Can You Fire Pottery In A Glass Kiln?

Pottery kilns and glass kilns are used for firing clay and glass pieces respectively to complete and finish the objects. Pottery kilns are designed to withstand the high temperatures needed to fire clay and ceramics. Glass kilns are made to handle the extremely high temperatures glass requires during firing. While both types of kilns operate by heating up to very high temperatures, they are optimized for their respective materials. This leads to the question – can glass be safely fired in a pottery kiln, and vice versa, can pottery be fired in a glass kiln?

Differences Between Pottery & Glass Kilns

There are some key differences between kilns designed for firing pottery versus those used for glass. According to Rocket Rose Art (https://rocketroseart.com/2021/11/21/what-is-the-difference-between-glass-and-ceramic-kilns/), a major difference is their maximum operating temperature. Glass kilns typically only need to reach about 1500°F to fuse or slump glass, whereas ceramic kilns need to reach over 2200°F to vitrify clay.

The heating elements also differ. Glass kilns usually have elements that heat from above to ensure even heating of a single layer of glass. Ceramic kilns have elements that surround the chamber to heat pottery from all sides as it is stacked on shelves (Skutt, n.d.).

Finally, glass kilns do not require shelves or kiln furniture since glass items are either slumped into molds or fused between layers of fiber paper or kiln wash. Ceramic kilns need sturdy shelves to hold unfired pottery pieces spaced apart so they can be surrounded by heat. The shelves are made from high-temperature materials like ceramic fiber board or cordierite.

Clay Chemistry

Firing clay causes a number of chemical changes to take place. As the temperature increases, clay loses its chemically combined water, organic matter burns off, and carbonates decompose. The clay particles begin to sinter together as they start to melt at higher temperatures.

Different types of clay are fired at different temperatures to achieve the desired results. Earthenware clays typically fire between 1800-2100°F. They fuse together but remain porous. Stoneware clays require higher temperatures between 2100-2400°F to fully vitrify and become impermeable. Porcelain clays are fired at temperatures upwards of 2300°F so the clay particles fully melt into glass (Ceramic Arts Network). Understanding how the chemistry of the clay changes enables potters to select the appropriate clay and firing method for their desired end product.

Glass Chemistry

Glass is made by heating ordinary sand (mostly silicon dioxide SiO2) along with soda ash (sodium carbonate Na2CO3) and limestone (calcium carbonate CaCO3) to very high temperatures between 1,300°C and 1,600°C. At these high temperatures, the chemicals combine and liquefy into a molten state that can be shaped and cooled into glass. The key chemical reaction is:

SiO2 (sand) + Na2CO3 (soda ash) + CaCO3 (limestone) → Na2O + CaO + SiO2 + CO2

The sodium oxide (Na2O) acts as a flux to lower the melting point of the silicon dioxide, allowing the sand to melt at achievable kiln temperatures. The limestone helps stabilize the glass. When cooled, the materials solidify into an amorphous solid with random molecular structure, rather than forming crystals. This gives glass its transparent, non-crystalline structure.

The melting point of glass varies by composition but is generally between 1,400°C and 1,600°C. Soda-lime glass melts around 1,500°C. Lead glass melts at lower temperatures around 1,200°C due to the fluxing effect of lead oxide. Borosilicate glass melts higher at over 1,600°C due to additions of boron oxide.

Temperature Needs

Clay and glass have very different temperature requirements for firing in a kiln. According to the Kiln Firing Chart from the Ceramic Arts Network, clay should be fired between 1100-1200°C or 2012-2192°F to fully fuse and vitrify the clay particles [1]. In comparison, most glass is fired between 1065-1260°C or 1950-2300°F to melt the glass mixture into a uniform material [2].

Firing clay at too low a temperature can result in weak, porous pieces that aren’t properly vitrified. Meanwhile, firing at too high a temperature can cause deformation, bloating, or melting. For glass, firing too low won’t fully melt the glass while too high can burn or scorch the glass. Using the wrong temperature for the material can lead to failed results.

Kiln Design Considerations

There are several key differences in the design of kilns intended for glass versus those made for firing pottery that need to be considered when attempting to fire clay in a glass kiln.

Shelves in a glass kiln are made of kiln-washed shelves, while pottery kilns use grog shelves. The smooth shelves in a glass kiln can lead to pottery pieces sticking. Modifications would need to be made to add grog shelves. Glass kilns also typically have fewer shelves than pottery kilns to accommodate taller pieces.

The heating elements in a glass kiln are usually made of kanthal wire, while pottery kilns use nichrome wire elements that can handle more thermal stress. Glass kilns also have fewer elements. To fire pottery, upgrades to the heating elements may be needed.

Glass kilns have thermocouples placed low in the kiln to measure glass temperature, while pottery kilns have thermocouples higher up to measure clay temperature. The thermocouple placement would need adjusting to properly measure clay temperature.

Ventilation is extremely important when firing clay to allow moisture to escape. Glass kilns are not designed for ventilation required for pottery firing. Significant modifications would be required such as adding vent holes and an external chimney.

In summary, firing pottery in an unaltered glass kiln can be problematic and pose risks. To properly fire pottery in a glass kiln, careful consideration and likely custom modifications to the shelves, heating elements, thermocouples, and ventilation would be required (source). It is not recommended for beginners.

Practical Experiences

There are some cases of potters experimenting with firing clay in a glass kiln. According to one post on WarmGlass.com, a potter tried firing stoneware clay in a glass fusing kiln and reported mixed results (https://warmglass.com/phpBB3/viewtopic.php?t=41769). The pottery did not fire evenly and ended up with scumming on the surface. They had better results using a dedicated ceramic kiln with proper kiln wash and firing schedules designed for clay.

On WetCanvas.com, a potter asked about firing clay beads in a glass kiln. One response said it was definitely possible to fire small clay pieces like beads in a glass kiln, though larger items may be more problematic. The key is adjusting the firing schedule and using kiln wash made for ceramics. With some experimentation, glass and ceramic pieces can be successfully fired together (https://www.wetcanvas.com/forums/topic/clay-firing-and-glass-kilns/).

Overall, these experiences show firing clay in a glass kiln is possible but challenging. The kiln temperature and schedule need adjusting to accommodate the different material. Special ceramic kiln wash is also recommended. While small ceramic pieces may fire well, larger pottery may be prone to uneven heating and scumming. With care and testing, though, some potters have managed to adapt their glass kilns for occasional ceramic firings.

Safety Concerns

Firing pottery in a glass kiln carries some potential risks that should be considered. The extreme temperatures inside a kiln can pose hazards if proper precautions are not taken.

One major risk is electric shock. As noted on the Skutt website, “The elements inside the kiln chamber will cause an electrical shock if touched. Never insert metal instruments or place any part of your body into the kiln while it is plugged in” (https://skutt.com/skutt-resources/resources-just-for-you/architectural-specifications/kiln-safety/). Proper kiln maintenance and operating procedures are essential.

There is also a risk of contamination between glass and ceramic materials during firing. As mentioned on WarmGlass.com, clay and ceramic glazes can introduce contaminants into the kiln that could negatively impact future glass firings (https://warmglass.com/phpBB3/viewtopic.php?t=41769). Using separate kiln shelves for ceramics is recommended.

Furthermore, the extreme heat poses a fire hazard. As Soul Ceramics notes, it’s essential to have proper ventilation and adequate clearance from combustible materials (https://www.soulceramics.com/pages/kiln-safety-tips). Having a fire extinguisher and smoke detectors nearby is also wise.

With training in proper firing procedures, adequate ventilation, and taking precautions like using separate shelves, many risks can be mitigated. However, firing ceramics in a glass kiln should only be attempted by those with experience in both mediums.

Recommendations

Firing pottery in a glass kiln can be done successfully, but requires careful consideration of the differences between clay and glass chemistry. According to warmglass.com, the key is using the proper kilnwash designed for ceramics, and consulting with clay suppliers for optimal firing schedules. The two materials have different thermal expansion properties, so firing schedules need adjustment.

For best results, use shelf cones designed for ceramics rather than glass to monitor firings. Ensure adequate ventilation is maintained. Limit pottery to the center of the kiln, away from heating elements. Use a slow ramp up and down, with a top temperature 25-50°F lower than typical for glass. Firing glass and pottery together requires even more precise control. Consider testing first with spare bisque pieces to dial in the right program. With care, beautiful results can be achieved combining these two mediums.

Conclusion

In summary, firing pottery and glass in the same kiln can be done but requires careful consideration to ensure optimal results. The differing chemical compositions and ideal temperature ranges for clay and glass mean that compromises will likely need to be made. While it’s not impossible, firing both mediums together makes it more challenging to achieve the desired characteristics for each material.

To successfully fire pottery and glass together, the kiln design needs to accommodate wide temperature variations. Slow ramp rates and prolonged soaks at critical points in the firing schedule can help normalize temperatures across the kiln. Strategic placement of ware and using kiln furniture to protect pieces are also advised.

With trial and error and close monitoring of results, artists can find the right balance to successfully fire both pottery and glass in the same kiln. However, firing them separately remains the ideal approach for ensuring the best outcomes for each medium.