Complete Pottery Glazing and Kiln Firing: From Raw Ash to Glossy Surface
Glazing transforms porous earthenware into waterproof, food-safe, beautiful vessels. This campaign covers glaze chemistry, ash glazes, kiln construction, firing schedules, and troubleshooting.
Chapter 1: Glaze Fundamentals
Component
Function
Sources
Percentage
Silica (glass former)
Creates the glassy surface
Quartz, sand, flint
55-75%
Alumina (stiffener)
Prevents glaze from running off
Clay, feldspar
5-15%
Flux (melter)
Lowers melting temperature
Wood ash, limestone, feldspar, borax
15-35%
Colorants
Add color
Metal oxides (iron, copper, cobalt)
1-10%
Opacifiers
Make glaze opaque
Tin oxide, zirconium
5-15%
Firing Temperature
Name
Cone
Glaze Type
Clay Body
1,650-1,940°F
Low fire (earthenware)
06-1
Lead, boron, alkaline
Red/white earthenware
2,100-2,300°F
Mid fire (stoneware)
4-7
Feldspathic, ash
Stoneware
2,300-2,400°F
High fire (stoneware/porcelain)
8-12
Feldspathic, ash
Stoneware, porcelain
Chapter 2: Ash Glazes (From Scratch)
Ash Source
Character
Color
Flux Strength
Availability
Hardwood (oak, maple)
Smooth, reliable
Tan to green
Moderate
Abundant
Softwood (pine, cedar)
Rough, variable
Brown to amber
Low
Abundant
Straw/grass
Smooth, high silica
Clear to pale
High
Agricultural
Fruit wood (apple, cherry)
Smooth
Green to amber
Moderate
Orchards
Bone ash
Smooth, opaque
White, creamy
High
Butchering byproduct
Seaweed
Variable
Green, blue-green
High
Coastal
Simple ash glaze recipe: 1) Collect wood ash (hardwood preferred). 2) Sieve ash through fine screen (remove charcoal chunks). 3) Wash ash: soak in water, stir, let settle, pour off water (removes soluble alkalis). 4) Repeat washing 2-3 times. 5) Dry washed ash. 6) Basic recipe: 40% washed wood ash + 40% feldspar + 20% ball clay. 7) Mix with water to cream consistency. 8) Sieve through 80-mesh screen. 9) Apply to bisque-fired pottery (dip, pour, or brush). 10) Fire to cone 8-10 (2,300-2,380°F). 11) Result varies with ash source (the beauty of ash glazes is their natural variation).
Chapter 3: Kiln Construction
Kiln Type
Max Temp
Fuel
Difficulty
Cost
Capacity
Pit firing
1,300-1,650°F
Wood, dung
Very low
Free
Small-medium
Sawdust kiln
1,200-1,500°F
Sawdust
Very low
Very low
Small
Updraft kiln (brick)
2,000-2,400°F
Wood
Moderate
Low-moderate
Medium
Downdraft kiln
2,000-2,400°F
Wood, gas
High
Moderate
Medium-large
Anagama (tunnel)
2,300-2,400°F
Wood (massive quantity)
Very high
Moderate
Large
Electric kiln
2,300°F
Electricity
Low (purchase)
High
Small-medium
Raku kiln
1,800-2,000°F
Gas, wood
Low-moderate
Low
Small
Simple updraft kiln construction: 1) Dig foundation trench (firebox below, chamber above). 2) Build firebox: brick-lined channel, 12-18 inches wide, 12 inches tall. 3) Build floor: grate or perforated shelf above firebox (allows heat to rise). 4) Build chamber walls: firebrick or dense clay brick, 24-36 inches diameter. 5) Walls: 4-6 inches thick (insulation). 6) Door: removable bricks on one side (loading/unloading). 7) Top: partially covered (damper controls draft). 8) Chimney: not required for updraft (heat rises through top). 9) Fire in firebox, heat rises through floor into chamber. 10) This kiln can reach cone 6-10 (2,200-2,380°F) with sustained wood firing.
Chapter 4: Firing Process
Stage
Temperature Range
Rate
Time
What Happens
Water smoking
Room temp - 400°F
Slow (100°F/hr)
2-4 hours
Remaining moisture driven off
Burnout
400-1,100°F
Moderate (200°F/hr)
3-4 hours
Organic matter burns out
Quartz inversion
1,063°F
Very slow (50°F/hr)
Pass slowly
Quartz crystal structure changes (cracking risk)
Sintering
1,100-1,650°F
Moderate (200°F/hr)
2-3 hours
Clay particles begin fusing
Vitrification
1,650-2,400°F
Moderate (150-200°F/hr)
3-6 hours
Glass formation, glaze melts
Soaking
Target temp
Hold
15-30 min
Even heat distribution, glaze smooths
Cooling
Target to room temp
Slow (natural)
12-24 hours
DO NOT OPEN KILN (thermal shock)
Bisque firing: 1) Load bone-dry greenware into kiln (pieces must be completely dry). 2) Leave space between pieces (air circulation). 3) Fire slowly to 1,800-1,940°F (cone 06-04). 4) Rate: 100°F per hour to 400°F, then 200°F per hour to target. 5) Hold at target for 15-30 minutes. 6) Cool slowly (do not open kiln until below 200°F). 7) Result: hard but porous (ready for glazing). 8) Bisque ware absorbs glaze like a sponge (the porosity is the point).
Chapter 5: Troubleshooting
Problem
Cause
Fix
Crawling (glaze pulls away)
Dusty bisque, glaze too thick, oil contamination
Clean bisque, thin glaze, handle with clean hands
Crazing (fine cracks in glaze)
Glaze contracts more than clay (tension)
Add silica to glaze, or use different clay
Shivering (glaze flakes off)
Glaze contracts less than clay (compression)
Reduce silica in glaze
Pinholing (tiny holes)
Gases escaping through glaze
Fire slower, soak longer at peak temp
Running (glaze flows off pot)
Too much flux, fired too hot
Reduce flux, lower temperature, apply thinner
Dunting (cracks during cooling)
Cooled too fast (thermal shock)
Cool kiln more slowly, don't open early
Bloating (bubbles in clay)
Over-fired, impurities in clay
Lower temperature, use cleaner clay
Reference Card
Bone dry before firing (any moisture in clay = steam explosion in kiln; dry completely before loading). 2. Slow through quartz inversion (1,063°F is dangerous; heat and cool slowly through this point or pots crack). 3. Never open a hot kiln (thermal shock cracks everything; wait until below 200°F to open). 4. Ash makes glaze (wood ash is a complete glaze flux; mixed with clay and feldspar, it creates beautiful surfaces). 5. Test tiles save pots (test every new glaze on small tiles before applying to finished work). 6. Glaze thickness matters (too thin = dry and rough; too thick = runs off pot onto kiln shelf). 7. Wax the foot (apply wax resist to bottom of pots before glazing; glaze on the bottom fuses pot to shelf). 8. Fire transforms earth (raw clay becomes stone through fire; glazing adds beauty and function to that transformation).
