Jump to content

Recommended Posts

I will be studying/testing stoneware clay body limits shortly: next month or so.

 

Input from stoneware users would be most appreciated. I not only need formula limits, links to recipes, or you can PM them: but also properties you find beneficial or undesirable.

 

From the research I have done so far on c10 bodies; the general range I am seeing ( in molar)

 

Alkali 2.5 to 2.75%  Total Flux 3.6 to 4.0   Alumina 19-20  SiO2 70-75   (would assume c6 is 10% higher in flux)

 

I did not list iron (Fe) or magnesium (MgO) ; which can vary widely pending on the clay used in formulation.

 

Stoneware can be groged or not: what % do you find most beneficial?  Light. med. or heavy. Do you think mullite grog works better than sand?

 

How important is color?  iron red, deep orange, tan to brown range...etc?

 

Is light speckling more attractive than heavy speckling?

 

Nerd

Share this post


Link to post
Share on other sites

I like combos of grog in one body. 5% red art in a reduction body or some gold art will respond well in a body reduction during the climb. 

Newman red in a stoneware body is courser that redart and gives a different shade.  The speckles are dependent on the firing process and reduction.

IMO.

Here are some stoneware clays all fired with a body reduction from Montana 1976 ^10 #4 and glaze speckles #5, 1990s ^6 #3, and a wood fired piece ^10 #3, and ^9 stoneware from Armadillo reduction/wood fired #1.

post-1954-0-65234900-1474756919_thumb.jpg

post-1954-0-76867300-1474756948_thumb.jpg

post-1954-0-23462600-1474756983_thumb.jpg

post-1954-0-01120400-1474757018_thumb.jpg

post-1954-0-27874500-1474757178_thumb.jpg

Share this post


Link to post
Share on other sites

Grog is like hot sauce: how much you like is a matter of personal taste, and it depends a bit on where you were raised. Mullite grog serves a different purpose than sand, depending on how you're firing (you want the silica chunks in a soda fire, for instance). Colour and speckling are all a matter of taste, and the type and style of work someone wishes to make. I would argue that they're both very important, but what I will find desirable (dark red, no speckle, enough tooth from grog to stand up on the wheel, fully mature, as opposed to vitreous, somewhere in the area of cone 6) is not what someone else is going to want to work with. I'm interested in visual texture and contrast, and I want to explore that within the framework of functional pottery. I moved away from white porcelaineous clays because they feel rubbery and weird (I'm looking at you, B-mix). Also, they don't look like clay to me. They're too refined, too "perfect" for my taste. I'd like to qualify this by saying I like it when other people use the stuff to make their work, I just don't like making my work with it.

 

Also, north of the US border, because gas-fired kilns are regulated under the heavy industry section of building codes, you can't build your own anymore, and they're becoming increasingly difficult to access even in academic settings. So I need to be able to fire to cone 6, so I don't murder the electric kiln I'm forced to use too soon.

Marcia Selsor and Juli Long like this

Share this post


Link to post
Share on other sites

Nerd,

 

Interested in your study and what you find out, should be informative for all.

 

I am interested in your angle on the "limits." Are you seeking to reconfirm the limits we commonly see in Insight, Matrix, etc.? Or are you proposing to identify new limits? Will you go oxide by oxide, or just keep it general? I think you have to have both Fe and Mg in there, as both are powerful fluxes at stoneware temps. Are you differentiating between oxidation and reduction bodies?

 

I assume you are looking at limits for both functional and non-functional clay bodies? If so, what criteria will you use to identify a limit or set of limits? Porosity? Shrinkage? LOI? Other measures?

 

Just wondering how you will approach it. One reason I ask is because the general ranges for fluxes, etc, that you have set out above seem to overarch both functional and non-functional bodies. For example, 4% total flux may be sufficient to hold a non-functional body together, but is it enough to produce a body that vitrifies to less than 2% porosity, which I believe is the generally accepted threshold for a functional stoneware body. Would be interested in how many recipes you get from "functional" potters which do not meet this porosity standard.

 

Particle size as you know will also have some impact on the "limits", and can affect how much flux is needed. Wondering how this will be factored in. Maybe as people provide their recipes this will become clearer.

 

Regarding grog, there seems to be many different types, but few chemistries available for those grogs as far as I can tell. I guess that does not matter as much if the grog does not melt at the temperature you are firing to, but grogs seem to run the gamut as far as melt temps go. Are you including the chemistry of the grog in your formulas like you would for a frit?

 

I am not asking these questions to be difficult, but rather because these are the kinds of questions I have when staring at the limits I have available to me now. They just stare back at me, silently, leaving all my questions unanswered. Pretty much leaves me on my own, with only the results of my own tests to give me comfort.

Share this post


Link to post
Share on other sites

Callie:

 

I was up to the wee hours researching and studying clay(s), not the formulation. Pretty much what I expected: courser particle size, higher alumina, slightly less silica: but the surprise was how much KnaO is already in the clay. The reason Grolleg is one of the premium porcelain kaolin: it has over 3% KnaO before you start adding any. Appears to be less choices: 3 fire clays, red/gold art, 2-3 primary ball clays, newman red as an option: and what ever choice of grog if desired. Formulation seems more centered around setting iron and magnesium levels more than anything: although titanium will automatically rise because stoneware clays are just naturally dirty.

 

Stoneware relies more on mullite development, rather than the glassy matrix of vitrification: from what I am seeing so far. So that would mean spinel production would need to encouraged; especially at cone 6. This would also mean that 980C would be a very critical temp range: when spinel forms in the body at the peak rate. Spinel is a formation of magnesium/alumina MgAl2O4, or Mg/Fe Al2O4 : which converts to mullite as the temps climb 1050-1100C. While mullite is a direct addition as a grog: my thought is kyanite might be a better choice at ^6. Kyanite has already been proven to produce needle/clusters of spinel in stoneware bodies: which would increase the mullite production.

 

Anyway, wrapping my head around: lots more reading to do first. Cone 6 is popular now, and will continue to grow as energy and element costs rise. My cone 6 recipes are just as vitrified as any cone 10 porcelain: I suspect more so. So I see no reason a ^6 stoneware cannot be the same.

 

Nerd

Share this post


Link to post
Share on other sites

Lol seems like we pressed the "Post" button at almost the same time on different sides of the world .

 

Adding this question to my post just above: Isn't mullite one product of the "glassy-matrix" vitrification process in stoneware during firing? Just had a look at Hamer and Hamer to refresh my memory, and I think it is.

Share this post


Link to post
Share on other sites

Curt: your reply popped up as I was typing the reply to Callie above.

 

I have not looked at Insight or Matrix: nor will I for some time because I do not want that information to bias my thoughts. I will be collecting recipes for c6 & c 10: and build a data base around them. Primarily median averages of molar percentages

 

Stoneware relies on mullite for maturity: more so than glass. So I think it would be advantageous to focus on increasing that production: in lieu of just straight flux additions. This is where Fe (iron) comes in as a flux: because spinel MgAl2O4 is the primary precursor to mullite production: but can be increased in the presence of iron. So I do believe that is where I am going to focus my attention first. If you recall in the U of I study: as little as 0.30% iron can significantly increase a melt.

 

I know I will be dealing with particle size: giving a couple of options some consideration. The easy choice is to fill in the blanks with silica: but I think in this case I will try some other options, in addition to blending.

 

Where I am going to start is firing some test bars and test cylinders of the Laguna and Flint Hills cone 6 stoneware samples I picked up this week. Do some break bars, check their shrinkage and absorption: even though that is already stated. Then look at them through the periscope and compare them to what I make later.

I am purposely avoiding much of the data out there already: just want to see what I see at this point: and will build from there. I do however suspect encouraging mullite production is going to be the key  ....

 

Nerd

Share this post


Link to post
Share on other sites

Curt:

 

Mullite is in that class yes: I have not read up on it entirely yet. Still cannot find my Hamer book: frustrates me to no end. However, if I remember correctly: the glassy matrix of porcelain is a silica melt.  Mullite is of a magnesium/alumina/silica melt: which is effected more so by the presence of iron. When spinel is formed: silica is actually ejected from that mass. Still have alot to read; and very well may have to adjust my thinking: but I think a solid stoneware body is going to rely on alumina/magnesium/iron primarily, and within a structure of silica/ Knao.

 

** the ratio of Fe/MgO is going to effect the KnaO level proportionately.

 

The other note: some serious thought needs to be given to ramp temps of stoneware. 980-1100C is the peak development period of spinel to mullite conversion.

 

Nerd

 

Edit: by the way- already noticed potassium is the naturally occurring flux in most clays used in stoneware. In seeing that, I now know why pin holing in stoneware is much more prevalent.

 

John: actually no, I would not be surprised. if there were clay abuse laws, I would be serving time.

Share this post


Link to post
Share on other sites

I'm betting you'll find little no potters who care about MgO in their stoneware bodies. The issues for most potters are simply vitrification, color and texture. Vitrification via how much feldspar they add, color via iron, and texture via fireclays or additives. More and more people are using white stoneware bodies, very often smooth bodies without any fireclays. Wood and soda/salt folks have a lot of different criteria than cone 6 folks. Reduction and oxidation bodies have different requirements. People add silica sand, grog, mullite, kayanite, etc, in a wide variety of particle sizes. Some really nice stoneware bodies also have kaolin in them. Stoneware formulas are a much, much broader range of materials, textures and colors than porcelain. You've begun on a very long journey......

Share this post


Link to post
Share on other sites

Neil:

 

You forget I started my clay journey with crystalline glaze: spending over 7 years unlocking that mystery. So spending a year or two developing formula limits for stoneware is just a short walk. Few more tests on porcelain: and I will write the final formula limits for that body. The parameters of clay formula limits has little to do with the clay varieties: but rather the molar % in combination: and in the case of stoneware: in conjunction with each other. Magnesium in the case of stoneware has nothing to do with color: but rather the development of spinel: which in turn forms mullite. While iron is one of the colorants of the body: in reduction and high oxidation: it likewise has to be calculated as a flux. So formula limits will only get you to a mature body: fall below them and you increase absorption, exceed them and you get to deal with pin holes, bloating, and dunting.

 

In reviewing porcelain vs. stoneware: there is not the much difference. You are still dealing with % of alumina, silica, and flux in both bodies. Stoneware tends have courser particle size: and much higher limits of Fe, Mgo, and TiO2: which mostly effects color. The primary difference is how they mature. Porcelain relies on vitrification through a basic silica/KnaO melt. Stoneware relies on a Alumina/Mgo/Silica melt that is accelerated by iron. Mullite is MgAl2O4 (spinel); the important distinction between the two. Both in the glass category; but morphology is different.

Nerd

 

http://www.ceramicindustry.com/articles/90219-ppp-influences-on-clay-color

curt likes this

Share this post


Link to post
Share on other sites

Nerd,

your original post included these 

How important is color?  iron red, deep orange, tan to brown range...etc?

 

Is light speckling more attractive than heavy speckling?

these are responses to the firing and types of clays mixed into the batch.

 

Marcia

Share this post


Link to post
Share on other sites

Well, I care about MgO in my stoneware, so there is at least one of us out there.  Mostly due to its flux potential (and maybe its color).  I don't think there is any doubt that vitrification is about much more than just the feldspars.  

 

Apparently, some other folks also think MgO is important.  Here is the chemistry for Limoge (molar) described as a "white, translucent porcelain." 

 

SiO2           73.3

Al2O3         19.55 

K2O            2.2

Na2O          1.65

Mg               1.38

CaO             1.43

Fe2O3         0.24

TiO2             0.14

BaO              0.08

 

These are not insignificant amounts, so I am not sure we can dismiss MgO (or CaO) from the list of porcelain fluxes just yet, either.  

 

I guess I am resisting drawing a hard line between porcelain and stoneware, since they both use the same fluxes, just in differing amounts.  And I don't think there is anything to say that stoneware cannot have fine particle sizes.  Put another way, isn't stoneware really just "dirty porcelain"?  Or is porcelain really just "ultraclean stoneware"?  :P

Share this post


Link to post
Share on other sites

Marcia:

 

Indeed it does, but like most threads: it took little time to get down to the chemistry of it all. Stoneware limits will end up being a set of values that produce a mature body, and the upper end of those limits before each creates problem. Color will end up being a different table of values all together:  and adjusting properties a different table as well. Ole Neil is trying to save me from spending the next two years pulling my hair out- bless his heart. The link I posted above does a fairly good job of detailing how to manipulate a stoneware body for color or speckling. Obviously that was studied and released because that is what potters care about the most. How to paint the canvas has already been tested, how to make a canvas has not.

 

Curt:

Porcelain originally started out being a basic 1/4 recipe:  25% kaolin, 25% ball clay, 25% silica, and 25% flux. Laguna now calls that their 50/50 porcelain recipe: more marketable I suppose. Most porcelain bodies are a light to medium gray color because they are 30-35% kaolin, and 15-20% FHC (foundry hills cream-ball clay). Again, price point driving the market. FHC is highly plastic, and once you get into the 12-15% recipe range: plasticizers are not even required. The vast majority of porcelain and stoneware bodies are using Nep Sy as the flux: again because it is dirt cheap. If I wanted to buy a rail car of Nep St, it would run me about 0.04¢ a lb.and FHC would run about the same.

 

Premium porcelains running from 0.80-0.95 a lb. include the good stuff like grolleg, or super standard (china clay), and higher amounts of KnaO are added to make it all perty and glassy. The recipe above notes much higher amounts of CaO than what is typically found in the States. Pot Clays out of England ran tests back in the 90's and found CaO made an excellent floucullant and added to the green strength. I am already seeing that the premium stoneware bodies are using kaolin: mostly #6 tile kaolin. They are also using the more expensive, but cleaner grades of ball clay. So deciphering clay bodies is really not that tough: just add up all the cheap stuff and you got it.

 

Any potter can make up their own clay, without going to the expense of blungers, puggers, or mixers. Just find an old kitchen aid mixer with a good motor and a dough hook: and go to town. You could easily mix 50lbs a day, in small batches. The only thing required is to set the moisture at 12-13%, then let it age for a day or so before you use it: so it will hydrate and bleed off the excess moisture content. Adjust to the amount of clay you use every day, and always stay one day ahead in your mixing schedule. Give me another year or so, and I will post some recipes as: 2 cups of kaolin, 1 cup of ball clay, 1/2 cup of Nep Sy, and 3/4 cup of silica: you will not even need a scale. Clay is not like glaze, exact amounts to the 100th percent is not required.

 

Nerd

Share this post


Link to post
Share on other sites

The low silica alumina ratios of those bodies makes me think they are mostly decorative, ie, not going to vitrify particularly well. And something is not quite right there because even the lowest combination of silica and alumina from the numbers you provide above gives you 98% molar, which is not going to fuse. Unless you are firing way above cone 10, which I doubt.

 

Is "white body" your description or is that the description provided by the various sources you got the recipes from? Are you thinking that any body with combined Fe and Ti below 1% molar is a "white body". And when does a white body become a porcelain? Just asking because it seems like you are already to starting to group body recipes into categories. It's your study, but as I said above, IMO you need to establish criteria for what objectives you are aiming at in establishing limits and do this sooner rather than later. Otherwise Neils "long journey" could turn in to a one way ticket to the insane asylum. ;-)

 

Insight automatically calculates Fe molar along with everything else molar. Doesn't Glazemaster do that? Not sure why you have done all that conversion?

Share this post


Link to post
Share on other sites

Curt:

 

All of the 20 recipes are from colleges and from clay trade magazines: being careful about just adding random recipes. All I am doing at this point is establishing current "accepted" limits: although I have little doubt formulation is based on looks mostly. You have already picked up on the molar levels indicating an immature body: again just establishing current trends. Alfred Porcelain (Alfred University) shows 3.37% total alkali (molar) for its porcelain body at cone 10. The median average for cone 10 stoneware is 2.52% (molar) : which speaks for itself in regards to maturity. Given stoneware utilizes clays with 40-60 mesh particles sizes just further expounds that issue.

 

I am citing "white" limits as what current recipes self define as white. More interesting, those citing "gray" also have higher limits of MGO: found that interesting. You already noticed "white" stoneware switches to kaolin additions: which in turn enters into the realm of porcelain. I would assume these kaolin additions are the basis of industry claims of being a "porcelainaus" body. That conversation of what defines a stoneware body and a porcelain body has already taken place awhile back. I will just reassert my statements from back then: the lines between the two are becoming increasingly blurred. Please remember, all I am doing at this point is establishing current industry standards: that does not mean I agree with them.

 

Yes, Glazemaster automatically calculates FE in unity, weight, and molar. The table above was to show you how to convert the molar FE % into the flux value that FE adds in a stoneware body. Glazemaster has a tab that converts all FE additions per source into a flux values for stoneware. It is calculating the fluxing ability of FE, when reduction firings are used.

 

All both one recipe is defined as cone 10 bodies. However, one claims to be a cone 6-10 body and yet is only has 2.71% total alkali molar. Ranks right up there with the claims made on some porcelain bodies. Now that I have a general sense of what alkali, FE, and MgO levels are in widely used bodies:I will gradually increase alkali until I reach a mature body.  I have already calculated my first test body with a total alkali of 3.12% molar. I will compare that with the 3 recipes that are closest in composition with 2.52% alkali (molar): and then compare the structures for maturity. .It's a starting point........

 

Nerd

Share this post


Link to post
Share on other sites

by the way:

 

 

And something is not quite right there because even the lowest combination of silica and alumina from the numbers you provide above gives you 98% molar, which is not going to fuse.

I am not going to automatically make that conclusion just yet. I have not studied it enough yet to make a blank statement: but I am finding studies that indicate the flux mechanism of FE is directly related to the melt of alumina, not silica. IF that holds to be factual: then the alkali is only present in direct relation to the silica melt. IF that holds to be factual: then not nearly enough to get the job done. However, then the other issue of MgO levels in regards to forming spinel, which in turns converts to mullite. So this study is going to take some time to sort out the pieces of the puzzle. Once I get that done, then I can play with the fun stuff like color and speckling.

 

Nerd

 

Fun fact: the link above studies stoneware in regards to color and speckling. They talk about granular manganese about two thirds of the way down. ...        " 5.5% granular manganese produces 200 speckles per square inch."

Share this post


Link to post
Share on other sites

OK, looks like you are going to ignore all the recipes you have been given and start your testing at a much higher alkali level.  I am wondering how you decided on that particular level of alkalis.  Also, will you include other fluxes (ie, the alkali earths?).

 

Understand your Fe flux adjustment now.  In Insight there is just a box you click for "reduction" which automatically turns the Fe into a flux for RO Unity calculations.  Although not sure about your Fe - Al link. 

 

Although, it seems like iron has plenty of potential to be a flux even without a reduction atmosphere once you get over 1220C (by liberating FeO via thermal decomposition).   So that suggests that we should be factoring it in to the flux package more often than we do.

 

Also, I don't think Mg is needed for mullite development.  Anything which melts silica will do, since it is the molten silica breaking down clay crystals that causes mullite to develop.

 

Interested to see where you go with this.

Share this post


Link to post
Share on other sites

 

Although, it seems like iron has plenty of potential to be a flux even without a reduction atmosphere once you get over 1220C (by liberating FeO via thermal decomposition).   So that suggests that we should be factoring it in to the flux package more often than we do.

 

+1

 

best,

 

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

Share this post


Link to post
Share on other sites

Well, I care about MgO in my stoneware, so there is at least one of us out there.  Mostly due to its flux potential (and maybe its color).  I don't think there is any doubt that vitrification is about much more than just the feldspars.  

 

Apparently, some other folks also think MgO is important.  Here is the chemistry for Limoge (molar) described as a "white, translucent porcelain." 

 

SiO2           73.3

Al2O3         19.55 

K2O            2.2

Na2O          1.65

Mg               1.38

CaO             1.43

Fe2O3         0.24

TiO2             0.14

BaO              0.08

 

These are not insignificant amounts, so I am not sure we can dismiss MgO (or CaO) from the list of porcelain fluxes just yet, either.  

 

I guess I am resisting drawing a hard line between porcelain and stoneware, since they both use the same fluxes, just in differing amounts.  And I don't think there is anything to say that stoneware cannot have fine particle sizes.  Put another way, isn't stoneware really just "dirty porcelain"?  Or is porcelain really just "ultraclean stoneware"?  :P

 

My comment was in regards to stoneware. Yes, people are much more aware of MgO amounts in their porcelain, because everything is more significant in porcelain. I don't think it's accurate to say that porcelain and stoneware are just dirty or clean versions. They are both clay bodies, yes, but they differ significantly in makeup. Otherwise we could just use the same limits for both.

 

I did no mean to imply that MgO isn't important in the formula, just that people don't generally consider it important because it's such a small amount. That's why what Nerd is doing here is so interesting. If he can show that it makes a significant difference in the vitrification of the body, that can change people's attitudes. The big question is whether it will be significant enough to matter.

Share this post


Link to post
Share on other sites

Nerd,

 

Thanks for laying out your approach.  I think it sounds quite sensible.  A few more questions:

 

You say that we do not want the high glassy matrix of porcelain in a stoneware body.  Why is that?  I can understand that as an aesthetic statement, but not as a technical one. I would venture to say that your own decision to start testing with a much higher alkali level than what you are seeing out there already suggests that you think more glass is needed.  Or more melt.  Or more something.  Not sure where the stopping point is on this before you wander into porcelain land, or if in fact there is one at all, and I think you agree.  But I like where you are going with it.  Body maturity is certainly a central objective.

 

You say you are surprised that silica is so high in your collection of stoneware recipes, but I think you would find that they actually end up with a lot of glass in them.   In fact not sure it can be avoided, since the fluxes start fluxing the free silica at around 800C, and then they start fluxing the silica produced by the breakdown of kaolinite not long after.  The mullite you are looking for only starts developing once the melt is already widespread around 1000C and up.  And all this melted silica turns into a glass which welds everything else which is not melted down already.  

 

So, in stoneware land, body maturity = vitrification = melted silica = glassy matrix.  And while I like your ex-ante rule of thumb for gauging flux levels against silica levels, the only convincing way to test body maturity ex-post has to be looking at porosity and similar objective tests.  Or?

 

Also, you quote an "industry standard" of 2.52% alkali.  Are you talking about the commercial ceramic industry?  Just wondering where that number comes from.

 

Finally, I think it would be worth clarifying the difference between

 

1) "spinel" which is the Mg flavour of a much larger class of minerals called the "spinel group" which includes things like zinc, iron, etc.. and I think is what you have been referring to,

 

and 

 

2) the spinel phase (which I think is what you really mean?), which happens around 925C or 950C when metakaolin is converted to aluminium-silicon spinel (also referred to as gamma alumina).  Not that this spinel phase does not involve magnesium or anything else - just alumina and silica, and heat.  Which is why bodies which don't have any magnesium present can still produce mullite.  They just need some kind of flux to get the silica melt going.  Which flux or group of fluxes you use to make this silica melt happen is up to you.  And yes, along the way there are probably some minor kinds of spinel produced when these fluxes (whichever they are) combine with the silica and alumina, but they are just passengers on the alumina silica train, not the main game.

Share this post


Link to post
Share on other sites

 

Well, I care about MgO in my stoneware, so there is at least one of us out there.  Mostly due to its flux potential (and maybe its color).  I don't think there is any doubt that vitrification is about much more than just the feldspars.  

 

Apparently, some other folks also think MgO is important.  Here is the chemistry for Limoge (molar) described as a "white, translucent porcelain." 

 

SiO2           73.3

Al2O3         19.55 

K2O            2.2

Na2O          1.65

Mg               1.38

CaO             1.43

Fe2O3         0.24

TiO2             0.14

BaO              0.08

 

These are not insignificant amounts, so I am not sure we can dismiss MgO (or CaO) from the list of porcelain fluxes just yet, either.  

 

I guess I am resisting drawing a hard line between porcelain and stoneware, since they both use the same fluxes, just in differing amounts.  And I don't think there is anything to say that stoneware cannot have fine particle sizes.  Put another way, isn't stoneware really just "dirty porcelain"?  Or is porcelain really just "ultraclean stoneware"?  :P

 

My comment was in regards to stoneware. Yes, people are much more aware of MgO amounts in their porcelain, because everything is more significant in porcelain. I don't think it's accurate to say that porcelain and stoneware are just dirty or clean versions. They are both clay bodies, yes, but they differ significantly in makeup. Otherwise we could just use the same limits for both.

 

I did no mean to imply that MgO isn't important in the formula, just that people don't generally consider it important because it's such a small amount. That's why what Nerd is doing here is so interesting. If he can show that it makes a significant difference in the vitrification of the body, that can change people's attitudes. The big question is whether it will be significant enough to matter.

 

Neil,

 

I don't think MgO is any less important in stoneware than it is in porcelain.  My point in posting the Limoge chemistry was to suggest that we dismiss the notion that somehow porcelain only uses potassium and sodium and stoneware uses those fluxes as well as others.  What fluxes are used in each of these bodies is pretty much wide open.  And although Nerd may prove me wrong, I believe we CAN just use the same limits for both bodies if we want but of course that has implications for the look, working properties, etc. of the body.

 

"Dirty porcelain" and "clean stoneware" are just euphemisms to challenge conventional thinking about these two types of bodies, and underscore the idea that porcelain and stoneware are - chemically - very close, and in fact depend pretty much on the same materials and mechanisms to get to their destination. We may select or deselect certain fluxes to pursue specific attributes like whiteness or translucency, but these are just choices.

 

That MgO makes a significant difference to body vitrification I would have thought was already a matter of general knowledge (see for instance Hamer and Hamer under Magnesium Oxide), but maybe I am wrong.  My guess is that more likely people are depending on derivations of the same handed-down main-stream recipes, and have not bothered to look that closely at their technical properties, assuming that they must be "OK".  If that is the case then Nerd can hopefully change some of this mentality.

Share this post


Link to post
Share on other sites

Good Study

 

"It is well known that MgO accelerates the densification process in alumina by modifying the lattice diffusion. Similarly, MgO enhances the densification of mullite. Diffusion of MgO and SiO2 from mullite grains favours the formation of a liquid phase along the grain boundary and thereby densification takes place. Thus, the addition of MgO accelerates the mullite grain growth and reduces the pore volume. This is the reason for the improved density and strength."
 
This is from a high fire study of the effects of Mgo on mullite production. However, I think it is in part applicable to lower temps. c6-10
 
NERD

Share this post


Link to post
Share on other sites

Nerd, I seem to recognize the picture you put above from some other thread, but I forget what it shows.  Seems like an overfluxed body?  No doubt your tests will stop short of this level of flux.  I would argue there is still plenty of glass in stoneware even if the surface looks much drier than in your picture, for all the reasons I discussed above.

 

But I am interested in how you will measure the amount of "glassiness" in your test bodies?    Visual assessment is a good rough starting point, but - being literally in the eye of the beholder - is fraught with difficulty as a consistent measurement technique. 

 

Regarding mullite, I suggest you take a look at a paper entitled "Mullite" by Duval, Risbud and Shackelford. The full pdf is available online if you google the words mullite duval.  I would link it here but I have tried several times and cant seem to do it.  It gives some very good background on the ceramic process and an overview of mullite formation, temperatures, phases, etc..  I found the XRD figures particularly compelling.  

 

No problem if you have a special thing for MgO.  I like it, too, and there is plenty of evidence laying around to say it is a powerful flux in both bodies and glazes.  But mullite formation can be enabled/assisted by many types of fluxes - not just MgO.  Each flux or flux group has its own personality and impact on the outcome.  Mullite can even form on its own from just alumina and silica if you heat them high enough (binary eutectic at around 1570C). 

Share this post


Link to post
Share on other sites

I don't possess any recipes that specifically mention adding molochite, but discussions did come up about adding it to recipes for mechanical purposes, eg adding some to a smoother stoneware to improve its "tooth" without shredding your hands. For optimum mechanical results, you needed to also add fine grog as well, for particle distribution size. No one was considering it for contribution to melt at the time.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×