Wood-fired kilns have been used for thousands of years to produce beautiful and unique ceramic art. The process of wood firing involves burning wood to fire ceramics at high temperatures in a kiln. This results in distinctive effects on the clay including flashing, ash deposits, and morphological changes. While the process is complex, the basic principles of how a wood kiln works are straightforward.
What is Wood Firing?
Wood firing refers to the process of firing ceramics in a kiln fueled by wood. The fuel is burned to produce flames and heat that fire pottery and other clay objects placed inside the kiln.
Wood firing dates back thousands of years and originated in ancient Asia. It involves loading the kiln with wood, allowing it to burn and produce extreme heat, then shutting off air supply to create a reduction atmosphere. The process leaves unique fingerprints on the clay through flashing, fly ash effects, and glassy surfaces.
Modern wood-fired kilns allow contemporary potters to achieve similar traditional effects. While gas and electric kilns are more popular today, many ceramic artists prefer wood firing for the serendipitous effects it can achieve.
How Does a Wood Kiln Work?
While wood kiln designs vary, they all work on the basic principle of burning wood as fuel to produce heat and fire the contents inside. Here is an overview of the main components and how they work together:
This is the area where wood is loaded and burned as fuel. Fireboxes are typically stoked through a front loading fire door. Primary combustion of the wood occurs here to produce heat and flame.
Channels direct heat, flames, and smoke from the firebox into the main firing chamber. Baffles may be used to control the direction and flow. The size and shape of the channels affects the movement of heat in the kiln.
Main Firing Chamber
This is the largest section where ceramics are loaded onto shelves to be fired. Flames, heat, and smoke enter from the combustion channels and fill this area during firing. Temperatures can reach over 1300°C.
The chimney releases exhaust smoke and draws air up and through the kiln. This creates the draft that pulls heat and flame through the firing channels. The chimney regulates air pressure inside the kiln.
Small openings allow the firing process to be monitored from outside. Ceramic plugs called wadding are used to seal peepholes.
Vents control oxygen intake. Closing vents restricts air flow, creating a reduction atmosphere in the kiln. This reduction environment affects the clay.
A thick brick lining protects the kiln structure from high heat. Heat radiates from the bricks into the firing chamber. High quality fire bricks withstand repeated firings.
Special ceramic glass windows allow the firing to be observed safely. These are positioned to see inside the chamber during the process.
This includes shelves, props, and posts used to hold and support ceramics in the kiln. Furniture is made from materials like brick and ceramic that can withstand high temperatures.
How Does Wood Firing Work?
Wood kiln firing involves several phases over the course of days or weeks to fully complete. Here is an overview of the firing process from start to finish:
1. Kiln Loading
Ceramic pieces are loaded carefully onto shelves inside the kiln. Pieces are spaced and supported to prevent contact during firing. The way pieces are stacked can affect the firing results.
Clay plugs seal up peepholes and fill any gaps in the kiln. This helps control air flow and heat distribution.
3. Firing Begins
A small starter fire is lit in the firebox and fed with kindling. As it grows hotter, larger pieces of wood are added.
Once the fire is established, the firebox is stoked with logs to produce flame and heat. Firewood is added periodically to maintain temperatures.
Flames spread from the firebox through the channels into the main chamber. Smoke and heat fill the kiln, which reaches over 1000°C.
6. Reduction Phase
Vents begin to close, limiting air intake. This reduction environment affects the clay through carbon trapping. Sodium vapor produces “flashing” effects.
7. Cooling & Unloading
Firing is complete once target temperatures are reached. The kiln is allowed to cool over several days before unloading finished pieces.
Wood Kiln Designs
There are many different types of kilns used for wood firing. Some major categories include:
One of the most common designs, these have a chimney in the center and firebox on one side. Heat travels horizontally across the kiln towards the chimney.
The firebox is positioned above the chamber in these kilns. Heat is pulled down through the ware before being exhausted near the bottom.
This ancient Japanese design consists of a single chamber kiln with firebox at one end. Heat and smoke slowly wind through the kiln and out the chimney.
Multiple fireboxes allow for precise control over multiple chambers connected like train cars. Atmospheres in each section can be controlled.
Fast Fire Kilns
Some newer gas kilns have a secondary firebox for incorporating a wood firing effect for only the end of the firing.
Kiln Construction Materials
Wood kilns are built from materials that can withstand repeated heating and cooling cycles at high temperatures:
- Fire Bricks – Dense, heat-resistant bricks line the interior walls and floor. High alumina and silica content provides durability.
- Refractory Cement – A specialized cement made from alumina and silica mortar binds bricks together. Withstands heat expansion.
- Ceramic Fiber Blanket – Insulating material around exterior helps retain heat and limit shell temperature.
- Steel – Used for exterior structure. Can withstand moderate heat from the shell.
- Ceramic Peephole Plugs – High alumina ceramic plugs control airflow through peepholes during firing.
Effects of Wood Firing
Firing in a wood kiln produces distinctive effects on the clay not achievable with other fuel types:
Flames produce areas of glossy sheen from liquefied silica and sodium vapor. Varied colors result based on temperature and atmosphere.
Fly Ash Buildup
Ash from the combustion process melts onto the ware’s surface creating textured deposits that appear cracked. Ash derives unique colors from minerals.
Reduction atmosphere allows carbon to penetrate the clay body. This causes dark smudges and trailing called carbon trailing.
High temperatures and wood ash act as fluxes to vitrify clay surfaces into glass. Melted ash can also flow in drips.
The intensity of heat and flame can warp and morph soft clay bodies. Kiln shelf marks also imprint onto the bases of pieces.
Chemical reactions produce unique colors and hues ranging from bright orange to metallic copper red. No two firings are ever exactly alike.
Benefits of Using a Wood Kiln
There are several key benefits that make wood firing a preferred choice for ceramic artists:
- Natural Effects – organic, one-of-a-kind results not achievable with electric or gas kilns.
- Traditional Method – valued cultural heritage dating back thousands of years. Celebrated in Japanese mingei pottery.
- Unique Atmosphere – reduction atmosphere and ash deposits affect the clay in a distinctive way.
- Experimentation – firing is never fully predictable allowing for serendipitous discoveries and effects.
- Aesthetics – many potters and ceramic artists prefer the visual qualities a wood kiln provides.
Considerations When Wood Firing
While rewarding, firing a wood kiln requires extra planning, effort, and considerations:
- Sourcing sufficient quality wood to fuel the kiln takes work and storage space.
- Firing is labor intensive, requiring round-the-clock stoking for days on end. Teams often work in shifts.
- It can be challenging to achieve precise temperatures and atmospheres. Each firing is unique.
- Fly ash and carbon trailing must be accounted for in glaze chemistry and clay body formulation.
- Significant heat work can limit usable clay bodies to stoneware and porcelain strength clays.
- Results are unpredictable requiring flexibility and allowing the process to contribute creatively.
Wood Firing Safety
Proper precautions need to be taken due to the hazards involved:
- Have properly trained personnel ignite and stoke firings under supervision.
- Ensure adequate ventilation to prevent dangerous buildup of carbon monoxide.
- Use protective eyewear when stoking and avoid direct exposure to flame.
- Let kiln fully cool before unloading to avoid burns.
- Check stability of ware on shelves to prevent pieces falling and damaging kiln lining.
- Keep fire extinguisher on hand in case a fire breaches the kiln exterior.
- Install heat shields around kiln to protect nearby structures and prevent fires.
Tips for Successful Wood Firing
Here are some tips for getting the best results from wood kiln firings:
- Test and prepare clay thoroughly to prevent flaws – use stronger stoneware and porcelain bodies.
- Control airflow carefully during firing by slowly closing vent holes.
- Use hardwoods like oak which burn evenly and produce good ash. Avoid resinous softwoods.
- Load ware tightly but allow space for heat circulation between pieces.
- Keep flames moving through the kiln regularly to prevent hotspots.
- Employ long cooling cycles of several days to a week to prevent cracking.
- Experiment with variations in wood placement, species, and stoking schedules.
- Embrace the unpredictability and allow the process to influence the final results.
- Keep detailed firing logs to improve consistency and understand how to modify atmospheres.
History of Wood Firing
Wood kilns originated thousands of years ago and continue today as both a traditional craft and contemporary art form:
- Ancient China – Early dates back 4,000+ years for utilitarian vessels and roof tiles.
- Japan – Anagama kilns for mingei pottery perfected during the Edo period 1603-1868.
- Mediterranean – Wood firing history dates back to ancient Greek and Roman pottery.
- America – Settlers brought wood kilns that fuelled 19th century stoneware production.
- UK – Train kilns adapted for industrial brick and tile production during the industrial revolution.
- Contemporary – Wood firing enjoys a resurgence as studio potters value its unique effects.
While technology has changed, the fundamental process of wood firing remains rooted in these long-standing ceramic traditions still treasured today.
Wood kilns allow modern artists to connect with part of humanity’s shared cultural heritage of clay and fire. The wood firing process bonds past and present through working in concert with the transformative power of flame.
From ancient pottery to modern artware, wood firing involves age-old techniques that produce beautiful ceramic effects. While complex, the fundamental components and process are straightforward. Stacking wood to fuel the kiln combined with controlling airflow transforms clay through flashing, fly ash, heat work, and reduction effects. It requires careful effort but rewards potters with natural aesthetic and artistic possibilities that are highly prized. Wood firing connects today’s ceramic artists with part of the long human history of clay, culture, and fire.
Frequently Asked Questions
Here are answers to some common questions about wood kilns:
What temperature do wood kilns reach?
- Wood kilns can reach cone 10-13 temperatures of 2300°F / 1260°C and higher. Maximum temperatures depend on the kiln design and firing duration.
What is the cost of building a wood kiln?
- Cost varies widely based on size and materials. Small hobby kilns can be built for under $2000. Large custom kilns may cost $8000 to $12,000 or more.
How long does a typical wood firing take?
- Firing and cool down lasts 4-10 days though some can stretch 2-3 weeks for very large ware loads. After cooling, unloading can take another full day.
What are the different wood kiln styles?
- Major types are crossdraft, downdraft, anagama, train kilns, and fast fire kilns. Each has variations in size, shape, and configuration.
How much wood is needed to fire a kiln?
- Depending on size, most firings require 5-10 full cords of hardwood. A mature tree yields 1-2 cords of wood.
Is wood firing environmentally sustainable?
- Using waste wood can improve sustainability. However, large fuel consumption and emissions make wood firing less eco-friendly than electric or gas-fired kilns.