Cristobalite

[glazenerd]

@Bill Kielb 

Thought I would move my response here from the pinging plate thread. Doubt they want to hear it.

you mentioned in that thread cristabolite would not form until cone .14.

Actually it starts to form by 1150C ( 2100F). It increases dramatically by 1250C, 

however, if feldspar levels fall below 7%, it increases by 31% at 1150C (2100F). For that reason, stoneware formulation states a minimum of 10% spar additions.  For the same reason Si:AL ratios can increase those levels as well. The conversion is metakaolin to spinel ( at 2050F), and from spinel to mullite. Spinel is alumina/silicate: with excess silica being ejected as it converts to mullite.  So if alumina/silica ratios start going above 4:1, then excess silica is ejected as crystallyttes: (highly divided silica). All stoneware bodies eject some: if ejected silica increases,then spars have to increase to incorporate that ejected silica into a melt. Cristabolite inversion occurs on the way down. 

Ougland & Brindley did a comprehensive study using x-ray defraction back in the 50's. 

2192F (1200C.). Glass 62.   Silica 21.    Mullite 19.       The 21% free silica increases COE, and increases the cristabolite.
2372F (1300C).  Glass 66.   Silica 16.    Mullite 21

the reason my posted firing schedules change at 2050F to a much slower ramp is to increase mullite. It also allows off gassing as spars. 

Tom

 

[glazenerd]

3 hours ago, Bill Kielb said:

cristobalite  not likely unless these went to cone 14 at some point.  I would guess just glaze crazing

 

[Bill Kielb]

I still would guess the glaze is not a great fit and crazing  as most likely.

The having significant cristobalite issue,  is just rare in today’s pottery. Maybe wood fires but we just don’t see this as typical with thousands of firings daily. Quartz inversion stuff is similar to this as well. Lots of theory and hoopla, not a lot of empirical data with respect to pottery produced today in my opinion.

[glazenerd]

3 hours ago, Bill Kielb said:

I still would guess the glaze is not a great fit and crazing  as most likely.   could easily be both. No idea if they buy or mix their own.

 

Here is a good general reference to cristabolite and glaze compression from a college in Kansas. 

http://www.kgs.ku.edu/Publications/Bulletins/211_4/bauleke.html

Tom

 

[Bill Kielb]

Thanks! Copy and paste says dead link as posted so I back tracked a bit through the site and got this:

 http://www.kgs.ku.edu/Publications/Bulletins/211_4/bauleke.html

The link looks identical except mine shows as a hyperlink. Strange!

I will read through, but again I did this for a year and realized there is a whole bunch of real world firings happening right now - no significant cristobalite issues.

[glazenerd]

4 hours ago, Bill Kielb said:

I will read through, but again I did this for a year and realized there is a whole bunch of real world firings happening right now - no significant cristobalite issues.

Fair amount of truth to that. Our generation are the beneficiaries of the research and corrections made by them before us. Problems from cristabolite inversion are rare; but not gone. Most commercial bodies have been formulated to deal with this issue. However, like all things pottery: still some squeakers once in awhile. I see it more when older recipes are used. Operator error as well. 

Tom

[Min]

14 hours ago, glazenerd said:

Problems from cristabolite inversion are rare; but not gone.

Potters refiring stoneware, especially forms like large flat platters, still have to take the cristobalite squeeze into account and slow the cooling down during the inversion.

[Bill Kielb]

I think good practice to fire and cool anything large slow. As far as Cristobalite squeeze very hard to form a significant amount in most firings in my opinion and as evidenced by the thousands of firings happening daily. It certainly is possible but as we see from the actual data not probable. A near 1% reasonably sudden change up and down in a nonlinear fashion should be a pot killer one would think.

If it were easy and present, that amount of change is going to likely ruin your pots. It really happens at a fairly specific point. During firing I would speculate pots differ in temperature from top to bottom or  inside to out by tens of degrees or more at the 450 degree’ish  point. Our gas kilns can be a hundred degrees different at the start

Much of this stuff to me is outlier, possible but not the norm, cool to know though. Most often when I read studies,  these esoteric issues are described as can and might but rarely stated as often ......

All good practice to go as slow as practical, grog shelves, reinforce holes cut in large flat shapes etc.... for common sense as well as esoteric reasons.

As far as refiring, I am always amazed it works actually. Ceramic stuff pretty tough!

just my opinion though.

[glazenerd]

5 hours ago, Min said:

Potters refiring stoneware, especially forms like large flat platters, still have to take the cristobalite squeeze into account and slow the cooling down during the inversion.

Just one more of the endless variables in pottery. 

[Bill Kielb]

I worry about analysis paralysis, actually.  In engineering school I can remember a friend who was adept at the abstract and very retentive of everything he read. Genius! If one needed to remember sythetic division simply ask Zack. He could almost recite his physics, thermodynamics, you name it word for word and actually solve the problems. Pure genious! One day while talking with him he agonized over changing his tires. I asked why and he proceeded to tell me that R X F ( radius cross product of the force) would yeld a resultant force out of the plane and his wrench would simply slip off.

I pondered this for a bit and asked him if he thought that there were folks who changed tires successfully to which he said, yes! I think I ended up helping him change his first tire to which I mentioned, you know sometimes the data is right in front of us and we fail to see it in favor of a cool nuanced explanation.

just sayin, good luck in life to Zack! Pure genious, hope he found his niche!