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Cone 6 Firing Schedule- Nerds


glazenerd

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  • 2 weeks later...
  • 2 weeks later...

Nerd,

Just tried this with my last firing. Slowing down to 60C/hr from 1120C up to top temp ("1240C" on my controller, but ^9 down so more like 1260C), 5 mins hold. Two observations, one about pinholes which I'll come to in a later post, the other about clay maturity.

 

Most of my work is on a smooth white body that's pretty mature at this temp (almost unmeasurable absorption), and I'm also testing some Doby's DSS which I bought in Cornwall last summer. DSS is a coarser iron-rich brown clay. It wasn't getting properly mature with my usual schedule (120C/hr straight through to ^9 with a longer soak), it had absorption of 1.7% but an unglazed cylinder was leaking, so I thought I'd see if slowing down from 1120C would improve its maturity by allowing increased densification as you describe above. I didn't know what to expect, and the answer is - it doesn't make any difference. Absorption is still 1.7% and a cylinder still leaks.

 

I think this body is just really wanting ^10 or above (dammit I want to lower my firing temps not increase them!). Mind you, as I write this I'm wondering if I've missed a trick. if my top temp "1240C" is really 1260 then perhaps my controller is also out at 1120C and I should be slowing from, say, 1100C. I'll try that next time.

 

Joe

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Joe:

 

Appreciate you reporting back in: info is always a good thing. My initial thought: have you calibrated your controller to match the results of the cones? In my large front loader, I had to do a thermocouple (TC) offset of nearly 40F- it was firing that  much cooler than the reading. I have noticed that the UK is grossly liberal in their application of cone ranges for clay. Let me post some factory tested info from Old Hickory Ball Clay Co.

 

 

FIRED PROPERTIES  (Absorption)   Cone 04  15.8%      Cone 3   12.7%      Cone 11   5.0%

 

Your assertions that it wants a higher cone is correct. In particular, the absorption falls from 12.7 at cone 3, to 5.0 at Cone 11: significant differences. Those wondering about over-firing stoneware: most ball clays are rated all the way up to cone 32. It is not the clay itself that causes slumping; it is silica/flux additions that are too high for the cone that causes it. So I would conclude from the data you have supplied: it is not the clay in the body: it is the flux level. The are only two remedies for clay makers to address weeping issues in a stoneware body when firing in lower and mid-range temps. 1. is to increase the % of sub micron (SAS 28) ball clays, and 2: raise the flux levels in the formula to increase the glass/mullite production.

 

So at this point you would have to fire to a higher cone to lower the absorption, or try a new clay body. There is one other remedy if you want to take the time to do it. You can wedge in 5-8% of potash feldspar to increase the glassy matrix within the body. A 1.7% absorption rating indicates to me that this formula has a higher % of large particle fire clay, coupled with a higher % of intermediate (120-200 mesh) clays. Let me know the pin hole results when you get them. Several potters have reported complete remediation of pin-hole issues using my firing schedule at cone 6. That would be up to them if they want to chime in: as those conversations where done in PM.

 

Nerd

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Correction: Doble's DSS. Apologies to them for mis-spelling their name! It's a clay pit on the north Cornish coast. To be fair to them I'm not sure what range they claim, certainly not as wide a range as some other UK manufacturers sometimes do. I've emailed for more info. No idea what they mix their own clay with but apparently it's a good thrower. Feels slightly rough, I've not used it yet apart from test tiles and cylinders. Here's an example of someone who uses it: http://www.oakwoodceramics.co.uk/Teabowls%20&%20Yunomi-John-Mathieson.htm

 

I might try your idea of wedging in some feldspar as I'm unlikely to be firing hotter in the near future. 

 

The main thing I thought people would be interested in is that giving more time to increase densification (even if I may not have slowed quite soon enough) does not seem in this case to have affected adsorption/porosity.

 

Joe

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Joe:

 

Read through the link you provided: he is firing to cone 10 with it.

 

You made an interesting notation in your remarks:

 

Feels slightly rough,

Which means they are using a higher percentage of large particle clay/s, of adding a measurable amount of grog: or both. Pending the formulation, you may not be able to overcome the 1.7% absorption rating. Would be curious to hear what the supplier rates this clay in cones? If you want to stay in the cone 6 range, you will have to add some potash feldspar and test.

 

Nerd

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I should have said "somewhat rough". The manufacturer sells this as 1250-1300C and acknowledges some seepage due to coarse particles even at the top end.

 

Please note my comments on this thread are about how slowing down firing above 1120C affects seepage and not particularly addressing the suitability of this body for specific purposes.

Joe

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  • 5 months later...

FYI to anyone using a skutt kiln:

they have gotten bartlett to design an electronic controller for them that does "cone fire" as a segment of a ramp/hold program, so you should get consistent results with every glaze firing, even if you do this slow ramp/hold to help the body mature and prevent micro bubbles.  Very cool, and takes out a bunch of the guesswork.

-jon

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  • 8 months later...

Cone 6-10 Firing Schedule for dark or red bodied clay bodies.

"Originally published in May 2018 issue of Ceramics Monthly, pages (68-69). http://www.ceramicsmonthly.org . Copyright, The American Ceramic Society. Reprinted with permission."

These bodies are typically high in iron; but all include higher levels of carbons such as sulfur and lignite coal particles.  White stoneware bodies are excluded. Firing defects include bloating, carbon coring, and blistering.

There is a division among researchers dating back to the mid 1920’s on the exact mechanism that causing these firing defects. Roughly half concluded that CO gases escaping from carbonaceous materials reduced the iron. The other half concluded that the sulfur (sulfides) from lignite coal and/or from iron sulfide reduced the iron. Due to modern milling techniques and my own experiments in this area: I tend to lean to the sulfide part of this equation.

Either way, bloating, coring and blistering is caused by the early reduction of iron: causing it to act as a vigorous flux which in turn begins to form glass as early as 1850F. This is nearly 200F degrees earlier than normally found in a firing cycle. The early formation of glass from reduction of iron creates an impermeable barrier which traps off gassing feldspars, and traps any remaining carbons.

Pyrite (iron disulfide) is the most common source of iron in raw clays. Iron is what gives stoneware that warm toasty color potters love, and the red/brown colors. However, the sulfur that is chemically bonded to the iron is problem child.  Heat too fast, and starve the oxygen in the kiln: and bloating, coring, and blistering will be the result. Reduction can occur in an electric kiln that is heavily packed, sealed, and heated quickly.

*Edward Orton “The Role Played by Iron in the Burning of Clays”. Amer. Cer. Soc. 7:112

A.G. Bole/F.G. Jackson “The Oxidation of Ceramic Wares during Firing. Amer. Cer. Soc. 7:183

Kramer/Fritz  “The Role of Oxidation in Porcelain and Ball Clay”  Amer. Cer. Soc. 12:13

While these researchers reported their findings mostly in the 1920’s: Brownell, West, and Lawrence found similar results in the late 1950’s and into the mid 1970’s.

 

 

From this research the following temperatures were reported  as carbon burnout phases:

Jackson reported that carbons began burning off at 800F, and accelerated to 1760F.  which others confirmed. Orton believed sulfurs were completely removed between 750-1110F. Others concluded from 750 to 1700F, all carbons were burnt off. The consensus among all the researchers was that high levels of oxidation were required to completely remove all carbons. In the research done nearly thirty years later: the consensus was carbon removal occurred mostly between 1200-1750F. It was Orton who concluded that ferrous sulfide began vitrification as early as 1800F; which resulted in carbon trapping, and trapping of off gassing spars. F.G Jackson performed that most extensive laboratory research of the effects of sulfur, including decomposition and the evolution of reactions on other materials.

Two primary results came from all of these studies.

1.      The kiln must be heavily oxidized, especially from 1200-1750F. Prop the lid open slightly, remove bunges if need be.

2.      The ramping temperature must be slowed down. The higher the iron level and carbon content: the slower the ramp needs to climb. (discussion to follow).

Orton originally recommended a rate climb of 75F an hour for high iron/carbon clay bodies: which was later adjusted to 108F an hour. Others suggested a rate climb of 125F an hour in the 1200-1750F burn out zone. Only testing will conclude which rate is correct for the clay body you use. However, as a rule of thumb: the higher the iron content- the lower the rate of climb.

Observations and conclusions I have made from testing:

1.      If carbon coring is occurring: oxygen needs to be increased and ramping rates lowered by up to 60 degrees an hour. Carbon coring indicates that heavy reduction of iron is occurring around 1800F and high levels of inorganic carbons are present.

2.      Bloating occurs when iron is only partially reduced: creating patches of glass instead of the hard shell associated with coring.  This means some off gassing spars are being trapped, while some is escaping. This indicates a partial starvation of oxygen and iron being reduced in the 1900-2000F range: just prior to off-gassing spars. Ramp speeds need to be lowered by 20-30 degrees, and pulling a bunge or propping the lid to add slightly more oxygen.

3.       Blistering (large craters) indicates that minor reduction in iron is occurring. A complete barrier has not formed, but the clay surface has become extremely dense causing escaping spars to push through under more pressure. Slowing down the ramp cycle by 10-20 degrees an hour during the burn out temperatures should resolve this issue. Pulling a bunge if the kiln is heavily loaded is also recommended.

I cannot give you an exact firing schedule because red bodied and dark bodied stoneware has such a wide variance of iron and carbon levels. I can tell you that ramping between 108-125F an hour from 1200 to 1750F, while supplying lots of oxygen inside the chamber will resolve most all of these issues. If they continue after these recommendations: you have a heavily contaminated clay body that is probably best just to discontinue its use. It is also advisable that you do not start any type of reduction until 2050F, when a normal clay body begins to seal up.

“Carbons” are most often used in an inclusive/broad sense when addressed in most pottery books. Organic carbons such as peat, twigs, bark, and other decayed plant matter burn off with little effort and no effect. Inorganic carbons such as lignite coal, iron pyrite, or other forms of sulfides are the primary contributors to coring, bloating, and craters.

 

Tom

Edit Note: permission is granted for educators to print and distribute this information as part of their instructional criteria.

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