Glaze Materials

Part of Pottery and Ceramics

Understanding the raw materials that form pottery glazes: wood ash, feldspar, silica, and natural fluxes available without industrial supply chains.

Why This Matters

A well-glazed pot is not merely decorative. Glaze creates a glass-like surface that is impervious to water, resistant to bacterial growth, and easy to clean. Without glaze, fired clay remains porous — it absorbs liquids, harbors mold, and leaches earthy flavors into food. For a rebuilding community, the difference between glazed and unglazed pottery is the difference between safe long-term food storage and vessels that spoil their contents within days.

Glazing also dramatically extends the useful life of pottery. An unglazed cooking pot develops micro-cracks from thermal cycling and eventually crumbles. A glazed pot can survive hundreds of heating cycles because the glassy surface seals those cracks and distributes thermal stress more evenly. When you are firing every pot by hand with fuel you gathered yourself, making each piece last matters enormously.

The good news is that effective glazes can be made entirely from materials found in nature. You do not need a ceramics supply catalog. Wood ash, crusite rock, quartz sand, and common clays can produce functional, food-safe glazes at temperatures achievable in a wood-fired kiln. The key is understanding what each material contributes and how to combine them.

The Three Components of Glaze

Every glaze is built from three functional groups. You do not need to know chemistry equations — just understand what role each group plays.

Glass Formers (Silica)

Silica (SiO2) is the backbone of all glazes. It forms the actual glass. Without enough silica, you do not get a glaze — you get a powdery or rough surface.

Source	Where to Find It	Notes
Quartz sand	Riverbeds, beaches, sandstone outcrops	Wash and grind fine; avoid calcium-rich beach sand
Flint/chert	Gravel beds, chalk formations	Calcine (heat to red-hot, quench in water) then crush
Diatomaceous earth	Lake beds, dried ponds	Very fine silica, dissolves readily in glaze
Rice hull ash	Burn rice husks	90%+ silica, excellent source in rice-growing areas

Grinding Silica

Silica must be ground extremely fine — 200 mesh or finer. Use a stone quern or mortar and pestle. Wet-grind for best results: add water to the mortar, grind for 30 minutes, let settle, pour off the fine suspension. The material that stays suspended for 30+ seconds is fine enough.

Fluxes (Melting Agents)

Silica alone melts at approximately 1,710C — far beyond any wood-fired kiln. Fluxes lower that melting point to achievable temperatures (900-1,300C).

Common Natural Fluxes:

1. Wood ash — The most accessible flux. Contains potassium, calcium, and phosphorus. Different woods produce different results:
   • Hardwood ash (oak, maple, ash) — high calcium, produces stiff, matte glazes
   • Softwood ash (pine, spruce) — more potassium, produces fluid, glossy glazes
   • Fruit tree ash — balanced flux, often produces beautiful celadon-like colors
2. Feldspar — A mineral found in granite and many igneous rocks. Contains sodium, potassium, and aluminum. Crush granite and the white or pink crystals are feldspar.
3. Limestone — Calcium carbonate (CaCOite). Crush to fine powder. Acts as a flux above 1,100C.
4. Bone ash — Calcined animal bones. Rich in calcium phosphate. Grind to powder.
5. Seashells — Nearly pure calcium carbonate. Calcine and crush.

Stabilizers (Alumina)

Alumina (Al2O3) keeps the molten glaze from running off the pot. Without it, your glaze slides to the bottom during firing and welds the pot to the kiln shelf.

Source	Alumina Content	Preparation
Common clay	15-40%	Dry, crush, sieve
Kaolin (white clay)	35-40%	Best source; often found near granite
Feldspar	15-20%	Also contributes flux and silica
Wood ash	1-5%	Minor contribution

Preparing Wood Ash for Glazes

Wood ash is the most important glaze ingredient for a post-collapse potter. Here is the complete preparation process:

1. Collect ash from hardwood fires. Avoid ash from painted wood, treated lumber, or trash fires. Use only clean wood ash.
2. Sieve dry through a coarse screen (window screen mesh) to remove charcoal chunks and debris.
3. Wash the ash (optional but recommended for consistency):
   • Place ash in a bucket, add 3-4 times its volume of water
   • Stir vigorously, let settle for 10 minutes
   • Pour off the water (this removes soluble potassium salts)
   • Repeat 2-3 times
   • The remaining material is “washed ash” — more calcium-rich and predictable
4. Dry the washed ash spread on a flat surface in the sun.
5. Grind fine using a mortar and pestle or stone quern. Sieve through the finest mesh available.

Caustic Warning

Unwashed wood ash mixed with water creates lye (potassium hydroxide). Wear hand protection when handling wet ash. Avoid touching your eyes. Washed ash is much safer to handle.

Simple Glaze Recipes

These recipes use proportions by volume (measuring cups, bowls, or any consistent container).

Ash Glaze (Simplest Possible)

The oldest known glaze — literally just wood ash that melts at high temperature.

• 50% wood ash (unwashed)
• 50% common clay (dried and powdered)

Mix with water to the consistency of heavy cream. Apply by dipping bisque-fired pottery. Fire to the highest temperature your kiln achieves (ideally 1,200C+). This produces a matte, earth-toned glaze. Color varies by ash source and clay type.

Feldspar Glaze (More Controllable)

• 40% crushed feldspar (ground very fine)
• 30% quartz sand (ground fine)
• 20% wood ash (washed)
• 10% clay (dried and powdered)

Fire to 1,200-1,250C. Produces a smoother, more glossy surface than pure ash glaze.

Low-Fire Glaze (For Simple Kilns)

If your kiln cannot reach 1,200C, use more flux:

• 60% wood ash (unwashed — the extra potassium helps)
• 25% quartz sand (ground fine)
• 15% clay

Fire to 900-1,000C. This produces a rough, semi-glassy surface. Not as smooth as high-fire glazes but functional for waterproofing.

Lead-Free Note

Avoid Lead

Historical potters often used lead oxide as a flux because it melts at low temperatures and produces brilliant glazes. Do not use lead. Lead-glazed pottery poisons food and water with cumulative, irreversible neurological damage. There is no safe level of lead in food contact surfaces. Stick to wood ash, feldspar, and limestone fluxes.

Adding Color

Natural colorants come from metal oxides found in minerals and ores:

Colorant	Source	Color Produced	Amount (% of dry glaze)
Iron oxide	Red clay, rust, bog iron	Amber, brown, black, celadon green	1-10%
Copper oxide	Malachite, native copper (heated in air)	Green, turquoise (oxidation); red (reduction)	1-5%
Manganese dioxide	Pyrolusite (black mineral in streambeds)	Purple, brown, black	2-8%
Cobalt	Cobaltite ore (rare)	Intense blue	0.5-2%
Titanium	Rutile sand (dark heavy sand in streams)	Cream, tan, crystal effects	2-8%

Testing Colors

Always make small test tiles before glazing finished pots. Apply the glaze to a flat piece of bisque-fired clay, fire it, and observe the result. A single batch of glaze may produce dramatically different colors depending on whether the kiln atmosphere is oxidizing (plenty of air) or reducing (smoky, oxygen-starved).

Applying Glaze

Glaze is always applied to bisque-fired pottery (pottery that has been fired once without glaze).

1. Mix glaze with water to heavy cream consistency. It should coat a dipped finger and show the finger’s color faintly through the coating.
2. Clean the pot — wipe off dust and ensure the surface is dry.
3. Dip method — Hold the pot by the foot and plunge it into the glaze bucket. Hold for 2-3 seconds, remove, and let drip. Rotate to ensure even coverage.
4. Pour method — For pots too large to dip, pour glaze over the surface while rotating the pot.
5. Brush method — Apply 3-4 coats with a soft brush, allowing each coat to dry between applications. Brush marks may show in the finished glaze.
6. Wipe the foot — Always clean glaze from the bottom 5mm of the pot and the entire foot ring. Glaze on the bottom will weld the pot to the kiln shelf.

Troubleshooting Common Glaze Problems

Problem	Likely Cause	Fix
Glaze crawled (pulled into beads)	Dusty bisque, glaze too thick, or too much alumina	Clean bisque before glazing; thin the glaze; reduce clay in recipe
Glaze ran off the pot	Too much flux, too little alumina	Add more clay to recipe; reduce ash or feldspar
Rough, sandpaper texture	Underfired or not enough flux	Fire hotter; increase ash content
Pinholing (tiny holes)	Gases escaping during firing	Soak kiln at peak temperature for 15-30 minutes before cooling
Crazing (fine cracks in glaze)	Glaze and clay body shrink at different rates	Add more silica to glaze; reduce flux

Glaze development is inherently experimental. Keep a notebook recording every recipe, firing temperature, and result. Over time, you will develop reliable glazes tuned to your specific local materials.