Foundations

Before layering, brushwork, or decorative techniques make sense, you need to understand what a glaze actually is — and why it behaves nothing like paint.

A ceramic glaze is a thin layer of glass permanently fused to a ceramic body through firing. Every glaze is built from three essential components:

• Glass-former (silica / SiO₂) — the backbone. Pure silica melts at ~1713°C, far too high for practical pottery.^[1]
• Flux — metal oxides that lower the melting point. Sodium and potassium work at low temperatures; calcium and magnesium at higher ones.^[2]
• Alumina (Al₂O₃)— the stiffening agent. Increases viscosity so glaze doesn't run off vertical surfaces.^[3]

This triad can be expressed using the Seger unity formula, which normalizes all fluxes to total 1.00. This makes it possible to compare glazes chemically and diagnose problems like crazing, where high-expansion fluxes are usually the culprit.^[4]

Paint dries by evaporation and sits on the surface. Glaze undergoes a permanent chemical transformation: raw materials melt, flow, and fuse with the clay at 900–1300°C+, forming an intermediate layer where glaze and clay intermingle. Once cooled, the glaze is glass bonded to ceramic — it cannot be removed without breaking the object.^[1][3]

This matters practically: glaze application depends on the porosity of bisqueware to absorb water from the suspension and deposit an even layer of particles. Treating glaze like paint leads to poor results.

Bisque firing is the first firing of raw clay. It drives off moisture, burns out organic matter, and passes through quartz inversion (573°C), giving structural strength while retaining porosity. Standard range: cone 08–04 (~950–1060°C). Cone 04 is the most common.^[5][3]

A properly bisqued piece retains >15% porosity — enough to absorb glaze quickly and deposit an even coating.

Low-fire context: For cone 06 glaze, Mayco recommends bisque 1–2 cones hotter (e.g., bisque cone 04, glaze cone 06) to ensure complete organic burnout. For mid/high-fire (cone 6+), bisque is typically lower than glaze temperature.^[3]

“Cone” measures heatwork (temperature × time), not temperature alone. The same peak reached quickly produces different results than reached slowly.

Cone 6 dominates American studio pottery — good durability, wide color range, efficient with electric kilns. Glazes must be fired to their labeled cone; underfired glazes appear cloudy and may not be food-safe.^[6][3]

Consistent results require consistent glaze. Specific gravity is the single most important measurement for repeatable application.

Specific gravity (SG) is the ratio of glaze weight to an equal volume of water. Water = 1.0, so a glaze at SG 1.45 weighs 1.45× as much.

The “correct” SG is always recipe-specific. AMACO DL-series glazes are recommended at SG 1.52–1.58 depending on formula.^[7][8]

Weight-per-volume (recommended): Fill a 100cc graduated cylinder with glaze and weigh it. If 100cc weighs 145g, SG = 1.45.^[9]

Hydrometer: Usable but limited — most glazes cluster around SG 1.4–1.5, so readings are compressed and hard to distinguish. Thick or thixotropic glazes can also prevent free floating. Weight-per-volume is more reliable for most potters.^[8][10]

Flow testing:SG alone doesn't fully describe behavior. Georgies recommends checking that mixed glaze keeps spinning ~5 seconds, or evacuates a drip cup in 15–25 seconds.^[11]

Deflocculants disperse particles (reduce viscosity at same solids): Darvan 7 and 811 (sodium polyelectrolytes). Dosage: 0.3–0.5% for porcelain. Powerful — over-deflocculation creates impenetrable hard pan.^[12]

Flocculants attract particles (create thixotropic gel): Epsom salts (most common — ½–3 tsp per 5 gallons), vinegar (temporary effect), calcium chloride.^[13][14]

Thixotropy is time-dependent behavior where a suspension is fluid when stirred but gels on standing. Ideal for glazing — moves easily in the bucket, stays put on the pot. Controlled by proper flocculation.^[15]

Sieve glazes to remove lumps and contaminants. 80-mesh is standard for stoneware/earthenware (Mayco: “80 mesh or finer”). 100–120 for finer work; 200+ for porcelain. Pass 2–3 times.^[3][16]

Rhodes notes that raw glaze materials are usually ground fine enough to pass 200-mesh — studio sieving is about breaking agglomerates and catching contaminants, not creating fineness from scratch.

Freshly mixed glazes benefit from sitting 24–48 hours minimum. Minerals hydrate, particles disperse, and the suspension equilibrates. Some potters age for weeks. Glazes with organic binders (CMC) can develop mold if aged too long — a drop of bleach prevents this.^[16]

How you prepare the bisque surface directly affects glaze adhesion, thickness, and defect risk.

Lower bisque = more porous = faster glaze absorption. Cone 08 (~950°C) is very porous but fragile. Cone 04 (~1060°C) is the sweet spot for most studios. Cone 02+ is “hard bisque” — may need dampening.^[9][5]

The clay body matters as much as cone number — a porcelain at cone 04 will be much less porous than coarse stoneware at the same temperature.^[17]

Wipe with a damp sponge to remove dust and skin oils — the most common cause of crawling. In shared studios, wash all bisqueware under running water.^[3]

Lightly misting very porous bisque slows absorption, giving more working time. Useful for brushing and large pieces.

Warning: Over-wetting can move the raw glaze layer and increase crawling risk. This helps with porous low-fire bisque but may cause problems on dense bisque (cone 02+). Mist lightly — do not soak.^[13]

Wax the bottom of every pot before glazing to prevent kiln-shelf fusion. Cold liquid wax (e.g., Mayco AC-302) is the studio default — easy cleanup, burns away in firing. Hot wax (melted paraffin) gives sharper edges but poses fire risk.^[3][18]

Tip: place the pot on a banding wheel, spin it, and hold a wax-loaded brush against the foot for a clean, even line.