Pottery firing: advice for potters and kiln owners

Kiln firing introduction

I started pottery in 2018, attending a workshop in the pottery of Islington Art centre in London. I developed my skills by doing a course at Camden Art Centre and classes at Turning Earth in London and Sundragon Pottery in Birmingham. These studios use electric kilns and a similar set of rules for firing pottery. Over these years, I learned to comply with these rules and experienced a limited number of failures.

In 2020 during the first lockdown, I set up my studio at the back of my garden and in November, I bought a Nabertherm 60 litre electric kiln.

Before choosing the kiln, I had to research what type of kiln was most suitable for my work, fire it safely, and use it. I found several videos on YouTube that were helpful to learn the basic operations, notably this series from AMACO.

Despite all my research, I experienced some issues when doing my first firings!

The most common problem I encountered was the glazes running on the shelves. When the kiln cools down, pots retract. If the glaze runs onto the shelf, it stops the pot walls from moving, and the tension causes the pots to chip. Not only does it break the pot, but it also damages the shelf, requiring extra work to repair it!

I was probably too excited to fire my pieces, rushing through the process and blindly trusting the commercial glazes that I was using at that time. Commercial glazes are great as they are formulated to provide reliable results, are stable and less likely to run. But they can run as well as there are many parameters that influence how a glaze melts in the firing: the clay used, how thick the glaze is applied on the piece, the maximum temperature reached and the optional soaking (hold) and the position of the piece in the kiln, etc.

My second most common issue was to adjust the kiln firing schedule and the maximum temperature. Each kiln has a different way of controlling the temperature, from the most basic kiln setters to the most advanced electronic programmers. My kiln is brand new, exceptionally well insulated and energy-efficient. When programming it for a glaze firing up to 1220°C, it reached the heat temperature of 1260°C as measured with Orton cones.

To conclude this introduction about firing my pieces, I would like to highlight that kiln firing is science on its own, given the diversity of kilns and firing methods. Kiln owners and studio technicians are the ones mastering this science and their kiln! They are ultimately the ones who should control the process, set up the rules to be followed to prepare your pieces for firing and guarantee you the best firing results, providing that you follow the rules.

Why are there different types of pottery kilns?

Today, the most accessible type of kiln is an electric one which will fire your pieces in an oxidising atmosphere. Most pottery studios use electric kilns, as they are more affordable, reliable and easier to maintain – be sure to read these top tips before buying your own.

Professional potters will opt for an electric or a gas kiln. Gas kilns will allow firing the glazed pots in a reduction atmosphere. Reduction firing partly removes the oxygen from the glazes’ metallic oxides and achieves denser colours or special effects.

Historically, pottery was fired inside kilns heated with wood, coal or gas. Today’s potters are still using wood or other alternative firing kilns to achieve a variety of results. Alternative firing techniques include raku firing, pit firing, smoke firing, salt glaze firing, etc.

If you want to understand better the science behind firing, I highly recommend the book Firing Kilns from Benedicte Bierley. The book covers the various types of kilns, oxidation and reduction firing, basic principles for firing a kiln such as kiln packing, heat work and cones, and firing schedules.

We suggest that you follow the tips below. By doing so, the kiln gods and the kiln owner are more likely to be good to you!

BEFORE MAKING YOUR POTS:

• Consider where you are going to get your pieces fired before making pots. If you are looking for kiln space to hire, search Kiln Share using your postcode / ZIP to find a kiln owner near you. Check the information on the kiln and the type of firing available. They may influence your decision to choose the clay and glazes you will be working with.
• If your pieces are fired with other pieces in a kiln load, they must be of a compatible clay body. When kiln sharing, always enquire what type of clay is fired in the studio (earthenware, stoneware, porcelain) and the firing temperature. 
• Oxides in the glazes will react differently depending on the firing atmosphere, giving different colour results. Electric kilns are fired in an oxidising atmosphere. Gas kilns can be used for oxidising or reduction firings. Choose a kiln based on the glazes you plan to use.
• You should find out the kiln’s internal dimensions and test how to load your pieces within that space before transporting them. Keep in mind that shelves are usually supported by three posts, restricting how the pieces can be set up on the shelves. It’s a good idea to mix small pieces with larger ones to fill the gaps (leaving enough space around to load and unload them with your hands).

WHEN BUILDING YOUR POTS:

• When making your pots, ensure that the walls are of a similar thickness as much as possible. It is mainly an issue when you are hand-building or throwing vessels, as walls can be uneven, thicker at the base and bottom and thinner at the rim. You need to trim them well to reduce the wall thickness where required.
• When throwing or building a piece with slabs of clay, check that there are no air bubbles trapped in the walls. If you find some, remove them carefully with a needle and repair the area. 
• If you make closed shapes, ensure that there is a small hole to let the air escape. Remember that some kiln owners will not fire pottery with organic material inside (such as newspaper) so double check first what is allowed inside enclosed forms for support during the building stage.
• Avoid working with clay in an area where it could be contaminated with plaster or other materials. They could be incorporated into the clay and cause cracks or bursts when fired.
• Compress well the bottom of your thrown pieces to avoid “S” cracks.
• Drying is critical! Let your pieces dry slowly and evenly. Consider wrapping the pots in plastic to slow and even the drying process. Halfway through, turn your pots upside down, cover the rim with plastic and let the foot dry slowly.

WHEN GLAZING YOUR POTS:

– Before deciding which glaze to use, ask the studio technician or kiln owner to confirm the atmosphere (oxidising or reduction) and the maximum firing temperature. Choose a glaze compatible with the max firing temperature and understand the result expected within the kiln atmosphere.

• Use preferably commercial glazes, which tend to have an extensive firing range and are more stable (less runny) than other glazes.
• Test fire the glazes on tiles or small pieces made of the same clay to check the result before decorating and firing your beloved pieces with them.
• To save lots of time wiping glaze off the bottom of your work, protect the bottom of your pots with wax resist before glazing them. It’s very important that you remove any trace of glaze from the bottom and the edge up to a distance of 2 to 4 mm from the shelf.
• Don’t apply your glaze too thick, as it will encourage it to ‘run’ when fired.
• Prepare “paddies” or “cookies”  to be placed under your glazed pots to protect the kiln shelves. 
• If you want to glaze your pieces up to their bottom, use three wadding balls to lift them from the shelf. Stick them with glue to the bottom of your pots.
• If you are using a runny glaze, such as the crystalline ones, you need to place a small dish with a pedestal under your pot to collect the excess of glaze.

BEFORE FIRING YOUR PIECES:

• Control your pots for cracks before firing. If you find one at the leather or bone-dry stage, repair it or recycle the piece. The cracks will always get worse after the first firing!
• Fire pots only when they are fully dry (this means when all the water has evaporated). Before the first firing, to test if a pot is bone dry, touch it with your hand or press it against your jaw. If it feels cold, it is not dry enough. During wet and cold weather it can take weeks for pots to dry. Using a dehumidifier in the room and occasionally candling your pots overnight in the kiln at 80°C can help speed up this process (you can also achieve the same result in a home fan oven).
• Pots are very fragile when dry, at the bone-dry stage, before the first firing. They should be manipulated with two hands and lifted by the bottom sides, never by the rim. 
• Unfired glazes are a layer of delicate materials that can be easily scratched from the pot surface. You should manipulate the unfired glazed pots with clean, dry hands avoiding scrubbing the glazes.
• If you have to bring your pieces for firing to a studio, pack them into a strong cardboard box or a plastic tray. Protect them with an old blanket or pieces of loosely compressed newspaper. Don’t stack them. Manipulate the unfired glazed pots as little as possible, using white cotton gloves, for instance.

AFTER THE GLAZE FIRING:

The kiln owner or studio technician should tell you what Orton cone was hit during the firing. It is critical to control the conditions of firing and melt the glazes at the suitable temperature range. It will also give you some indication to explain potential defaults in your glazes.

If you are not happy with the result of your fired glazed pieces, for instance, the glaze displays pinholes or is not the colour you expected, then consider refiring the pot or applying an additional layer of glaze and firing it again.

ADVICE FOR KILN OWNERS WHEN RENTING KILN SPACE TO POTTERS

We suggest that you follow the tips below to protect your kiln and help potters achieve great firing results when renting your kiln.

• Prepare a firing instruction booklet with information about your kilns(s) and how to prepare the pots for firing. Mention the kiln type and maximum temperature you are ready to fire to. It is not necessarily the maximum one your kiln can reach, as you may want to protect your elements. Specify the diameter of your shelves and the space available from the bottom shelf to the top of your kiln.
• Before agreeing a rental fee, discuss the type of pieces to be fired and the firing schedule. As it’s your kiln, it’s up to you what you fire!
• Confirm with the potter the clay and glazes they use and the appropriate firing range. If they want to fire pots made with different clays, check the firing compatibility of each clay type. Always check the firing range and type of glazes being used. Be particularly careful if the potter use glazes with copper that can produce flashes on other pots, or manganese oxide toxic when firing.
• If you rent your kiln for a full load, decide if you load and unload it yourself or let the potter do it. Your kiln is probably the most expensive asset in your studio. It can be damaged during the loading or unloading, for instance, by a shelf or a prop hitting an element or the wall. My preference is to load the kiln in the presence of the potter and unload it myself.
• If the potter renting your kiln will load and use your kiln, ensure that they are appropriately trained and have received a health and safety induction beforehand.
• If you fire a mixed kiln with your pieces, double-check the clay and the glazes used by the potter to avoid firing issues.
• When loading the kiln, inspect the pots to check that they are bone-dry and that glazes are not applied on the feet/bottom. A pot exploding in the kiln or a glaze running on a shelf will require extra work and cost you money to repair!
• To protect your shelves, always apply batt wash, and use “paddies” or “cookies”. You can make the cookies and sell them as part of the firing service you offer or request the potters to bring them with their pots.
• Programme the firing yourself. Do not let someone who is not experienced in using your kiln do it.
• For a glaze firing, always use Orton cones to measure the maximum heat and report it to the potter renting your kiln.
• Keep your kiln clean. Hoover it regularly!
• It is nice to provide feedback on the pieces fired and advice to potters renting your kiln. They will learn from your firing experience and improve their pots for next time.