Glazing from first principles

What a glaze is

A glaze is a thin coat of glass fused to a pot. Glazy, the open glaze-recipe database, calls it "a glass formula tuned to melt and fuse onto clay at certain temperatures". It goes on as a layer of powder and melts in the kiln into a glassy coat that seals the clay, so the pot is less likely to leak, stain or wear. At the top of the firing the melt also reacts with the clay, often forming a layer between them that locks the glaze on. On earthenware the glaze sits on top as a separate layer; on stoneware it melts into the body.

A recipe lists raw materials, such as feldspar, whiting and kaolin. In the kiln, part of each leaves as gas (water or carbon dioxide) and the rest becomes oxides: compounds of an element with oxygen, such as silica (SiO₂) or calcium oxide (CaO). The fired glass is made of oxides, so potters describe a glaze by them. Each oxide does one of three jobs.

• Silica (SiO₂) is the glass former: it builds the glass. Pure silica melts above 1700 °C (3100 °F), far hotter than a pottery kiln, and a glaze high in silica is generally more durable.
• Fluxes lower the melting point to one a kiln can reach. The alkalis, written R₂O, are the oxides of sodium, potassium and lithium (Na₂O, K₂O, Li₂O); they work from cone 06 up through mid and high fire. The alkaline earths, written RO, are the oxides of calcium, magnesium, barium and strontium (CaO, MgO, BaO, SrO), with zinc oxide (ZnO) grouped alongside; they work best from cone 5 to cone 10. The R stands for any metal in the family.
• Alumina (Al₂O₃) is the stabilizer. It stiffens the melt so it stays on upright walls, and adds hardness and durability. Glazy's warning: with too little alumina "your glaze may run; too much can stiffen the melt into a matte". Alumina comes mostly from clay and feldspar.

Boron (B₂O₃) blurs the line. Chemists class it as a glass former, but at studio temperatures it acts as a flux, and potters treat it as one. It is critical at cone 5–6 and not needed at high fire. (Cones, the way potters name a firing, are explained below.)

One way to hold it in mind: silica is the glass, the fluxes let it melt at a temperature your kiln can reach, and alumina thickens it so it stays where you put it.

Clay, coatings and the two firings

Clay bodies

The clay body is the clay a pot is made of. There are three main kinds.

• Earthenware: a low-fire body, about cone 06–02, that stays porous. Often red or buff, it needs a glaze to hold water.
• Stoneware: a mid- to high-fire body, about cone 5–10, that becomes vitreous or nearly so.
• Porcelain: a white, high-fire body rich in kaolin, often translucent.

Vitreous means glassy and non-porous; vitrification is a body becoming so as it fires. UK potters often draw the line by temperature instead: stoneware is fired above 1200 °C (2192 °F), earthenware below. Mid fire, cone 5–6, falls just either side of that line.

Coatings and drying

Slip is clay suspended in water, for casting, joining or decorating. An engobe is a slip-like coating, often colored, with more non-clay materials. An underglaze is colored slip or oxide decoration, usually covered with a clear glaze.

Unfired ware is greenware (sources disagree on whether the word covers all unfired ware or only dry ware). As it dries it becomes leather-hard, stiff but slightly damp, the stage for trimming and handles; then bone dry, with no free water left, very fragile and ready to fire.

The two firings

The first, lower firing is the bisque (UK: biscuit), usually cone 08–04 (most often 06–04), about 942–1063 °C (1728–1945 °F). It leaves ware strong but porous. Digitalfire, a glaze reference site, suggests generally more than 15% porosity, so the bisque draws water out of the glaze as it goes on. A soft bisque (cone 08–06) is more porous; a hard bisque (often cone 04) is sturdier but takes glaze less readily.

Potclays, a UK supplier, describes biscuit firing to around 1000 °C (1832 °F), about cone 06. Reading these sources together, we suggest cone 06–04 for a first bisque: 998–1063 °C (1828–1945 °F) at Orton's medium rate.

The glaze firing (British potters may say glost firing) melts the glaze and, for stoneware, matures the clay too. Once-firing glazes unfired ware and fires it just once. Digitalfire reports a production case where raw-glazed ware crawled on its rims until a bisque firing cured it, so begin with two firings. Firing a basic kiln gives schedules for both.

Cones and heatwork

Potters name a firing by a cone: a slender ceramic pyramid that bends when it has had a set amount of heat. Orton, which makes them, puts it this way: "Cones do not measure temperature alone. They measure heatwork, the combined effect of time and temperature."

Heatwork is the reason: glaze, clay and cone all respond to how long they are hot as well as how hot. A slow firing bends a cone at a lower temperature, a fast one at a higher. Orton's chart has cone 6 bending at 1185 °C (2165 °F) when the firing ends climbing 15 °C (27 °F) per hour, at 1222 °C (2232 °F) at 60 °C (108 °F) per hour, and at 1243 °C (2269 °F) at 150 °C (270 °F) per hour, the rate taken over the last 100 °C (180 °F). A cone is done when its tip has bent through 90°, which Orton calls the 5 o'clock position.

Cone numbers run from 022 to 42. A leading zero marks the cooler series, where a bigger number is cooler: cone 06 is cooler than cone 04, and cone 01 cooler than cone 1.

Name and program your firings by cone, and treat the temperatures as approximate. A witness cone on a shelf among the pots, or a cone pack of three (guide, firing and guard cones), shows the heatwork your pots really received.

Published temperatures vary. Glazy gives high fire, cone 9–11, as about 1280–1310 °C (2336–2390 °F), which matches Orton's fast column, and Digitalfire reports that most kilns reach cone 6 at about 1204 °C (2200 °F). Our advice: use Orton's medium-rate figures, as this guide does, program by cone, and let a witness cone settle it.

Outside North America, suppliers often give °C rather than a cone. Hermann Seger developed the modern cone, first used in Berlin in 1886, and the UK has had its own Staffordshire cones; their numbers should not be assumed to match Orton's, and we found no table to convert them. Firing a basic kiln covers cones in more detail.

From recipe to unity formula

A batch recipe lists raw materials by weight, usually scaled so the base adds up to 100. Additives (colorants, opacifiers and suspenders) go on top of the 100, usually as a percent of the base. To see what the glaze is, potters turn the recipe into a unity molecular formula (UMF, or Seger formula, after Hermann Seger, who first used its three-column layout).

Glazecalc's recipe page does all four steps as you type, and shows the result in three columns: fluxes (R₂O and RO) adding up to 1; stabilizers (R₂O₃: alumina, with boron often placed here); and glass formers (RO₂: silica). "Unification is just a scaling tool," Glazy says: two recipes made from different materials but with the same unity formula "behave almost identically once fired". So the 1 is a shared yardstick, not an amount you weigh. Digitalfire adds that the method "does not work as well at lower temperatures", where boron is not a plain flux, and that unity formulas are "comparative and best viewed in context". Compare lines up two recipes' formulas oxide by oxide.

The silica to alumina ratio

The most useful single number is the silica to alumina ratio: silica divided by alumina. In the example, 3.70 ÷ 0.41 is about 9.

A ratio of 5–7 predicts neither gloss nor matte on its own: the fluxes, the boron and the cone all move the line. Glazy gives a typical flux balance for stable glazes of 0.2 R₂O with 0.8 RO, up to 0.3 R₂O with 0.7 RO. More alkali makes the melt more fluid and raises its expansion, and so the risk of crazing.

Loss on ignition

Loss on ignition (LOI) is the weight a material loses in the kiln as water, carbon dioxide, organic matter or sulfur leave as gas. A fired glaze has none, so the unity formula leaves it out.

Gas that leaves after the glaze starts to melt must bubble through it, causing, in Digitalfire's words, "bubbles, blisters, pinholes, crawling". Talc keeps giving off gas until about 899 °C (1650 °F). Frits, calcined materials (heated beforehand to drive their gases off) and changes to the firing all help.

Older books do this arithmetic by hand, with molecular weight (the weight of one mole of a material or oxide) and equivalent weight (the weight of a material that supplies one mole of an oxide). By our arithmetic, about 100 g of whiting supplies one mole, 56 g, of calcium oxide. A calculator handles both.

Frits and raw materials

Frits

A frit is glass made in a factory: minerals melted, quenched in water and ground. Fritting makes soluble materials such as borax insoluble, locks toxic oxides such as lead and barium into a stable glass, gives a more consistent material than raw minerals, and drives off gases, so fritted glazes are "much more defect-free" (Digitalfire). Frits also melt early: a soda frit is active at cone 06, while feldspar "is only beginning to soften at cone 6". They cost more, they need clay to stay suspended (Digitalfire's example is 85% frit with 15% clay), and they are "theoretically insoluble, but in practice, they are not".

North American frits go by maker and number: Ferro Frit 3134 (Ferro is now Vibrantz) is in about 22% of Glazy's public recipes. UK recipes more often name generic frits, such as standard borax frit.

The common raw materials

Regional names

• Kaolin is china clay in the UK; Australian suppliers sell it as Eckalite.
• Flint is an older UK name for silica. UK suppliers still sell calcined flint as its own product, so "flint" in a British recipe probably means calcined flint, and in an American one probably ground quartz. Both are sold as sources of silica; check the analysis.
• China stone and Cornish stone (Cornwall stone) are the same material, related to feldspar.
• Whiting is calcium carbonate; German suppliers call it Kreide (chalk).

Judge a material by the oxides it supplies, not by its name. Glazecalc works that way: you can enter your supplier's analysis on the Materials page, and Try modern materials and compare, Suggest amounts and compare and Match with what I have all match fired oxides, not names.

Materials change, too. Custer feldspar ended in 2023. The original Gerstley Borate mine closed around 1999–2000; Laguna still sells a product under that name, and Gillespie Borate is the main substitute. Sources disagree on whether Gillespie replaces Gerstley Borate pound for pound (its maker and Potclays say yes, Digitalfire and others no), so compare the two in Glazecalc, then test a small batch.

Colorants and opacifiers

A colorant is a metal oxide or carbonate that colors a glaze; a stain is a factory-made colorant, more stable and consistent than raw oxides. Colorants are usually under 5% of the base, many under 1%, and a carbonate is about half the strength of the matching oxide.

Cobalt and chrome amounts are genuinely disputed. Glazy says 0.1% cobalt is enough for a blue; Glendale Community College gives ½–2% or ½–3% depending on the atmosphere; Birdie Boone, in Pottery Making Illustrated, says 3% or less. For chrome, Boone says under 0.5% and Glendale 2–3%. The safe course is to start low and run a line blend: a row of test tiles stepping one ingredient from a little to a lot.

The base glaze changes the color. A glossy, transparent base gives the brightest color; satin and matte bases, usually higher in alumina, mute it. Magnesium pushes cobalt toward purple, zinc turns chrome brown, and chrome with tin gives pink.

Opacifiers

An opacifier makes a glaze opaque, hiding the clay; a transparent glaze lets it show.

Glaze fit and defects

Clay and glaze both grow as they heat and shrink as they cool, each by its own amount: its thermal expansion. Glaze fit is how well the two match. A good fit leaves the glaze slightly compressed by the clay; Digitalfire says a little compression strengthens ware, while too much can weaken and even crack it.

Crazing and shivering

Crazing is a network of fine cracks. The glaze's expansion is higher than the body's, so on cooling it shrinks more, is stretched and cracks, usually because of too much sodium and potassium (Na₂O, K₂O). It can also appear later, in use, after repeated sudden cooling such as contact with cold liquids. When it is wanted, as in raku, it is called crackle. Digitalfire says "95% of the time the solution is to adjust the thermal expansion of the glaze"; slow cooling only delays crazing. Its test: heat the ware to about 149 °C (300 °F), plunge it into ice water, and rub felt pen over it to show cracks.

Sources differ on the first fix. Glazy suggests adding silica or alumina; Digitalfire answers that extra silica "only dilutes the KNaO" (the sodium and potassium); Linda Bloomfield adds boron or cuts feldspar, one material at a time. Our advice: in the recipe calculator, swap some sodium and potassium for lower-expansion fluxes (boron, lithium, magnesium, calcium), check the change with Compare, and confirm with the ice-water test.

Shivering is the reverse: the glaze's expansion is far lower than the body's, so it is over-compressed and flakes off rims and edges. Digitalfire warns that the flakes can be razor-sharp and could be swallowed, and Glazy calls shivering "even more hazardous than crazing on functional ware". The fix is a higher-expansion glaze or a lower-expansion body.

Gloss and matte

A gloss surface is reflective and smooth, satin has a subdued sheen, and matte is dull. Gloss needs a fully melted glass. Matte can come from a lot of alumina (a low silica to alumina ratio), from fluxes such as calcium, magnesium, barium or strontium forming tiny crystals as the glaze cools, or from underfiring.

Common defects

Change one thing at a time, and keep your test tiles.

Food safety

Leaching is metal dissolving out of a fired glaze into food or drink, and a glaze can look perfect and still do it. The legal tests soak the fired piece in an acid solution and measure the lead and cadmium that come out.

Lead-free is not the same as food-safe

The rules in the US, the EU, the UK and Australia limit the lead and cadmium a fired piece releases, not what the glaze contains. "Lead-free" removes one hazard; it is not a food-safety rating. Other oxides leach too: Glazy warns that high-barium glazes "can leach and are generally not food-safe", Digitalfire says copper "can make a glaze leachable; test it", and Glazy no longer advises lead glazes for functional ware.

A liner glaze is a stable, usually clear or white glaze for surfaces that touch food. In our view, one with no colorant, barium, lead or lithium, that melts fully, does not craze and wears well, is the lower-risk choice. Lower risk is not proof.

Limit formulas (Digitalfire prefers target formulas) give typical unity-formula ranges for stable glazes at a cone: Digitalfire's cone 6 targets put alumina at 0.285–0.64 and silica at 2.4–4.7. They guide you toward sound chemistry but prove nothing; Digitalfire avoids the word "limit" because it suggests glazes inside the ranges "are somehow safe". Glazecalc's tools for changing a recipe say when the new one goes past a recommended limit, such as a lot of boron at cone 6, and what the glaze will likely do.

The legal limits differ by country. The table shows those where you live, in the law's own units; choose another country to compare.

In the US, decorative ceramic ware that looks usable for food is treated as food ware unless it carries a stick-on label and a permanent notice molded or fired onto its base, such as "Not for Food Use. May Poison Food", or has a hole through the food surface. The EU sets lead and cadmium limits in Directive 84/500/EEC, and the UK keeps the same figures in its own law. From 29 May 2026 the Netherlands, Belgium and Luxembourg allow about 130 to 150 times less lead, and about 15 times less cadmium. Australia sets limits for imported ware; we found no official limit for ware made in Australia or New Zealand.

Testing

The FDA says home swab kits can show leachable lead, but Ceramics New Zealand warns that test pens are not designed for ceramics and give false positives. Treat a positive as a reason to stop using the piece, and a negative as proving nothing. A laboratory leach test of the fired piece (ASTM C738 in the US; EN 1388 or ISO 6486 in Europe) is the real answer.

Specific gravity and application

Specific gravity

Glaze powder mixed with water is a slurry. Its specific gravity (SG) is its weight compared with the same volume of water, which has an SG of 1.0: the higher it is, the more powder in the bucket. Older UK books say pint weight. Glazy calls 1.4–1.5 "a good starting point for most glazes". To measure it, weigh exactly 100 mL of slurry (Glazy suggests a syringe) and divide by 100: 145 g means SG 1.45. Hydrometers are unreliable in gelled glazes, and water evaporates, so check again before you glaze.

Targets differ by source. Digitalfire puts dipping glazes at 1.40–1.50 and commercial brushing glazes at 1.25–1.55, while Potclays gives 1.71–1.81 for brushing and 1.71 for spraying. Our advice: dip at about 1.45 with the glaze slightly gelled, follow the label for a brushing glaze, and write down what works.

A flocculant (Epsom salts, vinegar or calcium chloride) gels a slurry slightly, making it thixotropic: it flows when stirred and sets when still, for "even, drip-free glaze coverage". A deflocculant (sodium silicate or Darvan) thins it. How to make a glaze covers mixing and adjusting.

Putting glaze on

Clean dust and grease off the bisque first with a barely damp sponge rather than washing it, which soaks it. Don't blow the dust off with compressed air or brush it dry: that puts silica dust in the air (Safe mixing and ventilation).

• Dipping "generally gives a coating of even thickness" (Robin Hopper). The dip time sets the thickness: Digitalfire notes that commercial dipping glazes are made at about 1.4 for a 5-second dip and 1.5 for a 1-second dip.
• Pouring glaze should be "a little thinner than when used for dipping"; where pours overlap, the coat doubles and may run.
• Brushing glazes contain a gum such as CMC gum, which helps the glaze flow and stick, then burns away. Most need several coats, often two or three; follow the label.
• Spraying uses a much thinner glaze, put on in "several thin coatings, gradually building up the thickness".

Too thick a coat can crawl, blister or run. Suppliers' rules of thumb run from the thickness of a credit card to that of a US dime, by our estimate about 1 mm of dry glaze; test tiles at a few thicknesses are the reliable check. Wax resist on the foot repels glaze, so the pot cannot stick to the shelf, and kiln wash, a heat-resisting coating often of kaolin and silica, protects shelves from drips.

Oxidation and reduction

The gases in the kiln, its atmosphere, change how glazes come out. Oxidation means plenty of oxygen. "Electric kilns are synonymous with oxidation firing," Digitalfire says, and oxidation glazes are typically brighter than reduction ones, especially at lower temperatures. In oxidation, copper gives greens and turquoise, and iron browns, tans and reds. A poorly vented electric kiln full of ware giving off gases can run more neutral than intended; a vent connected directly to the kiln helps.

Reduction deliberately restricts the oxygen, usually in a gas, wood or specialized kiln. Copper turns red, and iron becomes a stronger flux and gives celadon (blue-green to gray-green, from about ½–2% iron) and tenmoku (temmoku: brown-black, about 5–6% iron). Glazy says celadons and copper reds "rely on reduction". The clay changes too: in one Digitalfire comparison, a stoneware body fired gray in reduction and yellowish in oxidation. Silicon carbide in a glaze can make small areas of reduction even in an electric kiln. For reduction from cone 010 to cone 3, Orton recommends its iron-free cones.

Expect a recipe written for a gas reduction kiln to come out differently in an electric kiln: a celadon or copper red that relies on reduction will not look the same in oxidation. Check which atmosphere a recipe was made for, and note your own, with the cone, in the recipe's notes in Glazecalc.

Every kiln, electric ones included, gives off fumes, so vent it. Safe mixing and ventilation explains how.

Glossary

Additive

A colorant, opacifier or suspender added on top of a recipe's 100.

Alumina (Al₂O₃)

The main stabilizer: stops running and adds hardness. From clays and feldspars.

Ball clay

A plastic clay with more iron and organic matter than kaolin.

Batch recipe

A glaze's raw materials by weight, usually adding up to 100.

Bentonite

A very fine clay added at 1–3% to keep glaze from settling.

Bisque (UK: biscuit)

The first, lower firing, usually cone 08–04 (most often 06–04), leaving ware strong but porous.

Boron (B₂O₃)

Formally a glass former but used as a flux; essential from cone 06 to cone 6.

CMC gum

A cellulose gum that helps glaze brush on and stick; it burns away.

Colorant and stain

A metal oxide or carbonate that colors glaze; a stain is a factory-made, more stable colorant.

Cone

A calibrated ceramic pyramid that bends at a set heatwork. A witness cone shows what the pots received.

Crawling

Glaze pulling back into beads in firing, leaving bare clay.

Crazing

Fine cracks in a glaze that shrinks more than its body; crackle when wanted.

Devitrification

Crystals growing out of the glass early in cooling, where they are not wanted.

Dunting

Cracks in a pot from cooling or heating stress, often at quartz inversion, 573 °C (1063 °F).

Earthenware

A low-fire body (about cone 06–02) that stays porous.

Feldspar

A mineral supplying alkali flux with alumina and silica: potash, soda or nepheline syenite.

Flocculant and deflocculant

Additives that gel a slurry (Epsom salts, vinegar) or thin it (sodium silicate, Darvan).

Flux

An oxide that lowers the melting temperature of silica.

Food safe

In law, releasing less lead and cadmium than the limits in an acid test. Not the same as lead-free.

Frit

Factory-made glass, melted, quenched and ground, that makes soluble or toxic oxides insoluble.

Glass former

The oxide that builds the glass, usually silica.

Glaze

A coat of glass that melts in firing to seal and decorate a pot.

Glaze firing (UK: glost firing)

The firing that melts the glaze.

Glaze fit

How well glaze and body expansion match; a good fit leaves the glaze slightly compressed.

Gloss, satin and matte

Reflective and smooth; a subdued sheen; dull.

Greenware

Unfired ware: leather-hard when stiff but damp, bone dry when no free water is left.

Heatwork

The combined effect of temperature and time in a kiln; what cones measure.

Kaolin (UK: china clay)

A pure white clay supplying alumina and silica; keeps glaze suspended. Australia: Eckalite.

Kiln wash

A heat-resisting coating that protects kiln shelves from drips.

Leaching

Metals dissolving out of a fired glaze into food or drink.

Limit formula (target formula)

Typical unity-formula ranges for stable glazes at a cone; a guide, not a guarantee.

Line blend

Tests stepping one ingredient between two amounts. A triaxial blend varies three on a grid.

Liner glaze

A stable, usually clear or white glaze for surfaces that touch food.

LOI (loss on ignition)

Weight lost in firing as water, carbon dioxide, organic matter or sulfur.

Once-firing (single firing)

Glazing unfired ware and firing it only once.

Opacifier

A material, such as zircon, tin oxide or titanium dioxide, that makes glaze opaque.

Oxidation and reduction

Firing with plenty of oxygen (electric kilns), or with oxygen restricted (usually gas or wood).

Oxide

A compound of an element with oxygen; an oxide analysis lists them by weight percent.

Pinholing and blistering

Small holes, or sharp-edged craters, where gas escaped through the melt.

Porcelain

A white, kaolin-rich, high-fire body, often translucent.

R₂O and RO

The alkali fluxes (Na₂O, K₂O, Li₂O) and alkaline-earth fluxes (CaO, MgO, BaO, SrO, with ZnO).

Shivering

Glaze flaking off edges because it is over-compressed; a hazard on functional ware.

Silica (SiO₂)

The glass former; also called flint or quartz.

Silica to alumina ratio

Silica divided by alumina in the unity formula: higher tends to gloss, lower to matte.

Slip and engobe

Clay in water; an engobe is a slip-like coating with more non-clay material.

Specific gravity (SG)

A slurry's weight compared with the same volume of water. Older UK: pint weight.

Stabilizer

An oxide, mainly alumina, that stiffens the melt and adds durability.

Stoneware

A mid- to high-fire body (about cone 5–10) that becomes vitreous or nearly so.

Thixotropy

Flowing when stirred, setting into a soft gel when still.

Underglaze

Colored slip or oxide decoration, usually under a clear glaze.

Unity molecular formula (UMF, Seger formula)

A glaze's oxides as molecule counts, scaled so the fluxes add up to 1.

Vitrification

A body becoming glassy and non-porous in firing.

Wax resist

Wax painted where glaze must not go, usually the foot.

Whiting

Calcium carbonate, the main calcium flux; loses over 40% of its weight in firing. German: Kreide.

Sources

• Glazy Help: Glaze
• Glazy Help: Oxides
• Glazy Help: Raw materials
• Glazy Help: Glossary
• Glazy Help: Analyses
• Glazy Help: Firing
• Glazy Help: Defects
• Glazy Help: Specific gravity
• Orton Ceramic Foundation: Pyrometric cone chart, °F (via The Ceramic Shop)
• Digitalfire: Bisque
• Digitalfire: Bentonite
• Digitalfire: Unity molecular formula
• Digitalfire: LOI
• Digitalfire: Frit
• Digitalfire: Limit formula
• Digitalfire: Glaze crazing
• Digitalfire: Glaze shivering
• Digitalfire: Specific gravity
• Digitalfire: Oxidation firing
• Ceramic Arts Network: Common glaze faults and how to correct them (Linda Bloomfield)
• Ceramic Arts Network: An introduction to color (Birdie Boone)
• Ceramic Arts Network: 8 ways to apply glaze (Robin Hopper)
• Glendale Community College: Glaze color
• Potclays: The difference between stoneware and earthenware
• US FDA: Compliance Policy Guide Sec. 545.450, Pottery (ceramics), lead contamination
• EUR-Lex: Council Directive 84/500/EEC of 15 October 1984 on the approximation of the laws of the Member States relating to ceramic articles intended to come into contact with foodstuffs
• eCFR: 21 CFR 109.16, Ornamental and decorative ceramicware