Wood Firing Tips: Clay, Loading, Atmosphere and Cooling

Wood firing gives you surfaces you can't get in an electric or gas kiln. Flame, ash and atmosphere all leave marks on the work, from faint blushes of flashing to heavy ash runs that look like frozen drips.

Some of that is luck, and that's part of the appeal. But planning and good firing habits make a big difference. A good wood firing and a disappointing one usually come down to timing, atmosphere control, and learning to work with the fire instead of fighting it.

This guide covers clay choice, wadding, loading, a sample firing timeline, and cooling, with tips you can use on your next firing.

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Why wood firing looks different

Three things create wood-fired surfaces.

Flame. Where flame touches the pot directly, you get flashing: areas where the clay changes color from the heat and the reduction atmosphere. Iron in the clay reacts strongly, turning orange, rust or deep brown.

Ash. Wood ash is basically a natural glaze. As wood burns, minerals in the ash get carried through the kiln on the flame and land on the pots. At high temperatures (cone 8 to 10) that ash melts into glossy drips, runs and pools.

Atmosphere. The balance of oxygen and carbon in the kiln affects clay and glaze color. Reduction (low oxygen) gives warm, rich colors and metallic effects. Oxidation (plenty of oxygen) gives brighter, clearer colors.

All three keep shifting over a firing that can last days, which is why the results have so much variation and depth.

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Choosing a clay body

Not every clay holds up to a long, uneven wood firing. The wrong one can warp, crack or bloat.

What to look for

• Grog or sand (10 to 20%). An open body handles thermal shock better as the temperature rises and falls. Grog also gives the surface texture that catches ash nicely.
• Stoneware that matures at cone 8 to 10. Most wood-fired work uses these bodies. They vitrify properly at wood-firing temperatures and color up well in reduction.
• Iron content. More iron means stronger flashing. Low-iron clays like porcelain and white stoneware flash more subtly, and ash deposits stand out against the lighter clay.
• Thermal shock resistance. The clay has to put up with uneven heating as flame moves through the kiln, and with different cooling rates in different parts of the kiln.

Porcelain in a wood kiln

Ash on white porcelain can be beautiful, but porcelain needs more care:

• Heat it slowly early on to avoid thermal shock and dunting (cracking from too-rapid quartz inversion).
• Support pieces well and keep them out of high-stress spots in the kiln.
• Expect some losses. Porcelain is less forgiving than stoneware, especially while you're learning.

When it works, a thin layer of ash on bright white porcelain is hard to beat.

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Wadding

Wadding is the clay mix that keeps pots off the kiln shelves. In a wood kiln it matters a lot, because melted ash will fuse a pot to the shelf anywhere they touch.

Wadding recipes

Basic recipe: 50% alumina hydrate, 25% EPK (kaolin), 25% coarse grog.

• The alumina stays stable at high temperature, so it won't melt or stick.
• The EPK gives it enough plasticity to form wads.
• The grog keeps the wads from shrinking and cracking.

Alternative: equal parts alumina, fire clay and sand.

Tips for using wadding

• Make wads big enough to hold the pot securely but small enough not to block ash. Usually that's 1/2" to 1" across, depending on the size of the pot.
• Set each piece on three wads, like a tripod. It's stable, and there are only three spots to grind afterward.
• Dust the tops of the wads lightly with alumina hydrate before setting pots on them. They'll pop off more cleanly after the firing.
• Mix fresh wadding for every firing. Old fired wads get brittle and can break while you load.
• Don't oversize them. Big wads keep ash off the bottoms of pots and mean more grinding later.

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Loading with the flame path in mind

Wood kilns depend on airflow. How you load decides where the flame goes, where ash builds up, and which pots get heavy effects versus light ones.

How flame moves through the kiln

Flame comes in at the firebox, travels through the chamber and leaves through the chimney. It doesn't move evenly. It swirls and eddies, and the pattern changes with the kiln design, the load and your stoking rhythm.

High-flame areas are near the firebox, along direct lines to the chimney, and in channels between pots where flame speeds up. Expect heavy ash and strong flashing there.

Quieter areas are behind shelves, in corners and in the shadow of other pots. You'll get lighter flashing, less ash and more predictable glaze results.

Loading tips

• Angle pots slightly instead of facing them straight at the firebox. Pots facing head-on can get a harsh, over-flashed front. At an angle, ash travels across the surface instead of piling up in one spot.
• Leave space between pots. Flame needs room to circulate and ash needs room to land. A tightly packed kiln fires unevenly and gives muddy surfaces.
• Put "ash catchers" in the hottest spots. These are simple pots you don't mind getting heavily ashed or even overfired. They protect the work behind them, and they often come out looking great.
• Use shadowed spots for porcelain and white stoneware. Heavy reduction and ash can overwhelm white clay, and moderate exposure gives a more refined surface.
• Vary shelf heights. Top shelves often collect the most ash. Lower shelves may run hotter but get less ash.
• Leave a few deliberate gaps where flame can speed up. Pots placed along those paths will get the strongest effects.

A well-loaded kiln fires better than a cramped one. If you're unsure, leave out one shelf and give everything more room.

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The firing timeline

Wood firings usually run 30 to 60 hours, depending on the kiln, the size of the crew and the effects you want. Knowing the stages ahead of time helps you plan stoking and crew shifts.

Early heat-up and body reduction

A slow, steady start protects the ware and sets up color development later on.

Hours 0 to 4 (room temperature to about 900°F)

• Ramp rate: 100 to 200°F per hour
• Atmosphere: neutral to lightly oxidizing
• Purpose: drive off mechanical water and burn out organic material
• Stoking: small, infrequent fires. You're warming up the kiln, not racing it.
• Watch for steam from the peepholes (moisture leaving) and light smoke (organics burning off)

Rushing this stage builds up steam pressure inside the clay, which can crack pots or blow them apart.

Hours 4 to 8 (900°F up to cone 010 to 08, about 1650 to 1730°F)

• Start light body reduction as the kiln reaches cone 010
• Purpose: the clay starts picking up color and flashing tones
• Atmosphere: visible flame at the damper with a little smoke
• In reduction, carbon gets into the clay body and creates the warm colors and flashing wood firing is known for

A few rules for body reduction:

• Keep it light. Heavy reduction this early can trap carbon where you don't want it and cause bloating.
• Aim for a lazy flame at the chimney, not thick black smoke.
• If the kiln stalls and stops climbing, ease off the reduction a little.

Mid-range climb (cone 08 up to cone 6, about 1730 to 2230°F)

Through the middle of the firing, a steady rhythm is the most useful thing you've got.

Hours 8 to 20 (approximate)

• Ramp rate: 150 to 250°F per hour, depending on the kiln, crew and wood
• Atmosphere: mild, steady reduction
• Stoking: moderate amounts every 7 to 12 minutes

Tips for this stage:

• Stoke small and often. Frequent smaller loads keep the temperature climbing steadily and avoid big swings in atmosphere. Occasional huge loads don't.
• Keep the reduction mild. You want an amber-orange flame, not roaring oxidation or thick black smoke.
• Pay attention to the flame. A strong, roaring flame means good draft and a steady climb. A lazy, smoky flame means the kiln needs more air (open the damper) or a hotter fire (drier or better wood, or more frequent stoking).
• Settle into a rhythm as a crew, with wood going in at regular intervals so the kiln responds evenly.
• Check pyrometers or cones in different parts of the kiln. If the front and back are far apart, change where you stoke to even them out.

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Glaze reduction

Heavier glaze reduction should start once the glazes begin to soften and melt. This is when you have the most influence on color, depth and how the surface moves.

Target: cone 6 to 9 (approximately 2232 to 2300°F)

Hours 20 to 28 (approximate)

• Atmosphere: steady, moderate reduction
• Purpose: develop glaze color and depth and get the ash melting
• Watch for glazes starting to gloss, stretch or pool

Reduction pulls oxygen away from glaze colorants, especially iron and copper, and that changes their color. Iron shifts from brown-orange to deep rust or celadon green. Copper goes red instead of green.

Stoking during glaze reduction:

• Keep the same steady rhythm.
• Adjust the air to hold moderate reduction.
• Don't let the temperature stall. Keep it climbing while you reduce.
• If part of the kiln is running cool, stoke more toward it. If part is running ahead, ease off there.

Signs to look for:

• Glazes getting a visible gloss
• Ash starting to melt and run
• Draw trials showing mature glaze

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Soaking at peak temperature

A soak means holding the temperature steady instead of continuing to climb. Done at the right time, it can improve the whole firing.

When: cone 9 to 10 (2300 to 2345°F)

How long: 20 to 45 minutes

What a soak does:

• Cooler parts of the kiln catch up with hotter ones, so results are more even.
• The ash gets more time to melt, so surfaces come out glossier and smoother with better movement.
• Glazes that were just starting to mature have time to finish.
• Pots that were close to underfired reach maturity.

How to soak:

• Stoke smaller amounts less often, just enough to hold temperature.
• If you need to, aim extra stoking at the cooler zones.
• Keep a close eye on pyrometers or cones. The goal is to hold, not climb.
• Get to know your kiln. Some hold temperature easily, and others want to keep climbing or drop off quickly.

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Oxidizing at the end of the firing

Switching to oxidation at the very end gives brighter, sharper color. A lot of wood firers skip this step, but it can make a big difference.

When: at peak temperature, after the soak

How long: 10 to 20 minutes

How to do it:

1. Open the damper slightly to increase the draft.
2. Stoke lighter, with smaller, drier pieces of wood.
3. Aim for a hotter, cleaner flame. More air means more complete combustion.
4. Hold that for 10 to 20 minutes.

Why it helps:

• Heavy reduction leaves carbon on the surface, which dulls color. Oxidation burns it off.
• Iron oxides turn richer orange and brown in oxidation, so iron glazes get sharper.
• Carbon can hide subtle flashing. A clean finish shows the full range of flame marks.
• Most glazes come out more saturated and lively.

It's often only about 15 minutes out of a 40-hour firing, and it has an outsized effect on the results.

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Cooling

Cooling affects the surfaces as much as firing does. Cool too fast and pots crack. Controlled cooling can also bring out certain glazes and effects.

• Let it cool slowly. Don't speed it up with fans or open doors. Natural cooling usually takes 24 to 48 hours from peak to safe-to-open (under 200°F).
• Keep the peepholes closed until 1000°F. Opening them earlier lets cold air rush in, and the thermal shock can crack pots.
• Only cool quickly on purpose. Some crystal glazes benefit from fast cooling at certain temperatures, but that's an advanced technique. For a normal wood firing, slow cooling gives stronger, more stable surfaces.
• Light reduction during early cooling is optional. Slight reduction from peak down to about 1800°F can help shino glazes, encourage carbon trapping on unglazed areas, and bring out some ash effects. It's experimental, and results vary.

Don't open it early to peek. Leave the kiln closed until it's fully cool.

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Sample atmosphere schedule

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More practical tips

Wood

• Hardwoods like oak, maple and ash burn hot and make good ash. Softwoods like pine and fir burn cooler, and their ash tends to be stringier and less glossy.
• Season wood for at least a year. Under 20% moisture is ideal. Wet wood smokes a lot, stalls the temperature and makes poor ash.
• Cut wood to consistent lengths that fit your firebox. Mixed sizes make it hard to keep a stoking rhythm.

Crew

• Long firings need shifts. Plan them so rested people take over from tired ones, especially overnight.
• Agree on clear signals for stoking, damper changes and temperature readings. Confusion leads to mistakes.
• Keep a firing log with stoking frequency, damper positions and hourly temperature readings. Over time it's the best record you'll have of how your kiln behaves.

Safety

• Heat exhaustion. Wood firing is hard physical work. Drink plenty of water, take breaks and watch for fatigue.
• Burns. Fireboxes are extremely hot. Use long tools and wear the right clothing (no synthetics, which can melt).
• Carbon monoxide. Make sure there's good ventilation, especially for indoor or partly enclosed kilns. CO has no smell and it's dangerous.
• Fire. Keep extinguishers close, clear anything flammable away from the kiln, and have an emergency plan.

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Wrapping up

Every kiln, crew and load fires a little differently. Good timing and atmosphere control will make your results more consistent, and the fire will still surprise you. You can read each finished pot like a record of the firing: where the flame hit, where ash collected, where reduction changed the clay and glaze. That's a big part of why potters keep coming back to wood firing despite the long hours and the occasional loss.

At Sheffield Pottery, we carry stoneware clays made to handle thermal shock and develop good flashing. If you're getting ready for a wood firing, we're happy to help with materials and advice.