Stoneware Limit Study

[Fred Sweet]

Nerd,

Is that OH4 and OH23 or OM4 and OM23? Just am not familiar with either of the first two ball clays. Would you confirm/clarify, please?

Fred

[Fred Sweet]

Thanks, Nerd. I've heard of Old Hickory, Just never looked at their numbering system since I've never used them.

Fred

[Callie Beller Diesel]

Might it not be a good idea to create a separate thread that summarizes these thoughts, if it's gong to be pinned and linked to? 7 pages of us all waxing nerdy is perhaps a bit of a chewy read to some initiates, and there's a bit of rambling to weed through. Im good at summary writing, if you want.

[Guest JBaymore]

John (Moderator)

 

Question:

 

Tony Hansen emailed me yesterday wanting to include the SAS Formula on his website. It is also coming out in Clay Monthly shortly: so I am bouncing this idea off of Julie as well. Tony wants a link, as does Julie to a place where all the formulation criteria is listed and explained. If I create a new thread, with several  lengthy posts explaining all the parameters and formulation criteria: can it be pinned?

 

Nerd

 

by the way: was just going to link this thread, to the pinned thread. For those with questions.

 

 

Looking into this idea.  Watch your PMs.

 

BTW........... Did you mean "Ceramics Monthly"  where you wrote "Clay Monthly"?

 

best,

 

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

[glazenerd]

Callie: it would be a new thread. I will keep you offer in mind. Been reworking the formula: I now have it where it gives particle distribution in numerical terms, rates density packing, projected green strength and gives a projected plasticity value: before you even mix it.

 

Sorry John..Ceramics Monthly...too many thoughts rolling around upstairs. Been working on conversion tables.

 

Nerd

[Guest JBaymore]

 

One of the assumptions about stoneware clay, was that all the clay particles melted to form a vitreous body.

 

 

Actually..... no.  I don't think that ever was the assumption by folks that are "into" this stuff.

 

Stoneware clay is more like fiberglass cloth with fiberglass resin.  The crystalline materials (cloth) are bound together with the glassy phase (resin). 

 

The fact that it is NOT fully vitreous (which would basically be glass) is the reason that no one has ever been able to develop a computer model for predicting the performance of a clay body for such stuff as CTE.

 

best,

 

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

[glazenerd]

Looking for alternatives to add plasticity to a clay body?  hectorite. In the same class as bentonite. Adds nearly the same plasticity as macaloid, for less than half the price. Works well with white body porcelain.

 

Nerd

[High Bridge Pottery]

I think part of the problem is calling ware 'vitrified' kind of implies that everything has melted into something noncrystalline when instead you have melted enough stuff to fill in the gaps.

[glazenerd]

Joel:

 

Formulation theory #2:  The Dart Theorem

 

Keep throwing crap at it until something sticks.

 

Nerd

[Fred Sweet]

Did I miss the announcement regarding the test!!!!😱

[glazenerd]

Well teacher: has the days of a pop quiz, come and gone?  No one needs to answer, just designed to make those interested think about it. It requires more fingers than you have: so you can also pluck some beard to keep count.

Nerd

[curt]

Here is a table from paper by Martin-Marquez et al. entitled "Mullite Development on firing in porcelain stoneware bodies"

 

The clay body in question was 50% kaolinitic clay, 40% feldspar and 10% quartz.  The weight % of different oxides is in the paper, but I note that alumina was 14.3% molar and silica was 79.3% molar.  Potassium was the main flux.  The body was sub-100 mesh.

 

What is interesting is the they have systemically identified the amount of mullite, quartz, feldspar and amorphous glass phase developed in the body at all the temperatures along the way we would care about.  

 

What I found most noteworthy was the mullite, which first appears at 1100C, and from 1230 on is pretty much stable.   At almost 15%, it is pretty clear that mullite (of all types) is a major feature of this rather standard porcelain body.   The SEM pictures in this paper are also excellent, because they identify the different phases visually at each temperature stop along the way.

 

Table II. Rietveld quantitative phase analysis results. including amorphous content for
porcelain stoneware body fired at different temperatures.
 
  Temperature (ºC)
                       1000       1100       1200       1230       1260       1300       1400
Mullite                -            3.6         12.2        14.4        14.5        14.0        13.7
Quartz             29.3        28.2        27.7        24.1        24.2        24.6        19.3
Feldspar          21.1        15.5         3.2  -  -  -  -
Amorph phase 44.2        52.8        56.9        61.5        61.2        61.4        67.0
Others               5.3  -  -  -  -  -  -
 

[Guest JBaymore]

Love the 1400 C (2552 F) data point... as the mullite crystals grown earlier on are starting to get melted back into the glassy phase and the crystalline silica is going rapidly into the melt as the glassy phase jumps up.

 

best,

 

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

[curt]

Love the 1400 C (2552 F) data point... as the mullite crystals grown earlier on are starting to get melted back into the glassy phase and the crystalline silica is going rapidly into the melt as the glassy phase jumps up.

 

best,

 

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

Yes, kind of gives some motivation why we would stop firing around 1300.   Still, kind of amazing that even at 1400 there is still a lot of mullite left in this porcelain.  

 

And even at 1400, 2/3 of the quartz present at 1000 is still there.  Tough stuff!

[curt]

Curt- great article... TY for sharing.

 

 

Cuboidal primary mullite crystals formed from pure kaolinite clay are surrounded by a highly viscous matrix.

Which I referred to as mullite platelets.. cubic crystal structures.

 

 

The growth of these crystals is easier, so they achieve a high aspect ratio (3-10:1). Finally, mullite needles formed from areas of  mixes of fine clay, feldspar and quartz show a very high aspect ratio (30-40:1) since they are surrounding by a more fluid liquid enriched in alkalis, which favours the fast growth of crystals

The usual mullite patterns found in porcelain.

 

This was the distinctions I was making: cubic or needle formed mullite. As your chart illustrates; they appear to be temperature sensitive, in conjunction with clay chemistry. Ron Roy and I got on a lengthy discussion at dinner one night over these differences. However slight it might be, it is a difference between a stoneware and porcelain body. Although, either body can and does produce them: the idea is to get stoneware to produce platelets: according to our discussions. In reading your chart: perhaps 1230C  (2246F) is the optimum firing target.

 

So in adding other articles to this equation:

 

Some suggest increasing calcium increases mullite production. same for magnesium.

Some suggest increasing alumina percentages will increase mullite production.

One article obtained 100% mullite by using nano-powders.   (way too expensive) but says something about meshes.

 

Give me another ten years of testing: I will figure it out.  Merry Christmas Curt.

 

Nerd

The main reason to post this table was to (hopefully) put to rest any idea that porcelain was distinct from stoneware because it did not contain mullite.  Clearly porcelain contains lots of mullite, just like stoneware. 

 

Although the table posted above appears to show nothing much happening between 1230 and 1300 (ie, Cone 6 to Cone 11), if you have looked at the paper you know that there is actually a great deal happening.  From 1230 on, the short, stubby, scaly Type 1 Mullite which is the first to appear is rapidly growing into Type 2 (long needles) of mullite, some of which will eventually turn into Type 3 (really long needles of mullite), some of which will join together and continue to lengthen into super mullite).

 

Nerd, I have not seen anything that says that small platelets of mullite (Type 1) are the best form of mullite.  In fact, the general thrust I have seen seems to be that longer mullite needles give a body more strength.

 

My take at this point is that alumina levels are the governing factor for mullite production in any given body, stoneware or porcelain.  Since some of the alumina being released from the deteriorating kaolinite is going to get sucked into the glassy melt process and turn into amorphous phase, there needs to be sufficient alumina left over to produce mullite.  Around 3 moles UMF alumina for each mole of flux (RO Unity setting in Insight) should provide enough for both purposes. 

[Guest JBaymore]

 In fact, the general thrust I have seen seems to be that longer mullite needles give a body more strength.

 

My take at this point is that alumina levels are the governing factor for mullite production in any given body, stoneware or porcelain.  Since some of the alumina being released from the deteriorating kaolinite is going to get sucked into the glassy melt process and turn into amorphous phase, there needs to be sufficient alumina left over to produce mullite.  Around 3 moles UMF alumina for each mole of flux (RO Unity setting in Insight) should provide enough for both purposes. 

 

 

 

This is where my understanding sits at the moment.

 

best,

 

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

[Guest JBaymore]

 

Yes, kind of gives some motivation why we would stop firing around 1300.   Still, kind of amazing that even at 1400 there is still a lot of mullite left in this porcelain.  

 

And even at 1400, 2/3 of the quartz present at 1000 is still there.  Tough stuff!

 

 

Quartz has a very high melting point.  Unless it is sub-micron particles...... the quartz crystals/grains look like boulders to the fluxing materials surrounding them.  Takes a while for the "water" to eat away at the boulders.

 

best,

 

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

[anchorman]

Glazenerd:

Do you have any more info re: calcium, magnesium, and iron and how these affect the overall melt?  And if one were to increase these, I would assume that the KNaO totals need to be decreased some amount?  I'm curious about calcium additions and how that affects green strength, but also how it does in the melt, but I'm not trying to reinvent the wheel here.

[anchorman]

I was curious because with my initial experiments that were on the low end of lacking in feldspar, I dropped a teacup from about waist height on the concrete floor 4-5 times and couldn't get it to break (I was curious what the interior of the body looked like after firing). I had to throw it at the floor in order to get it to finally break open. I wasn't sure it it was a fortunate shape, or if the body was stronger than normal. I'll be refining things a bit over the next month, and was considering minor additions of whiting or dolomite, and also how far I could push it with the redart before problems set in.

 

What are you using as a microscope? I'd be curious to get closer look at the particles in the fired body and see how it changes with different additions to it.

[glazenerd]

A cheap 200x5 USB microscope will work just fine: I use it most of the time. You can pick them up for under $25.

 

 

This is an SAS 23.12 stoneware fired to cone 6. It will give you some indication of what a mature stoneware body looks like. It has over 22.% molar alumina: you can see how much mullite was produced. Most of the 40m Hawthorne was also incorporated into the melt.

 

Nerd

[anchorman]

Modern technology is amazing. And so cheap!

[glazenerd]

I have been making trips to old brick mines across Missouri on and off all spring. This past week, I visited one that had been closed for over a decade. They made dark brown and nearly black bodied bricks: the reason it got my attention.

I collected nearly 600 lbs. directly from the adjacent clay pit. Going to fire this 100 gram test cylinder tomorrow. I suspect the iron level is above 10%, because I can see signs of oxidation on the sample already.  One level cup weighed over 400 grams, as a comparison OM4 weighs 253 grams per cup. This is a ball clay by the way, it is not fire clay.

nerd

[glazenerd]

Cone 6 firing to 2232F, no hold produced this:

 

 

50% raw clay, 25% silica! and 25% potash spar. One area has a coat of temmuko, then a raw area that self glazed, then a coat of white on the back that looks nasty. So there has to be a fair amount of spar in the clay: it self-glazed in the raw areas. Now I tend to think there is more than 12% iron! with a fair amount of titanium. At this point I am content enough to have a raw sample analyzed.

nerd

[terrim8]

looks like the old salt glazed field drainage things!

[Callie Beller Diesel]

It's a bit slumped, and I wonder if it's on the edge of bloating.