What Temperature Can A Clay Pot Withstand?
Clay pottery has a long history dating back over 20,000 years. Early pottery was made from clay fired at low temperatures in pit fires or open bonfires. Over time, potters developed higher temperature kilns and began mixing materials like sand into the clay to create stronger vessels. The maximum temperature a clay pot can withstand depends on the composition of the clay body and the firing process.
There are several standard firing temperature ranges for clay, from low fire (under 2,000°F) to high fire (over 2,300°F). The minerals present in the clay melt to varying degrees at higher temperatures, resulting in pots with increased strength, stability, and impermeability. However, too high of a firing temperature can cause defects. Understanding the characteristics of the clay body is key to determining the ideal firing range.
This article provides an overview of common firing temperature ranges for clay pottery and factors that impact the maximum temperature a given clay pot can withstand before defects occur.
Clay Composition
There are several main types of clay used in pottery, each with their own properties and characteristics:
- Earthenware – Made from clay containing iron and other minerals, giving it a more porous texture. Fires at lower temperatures up to 2,200°F. Examples include terracotta and red clay.
- Stoneware – Denser clay containing feldspar, firing at temperatures between 2,200°F – 2,400°F. Provides a non-porous and durable finished product. Examples include grey clay and brown stoneware.
- Porcelain – Made from kaolin clay, giving a white color and translucency when fired. Extremely non-porous and hard, firing between 2,300°F – 2,500°F. Also known as china clay.
- Ball clay – Added to stoneware and porcelain clays, high in kaolinite content providing plasticity and strength when thrown on a wheel. Fires up to 2,600°F.
- Fire clay – Withstands extremely high temperatures up to 3,000°F. Used to make kiln shelves and insulating firebricks. Resists thermal shock.
The specific clay composition affects the final strength, porosity, color, and firing temperature a pottery piece can withstand (Soul Ceramics). Most pottery combines multiple clay types to achieve the desired characteristics.
Firing Process
The firing process is critical for turning soft, unfired clay into a hard, ceramic material. There are several methods used for firing clay:
Kiln firing involves placing clay pieces into a kiln, which is an oven-like piece of equipment that can reach very high temperatures. The kiln brings the clay up to a high temperature, generally between 1000-1300°C depending on the type of clay, in a controlled manner. This allows the clay particles to vitrify and bond together through sintering. Kiln firing produces consistent results and allows firing at specific temperatures (Smith, 2022).
Pit firing involves placing the clay pieces into a fire pit or rudimentary kiln constructed from brick, stone or clay. Firewood is used to bring the temperature up to around 800-1000°C. The process is more variable but can produce unique effects on the clay. Specialized raku firing involves removing pieces from the fire pit at peak temperature to cool rapidly (Visual Arts Scheme of Work, 2022).
The firing process permanently hardens the clay through vitrification and sintering. The technique used impacts the maximum temperature and effects on the final ceramic piece.
Low Fire Temperatures
Low fire refers to pottery that is fired at temperatures between 1,812-2,102°F (990-1,150°C), typically cone 04-06 on the pyrometic cone equivalent chart1. At these temperatures, the clay does not vitrify but remains porous after firing. Common low fire glazes include terra cottas, red and brown glazes made from iron oxide, and blue celadons. Low fire clays and glazes are less expensive and more forgiving to work with. Pieces can be decorated with overglazes since the glaze layer does not melt and become glassy. Typical uses include flower pots, decorative bowls and tiles, and garden sculptures that can withstand some exposure to weather.
Midrange Firing Temperatures
Midrange firing temperatures typically refer to cone 4-7 with temperatures ranging from 2124°F to 2264°F (1162°C to 1240°C). This temperature range is commonly used for stoneware and porcelain clays (https://www.soulceramics.com/pages/guide-to-kiln-temperature-ranges-for-pottery).
Midrange firing is characterized by cone ratings in the 04 to 7 range. Cones are small pyramid-shaped indicators that bend and melt at specific temperatures to show when critical points have been reached in the kiln. Typical midrange cones include (https://www.lakesidepottery.com/HTML%20Text/Tips/Tempruturerange.htm):
- Cone 4 – 2165°F (1185°C)
- Cone 5 – 2236°F (1225°C)
- Cone 6 – 2277°F (1246°C)
- Cone 7 – 2285°F (1252°C)
Midrange firing is commonly used for stoneware clay bodies. At these temperatures, the clay vitrifies and becomes impermeable to water while remaining durable and resistant to crazing. Midrange firing is also used for some porcelains which fully mature and become translucent within this range. The ware is suitable for functional pottery, tableware, and artware.
High Fire Temperatures
High fire refers to pottery that is fired at high temperatures typically between Cone 8 and Cone 10 on the pyrometric cone equivalent temperature chart. This corresponds to temperatures ranging from 2300°F to 2381°F (1260°C to 1305°C) [1]. High fire temperatures are needed to fully mature certain clay bodies like stoneware and porcelain. At these high temperatures, the clay vitrifies becoming non-porous, hard, and glass-like.
Stoneware clay bodies are formulated to be fired at high fire temperatures. The silicates in the clay melt at temperatures above 2200°F (1204°C) creating a strong, water-resistant ceramic. Typical stoneware glazes are designed to mature at cone 8-10 temperatures resulting in a glossy or satin finish. When fired between 2336°F and 2381°F (1279°C and 1305°C), the stoneware clay and glazes become fully fused for a functional and durable final product [2].
Extremely High Fire Temperatures
Porcelain clays are fired at extremely high temperatures, typically between cone 10 and cone 13 which corresponds to 2345°F to 2462°F (1285°C to 1350°C) according to the temperature conversion chart (https://www2.bellevuecollege.edu/artshum/materials/art/husby/firingtemperatures.pdf). At these high heat levels, the clay vitrifies and becomes non-porous. Pieces fired at this range have very low absorption and high mechanical strength.
Cone 10, the most common high temperature for porcelain, corresponds to 2381°F or 1305°C. According to Soul Ceramics, porcelain clays can withstand firing up to cone 14 before deformation occurs (https://www.soulceramics.com/pages/guide-to-kiln-temperature-ranges-for-pottery). At cone 14, the maximum temperature reaches 2462°F or 1350°C.
Reduction firing, which produces darker colors in glazes, can enable porcelain clays to reach even higher temperatures before vitrification is complete. Kilnfrog states some porcelain bodies are formulated for reduction firing up to cone 12 or 13, achieving temperatures of 2427°F and 2462°F (1330°C and 1350°C) respectively (https://kilnfrog.com/blogs/frogblog/kiln-temperatures).
The extremely high firing temperatures enable porcelain clays to become fully vitrified with low porosity and high strength. However, exceeding the maximum temperature recommendations can lead to deformation, bloating, or melting of the clay body.
Firing Temperature Ranges
Ceramic clays are fired across a range of different temperatures depending on the type of clay and desired properties. There are standard temperature ranges categorized into cone levels that serve as a guide for firing:
| Cone Level | Temperature Range |
|---|---|
| Cone 06 | 1650°F – 1710°F (898°C – 931°C) |
| Cone 04 | 1945°F – 2050°F (1063°C – 1121°C) |
| Cone 2 | 2165°F – 2264°F (1185°C – 1240°C) |
| Cone 4 | 2190°F – 2290°F (1200°C – 1255°C) |
| Cone 6 | 2232°F – 2372°F (1222°C – 1300°C) |
| Cone 8 | 2345°F – 2415°F (1285°C – 1325°C) |
| Cone 10 | 2381°F – 2449°F (1305°C – 1343°C) |
| Cone 12 | 2409°F – 2483°F (1320°C – 1360°C) |
As shown, low fire clays are fired at lower temperatures up to cone 04, midrange firing goes up to cone 4-6, high fire is cone 8-10, and extremely high temperatures reach cone 12.[1] Keeping the firing within the recommended cone level range helps achieve the desired strength, porosity, color, and other properties for that clay body.
Factors Affecting Maximum Temperature
There are several key factors that affect the maximum temperature a clay pot can withstand:
Clay Composition
The makeup of the clay body is one of the most important determinants of firing temperature. Clays with higher amounts of silica and alumina can endure much higher temperatures before vitrifying or deforming. Earthenware clays usually contain lower amounts of fluxes and have a maximum temperature around 2,000°F (1,100°C), while stoneware and porcelain clays have higher amounts of fluxing agents and can be fired up to 2,500°F (1,370°C) or higher [1].
Glaze Ingredients
Glazes are made up of materials that flux, soften, and melt at certain temperatures. The ingredients in a glaze determine its melting range. Low-fire glazes contain more fluxing agents and melt at lower temperatures, while high-fire glazes are more refractory and can withstand much higher heat. Using a glaze that exceeds the clay body’s maximum temperature can cause defects like pinholing, cracking, and bloating [2].
Firing Method
The firing technique also impacts the maximum temperature. Fast firings tend to increase the thermal shock on pottery, while slower ramps and extended holds allow pieces to heat and cool more gradually. Fast firing can also cause defects like scumming, blistering, and crawling in glazes. Firing too fast risks exceeding the thermal stress limits of the clay body [3].
Conclusion
This article has reviewed the key temperatures that clay pots can withstand during the firing process. Low-fire clays can be fired up to around 1922°F (1050°C), while mid-range clays withstand temperatures from 2124-2264°F (1162-1240°C). High-fire stoneware clays reach maximum temperatures around 2300-2400°F (1260-1315°C), and porcelain can withstand firing up to 2645°F (1450°C) before melting.
Properly firing clay to an appropriate temperature is crucial for developing strength and durability. Firing too low will result in weak, porous clay that easily absorbs water and breaks. Conversely, exceeding the maximum temperature will cause the clay to melt, slump, or bloat. Careful control of the kiln temperature ensures the clay reaches an optimal fired state for its intended use.
With knowledge of the firing ranges for different clay types, potters can select appropriate clays and glazes to match their kiln capabilities and desired characteristics. This allows creating high quality, functional ceramic ware.
