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Zerostreet's Tiki Art! Tiki Zombie Cover!
danlovestikis
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Joined: Jun 17, 2002
Posts: 4867
Posted: 2013-08-07 2:09 pm   Permalink

Best wishes for a successful firing, Wendy

 
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zerostreet
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Joined: Feb 06, 2010
Posts: 1959
From: http://www.zerostreet.com
Posted: 2013-08-10 07:56 am   Permalink

Quote:

On 2013-08-07 14:09, danlovestikis wrote:
Best wishes for a successful firing, Wendy



Thanks Wendy...Alas....here are the pics.

Well. I'm happy with my glazes, but sad to see 2 of my mugs cracked in the kiln. Don't know why....


The Lone Survivor.




Love this green stone glaze.


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[ This Message was edited by: zerostreet 2013-08-10 07:56 ]


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MadDogMike
Grand Member (7 years)  

Joined: Mar 30, 2008
Posts: 7364
From: The Anvil of the Sun
Posted: 2013-08-10 08:17 am   Permalink

Sorry to see the mugs cracked but love the glazes. I wonder if your kiln is cooling too fast?
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zerostreet
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Joined: Feb 06, 2010
Posts: 1959
From: http://www.zerostreet.com
Posted: 2013-08-10 08:20 am   Permalink

Quote:

On 2013-08-10 08:17, MadDogMike wrote:
Sorry to see the mugs cracked but love the glazes. I wonder if your kiln is cooling too fast?




Something tells me it's the kiln. You may recall it runs kind of fast. I was recommended by the people at Paragon, a pcb controller, which will slow down my firing. That probably won't help with the cool down. The run the kiln outside and it's been pretty hot here.

R.
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danlovestikis
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Joined: Jun 17, 2002
Posts: 4867
Posted: 2013-08-10 09:55 am   Permalink

I sent you an email but most of what I wrote was about ramp and cool down but also water re-entering the cast when you glaze. So sorry, Wendy
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GROG
Grand Member (first year)  

Joined: Jun 21, 2006
Posts: 6910
From: Tujunga
Posted: 2013-08-10 10:24 am   Permalink

Be sure and let the kiln cool down completely before you open it. Slap some clear glaze on them puppies and fire 'em again. They're Moais, they can have cracks. They're supposed to look primitive.
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zerostreet
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Joined: Feb 06, 2010
Posts: 1959
From: http://www.zerostreet.com
Posted: 2013-08-10 12:50 pm   Permalink

Ok. TRUE STORY! I just had some guests over and they were looking at the cracked mugs on my kitchen counter. I explained how 2 of them cracked in the kiln but the third was fine. The person points at the ok mug mug and says "Is this the one that survived?" and at that SAME instant the mug shattered. No lie.



Grog, The seperation of the cracks are just over 1/8 of an inch...not sure that's do-able?

R.
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Gene S Morgan
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Joined: Jul 18, 2011
Posts: 342
From: Midwest the navel of the USA
Posted: 2013-08-10 10:35 pm   Permalink

I hope you don't mind me commenting here, but I thought those were really nice glazes. I showed my wife your cracked and broken mugs and asked her what she thought might have caused it. She taught ceramics for many years and fired her kiln thousands of times with poorly made student projects. She said the only time she ever saw that was when one of her gang snuck in a piece that they had glazed on greenware. No mater how dry greenware seems to be, there is always trapped moisture. I checked back earlier in your post and could not tell if you had bisque fired first. Please ignore this comment if you think it dumb. I just thought those glazes were really cool and it was a shame to lose them ..... Gene

 
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GROG
Grand Member (first year)  

Joined: Jun 21, 2006
Posts: 6910
From: Tujunga
Posted: 2013-08-10 10:42 pm   Permalink

Did you fire at the right temperature? Glaze fire is cooler than bisque fire.

 
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zerostreet
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Joined: Feb 06, 2010
Posts: 1959
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Posted: 2013-08-11 05:06 am   Permalink

Thanks Gene and Grog! I appreciate any and all help! I did bisque fire them first and they looked perfect to me at that point. I bisque ired at 04 then fired at 05 for the glazes. Grog you have me wondering now....maybe I accidentally slipped in a cone 02? But would that be reason enough for the third mug to explode after it sat on my counter for a few hours? I do keep the house pretty cool....
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danlovestikis
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Joined: Jun 17, 2002
Posts: 4867
Posted: 2013-08-11 05:17 am   Permalink

I won a rare mug on eBay. It was really old. We were standing next to it months later and heard a ping. We looked at each other and picked it up. Sure enough a crack ran down its side. We decided it was because the air conditioner turned on and was blowing on it. Sudden change of temperature on a thin mug. It couldn't handle the stress. So one more thing to add to the list!

Wendy
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MadDogMike
Grand Member (7 years)  

Joined: Mar 30, 2008
Posts: 7364
From: The Anvil of the Sun
Posted: 2013-08-11 08:52 am   Permalink

The official term is "Dunting", here is what Babalu had to say about dunting several years back


Hey guys,

I've had this happen before too. Glaze will continue to cure long after it has been fired. Dunting could be the cause when you see long vertical cracks like this. I have seen where this will happen when there is a thicker layer of glaze on the bottom inside of the mug than are on the inside walls. Anyway the below is a short explanation that I pulled off of the Internet:

Dunting
This is a special type of crack which occurs from stresses caused during firing and cooling.
This stress primarily occurs during two critical points of firing called silica inversions which occur at 1063 F or 573 C and 439 F or 226 C.
At these inversion points, the structure of the silica molecules rearranges.
It is important to fire slowly through these two temperatures and electronic kiln profiles often do this for you automatically while they are heating.
Most dunting however is caused during cooling.
These cracks appear as long, clean, body cracks with sharp edges.
If the pottery or ceramic is glazed, the glaze edges are sharp.
They can be vertical, horizontal or spiral.

There are three main reasons why cooling dunts occur.
The first occurs as you cool through the first silica inversion at 1063 F.
At this inversion the body contracts suddenly.
The more silica, quartz, in the body, the more contraction.
Since different parts of the pottery or ceramic reach this temperature at different times, it doesn't all contract together and that causes stress which can crack.
Take for example a tall pot.
The top will cool much faster than the bottom, because the bottom has the whole temperature of the kiln shelf keeping it warm.
The top will cool faster than the bottom, causing a crack around the bottom wall.

The second occurs as you cool through the 439 F inversion.
A similar thing happens as above.
But, pottery and ceramic artists sometimes like to open their kilns at about this temperature to see their pieces and this will make it much worse.

The third type of cooling dunt occurs months or even years after firing.
Sometimes the pottery or ceramic might split right in half after three months.
This is usually the result of thermal shock.
In this case the clay and glaze expand at different rates when exposed to temperature variations and this change causes the object to crack.
To be more specific, the body contracts more than the glaze.
If the glaze is weaker it will shiver.
If the clay is weaker the object will crack.

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zerostreet
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Joined: Feb 06, 2010
Posts: 1959
From: http://www.zerostreet.com
Posted: 2013-08-14 7:15 pm   Permalink

Thanks Mike, for that info!

I purchased a PCB-1 controller for my kiln. Wonder if you guys have any experience with one. You plug the kiln into it then it into the outlet. It is designed to speed up or slow down firing times by restricting the power to the kiln (turning it on and off) and is mainly for fats firing automatic kilns, which is what I have.

Gonna fire up again this weekend! I'll keep you posted.
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zerostreet
Tiki Socialite

Joined: Feb 06, 2010
Posts: 1959
From: http://www.zerostreet.com
Posted: 2013-08-15 2:40 pm   Permalink

Finally have a mold and the first cast of The Stowaway mug! Still battling these Mars Attacks aliens so updates may be sporadic!



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MadDogMike
Grand Member (7 years)  

Joined: Mar 30, 2008
Posts: 7364
From: The Anvil of the Sun
Posted: 2013-08-15 3:29 pm   Permalink

Nice Robert! Bold detailing. I'm interested in seeing how that controller works, I'd like to convert my kiln to auto
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