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kswan

Discolored Bisque

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I ran a load of bisque to cone 07.  I have a manual kiln with thermocouples, cone packs and a sitter so that I know what has happened inside.  But I stacked too many flat pieces together and instead of the pinkish color that my Standard 153 clay usually gets, it was white where they were too thickly placed together.  What might cause this?  Should they be rebisqued in case those areas didn't reach the same temperature? 

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D'oh!  Well, it's only in certain areas of the bottoms where the discoloration is, so they don't have the finished ring sound to them because the rest is the right looking bisque.  

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Do you have a photo? Because I've had this happen before too, but it was where a different clay had rubbed off from a working surface onto my work. Since they were both similar colors wet, I didn't notice till they were fired to bisque. One fires to bisque as slightly pinkish and the other one fires grayish white and they ended up looking kind of spotty. I don't think it affected the glaze but I can't remember. (I would probably remember if something disastrous happened!) 

Either way I wouldn't worry too much if it's just some places on the bottom. 

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Bisque or over fired should still have that glassy ring to it because it has been taken through the ceramic change. If there is a dull thud then maybe you need to refire.

 

A good look at the clay compared to greenware and the final fired look should also be an easy comparison to which one it looks closer to.

 

I had some bisque that went a nice blue where stacked inside each other but still overfired I think than underfired.
post-23281-0-24063300-1392495789_thumb.jpg

post-23281-0-24063300-1392495789_thumb.jpg

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Guest JBaymore

This situation is a VERY common issue when dense stackings are made like that one described AND there is inadequate air circulation within the kiln.  I bet that the "white" mentioned is actually a bit of a grey-ish white tone.

 

This is the impact of reduction of iron oxide in the body at low temperatures on the bisqued clay. It comes in the densely stacked areas because there is not enough oxygen circulating to burn out all the carbonaceous matter in the timeframe of the firing profile, and that production of CO in the body in the localized areas reduces the iron.  Iron in the reduced valiancy (black) state is a powerful low temperature flux on silica.  So it starts to flux a bit of the silica in the bisque.  Produces a grey-ish bisque color out of many clays. 

 

High reduced iron fluxed glass is also "punk" and brittle.  Usually causes the sound of the "ring" to change too from normal.

 

This is a common cause of defects that show up in the glaze firing... and people do not realize that the issue happened in the bisque firing.

 

best,

 

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

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John, as I was trying to read up on this topic, I did come across one site that mentioned the localized reduction, and yes it does have a little grey tinge to it. None of my books mention this problem, so I have been perplexed. Is it reparable with another bisque firing? Also, these pieces will be gas fired in reduction to cone 10, so will this localized reduction still matter if the whole thing will have much further body reduction later on? Or has it trapped the carbonaceous material permanently?

 

Giselle, they were all of the same clay body, so there was no contamination happening. Joel, the clay I use is a very dark grey after reduction firing, so it kind of made me think looking at it that it might have gotten some reduction happening because of the way the pieces were keeping oxygen out.

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Copied from digitafire:

 

"Black coring usually occurs during a reduction firing and is a result of fast firing and/or lack of oxygen in the kiln between 700 and 900C (usually in the bisque firing). If body carbon fails to oxidize to CO2 it steals oxygen from Fe2O3 (reducing it to FeO, a powerful flux.) This FeO will then flux the body, sealing it and preventing the escape of remaining carbon in the body. This produces the characteristic 'black core' you see on ware cross section. The more iron in a body, the greater the risk of this problem if firing is not right. Once iron is reduced to it is very difficult to reoxidize it back to Fe2O3."

 

In college we bisqued in a large gas kiln that was almost impossible to fire in oxidation because of the way the chimney drew. Black cored bisque that has grey and pink areas outside like the OP describes was common. You can glaze the stuff alright, but glazes can be prone to pinholing, especially if the glaze is already inclined towards that particular flaw. (I'm looking at you, rutile/floating blue!). The resulting wares will be weak and brittle as well. It'll work, kinda, but it'll smash more readily than it ought to if an item gets knocked in casual use.

 

We just did our best with the results because we were making so much stuff as students and smashing much of it at the end of the semester anyways.

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Thanks for all the input. I will be more careful from now on to keep air around the bisque. Luckily it was only two pieces!

 

 

We become so accepting of disaster! My last glaze load I was genuinely pleased that "only" three pieces were ruined and the rest came out well! 

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