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Blistering, Bloating, Coring.


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Blistering, bloating, and coring.

All three of these clay defects come from one source: sulfides. In raw clay, sulfides come from lignite coal particles and iron disulfide. There is a common association with iron when bloating or blistering occurs. Iron darkens raw clay, as does higher levels of organic and inorganic carbons. Dark grey, brown, and red clay bodies indicate higher levels of iron sulfide and inorganic carbons. The color of your clay fresh out of the bag is the biggest indicator of firing schedules.

Blistering can be caused by thick glaze applications, high levels of spar in the glaze, or by over firing. Bloating is almost exclusively related to inorganic sulfide content in raw clays. Coring is solely from inorganic sulfides, which have been improperly fired. Blistering, bloating, and coring occur just in that order pending sulfide content and firing schedule. Blistering means your off schedule a little, bloating a bit more, and coring means you are way off the proper firing speed. 

The same goes for your clay; the darker the body is: the more you need to slow down. In dealing with sulfides: 1800F (1000C) is the point of no return. If you have not burnt off the sulfides by this temperature: there is no further resolution.  Sulfides are inorganic: you must burn off these carbons from 1250F (677C)to 1750F. (955C) Unfortunately you do not realize the firing defects until you glaze fire. If you glaze fire under 2050F, you will never see them.  2050F (1121C) is when metakaolin converts to spinel: or as potters call it: vitrification begins.

All clays are actually expanding up to 2050F, after this the pores begin to close. Once the pores begin to close, then escaping spars do so under more and more pressure as glass forms in the body. Now your bisq firing program makes itself known. If you reduced the iron just a little: blisters form. If you reduced it more, bloating occurs. If you completely reduced it: black coring occurs. So the simplest way to figure it out is to break the defected piece open and have a look. Blistering is easy to identify because it is a surface defect. Bloating can occur sub-surface rather you see it or not. Coring is only identified on the surface by carbon trapping: dark splotches in your glaze that are not suppose to be there. Crazing is also an indication of sub-surface bloating and coring because the COE of the body rises exponentially with these body defects.

For the record: iron does not cause these defects. Iron gives all those warm toasty colors potters love. It is the chemically bonded sulfides attached to the iron that cause them. Potters associate iron with these because as the iron levels go up, so does the incidence of these defects. However, as the iron goes up, so does the sulfide content. The sulfides are directly responsible for reducing the iron: the reduced iron seals the body: the sealed body puts escaping spars under pressure. Spars under some pressure causes blistering, spars partially trapped causes bloating. Completely sealed traps both carbons and spars resulting in carbon coring. The remedy begins at 1250F, not at 1800F. (1000C)


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Bloating is often sub- surface and is not detectable visually. Carbon coring is also not readily visible. However, unexplained dark splotches in your glaze can be trapped carbons. Unexplained crazing is also an indication because theses clay defects cause instability in  clay body. Note: the upper right corner shows the true color of this low iron C6 glaze. A crazing line separates it from an area that continues to darken towards the left.

Note: photos are for educational use.


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Research, Citations,Documentation.

Remedial Firing Schedules.


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.



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



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