Glaze Firing

Part of Pottery and Ceramics

Firing glazed pottery to maturity — achieving a smooth, waterproof, glass-like surface coating.

Why This Matters

Glaze transforms porous earthenware into waterproof, food-safe, easy-to-clean vessels. Without glaze, water seeps through pottery walls, acids from fermented foods attack the clay body, and stains penetrate permanently. A glazed interior on a water jar or fermentation crock can be the difference between clean stored water and contaminated water, between preserved food and spoiled food.

Glaze is essentially a thin layer of glass fused to the clay surface during firing. Making it work requires understanding how glass forms from raw minerals, what temperatures are needed, and how firing conditions affect the final result. Get it right, and your pottery becomes dramatically more useful. Get it wrong, and the glaze crawls, crazes, blisters, or runs off the pot entirely.

For a rebuilding community, glaze firing represents a significant leap in pottery capability. It requires hotter kilns, more fuel, and greater skill than simple earthenware firing — but the payoff in vessel durability and functionality justifies the investment many times over.

Glaze Basics

What Glaze Is

Glaze is a mixture of minerals that, when heated to sufficient temperature, melt together and form a glass coating on the pottery surface. Every glaze contains three essential components:

Component	Function	Common Sources
Glass former (silica)	Creates the glass network	Quartz sand, flint, wood ash
Flux	Lowers the melting point of silica	Lead oxide, wood ash, borax, feldspar, lime
Alumina	Stiffens the melt so it doesn’t run off the pot	Clay (kaolin), feldspar

Pure silica melts at 1,710°C — far beyond any practical kiln. Fluxes reduce this to 900-1,300°C depending on type and quantity.

Glaze Temperature Ranges

Range	Temperature	Common Fluxes	Clay Body
Low fire	900-1,050°C	Lead, borax, sodium compounds	Earthenware
Mid fire	1,050-1,200°C	Calcium (lime), zinc, lithium	Stoneware
High fire	1,200-1,350°C	Feldspar, wood ash, calcium	Stoneware, porcelain

Lead Glaze Safety

Lead is the easiest flux to find and use — it produces beautiful, low-temperature glazes. However, lead glazes on food-contact surfaces can leach lead into food and drink, causing lead poisoning. If using lead glaze, apply it ONLY to decorative surfaces or non-food vessels. For food-contact surfaces, use lead-free glazes based on wood ash, borax, or feldspar.

Preparing Glazes from Natural Materials

Wood Ash Glaze (Simplest Lead-Free Option)

Wood ash contains silica, calcium, potassium, and other minerals — nearly a complete glaze by itself at higher temperatures.

Preparation:

1. Collect hardwood ash (oak, beech, maple, ash are best — they have higher calcium content)
2. Sieve through fine mesh to remove charcoal chunks
3. Wash the ash: mix with water (1 part ash to 3 parts water), stir, let settle for 10 minutes
4. Pour off the cloudy water (this contains soluble alkalis that cause crawling)
5. Repeat washing 2-3 times until the water runs relatively clear
6. Dry the washed ash

Simple Ash Glaze Recipe:

• 40% washed wood ash
• 40% feldspar (crushed and ground) or finely ground quartz sand
• 20% clay (kaolin or ball clay)
• Mix with water to a creamy consistency

This fires to a glaze at approximately 1,200-1,250°C. Below that temperature, it remains rough and underfired.

Feldspathic Glaze

If feldspar is available (common in granite-bearing geology):

• 60-70% ground feldspar
• 15-20% quartz (finely ground)
• 10-15% kaolin clay
• Fires at 1,200-1,300°C

Lime Glaze

Using limestone or calcium carbonate:

• 30% ground limestone (calcined to quickite first, then slaked)
• 40% ground quartz
• 20% clay
• 10% wood ash
• Fires at 1,050-1,200°C

Lead Glaze (Non-Food Use Only)

For decorative ware where food safety isn’t a concern:

• Galena (lead sulfide ore) or lead oxide from scavenged materials
• 70% lead oxide + 30% silica (ground quartz) makes a basic clear glaze
• Fires as low as 900-950°C
• Extremely easy to melt — often the first glaze a community can produce

Applying Glaze

Glaze Consistency

Mix dry glaze materials with water to achieve a consistency like heavy cream or thin yogurt. Test by dipping your finger — the glaze should coat evenly and not drip off immediately, but also not be so thick it clumps.

Application Methods

Dipping (Most Efficient):

1. Mix sufficient glaze to fill a container deep enough to submerge your vessel
2. Hold the bisque-fired vessel by its base (or use tongs)
3. Plunge into the glaze for 2-3 seconds
4. Remove and let excess drip back into the container
5. Rotate to ensure even coverage
6. Touch up any bare spots with a brush

Pouring:

1. Hold the vessel upside down over a catch basin
2. Pour glaze over the exterior, rotating to cover all surfaces
3. For the interior: pour glaze in, swirl to coat, pour excess back out
4. Works well when you don’t have enough glaze volume for full dipping

Brushing:

1. Apply with a soft brush in even, overlapping strokes
2. Apply 3-4 coats, each at right angles to the last
3. Most labor-intensive and least even method — but requires the smallest volume of glaze
4. Useful for applying different glazes to different areas of the same vessel

Glaze Thickness

Target thickness: approximately 1-2 mm when dry (about the thickness of a coin). Too thin and the glaze fires rough and incomplete. Too thick and it runs, crawls, or blisters.

Keep the Foot Clean

Glaze on the bottom of a vessel will fuse it permanently to the kiln shelf during firing. Wipe the bottom 5-10 mm of every piece clean of glaze before loading. You can also apply a kiln wash (50% silica + 50% kaolin mixed with water) to shelves as a release layer.

Bisque Firing vs. Raw Glazing

Two-Fire Method (Standard)

1. Bisque fire: Fire unglazed ware to 900-1,000°C. This makes it porous, hard, and easy to handle.
2. Apply glaze: The porous bisque absorbs the water from glaze suspension, leaving an even coating of dry glaze minerals on the surface.
3. Glaze fire: Fire again to the glaze’s maturing temperature.

Advantages: Bisqueware is strong enough to handle without breaking. The porous surface absorbs glaze evenly. Lower risk of glaze defects.

Single-Fire Method (Raw Glazing)

Apply glaze directly to leather-hard or bone-dry unfired ware, then fire once to the glaze temperature:

Advantages: Saves fuel and time (one firing instead of two). Requires half the kiln loads.

Disadvantages: Unfired ware is fragile and can break during handling. Glaze applied to raw clay may cause cracking if moisture absorption swells the clay. More skill required.

For a Rebuilding Community

Start with the two-fire method until your skills and kiln are reliable. Switch to single-fire once you have consistent results and need to conserve fuel.

The Glaze Firing Process

Kiln Loading

1. Leave space between pieces — glazed surfaces that touch will fuse together permanently
2. Stilts or wads: Small ceramic supports that hold pieces slightly above the shelf, allowing glaze to run to the bottom without fusing to the shelf
3. Stack efficiently but don’t overcrowd — air and heat must circulate
4. Place test tiles where you can see or extract them to monitor glaze maturity

Firing Schedule

Phase 1: Water Smoking (Room temp to 200°C)

• Rise rate: Slow (50-100°C per hour)
• Purpose: Drive out any residual moisture
• Duration: 2-4 hours
• Risk: Steam pressure cracking if too fast

Phase 2: Organic Burnout (200-600°C)

• Rise rate: Moderate (100°C per hour)
• Purpose: Burn out organic matter in clay and glaze
• Duration: 4-6 hours
• Signs: Smoky exhaust from the kiln

Phase 3: Quartz Inversion (573°C)

• Rise rate: Steady through this zone — don’t stall or rush
• Purpose: Quartz particles change crystal structure with a volume change
• Risk: Cracking if the temperature change is too sudden

Phase 4: Glaze Melting (900-1,300°C depending on glaze)

• Rise rate: 50-100°C per hour as you approach target
• Purpose: Glaze materials melt, flow, and fuse to the clay body
• Visual cue: Through a spy hole, the kiln interior glows orange to yellow-white
• Use draw rings or test tiles to check glaze maturity

Phase 5: Soaking

• Hold at peak temperature for 15-30 minutes
• Purpose: Allows glaze to mature fully and bubbles to heal
• Even heat distribution throughout the kiln

Phase 6: Cooling

• Close all dampers and air inlets
• Allow the kiln to cool naturally — do NOT open the door
• Pass back through quartz inversion (573°C) slowly
• Do not open the kiln until below 200°C — ideally, wait until you can touch the pots
• Duration: 12-24 hours depending on kiln size

Cooling Is Critical

Opening the kiln too soon causes thermal shock — pieces crack, and glaze crazes (develops a network of fine cracks). The most beautiful glaze fire can be ruined by impatience during cooling. Wait.

Troubleshooting Glaze Defects

Defect	Appearance	Cause	Solution
Crawling	Glaze pulls back from areas, leaving bare spots	Dusty or oily bisque surface; glaze too thick	Clean bisque thoroughly; thin glaze
Crazing	Network of fine cracks in glaze	Glaze contracts more than clay body during cooling	Increase silica in glaze; cool more slowly
Shivering	Glaze flakes off in chips	Opposite of crazing — clay contracts more than glaze	Decrease silica; increase flux
Pinholing	Small holes in glaze surface	Gas escaping through glaze; insufficient soaking	Soak longer at peak temperature; fire slightly hotter
Blistering	Large bubbles in glaze	Overfiring; gas from contaminated clay	Fire cooler; use cleaner clay body
Running	Glaze flows off the pot onto the shelf	Too much flux; fired too hot; glaze too thick	Reduce flux; lower temperature; apply thinner
Matte surface (when gloss intended)	Dull, rough glaze	Underfired	Increase temperature or soak time

Monitoring Temperature Without Instruments

Pyrometric Cones

Small pyramids made from glaze-like materials that bend at specific temperatures. Make your own:

1. Mix a glaze recipe known to melt at your target temperature
2. Form it into small triangular pyramids about 5 cm tall
3. Place them visible through the spy hole
4. When the cone bends and touches its base, the target temperature has been reached

Draw Rings

Small glazed rings placed near the spy hole on a long wire. Periodically hook one out with a metal rod and inspect the glaze surface. When the glaze is smooth, glossy, and fully melted, the firing is done.

Color Estimation

Kiln Interior Color	Approximate Temperature
First visible red (dark)	500-600°C
Cherry red	750-850°C
Bright cherry red	850-950°C
Orange	1,000-1,100°C
Yellow-orange	1,100-1,200°C
Yellow-white	1,200-1,300°C
White	1,300°C+

Glaze firing is the most technically demanding stage of pottery production. It requires a reliable kiln, consistent fuel supply, patient temperature management, and systematic troubleshooting. But mastering it gives your community access to waterproof, food-safe, durable vessels that last for years — a transformative capability that justifies every hour of learning.