How to prepare your pottery pieces for firing

Discover Bruno Vinel’s advise how to make pottery pieces, decorate and prepare them for a successful firing.

Bruno Vinel from Wychbury Ceramics and editor of Ceramics Today started pottery in 2018, by attending classes in a studio in London. In 2020, he set up his studio and bought an electric kiln.

Over the last three years, Bruno learned from his failures, asked for advice from experienced potters and studio technicians, and researched how to fire an electric kiln. He is sharing his knowledge in this article.

What can go wrong?

Based on my experience, the most common issues are:

• Unfired broken pots or patchy glazes. These issues can be caused by a lack of protection when packing, transporting and manipulating your pieces.
• Pieces not fitting in the kiln. This issue happens more often than you think, particularly when you want to fire several vessels together.
• Pots fired at the wrong temperature for the clay body or the glaze. Overfiring a clay body or a glaze will lead to the pot or glaze to melt on the kiln shelf. An underfired glaze will no melt properly and show defaults.
• Glazes fired under the wrong atmosphere won’t give the expected results.
• Pieces exploding in the kiln during the first firing. This could be caused by the clay not being dry enough or by air or other materials trapped into the clay.
• Pots cracking during the firing or S cracks at the bottom of your pots. They are mostly caused by a lack of compression of the bottom of a thrown pot, an unevent drying or large pieces that are firing or cooling too fast around the quartz inversion phase (573 °C).

Recommendations for making and decorating your pottery pieces

We suggest that you follow the simple recommendations below. By doing so, the kiln god and the kiln owner are more likely to be good to you.

Before starting making pots:

1. Before embarquing into making pottery, consider where you are going to get your pieces fired. If you cannot get access to a kiln closed to you, explore making pots with air-drying clay. This clay has limitations (you can’t make food-safe bowls), but you will be able to produce nice decorative pieces without firing them.
2. If your pieces are going to be fired with other pieces in a kiln load then they have to be of a compatible clay body. Enquire what type of clay is fired in the studio (earthenware, stoneware, porcelain) and the firing temperature.
3. Oxides in the glazes will react differently depending on the firing atmosphere, giving different colour results. Electric kilns are fired in an oxidised atmosphere. Gas kilns can be used for oxidising or reduction firings. Chose a kiln based on the glazes you plan to use.
4. You should enquire the kiln internal dimensions and test how to load your pieces within that space before transporting them. Keep in mind that shelves are supported by usually three posts which are restricting how the pieces can be disposed on the shelves. It’s a good idea to mix small pieces with larger ones to fill the gaps.
5. If you plan to build a large vessel, ask about the internal dimensions of the kiln to ensure that it will fit in (leaving enought space around to load and unload it by hands).

When building your pots:

1. When making your pots, ensure that the walls are of a similar thickness as much as possible. It is particularly 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 needed.
2. 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 neddle and repair the area.
3. Avoid working with clay in an area where it could be contaminated with plaster or other materials. They could be incorporated in the clay and cause cracks or bursts when fired.
4. Compress well the bottom of your thrown pieces to avoid S cracks.
5. Drying is critical! Let your pieces dry slowly and evenly. Consider wrapping the pots in plastic to slow and even the drying. Half-way through, turn your pot upside down, wrap the rim with plastic and let the foot dry slowly.

When glazing your pots:

1. Before deciding which glaze to use, ask the studio technician or potter to confirm the atmosphere (oxidising or reduction) and the top firing temperature that will be reached in the kiln. It is preferable to express the firing temperature in Orton cones, as they measure the heat received by the pieces (a combination of temperature and time).
2. Choose a glaze that is compatible with the firing temperature (or Orton cone) and will give the result expected within the kiln atmosphere.
3. Use preferably commercial glazes which tend to have a large firing range and are more stable (less runny) than other glazes.
4. Test firing 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.
5. Protect the bottom of your pots with wax resit before glazing them, and/or remove any trace of glaze from the bottom and the edge up to a distance of 2 to 4 mm from the shelf.
6. Don’t apply your glaze too thick as it will encourage it to run.
7. Prepare “paddies” or “cookies” to be placed under your glazed pots to protect the kiln shelves.
8. If you are using a runny glaze, such as the cristalline ones, you need to place a small dish with a pedestal under your pot to collect the excess of glaze.

Before firing your pieces:

1. Control your pots for cracks before firing. If you find one at leather or bone dry stage, repair it or recycle the piece. The cracks will always get worst after the first firing.
2. Fire pots only when they are fully dry (all the water has evaporated). Before the first firing, to know 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. With a wet and cold weather like the one we have often in the UK, it can take weeks for pots to dry. I found useful using a deshumidifyer in the room, and occasionally candling my pots over night in the kiln at 80 °C (you can achieve the same result in a home fan oven).
3. Pots are very fragile when dry, at the bone stage, before the first firing. They should be manipulated with two hands and lifted by the bottom sides, never by the rim.
4. Unfired glazes are a layer of fine materials which can be easily scratched from the pot surface. You should manipulate the unfired glazed pots with clean, dry hands avoiding scrubbing the glazes.
5. 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 losely 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:

1. The potter or studio technician should tell you what Orton cone was hit during the firing. This is critical to be able to control the conditions of firing and melt the glazes at the right temperature range. It will also gives you some indication to explain potential defaults in your glazes.
2. 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 fire it again.

We are wishing you great success in your pottery firing!

Any questions, feel free to reach our editor by email at info@ceramicstoday.org.

If you are interested in learning how to fire an electric kiln, check these excellent resources from AMACO on Youtube.