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Introduction of SAS Formulation - hypothesis

 

The industry standard has been formulation based on particle size distribution (PSD), which includes density packing. The Zameck article proposed formulation based on the PSD principle; which included emphasis on density packing. However, particle size is a measurement that only determines the plane (face) of a particle, but does not include the depth. A sugar cube represents a perfectly square particle: which has the same plane and depth equal. Clays however can have the same particle size on the face, but vary widely in depth. So a large grain of clay could be represented by a sugar cube shape: but in reality the depth can be anywhere from a thickness of hair up to a normal sugar cube shape. So using PSD as a determining factor for packing density is inaccurate; because it does not include the measurement of depth (platelet.)

 

 

Specific Area Surface (SAS) is:Specific surface area (SAS) is a property of solids defined as the total surface area of a material per unit of mass. The SSA can be simply calculated from a particle size distribution, making some assumption about the particle shape.

 

 

The SAS is far more accurate, in that it takes into account both the width (particle size) and the depth (platelet size) of clay. In common calculations: a potter will factor in a 200 mesh clay: but has no idea if that platelet size is  20-50-100-1200 mesh thick. So determining packing density solely on the basis of particle size is inaccurate, because platelet size is not factored. A 200 mesh clay could have a SAS as low as 18, and as high as 28: which this variance alone confirms the inaccuracy of using particle size alone.

 

 

In addition to using the SAS in determining accurate grain size; the SAS will also give the potter insight into the plasticity of the clay of choice.  The higher the SAS goes, the more plastic the clay will be. (Applicable to ball clays and bentones primarily). More importantly, the SAS becomes the basis of formulation because it factors total grain size. So from here, the SAS formulation needs to be applied..

 

Nerd

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PART 2:  Converting PSD to SAS

 

Post Removed under agreement with Ceramics Monthly. Information will be in upcoming issue.

 

The SAS Formula by Tom Anderson (GlazeNerd)  Copyright Nov. 20, 2016

Submitted to Ceramics Monthly & Technical Ceramics.

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I really like the logic and formulation of a of your SAS. If I was mixing my own clay I would be all over this. I have for years added OM4 to my reclaim (stoneware). I Keep a bowl of it on the wedging table and wedge the clay and set for a month and wedge again a use. Works great. I probably don't even use cup per 25lbs.

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How are you working out the SAS or surface area of 1g of whatever material? That is the part I am not quite understanding. WOPL is making sense and the comparison between particle and platelet size. Not made it all the way through the posts properly as it's all new thinking. Still outside the box one year on  :lol:

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Joel:

So far I have been using the SAS values supplied by the mines. There is a mathematical formula you can use; but even that requires certain assumptions be made. It is actually a very simplistic formula: I am omitting the (long hand/algebraic) math required. All the SAS formula does is assign definable numbers to the recipe: and then uses those numbers to track adjustments. If you formulate to the target of 23.00 to 23.50 SAS: it forces you to arrange the particle distribution to hit that goal.

 

If 18 SAS is the coarsest particle size, and 28.00 is the finest: the equal distribution then would be 23.00 SAS.

Then the rule becomes: % of large particle fire clay times 0.75 = minimum amount of fine particle clay  (cat 2)

Part of the minimum amount is ball clay for plasticity, and the rest can come from EPK, #6 Tile, or perhaps a coarser grained ball clay. You add the amount of high SAS ball clay you want for your desired plasticity: and fill in the remainder with Imco, Gold Art, EPK or something else.

 

If you attempted to apply the equation you are referring to: the formula would be very complex, and cover an entire page: with numbers into the millioneth". By simply using the SAS of the median particle size in any given bag: you come up with one very quick, easy to compute: SAS median average. I round numbers up and down to the whole: which makes it even more simple. Unlike glaze, 0.32 % is not going to have an effect on clay: although I do go to the next higher number.

 

Yes it is new to you and everyone else. Never did understand why someone long before me, tried to apply some formulation standard.

 

Nerd

 

Made my living outside of the box: why change now?

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If you had put in a high carbonaceous clay: you would have a bloat problem at cone 10. ...Not sure why this would be the case? Just need to take more care with organic burnout elsewhere in the firing process?

 

Two things are saving you from that: higher silica molarity, and OM4 is a kaolinitic ball clay. (27.90 % alumina) ...how do these things affect bloating?

 

I would expect to see a porcelain glassy matrix if this was cut open after firing. ...not sure how to interpret this since EVERY fired stoneware clay has glassy matrix.

 

This recipe is the classic 5/20 blend = 5 parts of 20% each. Typically stoneware is the 80/10/10 blend: 80 parts clay, 10 silica, and 10 flux.

 

...80% sounds a bit high on the clay fraction (and the flux seems too low). The stoneware body you are looking at here is 60%, and 60% to 65% clay in a stoneware body seems more typical to me.

 

Nerd

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Well Curt: I try very hard not to give overly complicated and scientific answers. I bore people to tears with my posts as it is. So in fairness to those reading, other than you let me say: Warning... boring post ahead.. Take a nap, drink coffee, and No Doz before you read.

 

Carbonaceous clay has several factors actually: the first being lower alumina and higher silica: coupled with the fact they supply very little naturally occurring flux. Of the carbons: just one to be specific: sulfur, which typically occurs in nature as a combination of sulfates and sulfides. One will burn off easily: the other will not. In the latter case: inorganic sulfides will rise to the surface of the clay and seal it: thereby causing bloat. The bloat is not caused by the sulfides, it is caused because they create a barrier, that escaping gases cannot penetrate.

 

Actually tested this over the past summer. I took some powered lignite coal (very high sulfur) and put some in a clear glaze to high cone 6. The surface was littered by dark brown/black specks: along with a bit of scummy looking areas. Although 90% plus was burned off: some remained. So the idea that bisq burns off all carbons is a bit of myth.

 

The higher presence of silica, and lower presence of alumina; does in fact make this, more of a porcelain body.  Even the fireclay added flux; and has a kaolinitic  ratio of alumina and silica. The added alkali from the Om4, the fireclay; coupled with the 20 % flux addition; created an amorphous body. You are forgetting at the time this recipe was written (when Neil was a tyke) custer had 10+ % potassium. The Si/Al  level of this recipe runs to the porcelain side.

 

Yes, there is a difference between porcelain and stoneware "glassy matrix"

 

Porcelain has the same amorphous structure as glaze: no discernible crystal lattice. The difference being material solids; and the requirement for higher alkali molarity (compared to stoneware) to fully incorporate solids into the melt. The mullite formations that do occur in porcelain are a completely different structure, compared to stoneware. Kyanite is often added to produce the needle-like mullite structures found in porcelain; and those are few.

 

The glassy matrix found in a porcelain body has no discernible crystal lattice: the mullite in stoneware does. The glaring difference between the two “glassy matrixâ€: one has a definable structure, and the other does not. What little mullite forms in porcelain is needle-like in structure, whereas the mullite developed in stoneware is orthorhombic (  cubic) in our case: “cubic platelets.† Platelet shaped because of the phase change of metakaolin to spinel. Stoneware starts with platelets in the clay, and ends with platelet shaped mullite. Which explains the different densities between the two clay bodies.

 

Mullite is Al(4)Si(1.5)O(9.75). Obviously we cannot reproduce that in our kilns because of temperature and pressures. However, stoneware to some degree does follow this chemical chain, in that stoneware clay has much higher alumina to silica ratios; as compared to porcelain.  This recipe had 17% alumina molarity, most stoneware bodies run between 19-20%, and the bodies I am developing: 20-22%. So the fact is: Neils’ recipe is a modified porcelain, more so than a stoneware body. The famous Coleman Grolleg;  has 16.4% alumina molarity.

 

So the 20% Custer addition is well in line with the Si/AL ratio of this body. The 10% flux additions made in classic stoneware bodies are typical: and in my opinion may be light. Of course, that is also dependent upon the amount of iron present as well. Flux is looked at solely as melt: which in stoneware is inaccurate. Porcelain requires high molarity because of amphorous structure: however mullite requires a certain level as well. Mullite in nature is formed in the presence of sodium and calcium: and yet the industry uses potassium, and little calcium.  Part of the reason my current testing includes 1-2% whiting additions.

 

My reference to Neils recipe as 5/20 and the other reference to 80/10/10 started with the term “classic.†IE: historically speaking. Did not say I agreed or disagreed. Although you could blend a body with all clay; if the right amount of alumina, silica, and flux was already present.

 

You say the glassy matrix in porcelain and stoneware is the same: I say they are not. One has no crystal lattice: the other has a definable lattice and structure. Apples and oranges are both fruit: but nothing alike. One requires a much higher alkali molarity, the other one does not. One has lower alumina levels, the other requires much higher alumina molarity.  One has a much denser mullite platelet structure, the other does not. I could go on.

 

I have not gotten to the chemistry side of this equation yet: will do a lot more testing before I make those assertions. Nor have I discussed the proportions of clay additions: that will come later as well. I do know of the hundred plus recipes I have downloaded, analyzed, studied, and categorized: nearly 75% of them are 80/10/10. Another 15% are 5/20 blends: the balance are “what the hell is this?† I say that politely of course: but when a recipe has 50-60% fireclay: one can only assume brick production.

 

For the record; I am not following the text books: because I think they are mostly wrong. It was the current thought at the time of writing: technology has come. There are some aspects that are correct: but if they had it right, then where are the formula limits? We spend a lot of time worrying about shrinkage: even though the nature of ball clay is to shrink. Who cares if it shrinks 12%, instead of 13%?

 

We disagree on some issues Curt: which makes it all the more interesting. I know you are trying to bring me to the dark side: not going to happen...

 

Nerd

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Hi Nerd, many issues here, I have replied in line.

Well Curt: I try very hard not to give overly complicated and scientific answers. I bore people to tears with my posts as it is. So in fairness to those reading, other than you let me say: Warning... boring post ahead.. Take a nap, drink coffee, and No Doz before you read.

 

On the contrary, if readers of a forum entitled "Clay and Glaze Chemistry" do not find these topics compelling they are probably in the wrong forum.   

 

Carbonaceous clay has several factors actually: the first being lower alumina and higher silica: coupled with the fact they supply very little naturally occurring flux. 

 

I think this is far too restrictive a definition, if not a mischaracterization.  As I said before, that is academic, unless it leads to the wrong conclusions, which is my concern here.

 

In any given bag of clay - of any variety - there is a mixture of kaolinite (the actual CLAY) and a bunch of other junk and contaminants which you can see identified in at least some form listed on the spec sheet.  For the actual kaolinite portion (the good platy stuff which is the main game for us potters) the ratio of alumina to silica is fixed by nature at something like 1.53 parts silica for 1 part alumina on a weight % basis.  In fact, knowing this ratio is really handy for discovering out how much actual kaolinite (as opposed to other junk) is in your bag, since any alumina found in the chemical analysis of a "clay" is almost certainly coming from kaolinite. 

 

Following this same logic, with a bit more simple maths and a straightforward process of elimination, you can also identify the weight % of FREE silica (quartz, ie, the silica not bound up with aluminosilicate clay kaolinite particles ) in the bag.   My experience with native materials in particular is that there is an astounding amount of sand already present in our dug materials, but that is going down another trail. 

 

Carbonaceous clay is simply clay (kaolinite) with (relatively) high levels of carbon (and other junk) mixed in with it in the bag.  If you dig any clay from a dirty source and do not clean it up or beneficiate it at some point during the process (because, lets face it that takes time and costs money), it is highly likely to have at least some carbonaceous material in it that goes up the chimney as LOI.  More specifically, as above, the amount of carbon you find in a bag has nothing to do with alumina, silica, flux or anything else. Any bag of clay - with any amount of kaolinite - likely has carbon in it (unless calcined of course)

 

Similarly, if the clay has not been beneficiated, iis not at all surprising (depending critically on its source) that it would have plenty of free silica (aka quartz sand) in it, possibly of many different sizes.

 

In any case, the point is that all clays are "carbonaceous" to some degree, but this has nothing to do with alumina, silica or fluxes, except to the very small extent to which carbon would otherwise replace all these materials by taking up volumetric space in the bag.

 

Have run out of time at the minute, but have more comments on what you wrote after this.  Will post more shortly. 

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Well Curt: I try very hard not to give overly complicated and scientific answers. I bore people to tears with my posts as it is. So in fairness to those reading, other than you let me say: Warning... boring post ahead.. Take a nap, drink coffee, and No Doz before you read.

 

Carbonaceous clay has several factors actually: the first being lower alumina and higher silica: coupled with the fact they supply very little naturally occurring flux. Of the carbons: just one to be specific: sulfur, which typically occurs in nature as a combination of sulfates and sulfides. One will burn off easily: the other will not. In the latter case: inorganic sulfides will rise to the surface of the clay and seal it: thereby causing bloat. The bloat is not caused by the sulfides, it is caused because they create a barrier, that escaping gases cannot penetrate.

 

Sulfur is not carbon of course. These are distinct elements on the periodic table.  I guess you mean predominantly carbon products like lignite that contain sulphur??   In any case, do you have a reference for the sulphides issue? I would be interested. 

 

Actually tested this over the past summer. I took some powered lignite coal (very high sulfur) and put some in a clear glaze to high cone 6. The surface was littered by dark brown/black specks: along with a bit of scummy looking areas. Although 90% plus was burned off: some remained. So the idea that bisq burns off all carbons is a bit of myth.

 

I am not sure anyone is expecting all carbons to be burned out.  Rather, just enough of them to minimize the likelihood of bloating.  I can tell you that in practice after having a major problem with bloating, instituting a one hour soak at 800C in the bisque with plenty of ventilation in the kiln has resolved all but the occasional bloat.

 

Regarding your lignite experiment, do you know for sure that what you saw left over was carbon?  Burning lignite (or its cousin coal, or anything organic - think wood in your fireplace) results in ashes which have their own chemical composition.  In fact the whole class of ash glazes depends on this.  Chemically, lignite is only about 60-70 carbon.  That means it has LOTS of other stuff in it, like silica, alumina, iron, etc..  See the following link as one example of what kinds of things are going on when lignite is heated up to 1200C.  Note in particular the iron-related phases, which are in evidence well below 1000C. 

 

http://www.sciencedirect.com/science/article/pii/0016236180901969

 

I would bet what you saw left over in that clear glaze were various phases of these by-products, most likely including iron.  Hard to know for sure until you sent it to the lab for chemical analysis. 

 

However, assuming for a second you are right and it was carbon, I would have to say it sounds suspiciously like the mechanism at work in a silicon carbide cratering glaze??  Combusting lignite in a glassy matrix seems abit akin to producing coke from coal, and if a similar process were to take place inside a clay body I guess we would call it black coring, which might also result in bloating.  Point is, if you try to combust organic materials inside a glassy melt, I guess this could be problematic.  Hopefully in our clay bodies we try to avoid this.

 

The higher presence of silica, and lower presence of alumina; does in fact make this, more of a porcelain body.  Even the fireclay added flux; and has a kaolinitic  ratio of alumina and silica. The added alkali from the Om4, the fireclay; coupled with the 20 % flux addition; created an amorphous body. You are forgetting at the time this recipe was written (when Neil was a tyke) custer had 10+ % potassium. The Si/Al  level of this recipe runs to the porcelain side.

 

I am sure Neil will be delighted to know that what he thought was stoneware was actually more porcelain!  :lol:  

 

Yes, there is a difference between porcelain and stoneware "glassy matrix"

 

Porcelain has the same amorphous structure as glaze: no discernible crystal lattice. The difference being material solids; and the requirement for higher alkali molarity (compared to stoneware) to fully incorporate solids into the melt. The mullite formations that do occur in porcelain are a completely different structure, compared to stoneware. Kyanite is often added to produce the needle-like mullite structures found in porcelain; and those are few.

 

The glassy matrix found in a porcelain body has no discernible crystal lattice: the mullite in stoneware does. The glaring difference between the two “glassy matrixâ€: one has a definable structure, and the other does not. What little mullite forms in porcelain is needle-like in structure, whereas the mullite developed in stoneware is orthorhombic (  cubic) in our case: “cubic platelets.† Platelet shaped because of the phase change of metakaolin to spinel. Stoneware starts with platelets in the clay, and ends with platelet shaped mullite. Which explains the different densities between the two clay bodies.

 

I think you may want to revisit the link below which I posted earlier in the stoneware limits thread, which is about porcelain.  Don't even bother with the text, just look at the pictures. 

 

http://www.ceramic-r...rticles_01.html

 

Porcelain is not like glaze.  Porcelain contains lots of crystalline structure in the form of mullite (and likely other crystalline phases as well depending on the flux cocktail).  And that mullite is exactly the same kind of mullite found in stoneware, including primary, secondary and tertiary mullite, depending on how long you soak it at the top temp.  

 

Yes porcelain has more mole % of flux so that more of that more of the fine silica can be turned into amorphous, translucent glass phase which is how porcelain starts to look more like porcelain and less like stoneware.  But unless you are firing to Cone 13 and soaking there for hours on end, there will still be plenty of crystalline mullite in your finished fired porcelain.   And all that mullite and glass will be surrounding plenty of unmelted residual large quartz particles (the boulders in the pictures).  If this was not true, if as you say porcelain was really like a glaze, it would all just melt down into a pool on the kiln shelf.  The Silica/Alumina ratios alone are evidence that porcelain is not like glaze.

 

 

Mullite is Al(4)Si(1.5)O(9.75). Obviously we cannot reproduce that in our kilns because of temperature and pressures. 

 

Plenty of mullite is formed in the kiln in virtually every stoneware firing

 

However, stoneware to some degree does follow this chemical chain, in that stoneware clay has much higher alumina to silica ratios; as compared to porcelain.  

 

Porcelain can have levels of alumina just as high as stoneware, it just wont be translucent. 

 

This recipe had 17% alumina molarity, most stoneware bodies run between 19-20%, and the bodies I am developing: 20-22%. So the fact is: Neils’ recipe is a modified porcelain, more so than a stoneware body. The famous Coleman Grolleg;  has 16.4% alumina molarity.

 

This is exactly the point I have been trying to make for a while now.  Porcelain and stoneware are arbitrary terms - until you start to define SPECIFICALLY what objective characteristics each one has.  You appear to have decided that any body with less than 17% molar alumina must be a porcelain body.  I don't agree or disagree, just wondering what is special about 17% molar alumina in terms of the actual characteristics of the finished, fired body?? 

 

So the 20% Custer addition is well in line with the Si/AL ratio of this body. The 10% flux additions made in classic stoneware bodies are typical: and in my opinion may be light.

 

OK, but why? I agree that 10% weight is probably light, but that is because my primary measure of functionality of a fired clay body is water absorption of the fired object below 2% and ideally somewhere around 1%, and my own testing of bodies with so little flux is that they do not achieve that 2%.  That is one of the main reasons I think many stoneware bodies (and body recipes) out there are, strictly speaking, not functional.  Because they have too little flux to create enough glassy phase to prevent them leaking or otherwise failing. 

 

Of course, that is also dependent upon the amount of iron present as well. Flux is looked at solely as melt: which in stoneware is inaccurate. Porcelain requires high molarity because of amphorous structure: however mullite requires a certain level as well. Mullite in nature is formed in the presence of sodium and calcium: and yet the industry uses potassium, and little calcium.  Part of the reason my current testing includes 1-2% whiting additions.

 

Mullite can be formed with help from many different fluxes. Nothing special about sodium and calcium, or magnesium or any other flux for that matter.  And, other helpful crystalline phases besides mullite may also be formed depending on the flux.  For example, calcium heavy bodies form anorthite.  I think this is what the guys who made Limoge figured out. (Limoge has 19.5% molar alumina by the way). 

 

My reference to Neils recipe as 5/20 and the other reference to 80/10/10 started with the term “classic.†IE: historically speaking. Did not say I agreed or disagreed. Although you could blend a body with all clay; if the right amount of alumina, silica, and flux was already present.

 

You say the glassy matrix in porcelain and stoneware is the same: I say they are not. One has no crystal lattice: the other has a definable lattice and structure. Apples and oranges are both fruit: but nothing alike. One requires a much higher alkali molarity, the other one does not. One has lower alumina levels, the other requires much higher alumina molarity.  One has a much denser mullite platelet structure, the other does not. I could go on.

 

Again, I disagree for the reasons laid out above.

 

I have not gotten to the chemistry side of this equation yet: will do a lot more testing before I make those assertions. Nor have I discussed the proportions of clay additions: that will come later as well. I do know of the hundred plus recipes I have downloaded, analyzed, studied, and categorized: nearly 75% of them are 80/10/10. Another 15% are 5/20 blends: the balance are “what the hell is this?† I say that politely of course: but when a recipe has 50-60% fireclay: one can only assume brick production.

 

For the record; I am not following the text books: because I think they are mostly wrong. It was the current thought at the time of writing: technology has come. There are some aspects that are correct: but if they had it right, then where are the formula limits? We spend a lot of time worrying about shrinkage: even though the nature of ball clay is to shrink. Who cares if it shrinks 12%, instead of 13%?

 

If you mean textbooks from the 70s and 80s I agree, technology has come a long way and allowed us to drill down into clay chemistry in a way that could not be done 40 years ago.   My own research into these matters is biased toward to the most recent papers I can get, usually from the last decade, from industry or academe, where they clearly have a lot more training, a lot more time to test, and a lot better equipment than I do.

 

We disagree on some issues Curt: which makes it all the more interesting.

 

Roger that.  For better or worse, I have learned quite a lot by having to go out and do my own research and get information to clarify why I disagree with you.  Perhaps that is the way of things.

 

I could written much more, but honestly a lot of this has already been covered in the stoneware limits thread.

 

I know you are trying to bring me to the dark side: not going to happen...

 

LOL, OK, so can I take off this dark black helmet now??  It is so hard to type!  :lol:

 

Nerd

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Yes: most of it has been covered. So I am trying to figure out what are the exact differences between stoneware and porcelain; because we both agree on paper there is very little. In looking at the chemistry of numerous porcelain bodies compared to stoneware bodies, two glaring differences: porcelain typically ran 14-17% alumina and stoneware typically ran 17-20% alumina. The second was alkali molarity: porcelain ran 3.50-4.50%, and stoneware ran 2.80 almost consistently. Lastly, some distinctions could be made on the basis of particle size, and carbon content. Porcelain does at least have a measurable standard of 50% kaolin, 25% silica, and 25% feldspar. Stoneware formulation consists of: throw whatever you got laying around together, and bake the """it out of it. At one time, the common flux of choice was potassium: but I suspect the dwindling supply of Ka played a role in the changes to sodium for porcelain. I also suspect that NEP SY being dirt cheap also played a role in porcelain switching more so to Na. A 2200lb bulk sack: Nep Sy runs about 8¢ a lb.

 

My reference to porcelain and glaze being the same was a qualified statement: the amorphous structure has no discernible crystal lattice, being a covalent bond. Obviously material mass is way different. Stoneware is suppose to produce more mullite: but that does not mean necessarily that it does. The 5/20 blend above is basically a porcelain body, with some big chunks (40 mesh) pieces of fire clay. If 20% FHC (high carbons) would have been used instead of OM4 (low carbon): this 5/20 recipe would have bloated. Currently, my testing indicates that 17% (or higher) high carbon ball clay, coupled with higher levels of alkali molarity results in bloat. (pictures and results later). Yes, I think that carbon levels need to be part of stoneware formula limits.And yes, I also think that as alkali molarity increases,: carbon levels must decrease to avoid trapping the excess gases produced by higher molarity. (still needs tested).

 

Obviously porcelain and stoneware have the typical amorphous structures: the only difference is crystal lattice. One has one, the other does not. Stoneware is suppose to produce mullite with a platelet structure: what it is suppose to do, and what it does are not the same. Mullite Al(4)Si(1.5)O(9.75).: is naturally occurring mullite found on the Isle of Mull (UK), My only frame of reference to it is the Si/AL ratio: which if we duplicated, we would be making kiln shelves instead of stoneware. However, for me that does confirm the idea that stoneware requires higher alumina molarity than porcelain. Like you, I have read more boring technical papers than I care to: but the most recent from Oregon State confirmed 100% mullite platelet at 1000C. However, a solvent bath was involved; which is not feasible for pottery: the Si/AL was noteworthy.

 

So we both agree porcelain and stoneware are basically the same beast. The problem is: our industry says they are not. Months back in the Stoneware thread I realized there were no set parameters for stoneware: that was discussed. Stoneware could be anything, as long it was not 50% kaolin, 25% silica, and 25% feldspar. Throwing 10% large particle fire clay in a batch does not make it stoneware: it makes it porcelain with big chunks in it. No reason to hash over fire clay: we both already know it is reasonably clean as clay goes: and most is kaolinitic in composition. (it just has a % of large mesh). Actually our fire clay is the basis of most brick recipes: who knew? So there still has to be some distinction made between the two bodies; even though they are similar. I doubt seriously if the clay arts is going to accept the terms: smooth porcelain and chunky porcelain. Now if we are talking peanut butter... never mind..

 

I also realized months back that some sort of standards of testing had to be set. Stoneware is all over the place: I have seen it with over 50% fire clay, and 10% fireclay. I have seen 80% total clay content, and 60% clay content. I have seen three, and up to seven different clay varieties used. Silica additions have ranged to none, and up to 15%. Even using particle size would create a booklet of values: every fire clay, ball clay, and kaolin has vastly different percentages of PSD. The only measurable (common) definition is SAS: Every clay has one, and most mines have already defined it: even though that info was never used in our industry. The paint, plastic, and rubber industries demand them: we do not. They formulate using them, we do not.

 

By using the SAS formulation method: I can now assign chemistry to the ascending values. An SAS of 23.00 is not based on how many clays,  or the percentages of PSD (although that is reflected in the values): it is the combined total of all the clays, regardless of the type, percentages, and number used. So now you do not need a database of PSD, and long hand equations to compute distribution: you just need one number. So now I have a definable number to test to and to set formula limits to: and is easy to compute.

 

22.00 SAS requires XYZ chemistry.

22.50 SAS requires XYZ chemistry.

23.00 SAS requires XYX chemistry,.... you get the drift.

 

The SAS value moves as the blending moves: does not matter how many clays are used: the SAS values detects the final distribution. However, you will find after running the math and playing around with it: as the SAS moves up the chain: so does the Si/AL values.To balance the SAS at 23.00: you have to make certain additions to hit that mark: and inevitably: that means you have to add higher percentages of kaolinitic material. We both already know that kaolinitic clays have higher alumina molarity. So now I do not need to test 100 samples with graduating amounts of fireclay, and another 50 samples with graduating amounts of ball clay, and another 50 samples of graduating amounts of kaolinitic additions, and another 50 samples of graduating silica/alumina percentages. All I have to do is figure out the chemistry limits for 22.00-24.00 SAS: 95% of existing recipes fall into that range.

 

 

We disagree on some issues Curt: which makes it all the more interesting.

 

Roger that.  For better or worse, I have learned quite a lot by having to go out and do my own research and get information to clarify why I disagree with you.  Perhaps that is the way of things.

 

I realize it is only clay: but the scientific community has this long held tradition. One puts forth a theory, and the rest check it, and challenge it. At the end of the process; a consensus ends up establishing the true baseline. So keep challenging: you are making me check, think, and at times: changing my thoughts. 

WAIT.. I am not changing my thoughts: that would be the dark side... :)

 

Nerd

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 So I am trying to figure out what are the exact differences between stoneware and porcelain;

 

 

I know.... I know.... I know............  (jumping up and down with hand raised).................

 

 

 

 

 

Porcelain is white and stoneware is not.   :ph34r:

 

 

best,

 

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

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Porcelain is white and stoneware is not.   :ph34r:

 

 

stains added to porcelain makes it stoneware then?  :ph34r:  ;)  :P

 

 

Nope.... then it's "colored porcelain".    Not the same.  Nah nah nah............ ;)

 

best,

 

..........john

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Nerd, is it not that porcelain is a stoneware recipe so you can't really compare them in the way you want to. That is why you have a bunch of different mixtures that can be called fired at the hotter end of temperatures (stoneware).

At least stoneware has always been more a temperature definition to me rather than a recipe.

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Joel:

 

Once upon a time, and in the classic sense that would be a true statement. I do know several crystalline glazers who take porcelain to the top of cone 10, and touching cone 11. I think the problem is; the classic definitions that separated the two bodies is vanishing. Chemistry wise, there is very little distinction between the two with the exception of alumina and flux levels: and some would argue that.

 

The biggest distinction is the amount of clay(s) in a recipe: porcelain usually caps at 50% of recipe, and historically stoneware reaches 80%. Porcelain has no more than two different clays; and stoneware can have up to seven. Mesh size is another obvious distinction; although chemical composition is not that dissimilar to kaolin. So while I am arguing the differences: Darth Vader (Curt) is actually correct when he says: stoneware is just dirty porcelain. I will still however, continue to at least bring some formulation standard to the whole process.

 

Nerd

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Perhaps it would be more palatable if we referred to porcelain as ultra-clean stoneware??  :D

 

I think the real arbiter of stoneware or porcelain is the end user and their classification of the finished, fired piece.  This may sound like a cop-out, but as I have said elsewhere, porcelain is in the eye of the beholder. It is not really about chemistry, firing schedules, etc.. 

 

I agree with John that white increases the likelihood that we will think of it as porcelain, and we know less iron and less titanium helps on that front.   I also tend to think that the generally smaller particle size of porcelain ingredients yields a finer fired surface, both raw and glazed.  I have some red coffee cups in my gallery that I call "red porcelain".  But are they really?

 

If we put 10 broken unlabled shards of fired clay on a table and asked people - both potters and non-potters (call them the "subjects") - to identify which ones were porcelain what do you think would happen?  Lets run the experiment a few different ways....

 

The first group of subjects are not allowed to get within in 10 feet of the shards, but they can see them. 

 

The second group of subjects are allowed to pick up the shards and feel them, but in a dark room so they can't see them. 

 

The third group of subjects are allowed to examine the pieces up close, but without touching them.  All pieces in this phase of the experiment are labeled "Bone China" :ph34r:

 

The fourth group of subjects can do whatever they want to identify them, including send pieces of the shards to their local lab for testing.

 

And so on...

 

I think would get all sorts of classifications from both potters and non-potters.  Everyone has their own standard.  That is why many different fired bodies can be either porcelain or stoneware, depending on who is looking at them.

 

As clay manufacturers - and dinnerware sellers - know: Their is a lot of salesmanship in the porcelain biz....   :P

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I think it is quite a subjective designation, really.

 

I think it is human senses that define porcelain, not chemical or physical analysis.  This is one case in ceramics where the "Mark One Eyeball Test" might actually give the definitive answer.  Heresy to my science geeky sidet!!!!!

 

I think that porcelain is a white and translucent clay-based material.  If it is not made from al least some clay... it is not porcelain.  If it is not white (sorry Chris :) ), it is not porcelain, and if it is not translucent to some degree, it is not porcelain.

 

COLORED porcelain is not somehow "less valid" or "worse"....... it is simply that... colored porcelain.  It is porcelain that has been adapted to the new idea / use.

 

best,

 

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

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The real truth is much easier: clay bodies are what the manufacturer tell us they are.

 

Secondly, the only reason mullite formation has become important in the last decades: is because the military is interested. It seems mullite glass is much stronger than borosilicate: hence thinner blast proof armored glass.

 

Lastly, my brain hurts from thinking about it. Perhaps it is time to just to pull my threads down and go back to the age old method of blending clay: throw some crap in a bucket and cook the crap out of it. Apparently clay chemistry is not of any relative importance.at the end of the day. In 2016, I should not be writing a book about clay chemistry, I should be reading one written 100 years ago.

 

Nerd

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I think the chemistry is critically important.  So is the array of particle sizes.  True for stoneware, but even more important for porcelain.

 

However, as I said way back at the beginning of the stoneware limits thread, if you don't define what end objective(s) you are trying to achieve by manipulating these variables, then you will never know when you have arrived.

 

My own focus at the moment is on developing FUNCTIONAL clay bodies from native materials.  Since all my native materials are full of iron, you know immediately I am not working on porcelain.  No, I am after a general purpose Cone 10 stoneware body that looks good in both oxidation and reduction.

 

As I said, perhaps the primary way I define "functional" at this point is porosity.  For me to label a clay body functional it must have water absorption under 2%, and ideally closer to 1%.  This is mainly an issue of flux amounts and silica, but also of particle size.  If particle size is too large, even unreasonable amounts of flux may not work to achieve my porosity target. So all the stuff we have been discussing with regard to chemistry must come to bear, along with particle size issues. 

 

And the ideal firing temperature and hold times to produce this ideal porosity given the flux and particle size packages will need to be identified. Or otherwise, the mix will need to be tweaked to make Cone 10 with a 30 minute hold (our standard community firing profile) be the "right" amount of heatwork.

 

And this body has to be good for throwing and handbuilding.  So it must be able to be wedged, bent and otherwise worked without falling apart, which in turn requires a carefully selected distribution of plastic and non-plastic materials of different particle sizes. 

 

Oh, and it cant have TOO much iron, don't want it too dark.

 

Oh, and I don't want it to look grey (magnesium!).

 

And...and....

 

Anywhoooo...you can see that all these requirements have specific implications for both chemistry and physical attributes of the raw materials.   In the end I don't think it is asking too much.  It can be done.  It is tricky but I am getting closer.  Just getting into the right porosity zone has taken more than two years, and many many many tests.

 

So it should be clear that not just any mix of stuff will do.  My porosity objective and related wish list is not going to happen by accident, or by just borrowing somebody else's 3rd-generation, hand-me-down recipe from who knows where originally developed for who knows what purpose.

 

This stuff matters.  Not very many people are out there doing it.  And when they get it right they probably ought to start a clay business....

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  Not very many people are out there doing it.  And when they get it right they probably ought to start a clay business....

 

Very true.

 

When I started in clay in the late 60's, just about every full time pro had his/her own clay body recipe.  If they were not mixing it themselves in something like a Bluebird Batch Mixer or a reclaimed commercial dough mixer, and pugging in a Walker pugmill or just wedging the crap out of it by hand... then they had the recipe being mixed in bulk by one of the few ceramics suppliers.

 

Fast forward to now... and so many folks just use the stuff that the suppliers sell... and have no idea or any control of what is actually IN the clay body that they use.  Myself included for a couple of the bodies that I use,..... but I know the additions that I put into them to change the look.  However some other bodies I use are of my own formulation.

 

The health and safety concept of "transference of risks" is of course a factor...and likely a good one for most folks.  Move that operation to someplace that has the equipment to do it safely and reduce your own exposure to the dust factor and the somewhat dangerous equipment operation.

 

best,

 

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

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I know more about clay, than I had planned on knowing. This whole process started because I had to figure out a porcelain body that would with-stand crystalline glaze, and a body chemistry that did not hinder development. I threw in dirty porcelain for fun.

 

Tom

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