How to load a kiln correctly

A good firing doesn't start at the controller, it starts with loading. If you load your kiln correctly, you protect the kiln and its shelves, get more even results and save electricity because more pieces fit into one firing. In this guide, we show you step by step how to load a kiln for bisque and glaze firings, how to plan your shelf levels and which mistakes happen most often.

Happy firing from the Formwerk team!

Contents

• Before loading: the check
• Using kiln furniture correctly
• Loading a bisque firing
• Loading a glaze firing
• Planning shelf levels: top loaders and front loaders
• Temperature distribution in the kiln
• Special cases
• The 8 most common loading mistakes
• Unloading after the firing
• FAQ
• Sources

How to build up a top loader level by level: bottom shelf on three posts, ware, posts directly above, next shelf. Graphic: Nabertherm (in German)¹

Before loading: the check

Before the first piece goes into the kiln, five minutes of checking pay off. They prevent most damage.

• The ware is bone dry. Damp clay can burst during firing. A simple test: if a piece feels cool against your cheek, there is still moisture in it.^2,3 How long drying takes is explained in How many days should clay dry before firing?
• The kiln and shelves are clean. Crumbs, glaze residue and flaked kiln wash have to go before loading, especially out of the element grooves.
• The heating elements are free and sit properly in their grooves.
• Cones are ready if you want to check the temperature. More on this below in the section on temperature distribution.
• You know which firing comes next. Bisque and glaze firings are loaded completely differently. Never mix both in one firing.

Using kiln furniture correctly

Kiln shelves and posts turn a kiln with one floor into a kiln with several levels. We have summarized which shelves, posts and kiln washes exist in Kiln accessories explained. This section is about how to use them.

• Three posts per shelf. Three points always stand stable, even if the shelf or floor is slightly uneven. With four posts, a shelf can wobble.⁶ For round shelves, three to four posts are common.³
• Posts directly above each other. The weight should travel straight down, not through the shelves. This matters most in large kilns and with heavy loads.^3,6
• Don't put the bottom shelf directly on the kiln floor. On short posts of about 1 inch (2.5 cm), heat can also circulate below the lowest level.² In the Nabertherm infographic, the first shelf also sits on three posts.¹
• Kiln wash on the top side only. Kiln wash protects the shelf from glaze drips, but it belongs on the top only and not over the edge. Many people leave a narrow border free all around.⁴ On the underside, kiln wash could flake off and fall onto the ware below.

A typical ready-made kiln furniture kit with shelves, posts and more. This example is from the manufacturer KITTEC.

Loading a bisque firing

In a bisque firing, the ware is still unglazed. That's why you can pack much more tightly here than in a glaze firing. Pieces may touch, they won't stick together.^2,3,5 You'll find the basics of the first firing in our article Bisque firing: What is it? Duration, temperature and costs.

• Stack and nest. Rim to rim, foot to foot or small bowls inside larger ones have proven themselves.⁵
• Heavy pieces at the bottom. Large, thick-walled pieces go at the bottom, light and delicate ones on top.
• Fire lids in place. That way the lid and the pot warp together and still fit afterwards.
• Not too tightly. Where one piece completely covers another, organic matter can burn out less easily.³ Flat pieces stacked without an air gap act like one single thick slab of clay in the kiln.⁵
• Keep your distance from the heating elements here as well, more on that in the next section.

Please don't do this: even in a bisque firing, ware shouldn't lean directly against the kiln wall with the heating elements, and flat pieces shouldn't stand close together without an air gap.

And this is how it's done: ceramic artist Petra Peter shows step by step how she loads a kiln for a bisque firing and sets the controller (in German).

Loading a glaze firing

In a glaze firing, the glaze melts into glass. Anything that touches during the firing fuses together as it cools.⁵ That's why stricter rules apply here. You'll find more about glazes and typical glaze defects in Understanding glazes: types, application and common mistakes.

• No contact between pieces. As a rule of thumb, leave about 1/2 inch (roughly 1 cm) between pieces.³ If a glaze runs a lot, allow more space.
• Keep the foot free of glaze. Glazed surfaces must not sit directly on the shelf, or they will fuse to it.³ Wipe the bottom and foot clean. If you apply wax resist to the bottom before glazing, you'll save yourself a lot of work.^3,4 With stoneware, many potters leave the lower edge of the piece unglazed so running glaze doesn't reach the shelf.³
• Pieces glazed all over need stilts. This is especially common in low fire.⁴
• Distance from the heating elements. Nabertherm recommends 1 to 2 cm (about 1/2 to 3/4 inch),¹ other sources recommend about 1 inch (2.5 cm) from heating elements and kiln walls.^2,3 Your kiln's operating manual is what counts. Glaze dripping onto a heating element can damage it.
• Keep the thermocouple clear. The temperature sensor sticks into the firing chamber and should have a few centimeters (1 to 2 inches) of space from ware and shelves.³

… and here is a clear video that shows the glaze firing step by step (in German).

Planning shelf levels: top loaders and front loaders

The number of levels determines how much fits into one firing. Here's how to proceed:

1. Sort by height. Pieces of the same height go on the same level, so you don't waste any space.
2. Choose the post height. Leave about 1/2 inch (1.3 cm) of space above the tallest piece on a level.³
3. Set the posts, three per shelf and directly above the posts below.
4. Place the shelf and check that it sits steadily and doesn't touch any glaze.
5. Don't put the top level too close to the lid. At the very top, it can get too hot or too cool.⁶

With a top loader, you build from the bottom up: load a level, set the posts, place the shelf, next level. From above, you can easily see whether pieces touch. With deep kilns, however, you have to lean far in, and lifting heavy shelves into the bottom level is hard work.

With a front loader, you load from the front, usually from back to front. This is easier on your back and simpler with large, heavy pieces. In front loaders with elements on the sides, heavy pieces tend to go towards the back, where they get more heat.⁶

Which design suits you depends on space, power supply and how much you fire. We compare this in detail in Buying a kiln: What size, power and technology do you really need?

The Kittec CB series are top loaders with 20 to 520 liters

The Nabertherm N Series are front loaders with 100 to 660 liters

Temperature distribution in the kiln

No kiln is exactly the same temperature everywhere. For its Arts and Crafts kilns, Nabertherm states a temperature uniformity in the usable space of plus or minus 10 °C (18 °F) above 850 °C (1,560 °F).¹ How even your firing turns out also depends on how you load.

• Distribute evenly. A very dense level next to an almost empty one heats up differently. The more densely the kiln is loaded, the longer it takes to reach the target temperature.³
• Fire full loads. A well-filled kiln uses the heat better and saves electricity.² What a firing costs is shown in How much electricity does a kiln use?
• Keep an eye on the weight. As a guideline, Nabertherm states a maximum load weight of 50% of the volume.¹
• Cones for checking. A cone on the middle level shows you whether the firing has reached the desired heat work.² For the first firings in a new kiln, we recommend placing one cone each at the top, middle and bottom. That way you get to know your kiln's quirks. We carry matching Orton cones, and the values are in our cone chart.

If you regularly see differences between top and bottom in a larger kiln, consider a multi-zone control. It controls several heating zones separately and thus evens out differences within the firing chamber.

Special cases

• Plates and tiles. Lying flat with full contact on the shelf so they don't warp. In a bisque firing, don't stack them without an air gap.⁵
• Large and heavy pieces go on the bottom level, centered if possible and with space on all sides.
• Lidded pots. In a bisque firing with the lid in place. In a glaze firing only if the contact surfaces of lid and pot are free of glaze.
• Painted bisqueware in classes. Only a glaze firing is done here, so the strict rules apply. More on this in Offering ceramic painting classes.
• Bisque and glazed ware together. Better not. Both firings have their own firing schedules and temperatures.

The 8 most common loading mistakes

Unloading after the firing

Patience at the end is just as important as loading. It's safest to open the kiln only once it has cooled to room temperature, otherwise pieces can crack from thermal shock.² Kiln manufacturers give different limits for this, roughly between 125 and 250 °F (50 to 120 °C) depending on the manufacturer.⁷ Your kiln's manual is what counts. After a glaze firing, the lower value is the better guide.⁷

Only take out the shelves once they have cooled down too.⁶ Carefully remove glaze drips from the shelf with a scraper or grinding stone, then touch up the spot with kiln wash.

FAQ about loading a kiln

How much space do pieces need in a glaze firing?

They must not touch. As a rule of thumb, about 1/2 inch (1 cm) between pieces³ and 1 to 2 cm (about 1/2 to 3/4 inch) from the heating elements.¹ More with glazes that run a lot.

Can you stack ceramics in a bisque firing?

Yes. Unglazed pieces may touch and stand inside or on top of each other.^3,5 Just make sure air can circulate and heavy pieces are at the bottom.

How many levels fit into my kiln?

Divide the height of the firing chamber by the height of your pieces plus about 1/2 inch (1.3 cm) of space³ and the thickness of the shelf. For mugs about 4 inches (10 cm) tall, that usually means three levels in a kiln with a chamber height of 18 inches (46 cm).

What do I do if glaze sticks to the shelf?

Carefully loosen the piece, remove glaze residue with a scraper or grinding stone and recoat the spot with kiln wash. Prevention is easier: wax the bottom and keep it free of glaze.³

Can you fire glazed and unglazed pieces together?

We advise against it. Bisque and glaze firings have different temperatures and firing schedules, so a combined firing is always a compromise for one of the two groups.

How full can a kiln be?

Full enough that the spacing rules are kept and heat can still circulate. You can pack more tightly in a bisque firing than in a glaze firing, but don't overdo it.³ As a guideline, Nabertherm states a maximum load weight of 50% of the volume.¹

When can I open the kiln after firing?

It's safest at room temperature.² Manufacturer specifications range roughly between 125 and 250 °F (50 to 120 °C).⁷ Follow the manual for your model.

Conclusion: careful loading pays off in the firing

The most important rule fits into one sentence: in a bisque firing everything may touch, in a glaze firing nothing may. Add to that dry ware, space from the heating elements, three posts directly above each other and an evenly distributed load. Follow these rules and you'll get more even results while protecting your shelves and heating elements.

Sources

1. Nabertherm GmbH, Arts and Crafts: Wie belädt man einen Brennofen richtig? (infographic, in German)
2. Ceramic Arts Network: Firing Clay: Ten Basics of Firing Electric Kilns
3. Big Ceramic Store: Loading the Kiln
4. Big Ceramic Store: Kiln Wash: What, Why, When, How, Where?
5. The Pottery Wheel: Can you Stack Greenware in a Bisque Fire?
6. Kiln Frog: How to Load a Kiln: Shelf Supports and More
7. The Pottery Wheel: What Temperature Can You Open a Kiln?

Note: All distances, temperatures and quantities are guidelines. Always follow the operating manual and safety instructions from your kiln's manufacturer.

Happy firing from the Formwerk team!