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Black Coring

A common fault in reduction gas fired ceramic ware made from iron bearing clays. The interior cross section of the clay turns black.

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For potters, who use periodic kilns and slow firing schedules, black coring almost always occurs in reduction firings with iron-containing bodies. But it can also occur in densely packed oxidation fired kilns (see below). In industry it occurs in oxidation firings (mainly in tile because production requirements demand fast firing).

There are two schools of thought about the cause in pottery. One blames a combination of fast firing, heavy reduction and trapped carbon. The view is that body carbon fails to oxidize to CO2 and steals oxygen from Fe2O3 (reducing it to FeO, a powerful flux). This FeO then fluxes the body, sealing it and preventing the escape of remaining carbon thus producing the characteristic 'black core' in ware cross-section.

However, black coring happens in bodies that have no carbon: Ones high in iron that have already been bisque fired. The higher the iron and the larger its particle size the more the black coring. Ware cross sections totally enclosed in glaze exhibit the most black coring. Thus, the characteristic black color is the reduced black iron, not carbon. This occurs even when kiln reduction is not heavy.

There will always be some degree of blackness (or gray) in the cross-section of reduction-fired ware that contains materials having any amount of iron oxide (including ball clays). Electric kilns can also produce this phenomenon (in the case of clays of high carbon and iron content, dense kiln pack and little airflow). Once iron is reduced to FeO it is very difficult to reoxidize it back to Fe2O3.

In the tile industry, where ware has not been bisque fired, the problem is organics in the body using up any available oxygen and thus preventing the oxidation of iron compounds. The high density of the ceramic coupled with fast firing make it amazing that it is even possible to fire tile without this issue. Black coring is not considered a functional issue (e.g. strength) as much as an aesthetic one.

Related Information

Close up of black coring: Is it carbon? Nope. Is it always bad? Nope.

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Closeup of black coring in a pottery shard

This is a closeup of a shard from the wall of a thrown vessel. The clay is an iron stoneware, Plainsman Fire-Red with added feldspar, fired in reduction at cone 10. The reduction was not heavy, the kiln was fired with enough air to burn almost all the gas leaving only a slight yellow (but mostly blue) flame at the damper. Is this black color carbon? Consider the following. Carbon is refractory, this is glassy. During bisquit firing the carbon was burned out of this. These black zones have a hole at their centers. This is black iron, a strong flux. The iron is coming from large particles (20-40 mesh) of iron pyrite. In the reduction atmosphere, the natural Fe2O3 is being robbed of oxygen (from both the decomposition of neighboring particles and the atmosphere of the kiln) and converting to FeO. That is melting and interacting with the feldspar to soak into the surrounding matrix. Contrary to what most people think this does not weaken the clay, it strengthens ware (provided the feldspar is present). Once the black has permeated the entire matrix of a piece it becomes very strong (even with a hammer it was unexpectedly difficult to break ware to get these shards). Note the right side is not glaze-covered, if it had been the entire matrix would be black. But still strong.

Black coring with L4168G and L4168F

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Block coring cross section

These are iron reduction test bodies. The L4168G (left) is far stronger yet it has a higher percentage of Fe2O3 and much more black coring. How is that possible? Because it has 10% added feldspar. The black FeO iron is staining the feldspar black (bleeding out from each pyrite particle), helping it do its job of producing a glassy black color is not coming from the kiln atmosphere? Because the buff-burning bodies in the same kiln did not have any of this. On the right, the iron is restricted by its ability to vitrify the body by limited glass development, ending up destabilizing it instead (by increasing body thermal expansion).

C-Red clay glazed and black coring at cone 10R

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Black coring failed cup

C-Red clay (also called Carbondale Red) has 12% iron oxide. It is also refractory so easily handles cone 10R. However with that amount of iron is it pretty well impossible to prevent black carbon coring in glazed ware. This cup simply fell to pieces when just touched. Each of these small pieces can be broken into many more with just light finger pressure. Yet the unglazed test bars were unbreakable by hand! The glaze-body bond is also strong. What is demonstrated here is the extreme compression under which the glaze is attached to the clay, breakage relieves it. That means the black coring is increasing the thermal expansion of the body, as the piece cools in the kiln the body is contracting thermally more than the glaze, putting the latter under more and more compression.

Carbon burnout in a ball clay

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A broken test bar of ball clay fired to cone 10 reduction. Notice the black carbon core. Ball clays commonly contain carbon, many have a noticeable grey color in the raw state because of this. Notice it has not burned out despite the fact that the clay itself is still fairly porous, the firing was slow and the temperature reached was high. Ball clay typically does not comprise more than 30% of a body recipe so its opportunity to burn away is sufficient. However some specialized bodies have a much higher percentage.

Inbound Photo Links

Two clack cored mugs
Feldspar saves this iron red, black coring reduction body

Black coring mug fracturing
Glazing ware only on the inside - the hazard


Articles Organic Matter in Clays: Detailed Overview
A detailed look at what materials contain organics, what its effects are in firing (e.g. black core), what to do to deal with the problem and how to measure the amount of organics in a clay material.
Temperatures Organic burnout (250-370)
Glossary Bloating
When clay materials and bodies bubble as they melt or over fire. This normally happens in raw materials that contain particulates that produce gases during firing.
By Tony Hansen
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