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  • Johannes Scott

FIRESTARTER: the art of pyrometry

Like in Stephen King’s 1980 novel Firestarter, where his character mysteriously manipulates fire to the amazement of huge crowds, the potter is often seen in similar light, as if in possession of pyrokinetic power. The ceramist is skilled in the art of pyrometry.

The sublime of ceramic aesthetic, for example, the brute physicality of sulphide acid bonding with alkali to form a beautiful lead-based red silicate, or the conversion of oxidation to reduction fire, is, to put it simply, founded by the convention of thermal manipulation.

Symbolic validation of ceramic aesthetic is a subliminal experience because the enigmatic, chemical bonding between these geological elements takes place, seemingly, beyond human experience. Inside the burning furnace, where the naked human eye turns into a ball of black ash, that is where primordial, molten glaze gestures its anamorphic natal.

Like the master chef, the modern ceramist practices thermometry for the precise measuring of kinetic energy. The ancient Greek noun qualifies, “πῦρ” (pyr) for fire and meter, meaning, to measure fire. The ceramist is a pyrometric analyst, measuring thermal expansion of solids – the more kinetic energy, the more the expansion.

Chinese master potters, almost one-and-half thousand years ago, developed a pyrometrical sophistication challenged only recently by modern physics, for example, by digital infrared radiometry. Premodern potters employed analogous pyrometry to register equivalence between incandescence and thermal radiation.

Variation in incandescence provides a complete thermal spectrum that enabled the traditional potter to register heating and cooling curves and gauge the effect of time on regulated combustion. By gauging through kiln spyholes, the thermal analyst measures incandescence for optical analogy between colour and temperature: blue-white for 1400 degrees Celsius; yellow for 1250; bright orange for 1200; dull red for 700, and so forth; with a limit of black for 550 degrees Celsius.

Like the pipe organist, operating multitudes of wind chambers, withholding here and shutting there, releasing by sequence, the master potter regulates thermal velocity with a system of spigots and bungs. Ventilator contraptions built into the kiln by mathematical engineering, regulate combustion. Through a sophisticated network of vents, a down-draught kiln can work inexhaustibly against gravity, building up a speed of kiln and chimney gasses, respectively, to 1 and 1.5 meters per second.

With fireclay saggers acting as radiators, accumulating and reflecting thermal projection, and together with the coordination of inlet and exit flues, fire-boxes, dampers, blowholes, and stoke-holes, a thermal blue-white incandescence can be achieved after thirty-five hours of wood fuel stoking.

In addition, by regulating the thermal combustion, the ceramist has full control of the atmospheric conditions inside the kiln. For example, when the kiln is firing at full combustion speed, peaking at its climax temperature, the ceramist simply shuts the inflow of oxygen to transform an oxidated refractory such as iron into a powerful flux. This starvation of oxygen, at a time when combustion is most ferocious, forces the reduction fire to purge both the molten glaze and clay body for fuel. This smothering causes metallic oxides to convert to their reduced metallic state.

Sophisticated pyrometry allows for down-firing, meaning, to fire while cooling. Down-firing regulates an extreme condition of slow-cooling that causes crystallisation. This is the sublime secret of the master potter for trapping large subliminal crystals inside the molten glaze surface. The magical reverse, fast cooling, produces shining, window clear, transparency when the molten flow of the glaze is suddenly immobilised, or ‘frozen’ to solidity.

Moreover, the art of pyrometry has significance in serial, thermal layering. Firing a ceramic piece consecutively, beginning at the highest and ending in the lowest thermal range, each sequence seals the glaze surface and renders it impenetrable and unyielding to the next overlay of glaze application.

In conclusion, the modern ceramist has exchanged analogous incandescence for remote-sensing pyrometers and thermocouples. Emitted thermal radiation is now determined from a distance and registered by a digital electro-meter.

At TAB Studios, we use a thermocouple that penetrates the kiln interior through a small spyhole. The thermocouple is a transducer consisting of two dissimilar metal wires, copper coupled with iron, and looping both inside and outside the kiln.

At the inside coupling, where the two metals are exposed to the same thermal expansion, kinetic energy calibrates with proportional difference between copper and iron. The proportional exchange of calibration is always a consistent, stable difference.

At the external loop, a pyrometer is wired sending a small voltage through the interior loop, from copper to iron wire. The pyrometer calculates the difference between input and output calibration as digital data.

For a true thermal reading, pyrometric cones are used as supplement to the digital data produced by a thermocouple. The cone represents the equivalence between true thermal expansion and a consistent digital chart.

Pyrometric cones are laboratory manufactured, slender pyramid shapes. These cones consist of precise formulas of clay and salt, each formula stamped with a serial number that represents a precise thermal identity. For example, in the Orton cone range, which is the manufacturing type we use at TAB Studios, Cone number 03 represents 1100 degrees Celsius; 05 is 1046; 06 is 1000; and 017 equals 747 degrees Celsius.

The formulated variations in clay and salt provides each cone category with a known resistance to thermal exposure. The scale of salt and clay mixtures determine at what precise temperature the cone will begin to shrink and collapse under its own weight. Heat and thermal time exposure softens the tip of the cone, after which it slowly starts bending and at its predetermined, known temperature range, the tip will complete a ninety degree fall, from straight up to finally meeting its base level.

The pyrometric cone registers the thermal equivalence for the digital data provided by the thermocouple, resulting in automated firings. Once the thermal equivalence for the digital data is charted, the electric pyrometer can be set, digitally, for unlimited, automated cycles of firings.

The modern ceramist can therefore be argued to manipulate fire at an automated distance, almost like Steven King’s pyrokinetic character. But unlike King’s fictional character, who is a product of nature, operating chaotically, and without symbolic convention, ceramic aesthetic represents a subliminal order of fire. J.J.S.7.6.20.

Pyrometric cone inside oxidation kiln.


Gracht Gallery


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South Africa

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