Glaze Firing

Part of Kiln Design

Higher-temperature firing techniques for producing glazed, waterproof pottery.

Why This Matters

Unglazed earthenware is porous. It absorbs water, harbors bacteria, and degrades over time. Glazing transforms pottery from a fragile, permeable material into a glass-sealed, waterproof, hygienic surface that can store food, hold liquids indefinitely, and be cleaned thoroughly. In a rebuilding scenario, glazed pottery is the difference between vessels that breed disease and containers safe for drinking water, cooking, and long-term food storage.

Glaze firing requires higher temperatures and more precise control than basic earthenware bisque firing. You must reach temperatures where the glaze ingredients melt and fuse into a continuous glass layer, typically 900-1,300 C depending on the glaze recipe. This demands better kiln design, more fuel, and careful attention to firing schedules. The reward is pottery that lasts generations and serves critical sanitation and food safety functions.

Understanding glaze firing also opens the door to producing tile, pipe, and architectural ceramics with waterproof surfaces — essential for building water systems, drainage, roofing, and hygienic food preparation areas. A community that masters glaze firing has a fundamental technological advantage in health, construction, and trade.

Glaze Basics for Kiln Operators

A glaze is a thin layer of glass fused onto a ceramic surface. Every glaze needs three components:

Component	Role	Common Sources
Silica (glass former)	Forms the glass network	Quartz sand, flint, river sand
Flux (melting agent)	Lowers the melting point of silica	Wood ash, limestone, feldspar, borax, lead (toxic)
Alumina (stabilizer)	Prevents the glaze from running off	Clay (kaolin), feldspar

Simple Survival Glazes

These recipes use materials available without industrial supply chains:

Wood Ash Glaze (fires at 1,200-1,300 C)

• 40% washed wood ash (hardwood best)
• 40% feldspar or crusite quartz
• 20% clay (kaolin or ball clay)
• Mix dry, then add water to cream consistency

Limestone Glaze (fires at 1,050-1,150 C)

• 30% powite limestone powder (calcined to quicklime, then slaked)
• 50% fine silica sand
• 20% clay
• Warning: this glaze has a narrow firing range

Salt Glaze (fires at 1,100-1,250 C)

• No glaze applied to ware — instead, throw common salt (NaCl) into the kiln at peak temperature
• Sodium vaporizes and reacts with silica in the clay surface to form a glassy coating
• Produces distinctive orange-peel texture
• Caution: releases hydrochloric acid fumes — fire outdoors with wind carrying fumes away from people

Avoid Lead Glazes

Lead oxide produces beautiful, low-temperature glazes (750-900 C) but is severely toxic. Lead leaches from finished pottery into acidic foods and drinks, causing chronic poisoning. In a survival scenario with no medical infrastructure, lead poisoning is a death sentence. Use lead-free alternatives exclusively.

Two-Fire Process: Bisque Then Glaze

The standard approach fires pottery twice:

First Firing: Bisque (800-950 C)

1. Load bone-dry ware into the kiln, pieces can touch and be stacked
2. Fire slowly for the first 2 hours (100-150 C/hour) to drive off remaining moisture
3. Increase to 200-250 C/hour through the mid-range
4. Hold at 573 C for 30 minutes (quartz inversion — volume change risk)
5. Continue to 800-950 C and hold for 30 minutes
6. Cool slowly — do not open until below 200 C

Bisque firing converts raw clay to a porous ceramic that is strong enough to handle but still absorbs glaze slurry. This porosity is essential — it draws the liquid glaze into the surface, creating a strong bond.

Applying Glaze

After bisque firing and cooling:

1. Clean the bisque ware — brush off dust, wipe with a damp cloth
2. Mix glaze slurry to heavy cream consistency (specific gravity ~1.45-1.55 g/mL)
3. Apply by dipping, pouring, or brushing:
   • Dipping: fastest and most even. Hold the piece, submerge for 3 seconds, withdraw, let excess drip
   • Pouring: pour glaze over the piece inside and out, rotate to cover evenly
   • Brushing: least preferred — tends to leave brush marks and uneven thickness
4. Leave the foot unglazed — glaze on the bottom will fuse the pot permanently to the kiln shelf
5. Apply wax or clay slip to the bottom 5 mm as a resist to prevent glaze drips from reaching the shelf
6. Allow to dry completely before loading into the kiln

Glaze Thickness

The glaze layer should be roughly the thickness of a playing card (0.5-1 mm). Too thin and it will be patchy and rough. Too thick and it will run, bubble, or crawl. Test on sample tiles before glazing important pieces.

Second Firing: Glaze Fire (Target Temperature Depends on Glaze)

The glaze firing must reach the temperature where your specific glaze melts and matures. This is typically 100-400 C higher than bisque temperature.

Glaze Firing Schedule

A typical glaze firing takes 8-14 hours depending on kiln size and target temperature:

Phase	Temperature Range	Rate	Duration	Notes
Warm-up	Room temp to 300 C	100 C/hr	~3 hrs	Drive off any absorbed moisture
Steady climb	300-600 C	150 C/hr	~2 hrs	Organic matter burns out
Quartz inversion	573 C	Hold 20 min	20 min	Slow through this zone
Mid-fire	600-900 C	150-200 C/hr	~2 hrs	Body continues to mature
High fire	900 to target	100-150 C/hr	2-3 hrs	Glaze begins to melt and flow
Soak	Target temp	Hold	30-60 min	Even maturation throughout kiln
Cooling	Target to 573 C	Natural, dampers closed	~4 hrs	Critical — do not rush
Final cool	573 C to room temp	Slow	8-12 hrs	Quartz inversion again on cooling

Controlling the Atmosphere

The kiln atmosphere during glaze firing profoundly affects color:

Oxidation (excess air, clean burn):

• Iron in clay/glaze produces warm browns, tans, and reds
• Copper produces greens and turquoise
• Standard approach for most functional pottery

Reduction (restricted air, smoky):

• Iron produces blue-gray, celadon green
• Copper produces deep reds (copper red, oxblood)
• Achieved by partially closing the damper and air inlets during the last 200 C of the climb
• Difficult to control — easy to over-reduce and produce muddy, dark results

Test Tiles

Before committing a full kiln load, fire test tiles with your glaze at the front of the kiln where you can observe them through the spy hole. Mark each tile with the glaze recipe scratched into the clay before bisque firing.

Kiln Modifications for Glaze Firing

A kiln designed for bisque earthenware may need modifications for successful glaze firing:

Temperature Capacity

Glaze firing requires reaching and holding higher temperatures. Ensure your kiln can reach at least 100 C above your target glaze temperature (thermal headroom for uneven spots):

• Improve insulation: Add an outer layer of clay-straw mix (8-12 cm thick)
• Seal air leaks: Cracks in walls leak heat exponentially at high temperatures
• Extend the chimney: A taller chimney (2-3 m minimum) creates stronger draft for higher temperatures
• Use better fuel: Switch from wood to charcoal for the final temperature push

Kiln Furniture

Glaze firing requires shelves and supports that do not fuse to the ware:

1. Kiln shelves: Flat slabs of refractory clay (high-alumina clay mixed with grog). Make them 2-3 cm thick, fired to the highest temperature you plan to use before loading ware on them.
2. Kiln posts: Support shelves at different heights. Make from rolled coils of refractory clay, fired hard. Three posts per shelf (triangular support is more stable than four).
3. Kiln wash: Coat shelf surfaces with a 50/50 mix of kaolin and silica powder in water. This prevents glaze drips from bonding to shelves. Reapply between firings.
4. Stilts: Small tripod supports that hold glazed pieces above the shelf, allowing the entire piece to be glazed. The stilt marks (three small rough spots) are ground smooth after firing.

Loading for Glaze Firing

Unlike bisque loading, glazed pieces must not touch each other or the shelves where glazed:

• Leave at least 1 cm between pieces (glaze can bubble and expand)
• Ensure no glaze on foot rings or bottom surfaces
• Place pieces so air can circulate around them
• Put larger, heavier pieces on lower shelves
• Leave spy holes unobstructed so you can observe cone packs and piece color

Troubleshooting Glaze Defects

Defect	Appearance	Cause	Fix
Crawling	Glaze pulls into beads, exposes bare clay	Dusty bisque, glaze too thick, oil contamination	Clean bisque thoroughly, thin glaze, handle with clean hands
Crazing	Fine network of cracks in glaze surface	Glaze shrinks more than clay body on cooling	Add more silica to glaze, or use clay with higher silica
Pinholing	Small holes like pinpricks in glaze	Gas escaping through glaze during melting	Soak longer at peak temperature, fire slightly hotter
Running	Glaze flows down and pools at base	Overfired, glaze too fluid, applied too thick	Reduce temperature, add alumina to glaze, apply thinner
Shivering	Glaze flakes off in chips	Glaze under compression (opposite of crazing)	Add flux to glaze or reduce silica
Blistering	Large bubbles frozen in glaze	Too fast a temperature rise, trapped carbon	Slower ramp through 600-900 C range, longer soak

Single-Fire Glazing (Advanced)

Experienced potters can apply glaze to raw (unfired) ware and fire once to both bisque and glaze temperature. This saves an entire firing’s worth of fuel but is riskier:

• Ware must be completely bone-dry
• Glaze must be formulated for raw application (different shrinkage characteristics)
• The firing schedule must accommodate both clay maturation and glaze melting
• Loss rate is higher — any moisture in the clay will cause explosions

Single-fire is worth attempting once you have reliable glaze recipes and consistent kiln performance. The fuel savings (roughly 40-50% less fuel overall) are significant in a resource-constrained environment.

Record Keeping

Document every glaze firing meticulously:

• Glaze recipes with exact proportions by weight
• Application method and thickness
• Firing schedule with times and observed colors at each stage
• Fuel type and quantity consumed
• Results on each piece — color, surface quality, defects
• Kiln position of each piece (top, middle, bottom; front, back)

Scratch recipe codes into test tiles before firing. Over time, this log becomes your most valuable technical document — it allows you to reproduce successes and avoid repeating failures. In a community setting, this knowledge must be preserved and passed down.