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Those experienced in firing ceramics know that kiln atmosphere can have a major influence on the reaction rates and properties of the ware. Since the point at which a cone will bend is brought about by thermochemical reactions, we would also expect that atmosphere also influences when a cone will deform. Fortunately, the atmospheric conditions that influence product properties generally have related effects on cone behavior.
The appearance of cones after they have been drawn from the kiln usually gives an indication of the kiln conditions during the progress of the cones through the fire. Cones going down in the wrong sequence or direction are often an indication of improper placement of the cones, improperly made plaques used to hold the cones, or some other atmospheric effect.
The environmental conditions that are most likely to influence cone behavior are listed below followed by a short description of the cause:
Presence of sulfur gases
Presence of water vapor
Drafts within the kiln
Reducing conditions are associated with gas kilns when the ratio of gas to air is adjusted so the oxygen content is very low. Under reducing conditions, the organic binder and any organic matter associated with the materials used in making a cone, cannot fully oxidize. As a result, the cones will develop a black core from trapped carbon. Most ceramics will mature at lower temperatures when fired in a reducing atmosphere.
In some processes, excess fuel is injected into the kiln to induce reduction, as is commonly done in the manufacture of bricks to produce a color effect. This process often called “Flashing” does not normally have an adverse effect on cone deformation.
Black coring can also occur if the temperature is increased too fast at the onset of a firing. Under such conditions, there is insufficient time for oxidation of the organic material within the cone to occur.
The presence of a carbon core (black coring) can induce bloating and change the bending behavior of a cone, making them inaccurate.
Typical appearance of cones
Cones exposed to reducing atmosphere
The presence of sulphur in kilns will have an adverse effect on cones that are manufactured with red iron oxide; those include cones numbered from 010 to 3. The “red” cones will have a green to black appearance after firing in a reducing atmosphere.
In kiln atmospheres that are either Sulphurous or reducing, cones can also develop a rigid “shell” with a softened interior that is partially melted. These “Hard-shelled” cones will not deform naturally. The surface of the cone will have a matte finish and appear rough with sharp edges. Hard-shelled cones do not bend in an arc but are stiff and will bend only at the base, like a felled tree. Hard-shelling may also be encountered due to the volatilization of boric oxide from the glass frit used to manufacture cones numbered from 010 to 3. Hard Shelling causes cones to deform at temperatures higher than normal.
In order to provide cones that will work in reducing conditions and in the presence of sulphur, Orton manufactures a twin line of cones that are formulated without red iron oxide. The “Iron free” cone series is identical in performance to the regular series of cones from 010 to 3.
The presence of water vapor, particularly in larger quantities, will influence cone behavior. The action of water vapor upon a cone will make it deform sooner than normal. The reason for this is that water vapor diffuses into ceramic materials and causes changes in the vitrification process. This accounts for different behavior seen in firings made in an electric kiln as compared to a gas fired kiln. Even though water vapor in the products of combustion can run as high as 20%, it does not take away from the value which cones can have because comparable effects are usually found in the properties of the ware.
Fly ash from the burning of solid fuel may settle on cones and influence thier deformation.
Likewise, volatile products such as salt fumes, lead, and zinc compounds may produce a surface glaze on the cone which may influence cone bending behavior.
The cutting tongues of flames may cause cones to develop melted tips, or cause abnormal deformation if the cones are placed too close to gas burners. If possible, cones should be protected from flame impingement and kiln drafts.
Radiation from hot surfaces, or, if the cones are placed close to cooler surfaces, will have some effect on cone deformation, and, so far as is practical, should be protected from such conditions.
Cone exposed to Flame Impingement
Radiant effect on cone to the right
From the above examples, it is evident that one can critically analyze kiln conditions through careful observation of the pyrometric cones drawn from the kiln. A properly responsive set of cones will invariably indicate a properly fired kiln, provided the heating schedule is suited to the ware.
More Information on Pyrometric Cones
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