Differences Between Cone Numbers in Kiln Firing Explained

Kiln firing is a critical process in ceramics, pottery, and glasswork, where materials are subjected to high temperatures to achieve desired hardness, durability, and aesthetic qualities. One of the essential tools for potters and ceramicists during firing is the pyrometric cone system. These cones help measure heatwork — the combined effect of temperature and time — inside a kiln. Understanding the differences between cone numbers is fundamental for achieving consistent results and avoiding costly mistakes.

In this article, we will explore what cone numbers represent, how they differ from each other, why these differences matter in kiln firing, and how to choose the right cone for your particular project.

What Are Pyrometric Cones?

Pyrometric cones are small, triangular-shaped ceramic devices designed to melt or bend at specific temperature ranges. Developed by Hermann Seger in the late 19th century, these cones are widely used to gauge heatwork within a kiln during firing cycles.

Each cone number corresponds to a particular melting point under standardized kiln conditions. Unlike a simple thermometer that measures temperature alone, cones account for both temperature and firing duration. This makes them invaluable for understanding the actual effects of heat on ceramic materials.

How Cone Numbers Are Classified

Cone numbers are divided into three primary ranges:

• Low-fire cones: Typically numbered from 022 to 011
• Mid-range cones: Numbered from 010 to 06
• High-fire cones: Numbered from 05 to 10 and beyond

The numbering system can seem counterintuitive because lower numbers often correspond to higher temperatures and vice versa, especially when considering the “zero” in the numbering. To clarify:

• Cones with two leading zeros (e.g., 022) melt at lower temperatures.
• As the number decreases toward zero (022 → 01 → 1 → 10), the firing temperature increases.
• For instance, Cone 022 melts around 1087°F (586°C), while Cone 10 melts near 2345°F (1285°C).

This numbering system has been standardized by organizations such as Orton Ceramic Foundation.

Differences Between Cone Numbers

Temperature Range

The most obvious difference between cone numbers is their melting temperatures. Each cone is formulated to bend or melt at a specific temperature range, reflecting different levels of heatwork in a kiln.

• Low-fire cones (022–011): These melt between approximately 1087°F (586°C) and 1828°F (997°C). They are suitable for earthenware clays and low-fire glazes.
• Mid-range cones (010–06): Melting between about 1828°F (997°C) and 2167°F (1186°C), these cones suit stoneware and some porcelain firings.
• High-fire cones (05–10+): Firing temperatures exceed 2167°F (1186°C), ideal for mature stoneware, porcelain bodies, and high-temperature glazes.

Heatwork Dependence

Beyond just temperature, cone numbers indicate different levels of heatwork — meaning how long a material is exposed to heat at a given temperature. For example, Cone 6 firing might reach approximately 2232°F (1222°C) over several hours. If fired too quickly or too slowly at that temperature, the same cone may bend differently.

This highlights why relying solely on pyrometers or digital thermometers can be misleading; they measure only instantaneous temperature. The cones provide a practical visual indicator of effective heat exposure.

Material Composition

Each cone is made from carefully balanced ceramic components that respond predictably at certain temperatures. Differences in material composition ensure that each cone melts or bends consistently under standard atmospheric conditions inside kilns.

For instance:

• Low-fire cones contain more fluxes that melt at lower temperatures.
• High-fire cones have more refractory materials that withstand intense heat before melting.

Application Suitability

Different ceramic materials require different firing temperatures for optimal maturation:

• Earthenware: Typically fired using low-fire cones like 06 or lower.
• Stoneware: Often fired using mid-range cones such as Cone 5 or Cone 6.
• Porcelain: Usually requires high-fire cones like Cone 10 or above.

Using the appropriate cone number helps ensure the clay body vitrifies correctly — becoming dense and durable without warping or cracking.

Why Choosing the Right Cone Number Matters

Consistency in Results

By selecting the correct cone number for your firing cycle, you can achieve consistent physical properties and surface finishes. Different clays and glazes require precise heatwork levels for predictable outcomes. For example, firing a glaze meant for Cone 6 at Cone 10 could result in glaze running or defects due to over-melting.

Energy Efficiency

Understanding cone differences also helps optimize energy usage. Overfiring not only wastes fuel but can damage ware by causing deformation or melting. Underfiring leaves ware porous and weak due to incomplete vitrification.

Kiln Safety

Incorrect cone selection may cause excessive kiln chamber stress or damage heating elements by pushing kiln limits unnecessarily. Using proper cones preserves kiln life and reduces maintenance costs.

Practical Tips on Using Cones

Placement Inside Kiln

Place multiple cones representing preheat, target fire, and postfire indicators inside your kiln strategically. This helps monitor progression:

• Guide cone: Placed before ware; bends first indicating approaching peak temperature.
• Firing cone: Located near ware; indicates when target heatwork is reached.
• Guard cone: Positioned after ware; shows when overfiring begins.

Interpreting Cone Bending

Cones bend progressively as they soften. The angle of bend indicates which stage the firing has reached:

• Slight tilt shows nearing target temperature.
• Complete bending or melting means target or even beyond has been reached.

Observing these changes allows timely adjustments during manual firings.

Digital vs Pyrometric Cones

Modern kilns sometimes incorporate digital controllers with thermocouples providing real-time digital temperature readouts. While convenient, these devices measure instantaneous temperature without accounting for total heatwork exposure time like cones do.

Therefore, many potters still use physical cones as backup verification tools during firings.

Summary

Pyrometric cones are vital indicators used in kiln firings that measure both temperature and duration of heat exposure — collectively known as heatwork. Different numbered cones correspond to specific melting points across low-, mid-, and high-temperature ranges suited for various ceramic bodies and glazes.

Key differences between cone numbers include their melting temperatures, material composition, sensitivity to heatwork duration, and applications in particular firing ranges. Understanding these distinctions enables potters to choose appropriate firing schedules with accuracy, resulting in quality ceramics with predictable finishes while maximizing kiln safety and energy efficiency.

Whether you’re a beginner ceramicist or an experienced professional, mastering pyrometric cone usage will significantly improve your control over kiln firings and overall craftsmanship quality.