Sovereignty Module: Glaze the Pot
Complete Pottery Glazing from Raw Materials: From Earth to Glass
Glazes are thin layers of glass fused to ceramic surfaces. This campaign covers glaze chemistry, raw material sourcing, recipe formulation, application, and firing.
Chapter 1: Glaze Chemistry Basics
| Component | Role | Source | Percentage |
|---|---|---|---|
| Silica (SiO2) | Glass former | Quartz sand, flint | 50-70% |
| Alumina (Al2O3) | Stiffener (prevents running) | Clay, feldspar | 10-20% |
| Flux (various oxides) | Lowers melting point | Feldspar, wood ash, limestone | 15-30% |
| Colorant (optional) | Adds color | Metal oxides (iron, copper, cobalt) | 1-10% |
Glaze is glass: 1) Silica (sand) is the main ingredient of glass. 2) Pure silica melts at 3100°F (too hot for most kilns). 3) Fluxes lower the melting point to kiln temperatures. 4) Alumina thickens the glaze so it does not run off the pot. 5) The balance of these three determines glaze character.
Chapter 2: Raw Material Sources
| Material | Provides | Where to Find | Preparation |
|---|---|---|---|
| Quartz sand | Silica | Riverbeds, beaches | Wash, grind fine |
| Feldspar | Silica + alumina + flux | Granite outcrops | Crush, grind fine |
| Wood ash | Flux (potassium, calcium) | Hardwood fire | Sieve, wash (optional) |
| Limestone | Flux (calcium) | Limestone outcrops | Crush, calcine, grind |
| Clay (ball clay) | Alumina + silica | Clay deposits | Dry, grind |
| Iron oxide | Red-brown colorant | Rust, red earth | Grind fine |
| Copper oxide | Green colorant | Copper ore, verdigris | Grind fine |
| Cobalt oxide | Blue colorant | Cobalt ore (rare) | Grind very fine |
| Manganese dioxide | Purple-brown colorant | Manganese ore | Grind fine |
Chapter 3: Simple Glaze Recipes
| Recipe | Ingredients | Temperature | Color | Character |
|---|---|---|---|---|
| Ash glaze (simple) | 50% wood ash, 50% feldspar | 2200-2300°F | Green-brown | Rustic, variable |
| Limestone glaze | 30% limestone, 30% feldspar, 30% silica, 10% clay | 2200°F | Cream-white | Smooth, reliable |
| Slip glaze | 100% high-iron clay | 2200°F | Dark brown-black | Simple, one ingredient |
| Albany slip (substitute) | 80% red clay, 10% feldspar, 10% silica | 2200°F | Dark brown | Smooth, reliable |
| Wood ash celadon | 40% ash, 40% feldspar, 20% silica + 2% iron | 2300°F | Green (celadon) | Classic, beautiful |
Wood ash glaze (detailed): 1) Collect hardwood ash (oak, maple, hickory preferred). 2) Sieve through 80-mesh screen (remove charcoal and debris). 3) Optional: wash ash (soak in water, pour off liquid, dry). 4) Washing removes soluble potassium (reduces flux, more predictable). 5) Unwashed ash is more variable but often more interesting. 6) Mix 50% ash + 50% feldspar (by dry weight). 7) Add water to make thick cream consistency. 8) Sieve through 80-mesh screen. 9) Apply to bisque-fired pot (dip, pour, or brush). 10) Fire to cone 10 (2345°F) in reduction for best results.
Chapter 4: Glaze Application
| Method | Best For | Thickness Control | Speed |
|---|---|---|---|
| Dipping | Small-medium pots | Excellent (timed dip) | Fast |
| Pouring | Inside of pots, large pieces | Good | Moderate |
| Brushing | Detail work, small areas | Variable | Slow |
| Spraying | Even coating, large pieces | Excellent | Fast (needs equipment) |
Application steps: 1) Bisque fire pots first (1800°F). 2) Clean pot surface (no dust or oil). 3) Mix glaze to thick cream consistency. 4) Stir glaze thoroughly before each use. 5) Dip method: hold pot by foot, dip into glaze for 3 seconds. 6) Remove, rotate, dip other end if needed. 7) Touch up bare spots with brush. 8) Wipe glaze from foot (bottom 1/4 inch). 9) Glaze on foot will fuse pot to kiln shelf. 10) Allow glaze to dry before loading kiln.
Chapter 5: Glaze Firing
| Atmosphere | Effect | How to Achieve | Result |
|---|---|---|---|
| Oxidation | Clean colors, predictable | Plenty of air in kiln | Bright colors |
| Reduction | Complex colors, variable | Restrict air (close damper) | Muted, earthy tones |
| Neutral | Balanced | Normal air flow | Standard results |
Firing schedule: 1) Load kiln carefully (no pots touching). 2) Slow heat to 400°F (100°F/hour) to drive off moisture. 3) Increase to 200°F/hour to 1800°F. 4) For reduction: close damper partially at 1800°F. 5) Continue to peak temperature (2200-2345°F depending on recipe). 6) Hold at peak for 15-30 minutes (heat soak). 7) Cool slowly: do not open kiln until below 400°F. 8) Total cycle: 12-24 hours firing + 24 hours cooling.
Reference Card
- Glaze is glass on clay (a glaze is a thin layer of glass that is fused to the ceramic surface during firing; it makes the pot waterproof, food-safe, and beautiful). 2. Three ingredients make a glaze (silica forms the glass, flux lowers the melting point, and alumina prevents the glaze from running off the pot; every glaze is a balance of these three). 3. Wood ash is a free flux (hardwood ash contains potassium and calcium that act as powerful fluxes; mixed with feldspar or clay, ash produces beautiful glazes from entirely free materials). 4. Wipe the foot clean (any glaze on the bottom of the pot will melt and fuse the pot permanently to the kiln shelf; always wipe glaze from the bottom 1/4 inch before firing). 5. Test before committing (always test a new glaze recipe on a small test tile before glazing important pots; glazes can behave unpredictably with different clays and firing conditions). 6. Reduction creates complexity (reducing the oxygen in the kiln during firing causes chemical changes in glazes that produce complex, earthy colors impossible to achieve in oxidation). 7. Consistency requires measurement (measuring ingredients by weight, not volume, ensures reproducible results; keep a glaze notebook with recipes, firing notes, and results). 8. Every glaze tells a story (the color, texture, and character of a glaze reflect the materials, the fire, and the potter's choices; handmade glazes from local materials connect the potter to the land).