Carbon coring or “black coring” can be an issue when firing clay bodies. While doing some experiments with reduction firings I had many pots that were cracking in the process and had no idea why. It seemed that the cracks were from fast cooling, as they were clean breaks through the glaze with sharp edges. Little did I know that this cracking was caused by carbon coring or “black coring”. I did some comparisons wth cross sections of the broken pots and noticed that this discoloration did not happen in my oxidation firings but only in reduction. I dug out my ceramic books and searched online to find out what this issue might be.

Shattered By Black Coring

While researching I found a post by the Lugna Clay company entitled ” Bloating and Black Coring”, which seem to suggest that I may need to bisque fire my clay body properly.  The theory behind the article suggested that not all the carbon was burned out of the clay body and the kiln also may need to be vented better in the bisque firing. The clay body I was using was AMACO high-fire warm brown 58-M stoneware clay. I only had issues with bloating when I accidently overfire it a few times on the bottom shelves while trying to reach cone 6 in the middle of the kiln.  The clay body was high in iron content which, I later found, created the issue with black coring when I reduced the updraft kiln to produce a body reduction. The iron in the clay and the reduction process was a bad combination and would produce a bad kiln load of pots.  Bowls shattered as they cooled.

Black Coring – The Cause

The answer to the problem came from an excellent research report entitled “Calcium and sulphur distribution in red clay brick in the presence of a black reduction core using micro X-ray fluorescence mapping”. by L.Gredmaier, C.J.Banks , and R.B. Pearce. These findings can be found on page 2 and 3 of this report:

“The following factors determine the extent of black reduction coring in red clay ware:

- Firing time – a longer ring time can eliminate the black reduction core.

- The oxygen atmosphere during ring. Lack of oxygen promotes the formation of black reduction cores.

- Iron oxide content in the raw clay.

- Carbon content and burnout or oxidation of carbon during firing of the raw clay.”

The research in this report stated also that the red iron oxide was converting to magnetite.

To the potter, according to “The Potter’s Dictionary of Materials & Techniques” by Frank and Janet Hamer, on page 26,  means this conversion created weakness to the clay body caused the clay to vitrify at a lower temperature due to the red iron oxide and carbon converting to black iron oxide and carbon dioxide, which creates an active flux . The pots become brittle and fragile.  One mug I took from this load popped apart while I poured coffee in it as a test, sending shards across the table, because it could not withstand the thermal shock due to black coring.

Black Coring – The Solution

The solutions to black coring from the article link to above would be to use a clay body with less iron content.  Also, it is suggested that bisque firings should be slower and to the correct temperature to allow carbon burn out.  I personally found that in my high iron clay body, if I skipped the body reduction of the firing and reduced the kiln towards the end of the firing, I still got reduction glazes to look great without black coring.

If any of you who read this have found this helpful or have your own findings, feel free to leave a comment!