Glazing

Part of Pottery and Ceramics

Glazing applies a thin layer of glass to the surface of fired pottery, making it waterproof, easy to clean, and chemically resistant. Without glaze, most pottery is porous — it absorbs liquids, harbors bacteria, and eventually deteriorates. Glazing transforms functional ceramics into durable, hygienic vessels for food and water.

Why Glazing Matters

Unglazed earthenware pottery is porous. Water seeps through slowly, oils soak into the clay, and cleaning is difficult. For cooking and water storage, this porosity is a serious problem — bacteria grow in the absorbed moisture, and acidic foods leach minerals from the clay. A thin layer of glaze — essentially a coating of glass fused to the clay surface — solves all of these problems. Glazed pottery can hold any liquid indefinitely, withstand repeated heating, and be cleaned thoroughly.

How Glaze Works

Glaze is a mixture of minerals that melts at kiln temperatures (typically 900-1,300C) and forms a thin glass layer on the pottery surface. All glazes need three components:

Component	Role	Natural Sources
Silica (SiO2)	Glass former — the main ingredient	Quartz sand, flint, feldspar
Flux	Lowers the melting point of silica	Wood ash, limestone, feldspar, lead ore
Alumina (Al2O3)	Stiffens the glaze so it does not run off	Clay, feldspar

Pure silica melts at 1,713C — far too high for most kilns. The flux brings this down to achievable temperatures. Alumina keeps the molten glaze from flowing off the pot like water.

Wood Ash Glaze

The simplest and most accessible glaze for rebuilding scenarios. Wood ash naturally contains all three glaze components in roughly the right proportions.

Preparation

1. Collect ash — Hardwood ash is best (oak, ash, maple, beech). Softwood ash works but produces less predictable results.
2. Wash the ash — Soak ash in water, stir, and let settle. Drain off the clear water. This removes soluble alkalis that cause crawling and pinholing. Repeat 2-3 times.
3. Sieve — Pass the washed ash through a fine mesh (woven cloth or fine basket) to remove charcoal chunks and grit.
4. Mix with clay — A basic wood ash glaze is 50-70% washed ash and 30-50% fine clay (the same clay you made the pot from works well).
5. Add water — Mix to a consistency of thick cream.

Application

1. The pot must be bisque-fired (fired once already at a lower temperature, 600-800C)
2. Dip the pot into the glaze for 3-5 seconds, or pour glaze over the surface
3. The porous bisqueware absorbs water from the glaze, leaving a thin powder coating
4. Let dry completely
5. Fire again at high temperature (1,100-1,250C) to melt the glaze

Ash Type	Color (Oxidation)	Color (Reduction)	Notes
Oak	Amber-brown	Green-gray	Most common, reliable
Pine	Yellow-green	Gray-green	Lower melting point
Straw/grass	Pale green	Celadon green	Very high silica content
Fruit wood	Warm tan	Olive	Subtle, pleasant colors

Test Tiles First

Every batch of ash produces slightly different results. Always test a new ash batch on small tiles before glazing important pots. Make test tiles at the same time as your pottery and fire them together.

Feldspar Glaze

If you can find feldspar (a common rock-forming mineral — pink, white, or gray crystals in granite), it makes an excellent glaze all on its own. Feldspar contains silica, alumina, and flux naturally.

Identification

Feldspar appears as:

• Rectangular crystals in granite rock
• Pink, white, cream, or gray colored
• Shows flat cleavage faces (smooth, shiny planes)
• Harder than glass — scratches a glass bottle

Preparation

1. Crush feldspar to a fine powder (use a hammer, then grind between stones)
2. Mix with 10-20% additional clay for adhesion
3. Add water to cream consistency
4. Apply to bisqueware and fire at 1,200-1,300C

Slip Glaze

A slip is simply liquid clay. Some clay deposits naturally contain enough flux (iron, calcium, sodium) to become glassy when fired high enough. This is the easiest “glaze” to produce.

Process

1. Find a clay that fires to a different color than your pot clay (for contrast)
2. Mix the clay with water to cream consistency
3. Apply to leather-hard (unfired) pottery by dipping, pouring, or painting
4. Let dry and fire
5. At 1,100C+, iron-rich slips become naturally glossy and somewhat waterproof

Iron-Rich Slip

Clay containing high iron content (red or dark brown clay) makes an especially effective slip glaze:

• Fires to a dark brown-black glossy surface at 1,200C+
• Naturally waterproof
• Ancient Greek and Roman pottery used exactly this technique

Salt Glazing

Salt glazing uses common salt (sodium chloride) thrown into the kiln at peak temperature. The sodium reacts with the silica in the clay surface to form a glassy coating.

Process

1. Fire pottery to maximum temperature (1,100-1,250C)
2. Throw handfuls of coarse salt into the firebox through the stoke hole
3. The salt vaporizes and circulates through the kiln
4. Sodium vapor reacts with clay surfaces to form sodium silicate glass
5. Repeat salt additions 3-5 times over 30-60 minutes
6. The characteristic “orange peel” texture develops on the surface

Salt Glazing Fumes

Salt glazing produces hydrochloric acid gas (HCl) as a byproduct. This is extremely corrosive and dangerous to breathe. Always salt-glaze outdoors, stay upwind, and keep everyone well away from the kiln exhaust during salt additions.

Results

• Produces a distinctive bumpy (“orange peel”) texture
• Color depends on clay body — tan, brown, or gray
• Extremely durable and waterproof
• Traditional method for stoneware crocks, jugs, and pipes

Glaze Application Methods

Method	Description	Best For
Dipping	Submerge the pot in glaze for 3-5 seconds	Even coating, fastest
Pouring	Pour glaze over the pot, rotating to cover	When the glaze bucket is too small to dip
Brushing	Paint glaze on with a brush	Decorative glazing, multiple colors
Spraying	Blow glaze through a tube (mouth or bellows)	Even coats on large pieces
Trailing	Squeeze glaze through a nozzle for lines	Decoration on contrasting slip

Application Rules

1. Consistent thickness — Glaze should be applied 1-2 mm thick. Too thin and it is rough and incomplete. Too thick and it runs, bubbles, or crawls.
2. Clean base — Wipe glaze off the bottom 5 mm of the pot. Glaze on the base fuses the pot to the kiln shelf.
3. No touching — Handle glazed pots by the unglazed base only. Fingerprints in wet glaze leave bare spots.
4. Dry before firing — Glazed pots must dry completely before going into the kiln. Moisture under the glaze causes bubbling.

Keep the Bottom Clean

Any glaze that touches the kiln shelf during firing will fuse permanently, destroying both the pot and the shelf. Wipe the bottom of every pot clean before loading. Use a small piece of broken pot or a disk of kiln wash (clay mixed with sand) under each piece as insurance.

Glaze Defects and Solutions

Defect	Appearance	Cause	Fix
Crawling	Glaze pulls away, exposing bare clay	Dusty bisqueware, too-thick application	Clean pots before glazing; apply thinner
Pinholing	Small holes in glaze surface	Gas escaping from clay or glaze	Fire hotter or hold at peak longer
Crazing	Fine cracks in glaze	Glaze contracts more than clay on cooling	Add more silica to glaze recipe
Shivering	Glaze flakes off	Glaze contracts less than clay	Reduce silica or add flux
Running	Glaze flows down to base	Too much flux, too hot, too thick	Reduce flux, apply thinner, lower temp
Blistering	Large bubbles in surface	Firing too fast, impurities	Slow down firing ramp

Making Color with What You Have

Without commercial colorants, natural minerals provide a range of glaze colors:

Material	Color in Oxidation	Color in Reduction
Iron oxide (rust)	Amber to dark brown	Green to gray-green (celadon)
Copper (ground verdigris)	Green	Red (difficult to control)
Manganese (pyrolusite ore)	Purple-brown	Brown-black
Cobalt (rare)	Intense blue	Intense blue
Rutile (titanium ore)	Tan speckles	Blue-gray speckles

Iron is Everywhere

Rust (iron oxide) is the most available colorant. Scrape rust from old iron, grind to powder, and add 2-10% to any glaze. Low iron (2-3%) gives amber; high iron (8-10%) gives dark brown or black.

Common Mistakes

1. Glazing unfired clay — Glaze must be applied to bisque-fired pottery (already fired once). Raw clay absorbs too much water from the glaze and collapses or cracks.
2. Glaze on the base — Any glaze touching the kiln shelf during firing permanently fuses the pot to the shelf. Always wipe the bottom clean.
3. Inconsistent thickness — Thick spots run and pool; thin spots fire rough and incomplete. Practice consistent dipping — 3-5 seconds submerged, with smooth entry and exit.
4. Not testing — Every new glaze recipe, every new ash batch, every new clay body can produce different results. Test on small tiles before committing important pottery.
5. Lead glaze for food — While historically common, lead-based glazes are toxic. Lead leaches into food and drink, especially acidic liquids. Use wood ash, feldspar, or salt glazes for food-contact pottery instead.

Summary

Glazing — At a Glance

• Glaze = silica + flux + alumina, melted onto pottery to form a waterproof glass layer
• Wood ash glaze is the simplest: 50-70% washed ash + 30-50% clay, fired at 1,100C+
• Feldspar makes an excellent natural glaze if you can find and crush it
• Salt glazing uses table salt thrown into the kiln at peak temperature — stay upwind (HCl fumes)
• Always apply glaze to bisque-fired pottery, never raw clay
• Keep the bottom of every pot clean — glaze fuses pots to kiln shelves permanently
• Iron oxide (rust) is the most accessible colorant: 2-10% added to glaze gives amber to dark brown
• Test every new glaze on small tiles before glazing important pieces