Understanding COE

[C.Banks]

1 hour ago, curt said:

Nothing magic about 4:1.  Many different ratios will work, including 3:1.  Depends very much on what you need your clay body to do performance-wise.

Much more important in my view is the ratio of silica+alumina to flux, particularly for functional bodies.

And what kind of fluxes you are using - all fluxes are not created equal in terms of melting/vitrification power.

And particle size of the constituent raw materials is also right up there in terms of importance.

And...and...     There is no one size fits all. 

thanks for this

I'm only beginning to develop an inkling of how important prticle size is.

[glazenerd]

Cb:

The 4:1 primarily comes from stoneware recipes: although  a standard cone 10 porcelain hits in that range. Stoneware  requires a minimum of 10% flux additions to help prevent cristabolite formation. The term you need to research is spinel: which is the precursor to glass/ mullite . Spinel is AL/Si based: with excess silica being ejected. The theory then being that alumina needs to be at the 4:1 to minimize ejected silica. The ejected silica  is what promotes cristobalite formation. The minimum 10% flux is there to ensure ejected silica is incorporated into the melt. Magnesium is also thought to minimize ejected silica: you will find recipe in Europe that blend in low levels of magnesium. Another theory suggest that 1-2% calcium will also incorporate free silica. That is the theories behind the 4:1 Si/al ratio.

T

[curt]

I think if you add 10% flux (molar %) to a standard cone 10 clay body you are going to be getting some serious bloating, warping/sagging and other evidence of overfiring.  That amount of flux is excessive at the relatively high firing temperatures of Cone 10, and IMO might only really (possibly) be suitable for a midfire or lowfire body.    But I don’t fire at those temps so will let others comment on that.

Take C Banks’ example clay recipe with 70% Silica and 24% alumina.  That leaves 6% for flux, which is much closer to the flux levels for the collection of cone 10 stoneware bodies I am familiar with.   In my experience 8% molar is just about the maximum flux a cone 10 stoneware body can handle.

Minimizing free silica (what nerd is calling “ejected”) is important for reducing the chances of cristobalite (which is just a specific form of silica).  But overfluxing is not an ideal way to achieve this.   Better to minimise the free silica in the first place by not adding more silica to the clay body than is needed (ie, can reasonably be absorbed in to the melt), and making sure the particle size of that silica is not too small (because smaller particles get fluxed into the melt more easily).

if you are not going with a (presumably pre-tested) commercial clay body, or a tried and tested recipe, than as usual the answer is...test, test, test!

[glazenerd]

"One must differentiate, at this time, between the silica which is added to the body and the silica that forms as a result of the change from metakaolin to spinel. This ejected silica is in the form of extremely small, highly defective crystallyttes."

"In the 7% feldspar body the lack of sufficient glassy phase to dissolve significant quantities of silica causes the proportion of crystal to glassy phase to be high: this results in high thermal expansion."

Ceramic Science for the Potter. W.G. Lawrence   PhD @ Alfred U  1972

There is chemistry behind clay. One of the more common mistakes is to equate the reactions of silica in glaze, to be the same in clay. Silica in glaze changes from crystalline to amphorous as part of an eutectic melt. In such cases, silica lowers the expansion properties. In clay, only a percentage of silica is incorporated into a glassy melt. The remaining free silica, along with ejected silica from spinel formation increase expansion. 

T

 

[C.Banks]

Thanks for this discussion.

I'd post the recipe but it's kinda' drifting away from COE and into claybody specific flux/al/si ratios.

Sounds like I might like to try to get away with less silica.

Again, I do appreciate the food for thought.

 

 

 

[glazenerd]

On December 13, 2018 at 12:35 AM, curt said:

I think if you add 10% flux (molar %) to a standard cone 10 clay body you are going to be getting some serious bloating, warping/sagging and other evidence of overfiring.  

Just realized I misread your post. I agree, 10% molar flux level would be a puddled mess. I was referencing 10% feldspar as the recipe %: which would land somewhere in the 2.70% molar range. 80% clay ( total of all used), 10% silica, and 10% feldspar: is a commonly used blend in stoneware. 

CB: perhaps after the holidays I will start a Clay Chemistry thread; where all things clay chemistry related can be discussed. There are many theories and variances of opinion on what makes a good body.

T