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glazenerd

Flux Formula Limits For Porcelain

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These kinds of papers are brain candy for a Nerd. Yes, have to temper a bit when a long period has passed: but the U of I paper is responsible for my quest of high quality, and micron particle sizes. It blows some holes in the sodium vs. potassium debate: only 10C +/- difference in melting temps. Only 10% free silica can lower the viscosity of a sodium melt, while potassium can handle double that before viscosity changes: that got my attention too. The glaring comments for me where; particle size effects melt/viscosity, and contaminant levels can alter viscosity.  .

 

With pure materials and equilibrium conditions the temperature of the eutectic formation would be 990 ± 20 deg C with a pure potash feldspar or 1050 ± 10 deg C with a pure soda feldspar, and the amount of melt formed would vary with the amount of feldspar present. In bodies which contain somewhat impure materials, the eutectic temperatures may be some-what lower.

 

There are some critical temps that I have not discussed yet: but I find them relevant to clay maturity.

 

Some technical info from the Brindley & Ougland researcg doc: "Quantitative Studies of High Temperature Reactions ofQuartz-Kaolin-Feldspar Mixtures." Trans. Brit. Ceramic society Soc. 61:599 (1962)

Cited and confirmed in Ceramic Science for the Potter  Lawrence & West 1982 (2nd Edition)

Also confirmed in the University of Illinois/ Campaign..Study above.

In direct relation to cone 6 firings- critical temps we need to be mindful of:

 

980C (1796F) Metakaolin changes to spinel with ejection of finely divided silica.

 

1050-1100C (1922-2012F) Spinel formed from metakaolin starts changing to mullite. The peak mullite conversion temps being 2012-2192F). Rapid decrease in feldspar and the appearance of the glassy phase.

 

1200C (2192F) All feldspar has melted and is no longer detected on X-ray diffraction. Porosity of the body decreases rapidly, and the pores of the body close tightly.

 

2282F-- cristabolite begins to form in high kaolin bodies. and 2192F for high ball clay bodies.

 

If you high ramp to 2180 F (just under 1200C): then you are blowing by the critical temps for body maturity. If you high ramp to 2050F, and slow ramp to 1200C: the clay develops maturity. I have also been wondering if the firing schedule is playing a role in COE issues, and pin holing because the above critical temps are blown by too quickly.

 

Nerd

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OK, just looked in Hamer and Hamer for the definitions of 'basic glass", "heavy glass" and "amorphous glass body". Can't find such definitions anywhere. I am guessing they are technical definitions from some other reference work?

 

If not, maybe good old fashioned absorption/porosity testing would provide a less subjective way to underpin conclusions.... although judging by your pictures I would not bother for the last sample, which I would describe more as "lumpy glass soup" (Technical definition from the book I will write one day)...

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Thanks for reporting the absorption results.    I find that information much more useful than your general descriptions of glassiness.  I am sure those terms have meaning to you, but they are not informative enough to me (and I am betting most others) for them to be helpful in my own investigations.  I have looked at enough sample bars myself to know that my eyes are not a reliable guide to objective test statistics.  Are you testing absorption with the "boil, soak and reweigh" method? 

 

The 3.90 molar flux level for the beginning of porosity is interesting as a kind of "line in the sand."  I am not sure true zero porosity is necessary (or even necessarily desirable) for porcelain, but I think most would generally agree that in porcelain land, less porosity is better. (?)   More to the point, pushing flux levels high enough to go past 0% porosity suggests a body which is perhaps overfired?

 

Applying your results to Cone 10, I tend to see the differences between porcelain and stoneware as more of "different points along a continuum" rather than a "hard boundary", I would be happy to have my porcelainous body have up to around, say,  2% porosity just like the general view of functional stoneware.  Did your tests reveal at what molar % porosity rose above 2% for porcelain?   Dont remember if you did porcelain flux tests at Cone 10 or not.   

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2180F with 30-45 minute hold..... commonly used cone 6 glaze fire.   I need to test this molar level further, was actually pursuing the holy grail of vitrified porcelain equals 0 absorption. In addition, was also searching for my own specific use: high translucency for a product line I have in mind. I never intended to fall this deep into the porcelain hole, but I did......onward.

Nerd

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Personally, I think you should test without any holds as a baseline. The most commonly used firing cycle does not have a hold time, nor do firings for people without digital kilns, which is still a very significant percentage of the population. I, for one, do not use a hold time. There's a lot of variability in firing cycles out there- 'Mastering Cone 6 Glazes' recommends 2190 with a 15 minute hold. Plus hold times will work with some glazes and not others. Just my 2 cents.

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Translucency The chemistry involved in producing translucent porcelain involves several parameters that work in conjunction to produce the final product. The very basic concept of translucency amounts to no more than producing a body that has a much higher glass content, with minimal amounts of colorants: in this case oxides. You simply need to think about “the melt,” how to achieve a body that melts completely; but yet has the mechanical structure to support itself. The basic recipe is silica, alumina, and KnaO: it is the proportions, heat, and particles size that determines how translucent the outcome.

The first is the amount of heat: you can achieve some degree as low as cone 3; but translucency as commonly defined by most potters usually begins at cone 6. As the temperature rises above this cone, then the corresponding chemistry changes because the additional heat is producing what flux levels produced at lower temps. The fundamental principles of particle sizes apply: higher heat equals lower mesh sizes, as does lower heat mean higher mesh sizes. This principle can be manipulated to some degree by compensating with higher flux levels, and using alternate minerals that increase glass content.

Si/AL ratios also become more important because higher glass content is desirable. Standard porcelain bodies average 68 to 74% molar and alumina average 15 to 19% molar. In applied terms: an Si/AL ratio of 4:1. To achieve translucency; the higher end of each is preferred. Again, some deficiencies in either can be overcome with increased molar levels of flux. Increased KnaO levels are imperative in this type of body, because increased glass content is what creates translucency. A typical cone six porcelain body runs between 3.50% molar on the low side, and up to 4.25% on the high side. (my preference) At 3.50% the body will have higher absorption, and at 4.25% it will be fully vitrified. At cone 10, these molar levels can be lowered by as much as ten percent. To achieve translucency; at cone six the flux level must increase by a minimum of ten percent. This would mean a molar level of 4.70% minimum, but can be increased to reach the desired level. It should be noted however, once you cross 5.25% molar level: the incidents of pinholing and blistering become problematic because of the off gassing of the flux.

Pure porcelain bodies are formulated from pure kaolin sources: without the addition of ball clay for plasticity. Most all kaolin fits within the 37% alumina, 45% silica, 12% LOI, with minimal amounts of KnaO, iron, magnesium and calcium. The iron and magnesium levels in most kaolin is so naturally low, these two are not a consideration when formulating this type of body. Titanium levels however can vary enough to cause problems in a translucent body. The molar level of titanium should not exceed 0.75%, but higher translucency occurs as this level drops. Titanium requires much higher temperatures to melt, and disperses a buff to cream color when it does: both parameters “cloud” the glass content thereby hindering translucency.

The cleanest kaolin currently available is New Zealand and Grolleg. While most recipes tend towards NZ kaolin, Grolleg has very low levels of iron, magnesium, and titanium. While the focus tends towards the kaolin; silica, fluxes, and plasticizers can add greater amounts of oxides than the kaolin. So it is just as important to choose clean ingredients across the entire spectrum. Plasticizers in this case become critical because typical bentonites have very high levels of some oxides. Macaloid is often considered, which is a premium grade of hectorite: but still has high oxide levels. V-gum T is the optimal choice; although it’s exact chemistry is a trade secret: it is processed and purified hectorite that removes much of the oxides.

The picture below was actually made from a combination of NZ and Grolleg kaolin. It is ¾” of an inch in diameter and the high glass content is obvious. In an earlier post, it was described as being over vitrified; but in reality it is highly translucent. Another secret is using Mica due to its high alumina and potassium content. If you research Mica; you will find it to be almost pure silica, alumina, and potassium with very low LOI values. In addition, Mica has a crystalline structure, with a refractive index conducive to translucency. (muscovite only.) Before glass windows, mica was used as window panes.

Copyright TJA 2017

gallery_73441_1250_4513.jpg

Edited by glazenerd
Usual grammatical blunders.

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Thanks. For all the information. Although I don't understand some of it. .. I have been working on a porcelain body for about 6 years now. It is cone 9 maybe 10. I have to do alot more testing still . it's been a long road, but a great drive. Anyway this is what I've been able to get

1005171954-1.jpg

glazenerd likes this

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Pat yourself on the back, nice piece. At the end of the day all the techno-blather does not matter; only matters if you are content with your work. Throwing/forming very thin is the tough part; and the part I struggle with the most. I might know the chemistry, but still cannot throw my way out of a paper bag. At cone 9-10, keep the alumina on the high end for structural strength.

nerd

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1 hour ago, glazenerd said:

Throwing/forming very thin is the tough part; and the part I struggle with the most.

Remember, that most of the 'famous' Chinese porcelains were thrown very thickly....and then trimmed at the dry state (yeah.... dust city) both inside and out with sharp metal tools to get them thin.  Saw this process first hand when I was in Jingdezhen.

best,

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

 

D.M.Ernst likes this

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But why throw thick if you don't have to. My porcelain is very very friendly. In my opinion it is the most throwable ( clay )    (any clay) I have worked with. This isn't the best example, it's an earlier test with a thin clear under a thin rutil blue . but it is 1/4 inch thick . with very little trimming . I have never seen or know from experience how thick or thin any other translucent porcelains are. I've never worked with southern ice or the cone 6 variation. I've heard the expansions of it and it sounds like it throws like cold bacon grease.  Anyway if any of you have first hand knowledge of thickness of Chinese porcelain I would appreciate it. Thanks.

1007171752-1.jpg

Joseph F likes this

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