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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!

 

 

20160512_185726
Al Wayman
Artist /Owner
Creek Road Pottery LLC

 

 

 

 

 

 

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Firing out the Amaco AG40 is great for raku , but I sure had issues with the higher temperatures.  When I was working towards my degree and also was an apprentice at a pottery when I  found a little Amaco AG40 updraft kiln in the classified section of the paper.  The kiln was owned by older gentlemen and hobby potter about two hours from where I lived.  This gentleman had the kiln installed above his garage he was using as a studio.  He had a ventilation fan that came with the kiln and everything was in great condition.  A family member and I loaded this little kiln onto a truck and carefully drove it home.

The kiln had no instruction manual, and we had to learn how to light it from the label on the side.  We had the gas company bring out two tanks and connected both with 2 lines running into one hose with a connection to the regulator.  After following the directions, we were able to fire the burner and bring the kiln to life. That summer, and for the next two years would use the little kiln to run a raku line of pots.  Those were the best summers.  We ran a Spring and Fall show with demonstrations for the public.  It was three days of fire, smoke, and pottery.

 

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Dusting off the AG40 Updraft

After graduating with a B.F.A. in Ceramics and leaving the pottery, I put all my equipment into storage. I had to find work to pay off student loans.  When my wife and I bought our home, I put in a small clay studio with the encouragement of friends and family who were into the wet shaving hobby.  These friends wanted lather bowls and brush handles.  I hooked up my little Amaco Kiln again and looked online for a manual. Amaco was able to send me an old manual.   I only fired this kiln to a midrange temperature with this kiln once but could not remember the schedule to repeat it.  The manual had a suggested firing schedule, so I started tinkering.

I had the gas company come and hook up a tank large enough to prevent freezing.  The first few firing were terrible simply because I was not able to properly regulate how much gas was going to the kiln’s burners and I had a broken gauge.  I started with a simple bisque fire but knew I had to make some repairs before I went to a full first firing.

At times I was almost ready to roll the kiln over the bank.  I had a whole cone or more difference in temperature between the bottom and top, with the bottom shelf being hotter.  The kiln would also stall out.  So I saved up some money and bought a converter kit.  The kit had a stand with an updraft burner which allowed hookup to a twenty-pound propane tank.  This helped out a great deal because I now had more control over the amount of gas and temperature of the kiln because this had a gauge that worked.

With the conversion kit, I now went back to square one and used the firing schedule out of the manual and completed a midrange firing.  I still had a large temperature difference between the top and bottom.  To fix this issue, I widened the glaze firing range so that the top of the kiln would not fall under fire.  I was able to get a few good pots using this method; that is until I accidently overfired on Christmas day.

Over Firing The Amaco AG40

Meet the Kilns

On Christmas Eve 2015 I prepped and glazed a load of shave bowls that I wanted to have completed as Christmas gifts.  I wanted to wrap those gifts right from the kiln for dramatic purposes and hand them out a few days later. Glazing ran later than expected and the firing stalled out climbing to cone 5/6.  I left it run for an hour longer than usual, but the cone was not bending.  I realized that the gas had dropped off.  After fixing the issue, I went back inside. It was now 3 am Christmas morning, and I had an alarm set to go off every 15 minutes, but instead it went off after another 45 minutes.  While I nodded off with”visions of sugar plums dancing in my head” the little Amaco AG40 was over firing.

I jolted awake and looked at the time and realized the problem, then rushed to the kiln and looked through the peephole.  The bottom shelf was running real hot, and the middle shelf cone was now all the way down.  Thinking I caught it in time I started the 2-hour cooling cycle the manual recommended. All was fine until I opened the kiln about 30 hours later.

The bottom shelf was severely warped with cone 5/6 clay pots melted to it. It appeared that the shelf might have gone to cone 9 or 10.  The second shelf of pots had blister marks in them, so I suspect this shelf reached cone 7/8.  The top shelf was perfect.  I was able the salvage five good shave bowls to give to friends.

All of this was great practice on how quickly things can escalate even if minor adjustments are made.  I am still working out temperature differences, and the little Amaco AG40 needs new bricks and repair work done.  But the kiln has grown on me, and I enjoy firing it out in both reduction and oxidation firings.

Click here for the manual for those who may need it. It’s quite old but had some good information in it.  If you would like to add your experiences or tips concerning the Amaco AG40 or updraft kilns in general, feel free to comment!

 

20160512_185726
Al Wayman
Artist /Owner
Creek Road Pottery LLC

Progress Over Perfection

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Back in August 2015, while pulling my kiln and wheel out of storage, I happen to find to see a YouTube video on the topic of learning a particular skill with deliberate practice. In the video “Learn To Kickflip in 5 hours And 47 Minuets.‘ The user Mike Boyd filmed his practice sessions from start to finish, landing a Kickflip. He had no skill of doing this at the start. After watching the video, I felt inspired and tried to focus on progress over perfection.

Progress Through Failure

When I first started making pots again after almost ten years or more, amazingly throwing the first bowl was easy but the next 25 were hard. They would end up collapsing, have thin bottoms, or simply go off center. I had to take a step back and work on the skills separately from step one to finish. First, I practiced centering, a very basic skill that one would learn in a beginner’s class. I found that centering one pound clay pieces were much more challenging than the five pound. Since most of the pots I would be throwing to fill my niche were created from one pound of clay I needed to practice this skill to cut down on time, material waste, and frustration. Needless to say, I ruined many pounds of clay before I was able to achieve my goal. But each time, with each step I was able to have a learning experience built on the failure, to have a centered mound of clay on the wheel. I had to repeat these learning steps through the whole process, from wedging the clay at the start to finishing with a nicely glazed pot. I have to admit the whole experience was quite humbling while remembering just ten years ago when my skill level was much higher. Some days I just wanted to toss the wheel, kilns, and pots in the creek. I had to set the goal of progressing a little each day with one thing, rather than being perfect.

Progress Through Practice

A person can have all the positivity and inspiration in the world, but if they are not progressing then it all is useless. Most ceramic artists and potters can create basic forms on the wheel rather quickly. But being away from ceramics so long I had forgotten how to throw most forms. I had to practice the essential forms that I would be throwing most, which was a simple cylinder shape, for cups, and wider cylinder forms for bowls. I made large amounts of ugly pots, wasted clay trimming heavy pots, cracked pots while loading the kiln, over fired pots, under fired pots, and melted a few kiln shelves. But with the view of progress over perfection I was able to have a somewhat enjoyable time experimenting with different parts of the process to find out what worked and what did not. I also had to practice with the raw materials to see what they could do and how far they could be pushed. One example where this took place was adjusting the glaze recipe for it to work correctly on the clay body I was using. I had to know how each material in the recipe reacted and how they all worked together. There were plenty of online resources to help with this process and having the small kilns I was able to fire out test tiles until I had a result that looked somewhat good. I then was able to practice putting a lot of ugly glaze on a lot of ugly pots. Friends and family thought the were nice and wanted to take them home, but I knew better. I needed more practice and progress over perfection.

Progress Through Time

After all the screw ups, piles of ugly pots, and glaze mishaps, I had to take the time to reflect on what I had learned and scrounge up what appeared to be the best-looking pot and have a drink, relax, and view where I was. Many time this is done easier with a friend, family, or even a stranger. A stranger has much better feedback in my opinion because they are allowed, to be frank and not worried about offending if they have no emotional or family ties. I was able to pass some of these ugly pots around to folks and ask them what they thought and took their opinions and feedback to heart and changed some things I was doing wrong and didn’t know it. Just because I may have thought a form looked correct, that did not mean others thought the same. At times, it can be a very sobering and humbling experience to receive constructive feedback, but over time, it will come, for the better or worse. I also took the time to view the works of other ceramic artists. Applications like Etsy and Instagram were excellent resources to view ceramic art and ideas. I was able to compare and contrast different forms to have a better understanding of what expectations artists and buyers were setting. Seeing the work of others forced me to take the time to look critically at the many processes it takes to create a finished piece and see if I could progress more by building skills in each area which in time would add value to the finished piece.

Unlike the YouTube user Mike Boyd, in the video referenced above, I was not able to complete a pot in five hours and forty-five minutes. I’m not even close after five months. I believe I have progressed. What I thought to be an excellent form last month, I now see maybe it was not. And what I am doing today, I see many opportunity areas in my skill level that needs developing, from wedging the clay to when the customer receives it. For a perfect pot can be created, marketed, and shipped. But if any part of the process is not of the highest quality, the whole chain can be easily broken. Creating a decent pot is only twenty percent of the entire process. I do not know what a perfect pot is, but I am beginning to know what it is not.

 

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Artist /Founder- Alford Wayman