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You Can Do It!

By firing manual you can get to know your kiln, better understand the firing process, and even manipulate the work by making changes to the firing schedule. This is good practice especially for beginners learning the firing process and firing techniques. It is my opinion that when firing out work in a kiln you need to be there through the whole process as it’s like giving birth. It is quite simple to do but just takes a bit of monitoring and being able to see the correct cones inside the kiln when it is hot and knowing when to shut the kiln off or start the warm down.

The Sermon

Even though many kilns are digital or use a cone sitter, I am a hardcore manual kiln fire guy. Which means I fire all my kilns , even bisque fire, with shelf cones. Now I will not preach at you like a baptist minister if you choose to fire digital shut off or use a sitter, but in my opinion you are depending on luck to shut down each load. Mistakes like putting in the wrong cone number digitally, wrong hold time, improper sitter set up, improper cone set up in the sitter, or the sitter cone melting in a strange way and not shutting down all can cause a kiln to over fire. I witnessed a kiln melt down when I was a young wart hog and ever since fired manual. Kilns can be like children playing in a sandbox next to the road. For the most part they will be fine but if you forget to check on them in no time flat they will be out playing in traffic. You do not want your kiln to be out playing in traffic. Forgetting your kiln could result in a melt down where all the work shelves and posts become like taffy stuck in there. If you simply do a google search for kiln meltdown you will see some amazing images of things that happened when folks had a bad time with their kilns. You will still have a bad time with your kiln even if monitoring it but those bad times will be less painful and disaster averted by taking corrective action. Amen.

Proper Setup

It depends on what types of shelf cones you have. Cones measure both temperature and work heat. If you use a pyrometer to check your kiln, you will only know temperature. Pyrometers do not measure work heat. Some shelf cones need to be placed in a cone pack while others are self standing. I like to use self standing cones in the peepholes and place them on a small piece of kiln shelf or post. When placing them in the cone pack, be sure to research the recommended angle by the cone manufacture. I set the cone almost to the edge of the shelf, so they are easy to see inside when things heat up, and it all looks the same color. At high temperature, you might be able to make out just a slight outline. Some say not to place it near posts or near elements, but I like it right out in front. Be sure your cone will not attach its self to a pot when it bends, cones like personal space.

Looking in There

When things heat up hotter than a goats butt in a pepper patch it might be hard to see the cone. When you look in there it is best to use welding glasses as you do not want to damage your eyes over the long run. Also, if firing with gas pull the plug and wait a bit as if the kiln is in reduction you may have flames shooting out the peephole. So when you look in there be aware of that flame or you can lose eyebrows or catch your good flannel shirt on fire. Now look in there real slow and find the outline. I try and place the cones in front of a pot to see it better. Another technique is to shine a flashlight in the and have the light reflect off the cone. You can also blow into the hole to slightly cool the cone to see the outline. At times if I was not sure and cold not see it I shut down the kiln to be surer than restarted.

Reading the Cone

Be sure to look at the chart put out by the cone manufacture. Orton has a free chart that you can download here.

Also, be sure to check the cone manufacture bend chart. You can see an example here.

Checking the Kiln

Each kiln is different But here is the schedule I use for both bisque and glaze fire to cone 5/6 when firing manual.

Hour 1 & 2 – Check each hour
Hour 3&4 – Check each 30 min
Hour 5 to Cone Drop – Check every 15 min

Below are some examples and tips on what the cone will look like inside the kiln and suggestions on when you might want to shut it down.

When to Shut It Down

Each kiln is different and the rate of the cone drop will change based on temperature and work heat.

If you have the cone that matches the firing, say a cone 5 shelf cones and you want a cone 5 firing, you want to shut it down at the slight bend.

Below is what a slight bend looks like base off the recommended bend chart of Orton.

If you have the cone that matches the firing, a cone 5 shelf cone, and you want a cone 5.5 firing, you want to shut it down at the half bend.
I go to the half bend for most of my firings. By going to the half bend you can properly bring to temperature shelves that may be running a bit cooler slightly.
A half bend for a cone 5.5 firing will look like this:

If you have cone 5 shelf cones and would like a cone 6 firing, you would do a full cone bend. It will look like this:

If you are all out of cones and only have a cone 5 shelf cones and would like to fire to cone 6.5 you would do a full bend plus 15 min.
It will look like this:

If you missed a check because you fell a sleep or the cows got out you can still tell about how much time you missed as you will have hard full bends and soft full bends. A soft full bend the cone will look like melted chocolate but be blistered and bubbled out but not a puddle. It will look a bit like below. Here I was out of cone 6 but used a cone 5 plus 30 min to reach a cone 7 for a bottom shelf. So I intentionally over fired to bring to temp a cone 7 glaze test on a bottom shelf.

All the checks might seem excessive, but help to catch mistakes early and also allow you to make correction. In my gas kilns I can see if the kiln is heating correctly, if the firing is going to slow or fast based on color. Also, when testing firing techniques or a new shelf configuration, how big or small the load is or doing holds. All the checks paid off and saved loads and prevented over fires. I have over fired, but only from falling a sleep and missing the last 15 min. Thank goodness my wife woke me and I only lost 1 shelf of pots to blistering. You can also check cone that did not bend to get an idea of how much off you were on the under fire. A cone that did not bend but at all might be like glass and have gloss say it was close to reaching temperature for example.

Do you fire digital or manual? Do you have any questions about firing manual? Let me know!

Al Wayman
Artist/ Owner

Creek Road Pottery LLC

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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!

 

 

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Al Wayman
Artist /Owner
Creek Road Pottery LLC