How to Fire Ceramic Pots in the Ground
Pit firing, or firing ceramic pots in the ground, borrows heavily from the earliest technique used to produce decorative and functional pottery. Archeological discoveries suggest the earliest ceramic wares were fired in bonfires on the ground and sealed with sap or wax for everyday use. Modern pit firing allows potters to return to the earliest roots of their craft to produce beautiful works of art marked by flame, smoke and stained by chemicals.
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Things You'll Need
- Grogged clay
- Electric kiln
- Copper carbonate
- Iron oxide
- Copper wire, fine gauge
- Sawdust
- Dry leaves
- Charcoal briquettes
- Rock salt
- Newspaper
- Wood
- Lighter fluid
- Corrugated sheet metal
- Soft cloth
- Beeswax or paste wax
Instructions
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Prepare your pottery. Create simple wheel-thrown or hand-built forms using a clay that contains grog, or particles of ground, bisque-fired clay. Stoneware clay or clays mixed for raku firing work well. If desired, apply a coat of terra sigillata, a very fine slip to your leather-hard pot and burnish it with a soft cloth until it begins to shine. Bisque fire your bone-dry, burnished pottery to 1,636 degrees Fahrenheit (cone 010) in an electric kiln. Fire other pots to 1,798 degrees Fahrenheit (cone 06).
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2
Decorate your pots. Undecorated pots will have black and grey markings after being fired. You can achieve brown and pink colors by making a wash of copper carbonate or iron oxide and water to your pot. Make a design on your pots by wrapping them with fine gauge copper wire.
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Excavate a rectangular fire pit. The size of your pit is determined by how many pots you want to fire. Dig to a depth of two to four feet, and keep the sides of your pit straight and vertical. A narrow pit offers greater convenience for you as you pack and unpack your in-ground kiln. Clear combustible materials away from your fire pit.
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Spread a 3- to 8-inch layer of sawdust, crushed charcoal briquettes and dried leaves or crumpled newspaper at the bottom of the pit. Add rock salt to the mix to encourage greater markings on your pots.
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Nestle your prepared pots onto this bed of combustible material. "Tumble stack" your pottery by randomly placing the pots laying their pots on their sides or placing them vertically. Leave adequate space around each pot. Spread rock salt, copper carbonate or even a common garden fertilizer, like Miracle Gro, on and around the pots to encourage interesting fire effects. Seaweed, vegetable peels and banana skins can also be mixed in with the pots.
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Place a 12-inch thick layer of crumpled newspaper over the pots. If desired, layer dry horse or cow dung, which burns slowly, on top of the newspaper. Carefully place pieces of wood cut to the same length over the newspaper. Uniform lengths of wood ensure the fire burns at a steady rate. Try to anticipate how the fire will burn. As the fire burns, the wood may shift and break the pots below. Create layers with more crumpled newspaper, wood, and kindling. Spray the wood with lighter fluid and allow it to soak in.
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Light the wood and allow the fire to burn for 45 minutes to 1 hour. If the wood burns too quickly, stoke the fire with additional pieces of wood. Place sheets of corrugated metal over the pit, leaving a small gap at each end of the pit. By covering the pit, you limit the oxygen available to the fire and create a reduction atmosphere. As the fire burns, it will consume oxygen in the salts and chemicals in the pit and leave orange, white, pink and brown marks on your pots. Depending on the size of your pit, the fire should burn for 1 to 12 hours.
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Wait until the fire has burned out to remove the corrugate sheet metal. Check the temperature at the bottom of the pit and allow it to cool completely. Remove your pots from the fire pit, and brush away excess ash and debris with a soft cloth. Apply a coat of beeswax or paste wax to seal and polish your pots.
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Tips & Warnings
Bisque firing limits the chances your pots will break while being packed into the pit. It also limits breakage from thermal shock.
Packing pots to maximize decorative effects of the fire and chemicals is a skill that is acquired through numerous firings.
Copper carbonate is a toxic material. Wear gloves and a respirator when mixing glazes, making washes or spreading it over the pit.
Stand downwind of the fire and avoid breathing the smoke and fumes. They are toxic.
Pit-fired ceramic pots should only be used for decorative purposes. They are not watertight.
