Understanding Pyrometric Cone Equivalent PCE

Pyrometric cones are indispensable devices in the ceramics industry, utilized to gauge the heat work involved in firing ceramic materials within a kiln. They play a critical role in determining when ceramics have achieved the desired state of maturity through a combination of time and temperature.

What Are Pyrometric Cones?

Pyrometric cones are specially shaped ceramic devices designed to bend and fall over at specific temperatures. Typically, they are placed alongside the wares in the kiln. Each cone in a set is formulated to soften at a different temperature, providing a visual cue for potters and ceramicists to assess the firing conditions. Rather than functioning as straightforward temperature measuring instruments, pyrometric cones offer a temperature equivalent, indicating the heat work performed during firing.

Design and Composition

The standard pyrometric cone resembles a pyramid with a triangular base. Crafted from a precisely proportioned and uniformly mixed batch of ceramic materials, the cone is engineered to bend and touch its base at defined temperatures. Pyrometric cones are produced in series, with the temperature interval between consecutive cones generally set at 20 degrees Celsius.

Types of Pyrometric Cones

Two notable types of pyrometric cones include:

Orton Cones: Developed by the Orton Ceramic Foundation, these cones are widely used in the ceramics community for their reliability.

Seger Cones: Named after the German ceramist Hermann Seger, these cones are used for similar purposes but may have different specifications.

Pyrometric Cone Equivalent (PCE)

The Pyrometric Cone Equivalent (PCE) refers to the number of standard cones that touch the supporting plaque simultaneously as a test cone made from a different material, when both are heated under standardized testing conditions. It is important to note that PCE is not an exact measure of temperature; instead, it serves as a comparative indicator of thermal behaviour relative to standard cones.

Objectives of the PCE Test

The PCE test serves several purposes:

Purity Determination: To evaluate the purity of refractory materials.

Usability Assessment: To confirm whether a refractory can withstand specified service temperatures.

Classification of Refractories: To categorize refractories based on their thermal properties.

Measuring Refractoriness

To assess the refractoriness of a material, the softening temperature of a test cone is compared to a series of standard pyrometric cones. The process involves heating a test cone in an electric furnace at a controlled rate (typically 10 degrees Celsius per minute). The temperature at which the apex of the cone bends and touches the base signifies its softening temperature. This value is then assigned a PCE rating. If the softening temperature falls between two consecutive Seger cones, the PCE is calculated as an average of the two.

Three-Cone System

In certain applications, such as firing porcelain and lead-free glazes, it is beneficial to operate within a two-cone range. The three-cone system consists of:

Guide Cone: One cone number cooler than the firing cone.

Firing Cone: The cone recommended by the glaze manufacturer.

Guard Cone: One cone number hotter than the firing cone.

This arrangement helps in assessing temperature uniformity and monitoring the performance of kiln controls.

Temperature Variability in Kilns

It is essential to recognize that kilns often exhibit temperature variations from top to bottom, influenced by design, age of heating elements, load distribution, and firing cone number. Cones should be positioned on the lower, middle, and top shelves to gauge temperature differences during firing. This information is crucial for optimizing kiln loading and firing strategies.

Factors Affecting Cone Bending

Both temperature and time, along with atmospheric conditions, can influence the final bending position of a cone, with temperature being the primary variable. The temperature is referred to as an equivalent temperature because actual firing conditions may differ from those used during standardization.

Cone Placement and Kiln Sitters

Proper placement of pyrometric cones is vital to ensure accurate temperature equivalency. Large cones should be mounted 2 inches above the base, while smaller cones should be positioned approximately 15/16 inches above. Maintaining an 8° incline from the vertical position is also crucial for reliable readings. Additionally, some kilns utilize a "kiln sitter," a device that automatically shuts off the kiln when the cone softens, providing an added layer of control.

Ensuring Consistency in Measurements

Pyrometric cones are sensitive instruments, and consistency in their performance is paramount for users. Manufacturers adhere to stringent quality control measures to minimize variability within and between batches. Various national and international standards, including ISO guidelines, have been established to regulate the production of pyrometric cones.

In conclusion, pyrometric cones are essential tools in the ceramics industry, providing crucial insights into the heat work applied during the firing process. Understanding their operation, types, and proper usage is vital for achieving successful outcomes in ceramic production. By effectively utilizing pyrometric cones, ceramicists can ensure their wares reach the desired maturity and quality, paving the way for exceptional craftsmanship.