What is Reduction Firing? The Process Explained
We all think we understand reduction firing, but do we? The process occurs inside of a kiln and the array of colors it produces may not be what you expect. Not all reduction firing is done with the Raku process. You can best control reduction firing using a gas kiln. Although electric kilns can produce a reduction firing, they are not designed to deliver a oxygen deprived atmosphere without a lot of work. In today’s article, we’ll take a look at what the reduction firing process looks like. Let’s get started!
Reduction Firing
Reduction firing is a specialized ceramic firing technique that occurs when there's insufficient oxygen in the kiln atmosphere for complete fuel combustion. This oxygen-deprived environment creates carbon monoxide at high temperatures, which then extracts loosely bonded oxygen from materials in clay, particularly iron and copper compounds. This chemical alteration process results in distinctive color changes and effects that are hallmarks of reduction-fired ceramics. The unique interplay between the kiln atmosphere and the ceramic material produces results that are highly valued by many potters and collectors.
A reduction atmosphere in a kiln is achieved by adjusting the ratio of fuel to air, introducing more fuel than can be fully combusted with the available oxygen. As a result, the excess fuel molecules "steal" oxygen from the clay and glazes, causing chemical changes that affect the color and texture of the finished ceramics. Common methods to create a reduction atmosphere include partially closing the damper to restrict airflow, increasing the fuel supply, or introducing additional combustible materials like wood or oil into the kiln. The level of reduction can be monitored by observing the color and behavior of the flames exiting the kiln, with a more orange or smoky appearance indicating stronger reduction conditions.
Preparing Bisque for Glazing
While raw-glazed, single -fired work exists, it's relatively uncommon due to several challenges. Bone-dry greenware is fragile and prone to breakage during glazing. Additionally, single-firing can lead to glaze pinholes caused by burning organic materials. There's also a risk of greenware bursting if fired too rapidly past 100°C / 212F (the boiling point), potentially trapping impurities within the glaze. To mitigate these issues, glazes that are meant to be used on dry greenware contain more clay than typical bisqueware glazes and must be carefully formulated to match the raw pot's shrinkage rate.
Most studio potters opt for a two-step firing process. They first bisque-fire their pieces to a temperature below the clay's maturation point, creating a porous yet sturdy form. This porous bisque is then glazed and fired to maturity. This method significantly reduces the risks associated with raw glazing while improving glaze adhesion and overall durability of the finished piece.
Related: What is Raku Firing?
Temperature Measurement Tools
Pyrometers
A pyrometer is a device used to measure temperature in a kiln, typically employing a probe inserted into the firing chamber. While pyrometers provide real-time temperature readings at the probe's location, they have limitations. They offer only a snapshot of the kiln's temperature at a specific point and cannot account for variations in heat distribution throughout the kiln or indicate whether the materials inside have reached their optimal firing state.
Pyrometric Cones
Pyrometric cones are the industry standard for measuring "work heat" in ceramic firing. Work heat refers to the combined effect of time and temperature on materials. These cones are crafted from a mixture of clay and glaze materials, designed to melt at specific temperatures. The temperature difference between consecutive cone numbers is approximately 20°C (36°F).
Cones come in large and small sizes, with slight variations in melting temperatures between sizes of the same cone number. Small cones are often used in electric kilns for visual monitoring through small peepholes and in kiln-sitters to automatically shut off the kiln at the desired temperature.
Using Cone Packs
Cone packs are essential tools for monitoring firing progress if you have a manual controller. This step is typically not used for kilns that have a digital controller. In gas kilns, place cone packs in front of the top and bottom peepholes using clay wads as a base. Each pack should include a guide cone, a firing cone, and a guard cone. Tilt the cones at an 8-degree angle for optimal observation of their softening and bending.
For high-fire processes like cone 10 reduction, use two sets of double cone packs:
- Lower cones (e.g., 011, 010, 09, and 4) help determine when to start reduction.
- Glaze maturity cones (e.g., 6, 8, 9, and 10) indicate when the glazes have reached their optimal firing temperature.
Pre-make and dry cone packs to prevent explosions in faster-heating glaze kilns. For higher temperature firings, use a large clay boat to catch melted cones, as they will liquefy and run.
Always complete a kiln chart and clearly label cone numbers in the packs to ensure accurate interpretation of readings during firing. This attention to detail in cone pack preparation and placement is crucial for achieving consistent and desired results in ceramic firing.
Related: Guide to Kiln Temperature Ranges for Pottery
How to Position Cone Packs
Cone packs are strategically placed in front of the kiln's peep holes. The reduction cones are typically positioned at the front, facing one direction and set to melt first. The glaze cones are placed at the back, facing the opposite direction. It's crucial to verify their visibility with the kiln door closed, using a flashlight or paper torch inside the kiln for confirmation.
Related: Best At-Home Kilns
Glazing
High-fire glazes are best applied through dipping for even coverage. To prevent glaze from sticking to pot bottoms, apply wax or create a dry foot. Waxing lid seats and edges helps reduce warping when firing with the lid on. High-fire glazes tend to run during firing, so leave a 3.175mm to 6.35mm (1/8 inch to 1/4 inch) gap between the pot bottom and the glazed area to prevent runoff onto the kiln shelf.
After firing, many potters deepen the foot bevel to accommodate Cone 10 reduction cone packs. Position cone packs back-to-front with cones 011, 010, 09, and 4 at the rear and 6, 8, 9, and 10 at the front for easier sighting. Placing a kiln shelf shard underneath helps manage glaze runs.
For thicker glaze applications or experimental combinations, use a clay waster or broken kiln shelf piece underneath to catch drips and protect the shelf. This precaution is important as fired glaze adhering to the kiln shelf can break when the item cools.
Loading
Before loading, ensure the kiln is free of debris around ports and behind the bagwall. Check the target brick behind the bag wall and remove any obstructions. Confirm all gas valves are closed and the blower system is off. If there are concerns about the kiln's functionality, test-light it before loading.
Apply kiln wash to each shelf, mixed to a cream consistency and applied in thin layers on the top surface only.
When stacking shelves, elevate the first level several inches off the kiln bottom for proper heat and airflow. Use shared tri-posted shelf ends for subsequent floors. Maximize shelf space by grouping similar-height items, placing taller components on the top shelf. Keep bottom shelf ware at least 15 cm (6 inches) high for adequate heat and gas circulation.
Firing
After loading, keep the damper open, turn off the air, and light the pilots. It's common to let the gas kiln burn on very low heat overnight for a convenient firing time the next day. Glazed ware holds less physical water than greenware and is less likely to explode at 100°C (212°F). However, gradually increase the temperature until surpassing red heat around 537°C (1000°F), which is also the point of quartz inversion.
Oxidation refers to the complete combustion of fuel in the presence of sufficient oxygen. This process is characterized by a blue, short, and bushy flame, clear kiln atmosphere, and a roaring sound. There should be no back pressure from the damper or peepholes, and the gas combusts at the burner tips, leading to efficient combustion and rapid temperature rise. It's recommended to oxidize until reaching about 010, and for smoother, brighter glaze surfaces, oxidizing for fifteen to thirty minutes towards the end of the firing (beginning around cone 9 soft for a cone 10 soft firing) can be beneficial.
Reduction occurs when fuel burning lacks oxygen, producing carbon monoxide. This method produces lengthy, gentle orange or yellow flames. Back pressure from incompletely burned gasses causes murky kiln air and orange or yellow flames at the damper and peek holes. Black smoke indicates gas overstock, causing clay body black-coring and poor glazes.
During reduction, oxygen deprivation turns the entire kiln into a burning region. This limits efficiency and slows temperature rise but even kiln temperature. Around cone 010, reduction takes twenty to thirty minutes. Starting decrease after cone 06 may miss effects.
Neutral or slightly reduced atmosphere in the kiln is characterized by greenish flames and creates conditions that lie between oxidation and reduction. Following reduction, it's typical to maintain a neutral atmosphere until reaching cone 9, after which oxidation is introduced.
Kiln Control
Kiln controls typically include a gas valve with a meter or handle position indicator, primary air (usually a blower on forced-air kilns), and secondary air controlled by the flue damper. To achieve an oxidizing flame, adjust the gas level appropriately. A blue, bushy flame can be obtained by opening the damper, increasing primary air, or reducing gas. For reduction conditions, partially close the damper, decrease primary air, or increase gas pressure.
The firing process typically starts in oxidation until cone 010, followed by reduction until cone 9. In the final 20 minutes, as cone ten moves, glazing ingredients are fully oxidized. At glaze maturity (cone 10), close gas valves and switch off the kiln.
Record the time on the kiln chart and close the damper to prevent cold air drafts. Slow cooling helps most glazes smooth bubbles and prevent quartz or cristobalite inversion stresses that can cause denting or cracking.
Safety Precautions
Before inspecting the kiln, ensure long or fluffy hair is tied back. Replace peep plugs from the side to avoid back pressure. If back pressure obstructs cone visibility, adjust the gas or slightly open the damper. When observing kilns at temperatures above orange heat, wear protective dark glasses designed for eye safety.
Resolving Issues
Kiln conditions must be assessed for changes, considering factors such as kiln type (downdraft or updraft), weather, and loading methods. These guidelines primarily apply to updraft kilns:
- If the damper is fully open, partially close it to maintain kiln heat.
- Reduce primary air intake if excessive turbulence is causing strong drafts.
- If the damper is too tight, open it slightly and reduce gas flow for better oxidizing conditions.
- To improve bottom reduction, increase primary air flow or slightly open the damper.
- If the kiln slows down near the end of firing, check and adjust gas settings for optimal combustion.
Make changes judiciously and record modifications in the kiln chart for future reference.
Kiln Shutdown Procedure
- Turn off gas valves (if applicable).
- Switch off the electric system.
- Shut down blowers to stop airflow.
- Close pilot valves to prevent gas flow.
- Close the damper to prevent cool air entry.
- Record shutdown time on the kiln chart.
Unloading
Slow cooling benefits crystal-surface glazes, but leaving the kiln damper open after firing until red heat may strain the kiln's structure and produce poor glaze surfaces. Rapid cooling around 540°C (1004°F) (quartz inversion point) and 225°C (437°F) (cristobalite conversion point) can cause dunting or cooling cracks.
Use the paper test through a spy hole to determine if the kiln is cool enough to open. After the paper stops igniting (usually at 230°C or 446°F), remove the peeps and slightly open the damper. Wait until the kiln is cool before unloading.
When unloading, note any anomalous reduction results and mark unreduced or underfired components on the kiln chart. Remove glaze runs from shelves using a chisel and hammer or black silicon carbide. Wear safety glasses during this process. After cleaning, reapply kiln wash and restack shelves face-to-face and back-to-back.
Finally, re-stack kiln posts, sweep if needed, and clean up the area. This systematic approach to unloading and maintenance ensures the longevity of your kiln and the quality of future firings
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