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Clay Body Or Glaze Fit Problem - Shattering Cups


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#1 AnnaM

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Posted 03 April 2014 - 09:47 PM

Hi all,

I'm testing some new recipes for the low fire porcelain type bodies and I've had a result that I've not seen before. These test cups' recipe is:

 

50 cullet

24 starcast/clay ceram (light firing ball clay)

25 kaolin

20 whiting

1 bentonite

 

I opened the kiln after glaze firing them to find they are shattered completely. There is what looks like crazing throughout the cups and if you can just pull the pieces off. They're like a 3D jigsaw puzzle.

 

The amount of cullet in this recipe is less than the proportion of frits I've included around before (usually between 50% and 65%).  And no boron in this recipe (have been using 4110 and 4131).

 

Fired to cone 05 in the first and the glaze firing.  Kiln was under 100C when opened and cups were still pinging away.

 

Commercial clear low fire glaze, which has fit nicely with the other compositions.

 

So my question to the brains trust is, is this more likely the result of not enough glass former and/or fluxes in the body, or have the cups been torn apart by the ill-fitting glaze?  

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#2 Tyler Miller

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Posted 03 April 2014 - 10:03 PM

Could it be cristobalite formation from too much silica?



#3 AnnaM

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Posted 03 April 2014 - 10:16 PM

Not sure.  Doesn't christoballite only form over about 1200C? I have been labouring under the (possible mis-) apprension that I wouldn't need to worry about that as I'm firing so low?



#4 Tyler Miller

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Posted 03 April 2014 - 10:23 PM

I think you're right, I missed the"low fire" part.  My only other thought is that you got things too glassy and it was too weak to suffer the stresses of cooling.  You could try a slow cooling between 760 C and 480 C to do a quasi-anneal.

 

John Baymore will likely know.



#5 AnnaM

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Posted 03 April 2014 - 10:41 PM

No, actually you could be right Tyler. Just reading up about it now.



#6 Min

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Posted 03 April 2014 - 10:45 PM

I believe your alumina level is way too low and silica is also low. I compared your clay to Val Cushings ^04 vitreous white dense plastic body. 

 

 

cullet

50.00 Glass Cullet
24.00 Ball Clay
25.00 EP Kaolin
20.00 Calcium Carbonate
1.00 Bentolite L

0.65* CaO
0.10* MgO
0.01* K2O
0.25* Na2O
0.00* P2O5
0.01 TiO2
0.35 Al2O3
2.28 SiO2
0.01 Fe2O3

Cost: 0.20
Calculated LOI: 12.87
Imposed LOI:
Si:Al: 6.56
SiB:Al: 6.56
Thermal Expansion: 8.21
Formula Weight: 230.95

 

 

cushing 04 vitreous white

25.00 Grolleg Kaolin
20.00 Tile #6 Kaolin
15.00 C & C Ball Clay
5.00 Talc
30.00 Ferro Frit 3124
5.00 silica
2.00 *macaloid

0.48* CaO
0.27* MgO
0.05* K2O
0.20* Na2O
0.04 TiO2
1.45 Al2O3
0.37 B2O3
5.65 SiO2
0.02 Fe2O3

Cost: 0.04
Calculated LOI: 7.35
Imposed LOI:
Si:Al: 3.91
SiB:Al: 4.16
Thermal Expansion: 5.81
Formula Weight: 573.30



#7 Biglou13

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Posted 03 April 2014 - 10:54 PM

Ok the pattern of breakage is similar to crazing, Albeit larger crazing.
My wild guess is mis match in clay to glaze
My WILD guess glaze contracted while cooling faster than clay. Or maybe (not so much) vice a versa.

If I knew what I was talking about I'd say you need to match the COE/ thermal expasion. Of glass to clay.

Please note I'm not a " brain trust" (I have stayed at holiday inn)
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#8 AnnaM

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Posted 03 April 2014 - 11:00 PM

Thanks Min. What program is that that you've used there?  Is there something on the net that shows how to read that result?

 

Thanks BigLou, Yes, I thought the glaze originally was the problem, but its just the sheer magnitude of the cracking throughout the whole body that made me wonder whether it is the composition of the body. There's definitely expansion issues, I just want to work out if they are originating from the body or the glaze.



#9 Biglou13

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Posted 03 April 2014 - 11:09 PM

Body or glaze?...
Chicken or egg?.....
You can fix either. But since it's a commercial glaze I'd try tweaking the clay?

I'm not sure either is to blame. Like in a good relationship there needs to be a good match, both sides need to work together. They need to communicate well each other, and support each other, But primarily (in clay) match each other's growth (expansion) and/or contraction. ( what do I know..... I'm single)

Or find other glaze that matches clay.
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#10 Min

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Posted 03 April 2014 - 11:14 PM

I use Insight from digitalfire, they have a free trial period before you have to buy it. The homepage is a confusing mess but the software is much cleaner. It takes a bit of effort to learn how to use it but it's priceless once you do, right biglou? Nudge nudge (can't put a grinning face in with my ipad)
digitalfire.com

Look at the alumina and silica levels in both glazes. Oops, both clays

#11 mregecko

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Posted 04 April 2014 - 12:24 AM

Might also be worth firing a few unglazed cups to see how the body holds up under the same conditions, without a glaze.

#12 AnnaM

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Posted 04 April 2014 - 02:12 AM

I've downloaded the trial version Min, thanks for the heads up. It seems to not have some Australian brands in it, I'm assuming its an American program?

 

If I increase the percentage of the cullet, will this address the low silica or do I need to add free silica?

 

And would I add alumina hydrate for the alumina?

 

UPDATE: I've answered my own question, I just pulled out another test that had an addition of molochite (15), that is calcined kaolin. This one has 15 more cullet and 50% less kaolin, but it has turned out fine, maybe because the molochite/calcined kaolin is, interestingly, alumina and silica.



#13 Min

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Posted 04 April 2014 - 10:31 AM

I've downloaded the trial version Min, thanks for the heads up. It seems to not have some Australian brands in it, I'm assuming its an American program?

 

If I increase the percentage of the cullet, will this address the low silica or do I need to add free silica?

 

And would I add alumina hydrate for the alumina?

 

UPDATE: I've answered my own question, I just pulled out another test that had an addition of molochite (15), that is calcined kaolin. This one has 15 more cullet and 50% less kaolin, but it has turned out fine, maybe because the molochite/calcined kaolin is, interestingly, alumina and silica.

I believe Hansen, author of Insight, is Canadian. Matrix would probably have more of your materials in it. http://www.matrix2000.co.nz/  I have never used that one but have heard good things about it.

 

I would look at increasing the plastic clay(s) in your recipe to increase alumina and silica, adding alumina hydrate is not going to help with the throwability of the clay.

 

Do you have target levels for an 04 body? 



#14 AnnaM

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Posted 04 April 2014 - 11:15 AM

When you say 'target levels' do you mean like the chemical composition for a body that is known to be vitrified and strong at cone 04? If that is what you mean, no I don't unfortunately. I have a few variations of recipes that work with certain ferro frits, but I'm trying to substitute recycled glass powder for frits, as the cullet is much cheaper (about $2/kg instead of nearly $20). BUT now that I'm getting my brain around this calculation program, I'll be able to work out what the levels of the fritted recipes are.

 

The body that has the molochite in it is not bad to throw, the star cast/clay ceram is what you guys call Tennessee ball clay? Its a very light firing, but still really plastic ball clay. I just have to see what that one does with the glaze on it. Its in the kiln now, so I'll know in the morning.

 

Min, thankyou for the info on the glaze calc programs, it never occurred to me to use one before, and now I'm starting to think I'll be using them a lot in the future. Brilliant invention!



#15 JBaymore

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Posted 04 April 2014 - 12:36 PM

Cristobalite forms in a body with insufficient fluxes present to act on all the silica at temps above 1200 C (2012F).  Not likely the culprit in a body that has so much in the way of fluxes...and firing to only 04.  But it is impossibe to know for sure what the chemical balance of the body is without an analysis of the particular cullet she is using.

 

Generally speaking "cullet" is typically a variable composition material....... unless your supplier has a specific analysis for the batches you are getting.  So from batch to batch.... that composition might be varying a lot.  Frits on the other hand are generally stable compositions.  Personally... unless I bought HUGE stable batches in advance... I wouldn't use it for a clay body.  Asking for variation issues in production.

 

As to Insight........ when you BUY the Level II version of Insight (well worth the access to the secure portion of the Digitalfire site) it has the ability to add in materials data for the MDT from around the world... just one of the reasons that I like it...  I'm frequently converting from American to Japanese and Japanese to American recipes.

 

If the glazes are pinging.. the COE of the glaze and the COE of the body don't match.  Remember you can't use the COE figures from ANY glaze calculation program for the BODY COE number.  It can't be calculated by any method known yet... it must be physically measured (dilatometer ... or the "known expansion glaze" method of approximation).  Calculation programs are of limited use in body development..... it is still unfortunately a much more empirical "testing based" approach....even for industry.  (They just have nicer "toys" to use when doing it -tools-.)

 

There are differences between "virterous" as we think of it traditionally for a body and "melted" as in potentially overrfired.  Generally you want an apparent poroisty approching zero in a vitreous type body....... but after you go past that mimum on the plot curve of porosity...... you will start having such issues as you are experiencing.

 

There is a quality of glasses (and vitrified bodies) that is called "brittleness".  That can be a significant factor here when combined wit hthe seriously crazing glaze.

 

To see this level of issue you likely have TWO issues synergistically combining.  A poor strength body and a non-fitting glaze.  I'd fix the body issue first and then work on the glaze issue.  Do one at a time (basic scientific method).  In fixing the body you MIGHT end up inadvertantly fixing the glaze.

 

best,

 

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


John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#16 Min

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Posted 04 April 2014 - 01:38 PM


If the glazes are pinging.. the COE of the glaze and the COE of the body don't match. Remember you can't use the COE figures from ANY glaze calculation program for the BODY COE number. It can't be calculated by any method known yet... it must be physically measured (dilatometer ... or the "known expansion glaze" method of approximation). Calculation programs are of limited use in body development..... it is still unfortunately a much more empirical "testing based" approach....even for industry. (They just have nicer "toys" to use when doing it -tools-.)



best,

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

Yup, I should of said I was using it just to look at the low alumina and silica amounts. Would be nice to have a program that worked for the COE of matte crystalline glazes and clays!

#17 Biglou13

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Posted 04 April 2014 - 05:54 PM

Dang I thought insight covered clay.......
Well since I'm making clays ....back to the emprical approach.
Well at least the program will get you in the closer to appropriate COE of glazes.
(No I haven't bit the bullet yet and bought insight)
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#18 AnnaM

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Posted 05 April 2014 - 07:14 AM

So do low silica and low alumina always result in a weak body? Or is it one or the other?


Can anyone give me some names of really good textbooks on ceramic chemistry?

#19 Min

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Posted 05 April 2014 - 08:09 PM

Clay and Glazes for the Potter by Daniel Rhodes, The Potters Dictionary of Materials and Techniques by Frank Hammer. The Val Cushing Handbook is good also but it might be hard to find. I'm sure John B knows of many more but those are my go to ones. You could try the Digitalfire site, I think you can access some of the info without buying level 2 access. Try this link, http://digitalfire.c...rial/index.html  it should take you to the materials page. 

 

It will be much easier for you if you have a really good knowledge base to work from when making a body from scratch. Otherwise it would be kind of like trying to bake a cake without knowing what the ingredients do and not having a recipe.

 

I would also suggest that you work only on the body first, and run it through a series of tests, slumping, porosity, plasticity etc before you start working on the glaze.



#20 AnnaM

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Posted 05 April 2014 - 10:20 PM

Yes, that's what I figured. When the cups came out brittle and shattered I had no idea which ingredients/ratio of ingredients/lack of ingredients had caused it.

 

I've been learning VERY slowly from trawling digitalfire materials and google, but it's like troubleshooting into space. Hence a good textbook or two would help to focus me on what I need to know. If only I could afford to go back to school and do a formal ceramics degree!






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