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Anytime you're working with a kiln to fire ceramic you'll need to select the right size and material for your kiln shelving. A kin shelf needs to meet the thermal requirements of your process, among other things. Moreover, you need to ensure it fits snugly within your kiln to remain stable and keep your product safe while it's being fired.
Kiln furniture of any variety can be difficult to choose due to the sheer amount of options available. Our guide will dive into the most common shelf materials and when they perform at their best so you can find the right choice for your use case.
Kiln shelves are most commonly made from advanced ceramic, which means there are various materials you can choose from when outfitting your kiln. Each material brings its array of unique properties, so it's best to understand them all to make an informed choice.
Great for: Gas & Electric
Temperature Limit: ~2,212F
Porosity: High; kiln wash is recommended for frequent cleaning
Cordierite shelves are made with a “hollow core”. Think of the inner-cells of cardboard and you are on your way to picturing what the inside of a cordierite shelf looks like; lightweight yet sturdy.
Great for: Gas & Electric
Temperature Limit: 2,284F
Porosity: High; kiln wash is recommended for frequent cleaning
Overall, high alumina kiln shelves have a negligibly higher heat resistance than Cordierite shelves. The inside of these shelves is solid, giving them a greater weight but a higher degree of durability.
Great for: Gas & Electric*
Temperature Limit: 2,284F
Porosity: Low; kiln wash is recommended, but not needed frequently
*Electric kilns should use Silicon Carbide shelves with caution due to their conductive properties; a kiln in disrepair could inadvertently conduct electricity and shock the operator
A lot of kiln operators struggle with the question of how to remove the old glaze from kiln shelves. Most often, the answer is to choose a kiln shelf material that is less porous. Lower porosity means your glaze will not seep deeply into the surface. Silicon carbide kiln shelves are a great example of this; sturdy but low porosity compared to almost any other material.
There is more to choosing a kiln shelf than its base properties. It's also important to understand your needs when you fire your product. We’ve broken down the most common considerations when choosing a kiln material to help you understand what else to look for in a material to ensure you choose a shelf that has the longevity you would expect with such an investment.
In the industry, it's common to forgo temperature ranges and instead refer to a material's "cone" rating when referring to its thermal durability within a kiln. The full range of the cone rating system can be confusing. Luckily, kilns typically operate within a cone rating range of 8-11, which equates to 2,212-2,284F. It's important to understand the temperature range you operate under.
Choosing a Cone 8 material like cordierite and firing it at temperatures upward of 2,284F could lead to a failure of the material’s durability and a serious snag in your production. Whether you go by cones, Fahrenheit, or Celsius you must choose a material that can withstand your typical heat index.
The average weight of your product or "ware weight" is important to consider. If you commonly fire heavy products in your kiln, then an extremely sturdy shelf will be needed to avoid excessive wear-and-tear, sagging, or complete breakage.
Cordierite, while lightweight and durable, is best used for lighter fare. Both Alumina and Hexoloy materials function well under heavyweight.
The length of your shelves, though obvious, is not what we are referring to when we say to consider the sizing of your shelves. Rather, we refer to the thickness of the shelves and the top and bottom spacing. Thinner shelves are easier to manage but will ultimately be more susceptible to thermal wear as well as regular wear-and-tear.
You may even want to consider half shelves if your product supports it; it will give you more room at the expense of a thinner shelf. If your product is heavy and cumbersome then we’d recommend a thicker shelf and fewer shelves overall.
Firing frequency is a big contributor to the shortened lifespan of your shelving if you do not choose a material that can deal with the constant fluctuations in temperature. If you are operating a kiln that is constantly being used, then a material like Hexoloy that has a very high thermal shock rating is recommended. Both cordierite and Alumina are great options for a kiln that stays at a steady operating temperature.
Kiln operators use a product called “kiln wash”, which is typically a concoction of kiln wash particulate (silica sand, alumina hydrate, and kaolin) that is spread across the surface of a kiln shelf to act as a barrier between the shelf and your glaze.
Once the powdered mixture of silica, alumina hydrate, and kaolin are mixed in a 50/50 ratio with water the kiln wash can be applied onto your shelf with a conventional paintbrush. We recommend three coats; let each coat dry between applications. Lastly, ensure you leave a ½” edge of unwashed shelf in front so the wash doesn’t flake, fall off, and adhere to future product while it’s fired.
Kiln wash will eventually become uneven and will need to be removed before it can be reapplied. An angle grinder, silicon carbide scraper (rubbing brick), or diamond rubbing wheel can be used. To put it simply, you need to grind it off in whatever manner is most convenient to you without grinding away your shelf.
A steady application of kiln wash is your primary war of ensuring a clean kiln shelf. As wash becomes uneven and glaze finds its way onto your shelves you’ll eventually need to grind that material away using the process we previously described.
