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Callie:

It has taken me several days to transfer the molar % of these recipes into a spread sheet. In doing that, I can follow FE, Mgo, or TiO2 down a single column: which always paints a picture. My conclusion? To date: stoneware is primarily formulated around Fe levels, and then MgO for gray in oxidation. TiO2 is not a player from what I can tell: in order to get Fe levels: you automatically get corresponding TiO2 levels.

 

John:

Four years ago I did an extensive interior remodel for a professor at SIU-E. He is a physics professor who use to work at the JPL lab developing laser guidance and tracking systems. It just so happens his wife has a wheel and a kiln-- I need say no more.

 

Curt:

Hold onto your chair: decided to make a hard left turn in my testing protocol. We both agree PSD is the primary target in stoneware bodies, and therefore formulation. After staring, and thinking, staring, and thinking, and more staring at the picture above: it occurred to me that most of the smaller meshes have melted into the body: therefore not distinguishable. So my initial series of tests will simply be 20-30-40-50 % additions of the three primary fire clays used. I will fill in the blanks with Om4, EPK, silica, and flux. I want to see how the large particles stack in the body as the % of each increase. However, I am only going to fire to 2100F and shut it down. I do not want a melt, I only want it to fuse. I should be able to see more clearly how the particles are stacking inside the body.

 

For instance: in a test bar with 50% Hawthorne FC (35m), if this large particle is resting against each other, clumping together, or over lapping each other: then it would be a safe assumption that the body would be weaker, and absorption would be higher. An absorption test would confirm that. So out of the gate, I only want to see what % of large particle size works best: setting a maximum % for that particle. Then I will step down to the 100-120 mesh size, and set that level,  then 200...etc. Keep working my way up the mesh size. I still think molochite or an extra fine ball clay needs to come into play at some point to fill in the micron particle sizes.

 

Something else I have noticed: stoneware relies almost exclusively on PSD for thermal values: which I will test that down the road as well. Apparently grog is being relied on for added thermal shock capabilities: which I have to question. The whole idea of adding 50m grog is to open up the body strikes me as moot: seeing as though the body already has 30-35m clay particles as it stands. It would play more into throwing and drying, than in thermal values.The argument would be made that it is adding mullite, which is again moot: because the primary function of a stoneware body is mullite. The one addition I have not seen in stoneware is pyrophyllte: I wonder why this has not been addressed before?

 

More questions than answers at this point: but a required step in testing.

 

Nerd

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And I know I sent you a recipe with Pyrophyllite! It is used in some circles.

Edit: If you use too much of it, you run into subsequent gaze fit problems because of the COE. Which, if you're an individual potter with a specific set of glaze recipes you work with doesn't have to be an issue, but in teaching situations or glaze shops with 15 glazes, it's a definite issue.

 

The discussions around grog in clay body formation tended not to be in terms of what the grog added to the melt (except in soda/salt firing), but its primary use was for workability in a clay body. More temper means the clay is more structurally sound while being made into large pots. It won't slump or warp so readily while drying or being formed.

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The discussions around grog in clay body formation tended not to be in terms of what the grog added to the melt (except in soda/salt firing), but its primary use was for workability in a clay body. More temper means the clay is more structurally sound while being made into large pots. It won't slump or warp so readily while drying or being formed.

 

This is a good example of what I meant in my previous comments. Stoneware clays are all over the board in terms of particle size and random additions intended to increase workability. I'm very interested to see the affects of these materials.

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The one addition I have not seen in stoneware is pyrophyllte: I wonder why this has not been addressed before?

 

Now that you mention it............. I have a body I am working on that contains pyrophyllte.  Dealing with forming issues... Not firing issues.  Sort of "proprietary" for me at the moment....... so won't say more here.  And it is not resolved to the point I want it anyway.  But yes..... you don't see it in clay bodies much if at all.

 

best,

 

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

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Callie:

The one recipe you are referring to is actually a porcelain body: no fire clays: all kaolin. So I did not include it in my tables.

 

Neil:

I have figured out a pattern in stoneware formulation: because 26 of the 30 recipe follow this pattern:

80% clay/s, 10% silica, and 10% flux.  So it is a simple 80 / 10 / 10 = 100.

 

The choice of clay/s is mostly based on iron content. However, several recipes have nearly 60% large particle distribution: which I highly doubt is remotely vitrified. The other issue , is using the "100" basis carried over from glaze formulation. You will never get a clay body to molecular unity: will not happen.

 

The minimum seems to be 4 clay types, and up to six clay types--- is also a paatern

 

Nerd

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Nerd, a couple of things:

 

Most mainstream stoneware bodies have a range of clays in them, and a range of particle sizes attaching to those clays.  This means that the PSD is spread out.   See Zamek on the "perfect" stoneware clay body in Ceramic Industry if you haven't already.  He discusses particle sizes, and the role of fireclays in the body in that article:

 

http://www.ceramicindustry.com/articles/87496-ppp-stoneware-clay-body-formulas-part-2-the-perfect-body

 

This makes sense, because as you seem to touch on above, smaller particles fit in the voids between larger particles.  If this is studied scientifically by replacing particles with metal balls, you get the McGeary article I referenced above.  Their punch line is this: 

 

With ONE size of round ball, the maximum packing density you can get in a cylinder is about 62%. (ie, 62% of the volume of their cylindrical container is metal balls, with the other 38% being air around the balls).  For FOUR different sizes of round balls, the highest packing density you can get is about 95%, the other 5% being air.

 

I think you will find that over reliance on large mesh fireclays will simulate the McGeary result, ie, packing density is not as good as it could be, and neither is sintering, absorption, etc..  I have seen other research which suggests that the packing densities deteriorate somewhat if the particles are not round (ie, have high aspect ratios), but the underlying principle about the RANGE of particle sizes still holds, ie, wider distribution of particle sizes is best.   

 

And it seems that a good range of particle sizes improves vitrification.  See Rahaman in "Ceramic Sintering", page 311.  Should be available on Google books I think.

 

Bottom line is you will not get any kind of realistic information on vitrification, body maturity or anything else by testing ONE size of clay particle at a time.  That is not how clay bodies work I think, or at least not any of the recipes I have seen. You need to start with a decent PSD and THEN start switching materials in different clay classes (eg, try out different fireclays to see their effect). 

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The same material can have both a MECHANICAL as well as CHEMICAL role.   Perhaps grog - as a pre-fired material - is more compatible to clay bodies than silica.  But Nerd, I agree that if you have large silica particles giving the mechanical tooth, why add grog?  However, how much of a 30 mesh fireclay is silica grains vs clumps of 2 micron clay particles?  Not sure, but my own testing of native clays suggests there might just be lots of silica sand at that mesh level...

 

I haven't seen any references to people adding grog to try and increase mullite but maybe some do.  Although I am curious to what extent grog is getting involved in the melt - and therefore the chemistry - of the clay body.   Either way, I disagree that "the primary function of a stoneware body is mullite".  Thought we and the rest of the ceramic and scientific community had already agreed that all stoneware and porcelain bodies have BOTH glass and mullite?  Maybe not.  Not disagreeing that porcelain leans on the feldspars to push glass-making to the extreme warping limit, but stoneware has plenty of glass as well, not just mullite, as even a visual test should reveal.  I found looking at Sohngens article posted by Tim T above to be a good reminder of just how much glass there is in stoneware and what the consequences of that are.

 

Nerd, 80% clay in a stoneware body sounds like a lot of plastic material.    I seem to recall Ron Roy saying that 60% to 65% plastic material is more sensible.  Unless some of those clays (say, fireclays?) are not very plastic at all...

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I went through all of my stoneware clay body recipes, and only about half of them contain added silica. Pretty much all of them contain 5-15% feldspar. Many of these were intended for the wood or salt kiln, where added silica is not always desired, as it causes the clay to gloss over too much and inhibit flashing.

 

I think a lot of formulas with grog don't take into account the refractoriness of grog. If you look at commercial clay bodies, they sell a lot of smooth bodies and grogged versions, however they haven't made any adjustments to the base formula- they just add grog. In terra cotta bodies, adding grog will increase the max firing temp by 2 cones. I imagine a similar effect would be had on mid range and high fire bodies, too, although maybe not as much.

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Neil:

That input helps; had two that did not have silica additions: now I know why. Hope everyone realizes the numbers I post are median values: not actual per recipe values. Still have not pinpointed a specific protocol for formulation: besides the 80/10/10 protocol, still nothing specific.80/10/10 does explain the consistent 2.50-2.75 KnaO levels.

 

Here is a question: does anyone know what this blend looks like?

15% 35 mesh, 10% 60 mesh, 20% 120 mesh, 40% 200 mesh 15% 325 mesh

 

I know you can conceptualize what it looks like, but have you seen it with your eyes? Reason I am doing particle size test starting out: I need to have a visual reference of what all those numbers mean. Curt, I have read Zamecks article a few months back. Most view porcelain as being tough to blend, I think stoneware is much more complicated because there does not appear to be any cohesive parameters.

 

Nerd

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Most view porcelain as being tough to blend, I think stoneware is much more complicated because there does not appear to be any cohesive parameters.

 

Nerd

 

Bingo. Porcelain tends to be more consistent because there are specific parameters that define porcelain and therefore fewer options on how to make it. Stoneware is pretty much anything that's not porcelain or earthenware, which is an incredibly broad definition. The possibilities for formulas are infinite, and it's very often formulated specifically for how it's going to be fired, which is not usually the case for porcelain.  A porcelain will work in either reduction or oxidation, but a brown stoneware intended for oxidation may be much too high in iron for reduction. Porcelain is good for some things, but awful for other things. Stoneware can be formulated to work in any situation. As mentioned before, wood and salt bodies tend to be lower in silica to encourage flashing rather than glossiness. Sculpture bodies tend to be very groggy to allow for thicker and less consistent construction, whereas white stoneware throwing bodies are smoother than porcelain. It's like dog breeds- they're all dogs, but each one has its own appearance and function.

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The first series in the PSD (particle size distribution). Merely looking at how the particle sizes distribute in the body: I have my thoughts which I will leave out.

 

Hawthorne 35m   US40 13.9%   US70 22.9%    US200 23.0%   Pan: 40.2%   (Can have less than 2% US20)

 

10 35m

    10% recipe
 

30 35m

    30% recipe
 

50 35m

    50% recipe
 
One recipe I imported had 55% large particle clay in the recipe.
 

6StoneLR

   commercial stoneware body.
 
Nerd
 
 

 

 

 

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Glaze Nerd: Question:

 

For those who have working experience with Gold Art and Imco 400: which one did you find to be more plastic?

 

I have been working with Imco 400 the last week or so, and find it to be more plastic than Gold Art.

 

Nerd

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Nerd or Nerds  ;)

I hope to start getting into the right brain space to read through your clay threads but ever since moving out the studio my thinking about ceramics has also gone down hill. I still have a little ponder here and there but it's not the same without things to test or a studio to inspire me...

 

I think you are a bit out there with all the clay body research, very few people seem to bother. That includes me, have always avoided the clay side of things. I hope you can share a good 'body' of research  :lol: so I can start joining in when I find a space to work in again.

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Joel:

 

Hopefully you can get your kiln up and running shortly. I have had the "how does it work" syndrome since I was a kid (55years ago). Took my dad's transistor radio apart ( got my arse spanked), and then pulled all the tubes out of the black and white TV ( got it spanked again.) I find clay to be a rather fascinating topic. It is not all that dissimilar to glaze: actions and reactions. Just as you learn what combinations of feldspar, silica, and modifiers work the best in glaze: clay is the same principle. The pitfall with clay is the lack of information available. Been having to build my own database of particle sizes, analysis (current), and then through testing the properties of each.

 

Nerd

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The light at the door is always on for the prodigal son, Joel. ;-). Come back, we miss you! :-)

 

Meanwhile, Nerd, you are doing a great job keeping this thread alive all by yourself (although I am not sure that is how this whole thread thing is supposed to work). LOL

 

Completely agree it is amazing how similar clay and glazes are chemically. In fact it took me a long time and a lot of testing to get beyond that similarity. Now I am much more focused on particle sizes AS WELL AS chemistry.

 

Unfortunately my particle size testing capabilities are limited to a few standard mesh sizes and a lot of hand sieving and I am worried that is not cutting it. Thinking....

 

Anyway, while I have not posted much on this lately I have nevertheless been dedicating not insignificant time to the topic. Will hope to share some results in the not to distant future.

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Curt:

 

I suspect no one answered my question, because no one had ever tested the difference. I am suppose to be testing clay differences, but find myself already picking favorites.

 

Nerd

 

I think most of the population doesn't mix their own clay bodies, so they don't have anything to add. I fall into that category. It's been so long since I've touched Goldart or Imco 400 that I just don't remember. I do remember preferring the texture of Imco 400 to that of Goldart, but that doesn't necessarily have anything to do with plasticity. And when it comes to fireclays, they're all pretty darn plastic so I don't know that plasticity is really the issue when selecting one. I think it's more about texture, color, size of iron spots, and price. I used to use a fireclay, a Muddox something, that gave great big iron spots. Super nice.

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Exactly. We formulate for color and texture, test to see if it will hold up to the type of firing- gas, wood, salt, electric- the cone range- 5-8 in electric, 9-11 in gas and salt, 9-14 in wood- then finally test it for absorption. Chemistry shmemistry. Chemistry is for glazes.

 

High fire stoneware bodies are probably usually under-fluxed, because they need it to work up to cone 11 or even 12. So at 9 or 10 it's looser than it should be. 'High fire' is a really broad topic.

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As has been pointed out many times now, there are no established limits because most stoneware bodies out there are effectively decorative, not functional.

 

Decorative bodies can be any crazy combination of stuff as long as it doesn't melt down in the kiln and kind of holds together after firing.

 

Vitrification? "What's it ever done for me? My stuff sells perfectly well after I paint it chartreuse green."

PSDs? Who cares!

Chemistry? Waste of time.

Crazed glazes? "All glazes craze." (Lol I love that one)

Porosity? "Huh?"

Chemical resistance? "What's that mean?"

Cutlery Marking? "What is cutlery? You mean that plastic stuff I get with my carry-out?"

 

I mean seriously, almost makes me laugh how many beginner potters I have met who tell me things like, "oh, I made this stoneware mug, but for some reason it leaks!" lol

 

Yes, funny if it weren't so sad. Clay vendors appear to have figured out that most of their clients know so little about the clay they are buying that they can make almost any outrageous claim on the bag and get away with it. I saw recently a bag that said "Terracotta - Earthenware - Stoneware". What does that even mean?

 

Why not just just use modelling clay when you slip cast that pair of giant sunglasses for your next exhibition?

 

OK rant over.

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I got that out of the WOPL test hey?  Thinking next spring I might actually have a "theorem" on formulation. Got these ideas rolling around in my head, but there is nothing subjective out there to rationalize or compare to. Maybe I should email the WOPL test to Tony Hansen: nothing on his site either. It would be nice to be a trail blazer, only problem; there are no trails out here.

 

Nerd

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I got that out of the WOPL test hey?  Thinking next spring I might actually have a "theorem" on formulation. Got these ideas rolling around in my head, but there is nothing subjective out there to rationalize or compare to. Maybe I should email the WOPL test to Tony Hansen: nothing on his site either. It would be nice to be a trail blazer, only problem; there are no trails out here.

 

Nerd

 

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