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Flux Formula Limits For Porcelain

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Curt: went out to the studio and pulled up my test sheets

 

Bar 2 : MgO 0.89   FE 0.36  TiO2  0.74       Bar 3:  MgO 0.78   Fe: 0.42  TiO2  1.16

 

 

Peter: understand that assumption about V-gum and Macaloid: but in other threads I was asking about their use recently. My order for both came in two weeks ago: so none of the test bars have any. Doing tile, I do not want either, or very little: although all of those have between 2-4% L10 bentonite. On another note: I found an old test bar from two years ago that has 7% pearl ash, before I decided to switch to technical grades. It has the same problem as above: I did titrate it with 1/2 liter of vinegar, but never did try a glaze fire afterward.

 

1000Gram batch.............. later titrated with 1/2 liter vinegar

 

#6 Tile kaolin   30%

EPK                 12%

OM4                8%

FHC ball          8%

silica                25%

pearl ash        7%

talc                 7%

L10 bentonite  3%     (recipe %)

 

which produced this

 

Porc#8

 
That should give you something to work on.. You can tell by the warped, cupped mess why that formula hit the trash. I have four batches with either macaloid or V-gum in greenware, ready to be bisque. I will say this: I now know why macaloid is preferred in porcelain. 2% macaloid increased the drying time by nearly double. Apparently macaloid does not like to cut loose of water.
 
Nerd

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Hi Nerd, thanks for the explanation. Not sure I completely follow, but the information is there all the same.

 

Just noticed that bars 2 and 4 are also comparable in that they have slightly higher but identical levels of iron to each other. If they also have identical levels of Ti (?), then the impact of increased Mg is more evident.

 

Regarding the testing methodology, is there a reason you did not just add, say, talc to adjust the Mg for the tests (as opposed to using the clay to adjust the Mg indirectly)?

 

As a general comment, I would say that molar amounts of these magnitudes for Fe, Mg and Ti are massive in the context of porcelain. For example, iron at the levels of bars 7, 8 and 9 is pretty far from what I would call porcelain. Similarly titanium at more than .50% molar is pretty much in stoneware land. I understand why you have taken the levels of these contaminants to this magnitude for the test, but if porcelain = whiteness, then every .05 molar lower for these contaminants is going to have a significant impact on whiteness, never mind on the glazes, just on the clay itself.

 

To put a finer point on it, I would say .50 molar Fe would be an absolute upper limit to even qualify as a porcelain. A truly white porcelain would have Fe in the mid to low teens, or possibly lower. And since titanium is usually the hardest to eliminate even with clean, high-quality kaolin and silica, the main game in white porcelain is about getting iron down as low as possible. Hence a very precise knowledge and tight control over these three contaminants is key.

 

Looking at your sample bars, I start to wonder if it is the total COMBINED molar percentages of these three contaminants that we should really be concerned about, as opposed to treating them each separately. More specifically, thinking about possible interaction effects between them. Do you have any thoughts on this?

 

Regarding your comments on iron, yes I test a lot of native clays, most of them with lots of iron. And since I fire mostly in reduction at this point, I am well-acquainted with the power of iron as a flux in reduction. So far I have not experienced problems with iron in reduction or oxidation around the .75% molar in your example, but I take your point about over fluxing the body. In fact my native bodies, and many reference bodies I have seen target around 1% molar Fe and sometimes more. In my experience it takes a lot more than that to really cause problems, although I don't have any MOR tests to back this up. However, too much iron certainly makes the body weak and honeycombed, so it does not really work as a major flux anyway.

 

I like your simple test for Mg and Fe. However, I often have other trace contaminants showing up in my native bodies (eg, Mn and Zr, and even Cu and V!) so If I really want to know these days I have to send my samples to the lab... But I will run a few of the clean ones this way just to try it.

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Nerd, just saw your most recent posts after I sent my last one. Thanks for the general discussion, plenty to think about there.

 

The alumina levels you are talking about seem high to me. Most of the recipes I have -particularly for porcelain - have lower alumina than this, usually lower than 20%. Silica contributes to good melt and glass formation. Too little and I would worry that porosity would become an issue - no problem for decorative bodies but fatal for functional ones. The commercial bodies I have used seem to err on the side of warping (overfluxed?) in order to get good glass and translucency. However, they do not seem to bloat or dunt and even tolerate some over firing, so maybe I am missing something in this story. Or perhaps others have had a different experience?

 

Also, I thought glaze run was mainly about the viscosity of the glaze itself, rather than leaching from the body. Put another way, I feel confident that I could find glaze recipes that would not run no matter how much - or how little - alumina is in the body. No doubt the nature of the glaze/clay interface has an impact on glaze run, and maybe this is what you mean by leaching.

 

And yes, particle size is critical in my opinion. The smaller the particles are, the better things melt. With the generally refractory native materials I use, sieving the materials to a smaller mesh size produces better body characteristics, especially porosity (true for the fluxes also). Haven't gotten down to designing bodies for packing density, but that is coming

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Porcelain Standards:

 

What constitutes a porcelain body over a stoneware body? Is it clay/s, Si/Al ratios, contaminant levels, particle size/s, or all of the above?

 

The industry defines the old standard of 1/4ths 25% kaolin, 25% ball clay, 25% silica, and 25% flux as 50/50 porcelain: although at one time that was deemed the porcelain standard formula. It is now suggested that 50% kaolin, 25% silica, and 25% flux is the porcelain standard. However, many commercial porcelains are blended such as 40% kaolin, 10% ball clay, 25% silica, and 25% flux. So would this type of blending be defined as a hybrid? Some blending of ball clay adds a variety of benefit such as plasticity, water retention, larger particle sizes, and higher alumina ratios. What board, what group, or what study determined the definition of porcelain standards?

 

Nerd

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Nerd,

 

Seems like you have sorted out your molar vs weight issue. I get the same numbers you do for the EPK recipe above. I prefer to speak in molar terms these days, but each to his own. Will have to just keep it clear which measure we are referencing.

 

No problem with the nature of your testing and I understand how these tests evolve - namely, in iterations. Few of us wake up thinking, "Oh, I think I will drop all my other projects and spend the next several days designing and executing a comprehensive test of specific thing X." No, more likely, you start off doing a quick, simple test on something that has been bugging you, or that you are curious about. Then, after looking at those results and realising they don't really answer all the questions, you do another test smarter or more focused than the first, and go again, and so on. Pretty soon (read: weeks later) you have a whole collection of somewhat related tests that can kind of be pieced together to create some general picture, maybe. That is the nature of the beast. Or at least that has been my experience.

 

I would like to see the Ti levels on those bars if you have time to dig them up and post them. Would be very useful in interpreting what we can see, while we wait for you to carry out your new and improved Mg test .

 

Will try to post some reduction related results soon.

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The Clay boss weighs in :  (part of the email was deleted- personal conversation)

 

 

 

Hi Tom,


I do agree that counting all the fluxes brought by all materials
should be taken into account - including iron if reduction fired -
including a dirty cone 6 oxidation with iron in the clays.

Being careful if using iron as a flux in high fired which will result
in cristobalite production without enough KNaO.

Your comments about silica - I do not believe there is much difference
between silica labeled as 200 mesh and 325 anymore - we did the
testing  and there was only something like 10%
200 mesh in the 200 mesh labeled bad. There was something like 5% 200
mesh in the 325M bag - almost all the rest was micron sized. It's the
result of improved crushing at the mines.

You should test to see if that is correct. It would mean buying sieves.

In porcelain bodies it is not a problem (fine silica) because you have
way over 10% spar.

Peter Sohgen talkes about this - see

Peter Sohngen in Studio Potter Volume 28 #1.
Cristobalite: The Hump - New Data on Silica at Cone Ten

...deleted.....

Graded silica - like 200M with few fines is not a problem.
The more iron involved the more complicated the problem becomes.

 

"It's theresult of improved crushing at the mines."

 

I have also addressed this issue in a few posts over the past months. Rotary crushers, air flotation, magnet milling: to name a few. The clay industry has based most of its formulation on 50-325 mesh: which is rapidly going by the way side.

 

Nerd

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 What board, what group, or what study determined the definition of porcelain standards?

 

A book you'll like is "The Arcanum".  ISBN-13: 978-0446674843  Historical fiction.... but you'll understand why I list this here when you see what it is about.  Steve Branfman's Potters Shop bookstore has it.

 

In China the recipe for porcelain was always a simple two part-er.  A kaolin and p'tunze, a silicaceous feldspathic rock.  Different potteries used different kaolins and proportions.... but that is the "core" of the starting point for all this stuff.  Chinese traditional porcelain is quite non-plastic.  You'd probably love it for tiles ;) .  It must be wet blunged to make it anywhere near workable.  If thrown... it is thrown thick and then trimmed all over (inside and out) to make it thin.  Usually DRY trimmed!!!!  With razor sharp tools.  I have some images from my travels in China....can send you a couple if you'd like. 

 

best,

 

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

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Please do John ......     kao'ling     I believe was the term Marco Polo used when he bought back the first porcelain pieces from China. Peter Issley in his book " Macro Crystalline Glazes "  (A.C. Black, London) discusses the history of porcelain and p'tunze.

 

 

 

More technical reviews---

 

http://www.marrowstonepottery.com/Images/A%20Discussion%20of%20Flameware.pdf    (studying cristobalite at ^ 10)

 

https://studiopotter.org/pdfs/sp28_1_sohngen.pdf

 

Nerd

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Interesting question on what defines porcelain, almost a whole nother thread.

 

I don't think there is any absolute definition. My take is that porcelain is identifiable by readily observable properties, rather than some specific chemical or material specifications. Whiteness is probably the most important property, followed not far behind by fineness-of-surface, with translucency sometimes also important. These properties or qualities have traditionally been judged with objects in a functional context, but there is plenty of decorative art using porcelain, so form is a wild card.

 

These observable properties derive largely from the quality of the raw materials we employ, and the chemistry we create when we blend and fire them. Oh, and a bit of skill in using the material! Non-ceramicists may not know that they are observing these properties, but I believe they nonetheless recognise them, and can compare between various objects based on them.

 

However, I think the word porcelain is overused these days, and the line between porcelain and stoneware has become increasingly blurred. Many vendors now claim that their product is porcellanous, alluding to the properties mentioned above but often not really providing them. Dirty clay when fired may have a fine surface and be somewhat translucent, and ring nicely when you tink it with your fingernail, but if it is not white it is not porcelain IMO. Even some clays that outright claim to be porcelain don't really look convincingly like it when fired.

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Curt:

Something occurred to me over the weekend about test bars 2 & 3. According to the contaminant levels: it is a real oddity that the grayish color is that pronounced. So I think I will fire those clay raw, and see what color/s they produce. My gut is telling me that one of the clay/s has much higher levels of impurities than the programmed values in the glaze calculator. Beginning to wonder if there is another "custer spar" type problem emerging in the clay biz.

 

Nerd

 

...TiO2 values have been posted, except one.

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2nd test verifying Mgo effects in porcelain.   ( 3rd extensive test end of next week)

 

Mgo Test 2

 

Bar 12 Mgo 0.23   Fe 0.32  TiO2  0.71      Weight % / not molar

Bar 14 Mgo 0.21   Fe 0.35  TiO2  0.73     

 

At this point Curt: I think your observation of all three interacting to produce the gray is correct. However, end of next week I will have more definitive results. Using NZ kaolin as the base line with Fe: 0.15 and TiO2 0.03: then with incremental % of talc additions, then finally straight TiO2 additions. (Fe excluded from test in this round)

 

Nerd

 

Edit*** a dirty bag of clay is not off the list either.

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Nerd, thanks for putting up the Ti values.  Had a look at them earlier but was too tired to keep all the changing factors straight in my head.  Will look at them agin, though.  Meanwhile have been trawling through my own library of tests to see if there is anything of that can contribute...

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Hi Nerd,

 

I have stared at your Mg test bars for a while, particularly in light of the new Ti info you added.   I have assumed a Ti value of .50 for the missing value on bar 5, for no good reason other than it seemed about right just looking.

 

It was too hard to skip around visually in the photo you provided, so I have taken the liberty of printing off a color laser image of your original photo and the cutting it up in strips so that I could play around with your test bars and put them in orders to ask questions I wanted to answer.  I may be asking too much of these samples, and possibly too much of your photo, but I felt these bars had more information to yield up and I wanted to extract it.

 

This first image is a picture of the bars arranged in order of increasing Mg content from left to right.

 

nerd magnesium test tiles in order of ascending magnesium

 
There is no obvious visual progression to me, and I think that is because the iron content is jumping around, so.... 
 
the next image is the bars arranged in order of increasing iron content.  There are three groupings generally based on low, medium and high iron content, as I thought this was more meaningful.  This groups bars 7, 8 and 9 in a visually satisfying way, and leaves us to contemplate the lower iron bars. 
 

nerd porcelain test tiles grouped by iron content.

 
I think bar 1 is getting short shrift in the whiteness stakes due to the uneven lighting in your original photo.  You can tell this by looking at the part of the photo which is NOT the bar - in a well lighted photo every bar should have the same color backdrop, clearly not the case here.  And given that bar has the lowest iron and overall combined contaminant level of any save the control tile, we need to adjust for that.  So Bar 1 gets a special whiteness waiver in my view.
 
Also, is it just me or do bars 1 and 4 have some kind of flashing effect going on around their edges. It almost looks like lithium or spodumene flashing to me.  Or possibly boron.  Am I just imagining that?
 
In the next image I have simply ordered the bars from lightest to darkest as my eyes saw them.   Then I looked back at the contaminant breakdowns for each bar in this order to try and see any progression. 
 

nerd test tiles ordered from lightest to darkest without regard to level of contaminants

 
For reasons just mentioned, bar 1 is probably not in the right place.  Beyond that, the order seems to correlate to a general increase in total contaminant levels, although it could well be just increases in iron, as both measures move up roughly together.  Not sure. 
 
The bars which seem to buck this order a little are 2 and 3, but again this could just be the lighting on 2, which is noticeably darker than 3.  But, bar 3's overall contaminant level is high really mainly due to Titanium, which I believe to be the weakest of these three contaminants. Hence these two are easily switched. 
 
Finally, I grouped the bars by the total combined level of the three contaminants (Fe, Mg and Ti).  The gaps between bars correspond to significant jumps in this total number.    For instance, the control tile has a combined total contaminant level of 0.73, while bars 4, 5 and 6 are roughly double this amount at 1.50.  Bar 9, at the high end, has a level of 5.50 (but did not make it into this photo for some reason!)
 
 

nerd test tiles in order of increasing TOTAL contaminants

 
Bars 2 and 3 probably the interesting cases here, because although they have relatively high contaminant levels, they still look white.  However, let it be said that iron is relatively low, and most of the contamination is due to high Mg and high Ti.  So maybe these two contaminants are not so bad together, even in relatively high amounts?
 
Conclusions?:  Organizing around iron makes visual sense to me.  Iron is far and away the dominant contaminant in these bars in my view.  More iron = more orange and brown, and I think this trumps magnesium gray every time.  However, the more I look at these and other samples the more I think I see the grey cast you are talking about.  Titanium, bringing up the rear, is really I think a whitener on its own, and a dirty-er, when combined with iron, so I would say its impact here is getting lost in the movement of the iron.
 
Happy for others to disagree with these interpretations, or indeed with my methodology! :lol:
 

 

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Like what you did with the arrangement per element; works better than what I did. I have this bad habit of "fonting" out loud and thinking people will follow my obsessive compulsion with clay. Bars 1 and 4 were actually fired upside down on a tile setter: so you are seeing rack lines.

 

Actually, like any testing: it is open to dissection. The whole point of posting my findings is to subject them to review and compare against the findings of others. Through a series of comparable tests, insights others add: a conclusion will be drawn. So I, like you welcome the scrutiny, questioning, and dissection: it will lead to the truth ultimately.

 

Nerd

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So Nerd is this a cross section of each tile? And have you ground it back to get a smooth surface? Looks that way, but could you please confirm.

 

I may not need 1000 words, but 50 or so would go a long way. Can you give a brief explanation of what you see in those pictures? Are the arrows pointing to flaws? And what is "IDP"?

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OK, a few thoughts:

 

I am troubled (in a good way) by your theory about potash and sodium volatilizing and building up pressure in the body.  More specifically, I am troubled by the idea that this is happening starting at around cone 5 and all the way through to cone 10.  The main concern is that these are exactly the alkalis are doing the heavy lifting in terms of getting the body to vitrify.  How can they be in situ to help vitrification if there are already gone (off-gassed)?  It is possible that, rather than disappear, they are getting involved in eutectic compositions with other clay body materials and hanging around for the whole vitrification process?  I can see from Mahavir's material description that it melts at about 1220C, and its LOI is less than 1%.  We also know that Potash Feldspar can be a glaze on its own.  Neph Sy is similar in terms of low LOI.  So it is not like half these materials are decomposing, volatilizing and escaping up the flue.

 

Anyway, not sure how to rectify your observations with this. I can see in the back of Hamer and Hamer, for example, that K2O and Na2O "volatilize" at around 1200 and 1100 respectively.  Perhaps this refers to the pure oxides (pearl ash) if we just put them in a bowl on their own and fired them to say, cone 10, maybe they would be gone when we opened the kiln up.  Not sure (nor am I likely to try and find out).

 

Another point regards the "migration" of flux.  I think you would have to get clay bodies alot hotter than we normally fire, and let them sit there at that high temperature for far longer than we normally fire (ie, 10's of hours not a 30 minute soak) to get any meaningful migration of flux materials.  I am not saying there is no movement - features like the glaze-clay interface at cone 10 show that there is some limited movement of constituents, but this is more localized intermingling rather than migration per se.  Certainly materials are not moving from one side of the clay wall to the other.  The simple viscosity of the clay body - even at these high temps - means that it is very difficult for anything to move far (more than a few millimetres?) inside it during the firing.

 

In your first picture above, I see what your arrows are pointing at, but I think you have to be splitting the clay body apart (delaminating it?) into thin sheets and then testing the various layers very carefully for porosity to know for sure that vitrification was different from the bottom to the top.   However, I can still come to terms with you observations, but by way of a different mechanism.  I believe what you are seeing in your three pictures above is simply that, while it is true that higher flux levels are giving different melting outcomes in each of the three tiles, flux is not migrating from the bottom to the top.  Rather, the heatwork is greater for longer where the tile makes contact with the thermal mass of the kiln shelf, and this means that the melt is more complete at the bottom of the tile (which is touching the kiln shelf) than at the top of the tile.

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Curt:

All of your thoughts on the vitrification process are valid. I have the same ones: except I having problems explaining certain points. I have read articles about silica melt, phase changes ( liquids to gas, or solids to gas) and have not found anything within the range we fire. They talk about pressures inside magma chambers, or other intense pressure/heat situations. I know the gas moves, and I know the fluid builds: but how much, and how much volatilizes I cannot pinpoint. Most graphs show 20% loss: but again under extreme pressure and heat.

I have been making cylinders from 3/8 up to 1 inch round by 5" long. I prefer them over bars to check flux flow and melt. I see the same occurrence in these cylinders as I do in the bars. I trust bars to a certain extent: but I also believe the external part of the bar is subjected differently. I can snap these cylinders to view what is happening inside. I have cut them elongated, but the blade seems to burnish the edges: and distorts the view.

I agree with the thermal mass: the shelf contact super heating the bottom. Yet these bars are only 1/4 to 3/8" thick: there should not be that much difference between the top and bottom: but there is. The other issue is the porosity, voids, and fissures that form almost across the board in under-fluxed samples. If the KnaO was equally distributed, I am not sure that amount of variation would be present.

My thought at this point is to make 6-8 bars out of the same batch with the same molar levels. In this case stay at 4.00% molar range, repeating my standard firing cycle to cone 6 with four bars.  Keep two as benchmarks: using the other two, with two new bars and fire again to cone 6, but with a 2 hour hold. Note difference, and do a final firing up to a soft 7 with a hold: and note vitrification differences. I cannot think of another way to pinpoint rather it is lack of heat, or movement of flux.

The other puzzling issue is even with 13% more molar flux levels: there is less traces of gas than lower levels. The only explanation I can come up with, is the extra pressure that would build up in the wall cavity. Perhaps grasping at straws, but I just cannot come up with a conclusive answer for that, other than movement. I can see better with a usb microscope: but it would take x-ray diffraction, or other super hi tech gadgets to really view it.

 

Nerd

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I have tried to share parts of this documentary before and failed. Let's see.

 

At 10:30 there is a interesting animation firing porcelain on the inside, lots of liquid phase. I agree that you could be putting too much weight into gassy KNaO. Surely most of it is staying in the clay body or glaze? 

 

Good documentary.

 

 

http://player.youku.com/player.php/sid/XMzkxNTQ2MDcy/v.swf

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Actually the answer is all of the above ( I think). It looks like I have inadvertently stumbled across a better cone 6 firing cycle. I ramp @ 400F up to 2100F in my glaze fire. Then I slow down to 120/125F an hour to 2225-2230F, with 10 min. hold. From 2012-2180 is the critical range for mullite formation, feldspar melt (which would include the ejection of Co2, and the formation of oxides), and the glassy phase (matrix). By stopping a high climb at 2100, I am hitting right at the peak of all three functions required for vitrification. However, this is also when off gassing isat its peak: and because the glassy matrix is not fully formed: this allows the Co2 to pass through the body more quickly. Once you climb into the 2170-2180 range: then the pores of the glassy matrix close: trapping Co2, instead of allowing it to pass.

It takes 40 minutes at this schedule to climb from 2100F to 2180F: which is the peak range to vitrify a cone 6 body. Using the firing cycle of hitting 2170-2180, with long holds to produce c6 results may be counter-productive: because you are ramping past the critical temps required for clay vitrification.

 

Edited: additional info.

 

Critical Temp: 1200C (2192F)  Feldspar has completely melted and is no longer detectable on X-Ray diffraction.

                       1100-1200C is when rapid mullite production occurs and the glass phase begins.

 

My firing schedule lands in the middle of these critical temps: so the mason/dixon line I keep seeing (could be) the glassy matrix developing during the time in this critical period. So I am going to ramp to 2050F, and climb 120F to 2230F to see if full development occurs.

 

There is however a loss of flux weight due to off-gassing. I have not been able to pinpoint that amount: information is too vague. There is also a conversion to an oxide, creating a liquid melt. Still wrapping my head around that one. However, that still comes back to molar levels of KnaO: enough has to be present to convert to oxides: which in turn creates the liquid melt. Norm on another forum is wrestling up information for me on that issue. I did however read some informative articles of how FeO effects the melt in stoneware/reduction or hi fire ox. From the initial reading of it, FeO melts alumina, which reacts with silica: and creates mullite. So mullite seems to be the key in stoneware for vitrification.. Guess I will be reading a lot in the next few months, wrapping my head around that one. From reading" the glassy matrix is the key in porcelain for vitrification, and mullite in stoneware. Which explains why stoneware bodies have more alumina, and less silica. I see another learning curve in my near future.

 

Nerd

 

Joel: I have watched that one before when you posted it.

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The most comprehensive and complete study on feldspars/potash that I have read. Explains reactions, off gassing, liquid phase, reactions to silica and kaolin..etc. I read this about 3 years ago, and forgot to book mark it. From Marcia Selsors old university -stomping grounds.

 

- reason I went with potassium, instead of sodium as a body flux.  In their study, potassium interacted with silica; creating translucency.

 

http://r.search.yahoo.com/_ylt=A0LEVry5SeNXBdUAPEYnnIlQ;_ylu=X3oDMTEyaDNrMXJrBGNvbG8DYmYxBHBvcwM5BHZ0aWQDQjI0MjdfMQRzZWMDc3I-/RV=2/RE=1474542137/RO=10/RU=https%3a%2f%2fwww.ideals.illinois.edu%2fbitstream%2fhandle%2f2142%2f4264%2fengineeringexperv00000i00422.pdf%3fsequence%3d3/RK=0/RS=affRis4ucX7yWlLV2wT9EcoOzBw-

 

Nerd

 

Link revised: first link would not open.

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Kind of amazing that a 70 year old paper is still hot reading on these topics today...  Not sure how to feel about that...

 

edit: .... but just having dipped into big chunks of it, have to say there is some pretty good stuff there.

 

Most interesting tidbit?  They found out by firing several of their test bodies in an X-Ray furnace (huh? wow)  that Cristobalite forms during THE COOLING CYCLE between 1100C and 1000C.    And suddenly the cooling cycle in my firings takes on a whole new level of importance....

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