bodies that have more than that, and have bodies with less.  Fixing porosity is about the amount of flux as I think you know.

 

Finally, relieved to hear you will be doing absorption/porosity tests.  I belive such testing is very informative re maturity, and I cannot see any other objective way to measure at this point without resorting to much more time-consuming and expensive lab tests.

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LOL, OK Nerd, but those were "yes or no" questions, not really "post a picture as an answer" questions. I think the answers are important because there still seems to be some confusion about what is porcelain, what is stoneware, what is mullite, etc, what is glass, when and where do these occur, etc.. I am willing to accept if I am the one confused, but if I am will someone please set me straight?

 

While you (and anyone else reading this thread) are thinking about whether those 4 stylized facts I set out are right or wrong, you may want to have a look at the following article. It is (refreshingly) not written for scientists or academics, but more for normal people who have an interest in these topics. It addresses mullite in porcelain, how vitrification happens, and best of all has those cool mullite pictures you are craving, Nerd. In fact it shows both primary and secondary mullite up close in 1 and 10 micron SEM photos. Their photos bears little resemblance to your picture above but maybe that is a magnification problem. I also found it interesting from the pictures how much of what we put in our clay bodies does not get fired at all. At the SEM level, it all really looks more like boulders surrounded with glue!

 

http://www.ceramic-research.com/articles_01.html

 

I think those things in your picture are just pores.

 

Also, if my characterisations above are accurate, the amount of mullite in your fired body will have little to do with how much alumina is in your raw materials. Rather it has much more to do with the effectiveness of your flux package and your firing process in getting kaolinite to liberate its locked-up alumina, since most of the alumina never gets accessed in a firing.

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Regarding all these "mainstream" clay bodies you are seeing being relatively under-fluxed and with big particle sizes, I think there is a simple explanation.

 

The bodies being used by universities, schools and other teaching institutions are mainly geared toward teaching people how to make things out of clay. The main requirement for such bodies is that they are easy to throw with, hand-build with, etc.. This means the incentive is to load the clay bodies up with plenty of coarse materials so that the bodies are open, stand up well on the wheel even after over-wetting, and dry well without too much cracking. Oh, and did we mention that coarser, less processed materials are cheaper?

 

How they fire is secondary, as long as they don't melt down in the kiln or spit glaze on kiln shelves or other similar inconveniences for the overworked kiln technician. To put a finer point on it, PSD, porosity, durability and other functional attributes are a distant second priority to just getting the students some work to be graded on and to take home to show family and friends.

 

Since the "customers" normally have only limited understanding of the technical properties of the clay bodies they are using, there are few complaints.

 

Conclusion? These "mainstream" bodies may or may not be good benchmarks to which to compare your own tests.

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1.  EVERY ceramic object fired to stoneware temperatures - and lets be clear that means BOTH stoneware and porcelain - have BOTH mullite AND glassy phase in them when the firing is over.

Yes, where alumina is: there is mullite also- unless you drop below the metakaolin-spinel temps.

 

2.  The amourphous (glassy) phase is mostly melted silica, while the mullite phase is mostly alumina, but in the finished, properly fired object, these two solid phases exist side by side simultaneously in the fired object, intermingled.

Mostly agreed: I would like to "see" how they intermingle before I make a final conclusion.

 

3.  Mullite forms after (as) fluxes suck the silica out of kaolin (metakaolin) particles, leaving a high alumina spinel phase.

Indeed- still curious about how MgO is involved in that. --keep seeing those references in the tech papers.

 

4.  Any combination of alkali and alkali earth fluxes can be used to do the fluxing, and will produce (irreversibly) both mullite AND glassy phase when fired to stoneware temperatures.

Certainly agree in regards to porcelain; mostly agree in regards to stoneware. I expect to find certain blends of fluxes will be more effective in stoneware, whereas porcelain will always favor straight KnaO.

 

Stoneware in blended according partly to price, partly due to availability, and partly due to mechanical: I agree. However, what I see the most is blending to achieve a certain Fe or MgO level: and in one case ( maybe accidental) TiO2 level.

 

My theoretical differences between porcelain and stoneware at this point are:

 

1. Stoneware relies on particle size for mechanical properties, and porcelain relies solely on plasticity supplied by additions. Stoneware is large particle size, and porcelain is intermediate to super fine.

 

2. Stoneware relies more on mullite development, and porcelain relies on amorphous glass: although both exist. While both bodies have KnaO, stoneware appears to rely on Fe and other earth alkali to produce additional mullite.

 

For certain however, there is very little difference in the silica/alumina ratios between the two bodies. I still believe PSD is one of the big issues with stoneware: which creates porosity/absorption issues irrespective of flux additions. If a ^10 firing still has minor absorption issues: then I would imagine ^6 is a nightmare.

 

Nerd

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Nice article to read there Curt. Am I right in thinking that the quarts grain is 1/5 the size of a 120 mesh hole? It looks like 10um~ on the photo and 120 mesh is 50um right? Seems very small and no melty. 

 

That gas pressure quote seems to be talking about 500-600c when you are getting rid of the water in clay.

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........................ it all really looks more like boulders surrounded with glue!

 

 

Bingo!  And this is why no one has yet been able to model the behavior of clay bodies for computer simulation.  The impacts of so many variables comes into play....... including the size and composition of those boulders and the hlue that is sticking them together and eating into them.

 

best,

......................john

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2nd time this week when I hit post, kicked me out of the forum.. Whats up with that?

 

Joel: misread 500C for 900C--- but I still believe gas pressure in the body is creating some mechanics...

 

 

Curt said: since most of the alumina never gets accessed in a firing.

Noticed that myself... which is why I am wondering if MgO plays a role in that.

 

 

The impacts of so many variables comes into play....... including the size and composition of those boulders and the hlue that is sticking them together and eating into them.

Which has been my argument for many years: how many of the variables are really variables? Or are they just been long accepted as such?

 

Curt:  Fig.5 in the link you posted, is very telling to me. If that is just one among many areas of un-melted silica: that alone tells me that the KnaO levels in recipes are too low. I keep seeing 2.5 to 2.75 total KnaO (molar) levels in ^10: who set that formulation rule? I also keep seeing Custer: is that 1995 custer or 2000 custer?  The very fact that raw silica is showing up on that scale says levels are too low. Not studied it yet, but I suspect many dunting problems are the result of these low levels as well.

 

Not satisfied yet: my gut is telling me something is missing here. Did the more modern formulation just get spun off from the 50-60's? Not passing judgment but rather making an observation: I do not see where much thought was given to PSD criteria. I do not see where that is possible: some of these recipes rank up there with Campbell's Chunky Soup.

 

An analogy: I see softballs, baseballs, and golf balls: where is the marbles, BB's, sand and dust?

Nerd

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They are variables ... in that we (studio ceramists) do not typically control them.  In many cases we do not even know that certain parameters impact the results we get.  So we tend to blame "phase of the moon" or "black cats" for things that don't work out as planned.

 

You need to make a field trip.... up to Alfred University in NY.  You'd like a guy named Matthew Katz.  (http://art.alfred.edu/academics/glaze-formulation.cfm  )  He's the "bridge" between the engineering department and the art department.

 

best,

 

..................john

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Nerd, thanks for taking the time to respond to the stylized facts above.  Seems we are mostly on the same page, and so far no one else watching here appears to have found any gaping holes in the story.  Reassuring, and gives confidence in the results to come.

 

Like you I think once the ideal combinations of fluxes become apparent, the whole story will quickly boil down to particle size issues.

 

With silica, important to remember that, as well as serving as food for fluxes and an eventual glass former, it is also plays an important role as a filler and/or grog.  This means that even if some of the surface of quartz grains gets absorbed into the melt, it may be just as important for some to stay behind intact.  One theory I have seen is that these larger remaining quartz particles actually prevent/stop major cracks developing in the piece, by creating terminal points for the many smaller micro-cracks which originate out of compressive stresses throughout the body.  Note the space AROUND that silica grain in Figure 5.

 

Question is, how much quartz needs to melt? I don't think the answer is all of it.  I think it rather depends very much on what you need the body to do.If all you need is some glue to stick together a bunch of coarse materials for your decorative piece, then not much glassy phase is needed.  However, if you are looking for most or all of the voids between the particles in the body to be filled (i.e., between 2% and 0% porosity) so that the body is functional, then you need a lot more silica to be melted.  Yes, some of those voids will be criss-crossed and partially filled by needles of mulllite, but some glass will still definitely be needed.

 

Regarding all this increasingly suspect recipes you have gotten, I don't think KNaO levels are too low.  I think FLUX levels are too low.  However, if you want to pick on KNaO, I don't think there is any "rule" about how much to add to a body.    Nor could there be, since a wide range of flux levels will produce some kind of melting, and even if you and I think it is inadequate, someone else will say "good enough."  The 2.5% to 2.75% KNaO you are seeing is, I believe, simply the level that has been handed down for decades because it works for that class of mainstream recipes and their users, for the reasons I outlined above.  They don't have to be be functional in the way that you and I would measure it - they simply need to hold together enough to deliver their decorative content.  Just don't put them in the dishwasher too many times....

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 The 2.5% to 2.75% KNaO you are seeing is, I believe, simply the level that has been handed down for decades because it works for that class of mainstream recipes and their users, for the reasons I outlined above.  They don't have to be be functional in the way that you and I would measure it - they simply need to hold together enough to deliver their decorative content.

Agreed= however, it is very common to see threads or comments about commercial stoneware weeping. So on some level: rather accepted or not/ or intentional or not: there is some kind of "informal" acceptance of the KNaO levels in those bodies as well.

 

Now after reading your comments, research links, and obsession with clay: it is my great honor to bestow upon you: the title of NERD.

 

Nerd

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LOL, thanks Nerd! Er, I mean, fellow Nerd! Now all I need is the T-shirt! Although I think there a few others around here that may deserve such an honor more than I, even if they have not been quite as obsessed as I have with this particular topic.

 

I think what you are looking at here is tremendously valuable, and since I am a highly-likely beneficiary, I want - no, I NEED - to be completely convinced that the foundations are sound. Hopefully you appreciate that my questions, challenges, counter-proposals etc are intended to create dialogue which gets home truths, sacred cows and all other preconceived notions about these topics out in the open (including my own) for verification testing. I know from my non-potting life that such artefacts can taint the research process and send one down false trails forcing laborious back-tracking later. I (selfishly) want all your tests to be as productive and meaningful as possible, and I honestly believe this process will make your results stronger and more robust.

 

Plus I think just watching the discussion delivers real values for others. Hopefully they will stick their thoughts and concerns in as well!

 

Either way, once you go down the rabbit hole and start looking at your clays and glazes as closely as you have, there is no turning back. There are things learned that, once known, cannot be unknown, and forever change your process.

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Test Protocols      

 

1. Blend according to the most commonly used clay varieties found in the majority of recipes: Gold Art, Hawthorne, #6 Tile, OM4 Ball, (some Red Art).

   ** most recipes have between 76-80% clay, balance is silica and feldspars, with a blend of 4-6 clay varieties.

2. PSD study to determine how particle size creates/or minimized absorption by the following standard:

    6 (clays) @ 13% each = 78%      6/17 blend   4 combinations of differing PSD levels   6/17 1-2-3-4

    5 (clays) @ 15% each = 75%      5/20 blend   4 combinations of differing PSD levels   5/20 1-2-3-4

    4 (clays) @ 19% each = 76%      4/19 blend   4 combinations of differing PSD levels   4/19 1-2-3-4

 

All test bars will be visually inspected ( up close) and absorption tested.  2 bars from each blend, with the best absorption values will move on to round 2.

 

3. Two bars from each blend will then be manipulated by switching out ball clays for finer particle size, altering PSD groupings, and the addition of molochite and other extra fine grain clays. Retested for absorption; with one bar from each blend moving forward to round 3.

 

4. One test bar from each blend will then be increased with KnaO levels ranging from: 2.50-2.75-3.25-3.50-3.75- and 4.00 molar. One bar from each blend will be selected according to the amount of glass present. Glass content will be just "barely" evident, but not fully vitrified, or overly abundant. One test bar from each round will move to round 4.

 

5. One test bar from each blend will then receive magnesium oxide additions of  1/2, 1, 2, 3 and 4%  and visually inspected for additional glass/absorption properties. I realize those are radical jumps in MgO additions: but it will make the process of quantifying amounts go faster. Once a general number is assessed: then I will drop back to 1/4, 1/2 percentages. The level of MgO that shows accelerated mullite production (if any), one bar from each blend will move to round 5.

 

6. Fe is already known to be a flux in reduction and in higher firing oxidation. This round is to simply establish the minimal limits producing the best results.

 

The the fun stuff of adding or blending for color or effect. Actually the testing is the fun stuff if you are a Nerd.

 

Of the final three test blends, one will be selected for further testing to develop a "functional" stoneware recipe for ^6 and for ^ 10. Actually I expect this body to be a hybrid porcelain with stoneware qualities or a hybrid stoneware with porcelain qualities: take your pick. 

 

Let the testing begin... should not take more than a year.

 

Nerd

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The starting point:

 

6StoneLR

 
L& R white stoneware c5/6 fired to 2228F, with 10m. hold. Black arrow indicates glass formation, red indicates (particle size?) and the blue indicates the pitting and roughness due to particle size: piece was snapped, not cut.
 

 

Same view as above except with a filter to show the texture: course and pitted.

 

 
Nerd

 

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Hey Nerd did your digital microscope come with a little ruler or measuring tool which is transparent and can be included in your shots to proved scale? If not, you might want to pick one of those up. If you look at some of the pictures in the "bubbles" section of my gallery you can see what I mean.

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Yes Curt; but it also came with measurement software. It is suppose to measure once I calibrate it, but I had to put in a request for a new disc: it is not working. It is rather cheap as microscopes go: but it does what I need for now. For the moment, I am counting the grains trying to determine the PSD in a commercial body.

 

Nerd

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Not wanting to complicate things too much, but I don't think you can just look at the formula without also looking at particle sizes - yes for a common set of raw materials (and so particle sizes) the formula will give a comparison, but for example if you changed to coarse sand for the silica that would make a big difference to the dynamics of the firing, and so the result. This is a good discussion on the topic: https://studiopotter.org/pdfs/sp28_1_sohngen.pdf

Because particle size is so important to the handling properties of the clay, there is inevitably a compromise between what is wanted for good making and what is wanted for good firing.

I've avoided the temptation to play with my own bodies - I'd never get anything actually made - but I've tested a load of commercial clays, and it is quite informative getting the particle size distribution from sieve tests.

Also, the rate of firing can be very important - if you fire fast enough the silica transformations are different, as things are changing too fast to follow the equilibrium states. I have a paper on this somewhere, but at the moment it is lost in the files.

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Hey Nerd,

 

what does "6/17 blend".. "5/20 blend" etc mean in your protocol above.  

 

Also wondering how you will define the 4 PSDs?   When I see the word "distribution" I am immediately thinking of both average and standard deviation. Are you thinking of different ranges of mesh sizes for each PSD, and will you exlude particle sizes above and below those meshs?  Or, are you going for, say, an average particle size as a proxy for the PSD for each blend (and not worry about the tails of the distribution).  Or some other method?  From a practical perspective (ie, actually getting some tests done as opposed to talking about them), seems like above this mesh, below that mesh is easy.... 

 

Also, are you going to try and keep flux levels constant across the clay blends by recognizing the natural flux content of each clay and then adjusting?  Or will you let flux levels differ?

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6/17 blend".. "5/20 blend          6 clays/ 17% each        5 clays . 20% each.

Just sitting a base line. Variations of the recipes I have already studied. Just want to see visually how they compact. Will use the stoneware pic above as a gauge. I have 3 fire clays coming, and some smaller units of mesh sizes 35m to 60m.   Just getting a feel for it at this point: seeing how they interact, etc. My nerdiness wants to sieve out particles: but that defeats the purpose. I have to formulate around available clays as they come out of the bag.

 

Nerd

 

Was serious about getting a bigger/more professional microscope: 2500x with florescent filters that highlight the various materials with color. Only way I am going to be able to see how mullite, alumina, Fe, MgO are being distributed. Very pricey.... thinking about it. Can also use it to determine particle size, median mesh, etc: it does measure professionally.. beats the He*** out of sieving! Finding accurate info about the exact particle distribution in a bag is turning out to be a tough cookie.

 

Starting out, will keep all the flux levels as close as possible: flux variations will be down the road.

By the way: the initial 12 bars are designed against the range of blends I have seen in recipes. Trying to understand why there seems to be such an extreme range in formulation: other than the obvious.

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OK, I see about the 5/20 etc.  Was getting confused with all the percentages. 

 

So if you are not sieving yourself, I guess that means you are using the PSD's as provided in the specs from the manufacturers, which is fine, just wanted to know.  Still leaves the question of how you will characterize the PSD of a given clay - or blend of clays.  

 

I am imagining a grid.  Each clay down along the left column, with the overall blend at the very bottom row cumulating everything.  And mesh sizes across the top, like buckets for particle sizes.  So that each clay has a certain percentage of itself between mesh x and mesh y, and all those %'s sum to 100. 

 

Is that what you are thinking? 

 

If so, I am thinking that provides the ability to approximate a distribution for each clay "(and ultimately for the overall blend as well) which provides both an "average" mesh size for each, as well as a standard deviation around this average.    As I think about median may work better than an average, as that median is going to end up being in a given mesh size "bucket".

 

I guess another way to do it, although not really consistent with your overall methodology above, is to "target" a certain distribution of mesh sizes for the final blend, and then work backwards going upstream to get the right blend of individual clays to achieve this targeted distribution....

 

Re the fluxes, I guess they will be kind of hard to control while tinkering with PSDs.  But you can account for them at the KNaO stage and adjust KNaO and other fluxes accordingly to account for what fluxes are already in the clays naturally.

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I am hoping that you will absorption test ALL of them before making any decisions about which ones to proceed with.  Looking at them to decide if they are any good is just too subjective IMO.  Plus having a well-defined series which includes all the members (even the unviable ones) will aid understanding about what is going on.  Up to you of course. 

 

With regard to what particle size distribution to aim for, there is an old paper by McGeary called "Mechanical Packing of Spherical Particles" which is worth a look.  It basically sets out the ratio of particles of different sizes to get the maximum packing density, based on a series of experiments they did with metal balls.  They go up to four different particle sizes if I remember.  I think there are some other references as well, including some addressing particles that are not round.  Have to think this whole topic is relevant.

 

With the fluxes, I am thinking of the flux which is already in the clay itself, not the flux you would add.  Those natural amounts will vary from clay to clay.  Not sure how you control for that.  Maybe what you mean is that you will add a little flux to make them all the same? 

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With regard to what particle size distribution to aim for, there is an old paper by McGeary called "Mechanical Packing of Spherical Particles" which is worth a look.  It basically sets out the ratio of particles of different sizes to get the maximum packing density, based on a series of experiments they did with metal balls.  They go up to four different particle sizes if I remember.  I think there are some other references as well, including some addressing particles that are not round.  Have to think this whole topic is relevant.

 

That paper and this concept also relates to developing good glaze qualities for both application as well as for the durability of the dried (but unfired) glaze coating on the ware.  Helps to make it less prone to dusting and chipping.

 

Nerd.... you are going to have to become friends with someone at a nearby university and get access to a scanning electron microscope.  Maybe, as a retired contractor, you can do someone a new kitchen for free... and they'll give you some access ;) .

 

best,

 

.......................john

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Sorry for the late information here. I was a bit behind on this thread as I was out of town.

1) Regarding some of the recipes that I sent you, there was more than one that was indeed, formulated for aesthetics over function. If it looks out of whack at a glance, it most certainly is. The guy who built the recipe with half Helmer was after wood fire flashing, and really didn't care about absorption rates. He was making art pots.

 

2) Custer formulations should indeed be based on 1999 levels.

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