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Cone 6 Body Or Cone 6-10 Body: Much Difference?


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#1 NancyAmores

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Posted 10 April 2014 - 01:09 PM

I recently switched from low-fire to mid-fire and am having some difficulties with pinholes and weak finished wares. The body I currently use is Standard #181 white/smooth stoneware Cone 6-10 clay. The absorbency is 2.75% at Cone 6.

I have tried the #181 with both Cone 06 bisque and Cone 04 bisque (both slow, approx 12 hours). I’m using Coyote glazes. The first glaze firing I did was a Cone 5 with 20 minute hold (Coyote recommends a Cone 5 with 15 to 30 minute hold so that the 6 cone is bent 3 to 5 o'clock after firing. My #6 cone bends to 2 o'clock with this schedule). The colors of the glazes came out nice but there were pinholes on all pieces.  I thought it may be because I brushed on too many coats, so on glaze firing #2 I reduced the number of coats to 2 for all glazes. Still had pinholes but once again the colors were ok. And so on glaze firing #3 of these same glazes (different pieces each time) decided to just take them to Cone 6; they were overfired and looked really bad and washed out, and still had pinholes. Grr.

Bottom line: I think I have to fire to Cone 5 with a 20-30 minute soak in order for these glazes to look decent, and the clay body I’m using doesn’t start maturing until a Cone 6.

After throwing a moody this morning over another botched load, I’m back at the glazing table and ready to try again. Now I’m re-thinking my clay body. Even at cone 6, the #181 feels open and porous, has a dull ‘thud’ instead of any ‘ring’, and is quite easy to break (I know this because I threw a few ;) ); the glazes also seems to ‘float’ above the texture even when applied thin, instead of becoming one with the surface. Altogether, the results have not been that much different from the white earthenware I’ve been using. The pieces feel light for their size. I don’t make functional ware (is jewelry functional?) so it’s mostly an issue of aesthetic, but I was hoping for increased strength and  weather-proofing as benefits to mid-fire. Maybe I’m looking/hoping for a dramatic difference that doesn’t exist, or maybe I’m just doing it wrong :(

So, finally, my questions: Are there advantages to using a Cone 6 clay over using a clay with a wide range of Cones 6-10? Is there a better chance of improved glaze ‘fit’? Would a clay fired closer to maturity produce a noticeably less porous and stronger piece/have a ‘ring’ after glaze firing? Is there something else that could be causing the pinholes?

What, if any are the additional challenges to firing closer to the clay’s maturity?

Thanks much for any wisdom you can give. I’ll be firing another glaze load tomorrow and would love to open that kiln the next day with a ‘ahhhh’ rather than a ‘uhhhh...?’.



#2 bciskepottery

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Posted 10 April 2014 - 01:28 PM

If you are firing to mid range, use a clay body rated cones 4 to 6.  Standard has some nice Cone 6 clay bodies that will work. 

 

At cone 6, a clay body rated 6 to 10 is not mature and vitrified, creating the weakness in strength you mention.  You may also have some glaze fit issues at cone 6 with a clay body rated 6 to 10.  Check the Coyote web site to see if anyone else has reported pinhole or other problems with Standard 181 clay. 

 

Pinholes could be caused by dust or oil on the bisque surface,  or by organics not fully burning out during bisque -- although bisque firing to cone 04 should be high enough  temperature wise.  Maybe add a 10 minute hold at top temperature to your bisque and see if  that helps.  And wash items before glazing to remove dust, oils, etc. 



#3 NancyAmores

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Posted 10 April 2014 - 02:46 PM

Thanks, I'm heartened by your response as changing the clay body and soaking the bisque narrow down the variables to a better firing quite a bit.



#4 JBaymore

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Posted 10 April 2014 - 03:28 PM

There is no such trhing as a cone 6 to 10 body.  Can't be due to basic ceramic chemistry issues.

 

It is one of three things:

 

1.)  Overfired at cone 6 and mature at cone 10.

 

2.)  Mature at cone 6 and overfired at at cone 10.

 

3.  Underfired at cone 6, mature at cone 8, and overfired at cone 10.

 

CAN'T work too much different than that above scenario I described.  Likely it is really a cone 8 body if the manufacturer is smart.  Then is is only "slightly screwed up" at the typical cone 6 and cone 10 that most folks fire.

 

This is the manufacturers keeping their clay body inventory needs in check.  They trade off the problem to the end user.  Caveat emptor.

 

best,

 

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


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#5 Pres

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Posted 10 April 2014 - 03:35 PM

Bruce's reply is right on! I had used a 6-10 stoneware at the HS where I taught the first year. I decided after the first year to go to a tighter range clay to settle some issues I had with the 6-10. These included brittle not vitrified ware, not being able to figure out where to bisque to because no temp seemed to work, and some pinholing. Most of the time pinholing comes from dust, or organics in the clay, but you can get it from a glaze being too thick, or the glaze temp being too short on soak.

 

I think you will be much happier with something like the 240, 240G, or the 553 or 563. Good luck!

 

Best,

Pres


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#6 Biglou13

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Posted 10 April 2014 - 08:28 PM

Glaze fit has to do with matching COE with COE of glaze. ( yes I need to get on insight ), not all glazes will work with all clays.

I've read less than 3% is vitrified.....

Ok lets say I have a clay that has 2.5% @ cone six. Yet it performs well in higher firing (woodfire). How does one tell if its cone 6 is under fired , or cone 13 is over fired ?

What characteristics determine "mature" over fired, under fired?

This ring, or sound value of clay what does this tell us?? I brought some yunomi to a japanese restaurant the manager and waiters were appreciating them, Every one of them looked at them like a potter (seeing foot...). The proceeded to ping test. The stone ware I used for those didn't ping like my porcelain ware?

Great questions nancy.
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#7 neilestrick

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Posted 10 April 2014 - 08:35 PM

If you want a smooth white cone 6 body from Standard, try 240. If you want something that is more forgiving but mostly smooth try 630. Throws like a dream but has some fireclay for a tiny bit of tooth.


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#8 JBaymore

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Posted 10 April 2014 - 10:22 PM

How does one tell if its cone 6 is under fired , or cone 13 is over fired ?

 

Plot the Apparent Porosity curve and look for the minimum.  It'll be a U shaped curve.  That is the cone that the body is "mature at" and should be the firing cone.  The flatter the bottom of the curve, the more "forgiving" the body will be to firing variations within the kiln as well as for general under or over firing.

 

You also then align this information with a MOR test curve for strength.  You are trying to get the minimum porosity and the maximum MOR to basically overlap on the same cone. 

 

3 percent porosity is a LOT.  Asking for issues in functional tablewares, in work destined for outdoor installations, or things like pool tile, and sinks and such.  Shoot for something below 1%.  The closer to 0 the better in general.  0.5% is a nice optimal target..........hard to hit without dropping the MOR.

 

best,

 

....................john  


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#9 Biglou13

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Posted 11 April 2014 - 07:51 AM

^^^

Homework again......
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#10 JBaymore

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Posted 11 April 2014 - 07:57 AM

This unfortunately takes LOTS of test firings.  You need a small test kiln that matches the firing type of your "normal" firings (oxidation/ reduction/soda/wood etc.).

 

To plot the "cone 6 to 10 clay" for Apparent Porority .... you'd likely fire to cone 3, 4, 5, 6, 7, 8, 9, 10,  11, 12, 13 ...and for accuracy... put in the half-cone firings as well.

 

best,

 

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


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#11 Stephen

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Posted 11 April 2014 - 10:39 AM

wonder how they pulled off pottery a couple thousand years ago?

 

Maybe they stuck their finger in the fire for x number of seconds (1 big bird, 2 big bird etc.) and inspected the burn?



#12 JBaymore

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Posted 11 April 2014 - 12:17 PM

wonder how they pulled off pottery a couple thousand years ago?

 

 

To start with.... things like 7 year full time apprenticeships. ;)

 

best,

 

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

 

PS:  It only matters if you THINK it matters.  Depends on what you are making and what your goals are.  For example...... I WANT crazing in many cases.  I have a heavy technical background.... that lets me let go of that and work intuitively.  HAMADA Shoji was my role model in that approach.


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#13 Biglou13

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Posted 11 April 2014 - 03:54 PM

This unfortunately takes LOTS of test firings.  You need a small test kiln that matches the firing type of your "normal" firings (oxidation/ reduction/soda/wood etc.).
 
To plot the "cone 6 to 10 clay" for Apparent Porority .... you'd likely fire to cone 3, 4, 5, 6, 7, 8, 9, 10,  11, 12, 13 ...and for accuracy... put in the half-cone firings as well.
 
best,
 
....................john


I don't actually think ill actually do the tests.......
The homework was ...... I spent hours on the Internet studying MOR, and porosity curve....
I will apply the knowledge to emprical results. And guestimations.

Me likes crazed!!!
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