Black Coring

[preeta]

I don’t get it.  I’ve read a lot online and digital fire but I still don’t get it!

i understand the chemistry but how does it weaken? By bloating? Cracks? Crumbly breakup like Frank Matranga’s pot in the other thread? Black glass?  

in what way does black coring affect your pot. How is it bad?

I like using red clay. And other clays rich in iron. In gas reduction ^6

ive broken pots to see but I don’t understand what to look at? Even the pictures on digital fire didn’t help. If I am to believe their illustration if the inside of the red turns black then it’s black coring. Is it as simple as that?

But ... then what about other cultures who have done reduction with red clay body from ancient times where they only had access to red clay.  Both low fire and high fire. 

Then does degree of fire make a difference? 

Would grog make it stronger? White grog? 

Some of my favourite soda (^6R) and wood salt (^12 but not too much reduction ) pieces are my often used at home - porcelain slip covered black mountain/IMCO 811 Red clay.  It’s been a year for soda and 5 months of salt - regular everyday use.  Not had any problems so far. Luck?

[glazenerd]

Preeta:

porcelain firing is page one. Near the end of page two is a lengthy post about coring/ bloating and how to resolve them.

In the May issue of Ceramics Monthly is an article I wrote on these issues. I will spare you lengthy paragraphs on the effects of coring and will sum it up as: brittleness.

Tom

[preeta]

Succinct! Perfect.  What makes the brittleness? Bloating? Glass... or all of the above? 

Yes or no succinct would be fine too.  I’ll check out your article and reread your thread  again. I’ve faced issues last fall since I read your thread.  

[glazenerd]

Just now, preeta said:

Succinct! Perfect.  What makes the brittleness? Bloating? Glass... or all of the above? 

Yes or no succinct would be fine too.  I’ll check out your article and reread your thread  again. I’ve faced issues last fall since I read your thread.  

Coring is actually a form of early vitrification: useful for bricks, bad for pottery. In western clays; iron comes in the form of iron disulfide (pyrite, better known as fools gold.)  Iron is good, all kinds of warm, toasty colors. Sulfides not so good: blisters, bloating, and coring. All three of these defects are caused by inorganic carbons (sulfides),  it is the degree of reduction which results in each. When the kiln is oxygen starved in the kiln: iron disulfide emits sulfur dioxide gas: which reduces iron as early as 1750F, resulting in early vitrification ( black glass). When oxygen is abundant: iron disulfide emits sulfur trioxide gas which passes without reduction occurring. As iron levels increase, so does the sulfide: once you cross a given molar %: coring is certain to occur.

when dealing with brittleness: "yield point", flexture", and other technical terms come into play. MOR (modulus of rupture) was discussed recently in another thread. Fine particles  clay usually run from 500-750 psi ( pounds per square inch)  When clay is under fired: these numbers go down. Bisq for instance never survives a fall because it is weak ( low MOR value). When clay is over fired, or coring occurs: then MOR values go way up. Once you cross a certain point: chipping occurs if dropped.. ( brittleness). 

Tom

 

[neilestrick]

14 hours ago, preeta said:

I like using red clay. And other clays rich in iron. In gas reduction ^6

 

Most cone 6 brown clays are formulated to be fired in oxidation, which requires a lot more iron to get the brown color than in reduction. So if you start having issues, you may need to find a clay that is lower in iron.