What are Orton cones used for? Understanding Pyrometric Cones in Ceramics
If you've ever admired a beautifully glazed piece of pottery or a precisely fired ceramic sculpture, you might not realize the silent heroes behind its perfection: pyrometric cones, often referred to as Orton cones. These humble-looking ceramic objects play a crucial role in the firing process of ceramics, acting as reliable indicators of heat work. But what exactly are they used for, and why are they so important?
The Core Function: Measuring Heat Work
At their heart, Orton cones are used to measure and indicate the amount of heat work that has occurred within a kiln. This is a critical distinction. Kilns don't just reach a certain temperature; they reach that temperature over a specific period, and the rate at which the temperature changes also matters. This combined effect of temperature and time is known as "heat work."
Think of it like baking a cake. You can set your oven to 350 degrees Fahrenheit, but if you only bake it for 5 minutes, it won't be done. If you bake it for 3 hours, it will be burnt to a crisp. The oven's temperature is one factor, but the duration is equally important. In ceramics, pyrometric cones visually demonstrate whether the "baking" – or firing – has been sufficient for the clay and glaze to mature properly.
How Do They Work?
Orton cones are specifically formulated ceramic cones that bend and deform at a predictable combination of temperature and time. They are made from a precisely controlled mixture of clays and fluxes, similar to the materials used in actual pottery. Each cone is designed to soften and bend at a specific "cone number," which corresponds to a particular range of heat work.
When a kiln is fired, cones are placed inside, usually in a prominent spot where they can be easily observed (or on a shelf where they are visible through a peephole). As the kiln heats up, the cones begin to soften. The cones are designed to bend over until they lie flat, indicating that the desired heat work has been achieved. The degree to which a cone has bent provides a direct visual cue to the kiln operator.
Why Are They So Important for Potters and Ceramicists?
The accurate firing of ceramics is paramount for several reasons:
- Clay Maturation: Clay needs to reach a certain level of heat work to become vitrified, meaning it becomes dense, strong, and non-porous. If fired too low, the clay will remain brittle and porous. If fired too high, it can become distorted or even melt.
- Glaze Maturation: Glazes, which are essentially glass coatings applied to clay, also need to reach a specific heat work to melt, fuse with the clay body, and achieve their intended color, texture, and durability. An underfired glaze will be dull and may craze (develop fine cracks), while an overfired glaze can become runny, discolored, or even burn off.
- Consistency and Reproducibility: For professional potters and studios, achieving consistent results from firing to firing is essential. Orton cones provide a reliable, objective measure to ensure that each batch of work is fired to the same standard.
- Safety and Efficiency: Overfiring can damage kilns and waste energy. Underfiring can result in unusable pieces. Using Orton cones helps prevent these issues, leading to more efficient and safer kiln operations.
Types of Orton Cones and Their Applications
Orton cones come in a wide range of cone numbers, each corresponding to a different softening point or heat work value. These are typically categorized as:
- Low-Fire Cones: Used for firing earthenware and low-temperature glazes, generally in the range of cone 022 to cone 06.
- Mid-Fire Cones: Used for stoneware and mid-range glazes, typically from cone 05 to cone 6.
- High-Fire Cones: Used for porcelain, stoneware, and high-temperature glazes, ranging from cone 8 up to cone 10 and beyond.
Potters select cones based on the specific clay body and glaze they are using, consulting firing schedules and charts provided by clay and glaze manufacturers. Often, a "cone pack" – a set of three cones with progressively higher cone numbers – is used to monitor the firing more precisely. The first cone in the pack is expected to bend, the second to lie down, and the third to remain upright, indicating that the firing has reached the target heat work without going significantly over.
Beyond the standard cones, Orton also produces specialized cones, such as self-supporting cones which don't require a cone plaque. However, the fundamental principle of measuring heat work remains the same.
In Summary
In essence, Orton cones are invaluable tools for any ceramic artist or hobbyist. They are not just temperature indicators; they are sophisticated monitors of the complex interplay of heat and time that transforms raw clay into durable and beautiful ceramic objects. Their simple, visual representation of heat work makes them an indispensable part of the ceramic firing process, ensuring that your creations achieve their full potential.
Frequently Asked Questions (FAQ)
How do I choose the right Orton cone for my firing?
You should choose an Orton cone based on the firing temperature recommended for your specific clay body and glaze. Manufacturers of clay and glazes provide recommended cone numbers for their products. It's also common to use a "cone pack" of three cones to monitor the firing more precisely.
Why do Orton cones bend instead of melting like metal?
Orton cones are made from carefully formulated ceramic materials, not metal. These materials soften and deform gradually under heat due to the melting of their glassy components. This gradual bending allows them to accurately indicate the cumulative effect of temperature and time, which is heat work, rather than simply melting at a single point.
Can I reuse Orton cones?
No, Orton cones are single-use items. Once a cone has bent and deformed, it has served its purpose as an indicator. They cannot be reliably recalibrated or reused for subsequent firings.
What happens if my Orton cone doesn't bend?
If your Orton cone doesn't bend, it means the kiln did not reach the intended level of heat work. This could be due to insufficient temperature, insufficient time, or a combination of both. The clay and glaze may be underfired, potentially leading to issues like poor strength, porosity, or glaze defects.
Why are they called "Orton cones"?
They are named after Edward Orton Jr., who was a prominent ceramic engineer and educator in the late 19th and early 20th centuries. He established the Edward Orton Jr. Ceramic Foundation, which pioneered the development and standardization of pyrometric cones for the ceramics industry, making them a reliable tool for controlling kiln firings.
