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SAS Formulation stoneware WOPL Plasticity Green Strength particle distribution paxking density

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#61 anchorman

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Posted 03 May 2017 - 11:50 PM

Thanks!  that all makes sense looking at it now.

I forgot to mention that we primarily fire this to cone 10 as you gathered.  We've used it at cone 6, and it makes usable ware.  With no feldspar added it dunts at cone 10.  It did less so when fired very quickly and less cristobalite was able to form, but eventually would crack nonetheless.  With even just 5-6% feldspar added, that problem goes away.  I haven't pushed the upper limits for how much one can add before it starts bloating and doing other bad stuff, but additions of upwards of 20% or more didn't cause obvious problems with glaze fit, so I split the difference until I could do proper absorption testing.

I'm not sure what constitutes high levels of sulfur to you.  I haven't had the FHC tested, but I did have the goldart and fireclay tested a while back.  The goldart had a mean sulfur content of 7.5 ppm , and the hawthorne bond fireclay had mean sulfur content of .67ppm.  Getting goldart out of the mix changed it from being intolerable to be in the kiln room when firing during the sulfur burnout stage of things to being able to be in there even if the vent were off.  We of course always run the vent when the kilns are firing, but it was amazing to not the difference.  I can now just barely not a hint of sulfur when the kiln is late in the bisque firing.  7.5 ppm doesn't seem like much, but when you have a kiln full of 100 lbs of clay getting fired, that's still close to 37 grams of sulfur getting burned out in a clay body made up of 50% goldart... and that ends up making a rather good volume of sulfur gases eating away at things.

I noticed the same thing Regarding the fireclay with the 40 and 50 mesh clay, I was looking at the 20 mesh, but perhaps that is a little too big for a throwing body?

 



#62 glazenerd

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Posted 04 May 2017 - 03:31 PM

Cone 10 should run in the 2.85-2.90 range for alkali molarity: cone 6 - 3.10 - 3.15 or so. Avoid using high sodium fluxes in stoneware: they increase drying issues.

 

I would avoid the 20 mesh:

30 35m

 

As the lower spectrum particle size ( SAS ) increases; the upper end has to increase to fill in the blanks. As you can see in the above pic: lots of chunks as the particle sizes goes down. That is part of the SAS formulation standard: supplying sufficient sub micron particles to encapsulate the large mesh: which produces a functional body.

 

10 1200m

 

The SiC tracers in this pic shows how sub micron particles will encapsulate large particles (20-80 mesh).

 

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#63 anchorman

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Posted 05 May 2017 - 05:12 PM

So if I'm understanding correctly, we should likely increase the amount of feldspar in this recipe to raise the KNaO molarity from ~2.3 to closer to 2.85/2.9 in order to get a better overall melt?  I'm going to do absorption tests before fooling with things much more.

Also I want to be sure that I'm getting the molar percents right.  I've got a unity formula, so I'm adding the total RO, R2O3, and RO2 numbers together, and then dividing each individual chemical by the total x 100 to get it's molar percent?  this is an interesting way to look at it.

 

Many thanks again for looking into this and explaining it all so well. 



#64 glazenerd

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Posted 05 May 2017 - 05:59 PM

Molarity in clay formulation is not applicable, as with glazes. In fact, formula limits have not been written other than si/al values. Porcelain requires a higher level of Knao, as compared to stoneware. Although both bodies form a glassy matrix: stoneware is more dependent upon mullite production. This is the reason you see much higher molarity values in porcelain as compared to stoneware. The precursor to mullite is spinel ( meta kaolin) which needs more alumina in lieu of Knao to form. Excess silica is ejected from spinel; which is why the silica in stoneware is lower than porcelain. If too much excess silica is present, then the problem of free silica, and as the temps climb over cone six:cristobalite formation. A good stoneware body fired to c six should have at minimum 19.5 percent molar alumina.
This level will produce much better green strength, and in addition absorption under one percent provided other formulation criteria is met. At cone ten, a Knao level of 2.90 is more than enough to ensure a melt. The general rule of thumb is: stoneware needs more alumina and porcelain needs more flux. The flux levels in porcelain are nearly comparable to the flux levels in glazes. Porcelain is less dependent on alumina, although alumina does provide better strength to both bodies.

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#65 glazenerd

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Posted 09 May 2017 - 08:42 PM

Anchorman
I finally had time to look at your recipe on page 3.
Alkali 2.73. Al 20.85. Si. 73.43. Si/al. 3.52
The silica level is slightly elevated for stoneware. Usually not critical unless higher carbon clay is used in percentages above fifteen percent. Alumina levels in stoneware are of equal importance as flux because spinel forms from silica/alumina.

I made this simple adjustments:
Hawthorne 30. Fhc 20. Epk 12. Om4 12 Red art 4. Custer 14. Grog 8
Alkali 2.92. Al 22.08. Si. 72.14. Si/al 3.27

You will find little to no difference in plasticity. Green strength will improve somewhat, and absorption will be lowered. The balance between sodium and potassium is good: sodium is reactive when you first pug the recipe, and then later causes other problems. The term is electrophoretic: meaning there is an instant reaction to sodium anions (positive charges) . This is the same reaction that occurs when sodium bentonite hits water: gelatinous . Higher sodium in clay recipes causes a thinning (think sodium silicate) at first, giving the impression/ feeling of softer moist clay. As the reaction reverses ( over weeks) the clay then stiffens because the moisture content is low. Sodium overall is a rather caustic flux, after all hydrochloride acid is made from sodium chloride. Same reason salt firings eat bricks for lunch.

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Also tagged with one or more of these keywords: SAS Formulation, stoneware, WOPL, Plasticity, Green Strength, particle distribution, paxking density

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