Kiln Firing 101: Oxidation vs. Reduction Explained

Kiln Firing 101: Oxidation vs. Reduction Explained

Master the two core kiln firing techniques that define ceramic glaze outcomes

🔥 The Science of Kiln Atmosphere

Kiln firing isn’t just about heat—it’s about controlling oxygen levels to trigger chemical reactions in clay and glazes. The two primary methods are:


  • Oxidation Firing: Abundant oxygen (air-rich environment)
  • Reduction Firing: Limited oxygen (fuel-rich environment)

🔀 Oxidation vs. Reduction Firing: Key Differences

"oxidation vs reduction firing comparison"


Parameter Oxidation Firing Reduction Firing
Oxygen Level 21% (normal air) <10% (oxygen-starved)
Temperature Range 25–1000°C (bisque + glaze firing) 900–1300°C (glaze-only transformation)
Glaze Reactions Oxidized metal oxides (e.g., Fe²O³ = red) Reduced metal oxides (e.g., FeO = black)
Typical Uses Transparent glazes, white porcelain, blue-and-white ware Copper red glazes, Jun kiln variegation, celadon

🚀 How Oxidation Firing Works

"oxidation firing process breakdown"

Stage 1: Low-Temperature Oxidation (25–600°C)

  • Goal: Remove moisture and organic matter (wax, resins) from greenware
  • Observation: Water vapor exits through the kiln vent; clay turns from earthy yellow to grayish white

Stage 2: High-Temperature Oxidation (600–1000°C)

  • Glaze Reaction: Silicates melt to form a glassy layer
  • Classic Examples:
    • Jingdezhen blue-and-white porcelain: Cobalt oxide stabilizes to blue in oxygen
    • Bone china: Calcium oxide enhances translucency

🌫️ How Reduction Firing Works

"reduction firing process breakdown"

Stage 1: Oxygen Starvation (900–1000°C)

  • Operation: Close air vents, introduce fuel (pinewood, natural gas)
  • Chemistry: Fuel consumes oxygen, producing CO (carbon monoxide)

Stage 2: Metal Reduction (1000–1300°C)

  • Copper Red Glaze: CuO → Cu (metallic red) in CO-rich air
  • Jun Kiln Variegation: Random precipitation of iron/copper in reduction creates "10,000 colors"

Critical Controls

  • Fuel Flow: 0.5–1.0 L/min for gas kilns
  • Kiln Pressure: Slightly positive (10–20 Pa) to block oxygen ingress

🎨 Glaze Selection by Firing Method

"which glaze works for oxidation or reduction"


Glaze Type Ideal Atmosphere Color Mechanism Example
Cobalt Blue Oxide Oxidation Stable Co³+ ions in oxygen Chinese blue-and-white ware
Copper Red Glaze Reduction Cu²+ reduced to metallic Cu⁰ Ming Dynasty sacrificial red
Ash Glaze (Wood-Fired) Reduction (wood kiln) Potassium oxide (K₂O) in wood ash fluxes silica Japanese Bizen ware

⚠️ Common Firing Mistakes and Solutions

Issue Oxidation Cause Reduction Cause Fix
Pinholes in Glaze Rapid heating trapped gases Early reduction Carbonized organic matter Soak at 600°C for 1 hour
Dull Red Glaze Residual oxygen in oxidation Inadequate reduction atmosphere Increase fuel, extend reduction phase
Warped Greenware Overfiring in oxidation Excessive kiln pressure in reduction Calibrate thermocouple, control pressure

🌍 Cultural Case Studies

Oxidation Icon: Chinese Blue-and-White Porcelain

  • Process: Hand-painted cobalt on bisque → transparent glaze → 1300°C oxidation firing
  • Advantage: Stable blue color, ideal for mass production

Reduction Legend: Song Dynasty Jun Kiln

  • Process: Copper-iron glazes fired in reduction for random color variegation
  • Uniqueness: Each piece’s color is a one-of-a-kind result of natural reduction

🔧 Kiln Type Selection Guide

"choosing the right kiln for oxidation or reduction"


Kiln Type Atmosphere Control Typical Uses Cost Range
Electric Kiln Oxidation (precise temp control) Blue-and-white, white porcelain $1,500–$5,000
Gas Kiln Reduction (burner-adjustable) Copper red glazes, celadon $5,000–$15,000
Wood Kiln Natural reduction (ash + fire marks) Bizen ware, Taoxichuan wood firing $10,000+ (custom built)

🎯 Pro Tips for Successful Firing

  • Test Tiles: Fire glaze samples in every kiln load to document atmosphere effects
  • Kiln Zoning: Oxidation kiln tops are 20°C hotter (ideal for light glazes); reduction kiln bottoms have higher CO (dark glazes)

🔗 Internal Link 

Back to blog

Leave a comment