Whether you’re using a kiln for industrial purposes or personal projects, understanding your kiln’s firing temperatures is essential to get the right results. At HeatTreatNow, we’ve written this article to help you figure out your temperature ranges, whether it’s for heat treating, pottery, or glass. Let’s get started!
The Significance of Kiln Firing Temperatures
Temperature changes can greatly affect the properties of materials by altering their molecular structure. For instance, heating can make a material harder or softer, change its color, or affect its flexibility and resistance to water. These changes are crucial in various processes, such as firing clay in a kiln, tempering metal blades, or melting glass for molding.
Different materials respond to heat in unique ways, so controlling the temperature precisely is essential. Each material has specific temperatures at which it undergoes particular physical or chemical transformations. Understanding and managing these temperatures ensures that materials achieve the desired characteristics and performance for their intended use.
Classifying Kiln Firing Temperatures
There are several ways to categorize kiln firing temperatures, including temperature range, material, procedure, and cone temperature. In general, artists in various media categorize kiln temperatures differently.
- Kiln-Formed Glass: For glass artists, temperature settings are categorized by the technique being used—such as slumping, casting, fusing, or annealing—and by the glass's coefficient of expansion, which measures how glass expands with temperature changes. The thickness of the glass also affects the process, influencing hold times and ramp rates more than temperature alone.
- Pottery and Ceramics: Temperatures are commonly organized based on cone firing ranges on the Orton Pyrometric Chart. Each ceramic material is designed to withstand specific cone temperatures, which provides a more standardized framework for firing. This system allows for consistent results across various types of ceramics, as each cone represents a range of temperatures essential for achieving optimal results.
- Heat Treating: Temperatures are primarily classified by the types of treatments such as hardening, normalizing, or tempering, and the specific material being treated. Unlike some fields, there are no universal temperature ranges or schedules that fit all scenarios—each treatment requires precise adjustments tailored to the material and the desired outcome.
Pottery vs. Glass vs. Heat Treatment
The main differences between kiln temperature ranges for pottery, heat treating, and glass are primarily based on the materials' firing requirements and the intended outcomes of the processes. Pottery kilns typically operate at higher temperatures compared to glass kilns. For pottery, the firing temperatures can be categorized into low-fire, mid-fire, and high-fire ranges. Pottery kilns have longer cycles and controlled heating and cooling phases to prevent thermal shock. Pottery kilns can use different atmospheres, such as oxidation or reduction, to achieve various finishes.
In contrast, glass kilns operate at lower temperatures tailored to different glassworking techniques. The firing temperatures can be categorized into slumping, fusing and casting ranges. Glass kilns, however, generally operate in an oxidation atmosphere to maintain the clarity and color integrity of the glass.
Heat treating differs significantly from both pottery and glass firing. It typically involves using an industrial kiln to heat metal to specific temperatures and then cooling it in a controlled manner to alter its physical and chemical properties. Heat treating temperatures vary widely depending on the metal and desired outcome, but they can range from relatively low temperatures (around 300°F/150°C) for some aluminum alloys to very high temperatures (over 2000°F/1093°C) for some steels. Unlike pottery and glass firing, which are often about creating a finished product, heat treating is usually an intermediate step in metalworking to enhance properties like hardness, strength, or ductility. The atmosphere in heat treating furnaces is also carefully controlled, often using inert gases or vacuum to prevent oxidation.
Temperature Ranges for Heat Treating
Heat treating involves using a heat treatment furnace to heat and cool metals to achieve desired properties such as hardness, strength, or ductility. The temperature ranges for heat treating vary depending on the metal and the specific process being used. For instance, when heat treating a knife, annealing, which softens the metal and relieves internal stresses, generally occurs between 1,200°F and 1,600°F (649°C and 871°C).
Hardening, which increases the metal's hardness, typically takes place at higher temperatures, ranging from 1,400°F to 1,600°F (760°C to 871°C) for steel. After hardening, tempering is done at lower temperatures, usually between 300°F and 700°F (149°C and 371°C), to reduce brittleness and achieve a more balanced toughness. Each metal, such as steel, aluminum, or titanium, has its own specific temperature range for these processes, so it’s crucial to follow the recommended guidelines for the material you're working with to ensure optimal results
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Related: The Role of Heat Treatment in a Sustainable Future
Kiln Temperature Ranges for Pottery
Low Fire
The kiln temperature ranges for pottery vary depending on the type of clay and glaze used. Low-fire pottery typically uses earthenware clay, which is known for its porous nature and affordability. The temperature range for low-fire pottery is between 1,828°F (997°C) and 2,116°F (1,102°C), which corresponds to cone ranges from cone 06 to cone 02. When fired, the kiln color will usually glow red to orange.
The firing results for low-fire pottery often include bright, vibrant colors and a glossy finish due to the low-fire glazes, which are formulated to mature at lower temperatures. These glazes can produce a wide variety of effects, from transparent to opaque and matte to glossy. Additionally, low-fire ceramics tend to be less durable compared to higher-fired pieces, making them more suitable for decorative rather than functional use.
Electric kilns are used for low-fire ceramics due to their ease of operation and accurate temperature control. They help low-fire glazes achieve optimal outcomes by maintaining consistent temperatures. Electric kilns offer more control and consistency, therefore gas kilns are less prevalent for low-fire pottery. Using raku kilns, pottery can be removed when red-hot and placed in combustible materials to create unique textures and finishes.
Mid-fire
The kiln temperature ranges for mid-fire pottery are slightly higher than low-fire, typically using stoneware or porcelain clays known for their strength and durability. The temperature range for mid-fire pottery is between 2,124°F (1,162°C) and 2,262°F (1,239°C), which corresponds to cone ranges from cone 4 to cone 7. When fired, the kiln color can range from orange to yellow.
Firing results for mid-fire pottery include a harder, more durable ceramic body compared to low-fire pieces, making them suitable for functional ware like dishes and mugs. The mid-fire glazes used in this temperature range can achieve a wide variety of finishes, from matte to glossy, and are often more stable and less prone to crazing than low-fire glazes. Additionally, mid-fire glazes can produce beautiful, earthy colors and effects that are highly valued in functional and artistic pottery.
Kiln options for mid-fire pottery include electric and gas kilns, with many potters favoring electric kilns for their precise temperature control and ease of use. Gas kilns are also popular for mid-fire pottery due to their ability to create reduction atmospheres, which can result in unique glaze effects and colors that are difficult to achieve in electric kilns. For those looking to experiment, wood-fired kilns offer another option, providing distinct textures and surface qualities influenced by the interaction between the wood ash and the pottery.
High-fire
High-fire pottery is characterized by its use of high kiln temperatures, making it the hottest among pottery firing types. Typically, this process involves stoneware or porcelain clays, which are known for their exceptional strength and vitrification properties. The temperature range for high-fire pottery spans from 2,345°F (1,285°C) to 2,646°F (1,450°C), corresponding to cone ranges from cone 8to cone 10. During firing, the kiln often glows bright yellow to nearly white.
The results of high-fire pottery are incredibly durable, producing non-porous ceramic bodies that are ideal for practical items like dinnerware, bakeware, and sculptures. High-fire glazes tend to have a subdued, earthy palette with subtle variations in texture and color, creating a refined finish that can withstand high temperatures without excessive running or melting. Porcelain glazes, in particular, can achieve a range of finishes from glossy and translucent to matte and opaque, enhancing the natural beauty of the porcelain.
For high-fire pottery, kiln options include gas and wood-fired kilns, which are capable of reaching the necessary temperatures. Gas kilns are favored for their ability to create reduced atmospheres needed for certain glaze effects, while wood-fired kilns produce distinctive pieces with unique textures and color variations due to the unpredictable nature of wood ash Cone 10 electric kilns are usually built with 3-inch brick and heavy-duty elements to withstand the rigors of higher temp firings...
Kiln Temperature Ranges for Glass
Slumping
For glass slumping in a kiln, temperatures typically range from 1,100 to 1,350 degrees Fahrenheit (593 to 732 degrees Celsius). The specific temperature you choose depends on the type of glass and the desired outcome. Slumping is a process where glass is heated until it becomes soft and flexible, allowing it to conform to the shape of a mold.
To achieve this, the kiln must reach a temperature where the glass begins to soften but not so high that it melts completely. Generally, you'll start by heating the kiln gradually to avoid thermal shock and then hold the temperature for a specific period to ensure the glass fully conforms to the mold's shape. After reaching the desired slumping temperature, the kiln is slowly cooled, in a process known as annealing, to prevent any cracking or warping of the glass.
Slumping uses float, fused, and art glass. Melting points and properties vary by type, affecting slumping temperature and process. Art glass may require slightly different temperatures than float glass. To avoid thermal shock, the kiln is gently heated during slumping, and the glass is maintained at the slumping temperature long enough to conform to the mold. Then, the kiln is progressively cooled to prevent glass cracking or warping.
Fusing
Fusing glass involves heating it in a kiln to temperatures where the glass softens and melds together, but doesn’t fully melt into a liquid. Typically, the kiln temperature for fusing ranges between 1,300 and 1,500 degrees Fahrenheit (700 to 815 degrees Celsius). At these temperatures, glass pieces, whether they are small chips or large sheets, will fuse into a single, cohesive piece. This process is used to create everything from intricate art pieces and jewelry to functional items like tiles and coasters.
Common materials for fusing include compatible glass types such as Bullseye or Spectrum glass, which are formulated to fuse well together. The kiln heats the glass slowly to avoid thermal shock and to ensure even melting, then it cools down gradually to solidify the fused glass into its final shape. Proper temperature control and timing are crucial to achieve the desired result and avoid issues like cracking or incomplete fusion.
Casting
For glass casting in a kiln, temperatures typically range between 1,400°F to 1,600°F (760°C to 870°C). This process involves melting glass in a kiln to form intricate shapes and designs. The specific temperature can vary depending on the type of glass being used and the desired outcome. For example, a lower temperature of around 1,400°F (760°C) might be used for softer, more delicate glass forms, while higher temperatures, up to 1,600°F (870°C), are often employed for more robust, detailed castings.
Glass casting requires specialized materials like silica sand, soda ash, and lime, combined in precise ratios to create the desired glass composition. The kiln itself needs to be capable of reaching and maintaining these high temperatures accurately to ensure proper melting and forming of the glass. Additionally, the kiln must have a controlled cooling phase to avoid stress and cracks in the final piece. This process is used to create everything from small decorative objects to large sculptures, showcasing the versatility and beauty of glass as a medium.