Ceramic Glazes & Sanitaryware: Green That Survives the Kiln
A glaze colourant has to do three things: dissolve into the melt, keep its oxidation state, and not blow bubbles while doing it. Most greens manage one. Chrome oxide green manages all three — at every temperature the kiln throws at it.
Glaze greens that reduce or volatilise in the kiln come out grey or brown; gas evolution pinholes the surface; shade drift between kiln loads forces re-matching on every firing. In sanitaryware, a pinhole is a scrapped basin.
Stable through the whole firing curve
Cr₂O₃ is stable from 1000 °C to 1300 °C across oxidising, neutral and mildly reducing atmospheres. It dissolves into the glaze as Cr(III) and stays there — no reduction to gas, no volatilisation, no colour drift. The fired shade is the designed shade.
Strength and consistency
With colour-card lightfastness 8/8 and complete insolubility, the fired glaze keeps its green for the life of the sanitaryware — through cleaners, abrasion and steam. Batch consistency is SPC-controlled, so the glaze recipe runs unchanged from kiln load to kiln load.
Pinhole-free, drift-free, reduction-proof green — the fired shade your designer approved, every load.
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Glaze behaviour by firing range
| Firing range | Typical body | Cr₂O₃ stability | Use |
|---|---|---|---|
| Low fire (900–1000 °C) | Earthenware | Excellent | Wall tiles, decorative |
| Mid fire (1100–1250 °C) | Stoneware | Excellent | Sanitary ware, tableware |
| High fire (1250–1350 °C) | Porcelain | Excellent | Vitreous china, tiles |
| Reduction firing | Stoneware | Shifts to green-grey | Craft effects |
Firing to 1000–1300 °C: what happens to the green in the kiln
A glaze kiln is a brutal filter for colourants. Every organic pigment in the studio burns out before 600 °C, most inorganic colourants bleach or shift by 1,000 °C, and only a handful of oxides come out the other side recognisably the same colour. Chrome oxide green is the exception among greens, and the reason is written in its physical constants: melting point 2,435 °C, decomposition beginning only near 2,266 °C. A cone 6 stoneware firing peaks at 1,220 °C; cone 10 porcelain at 1,280–1,300 °C. The kiln is hot enough to melt the glaze, and still leaves the Cr₂O₃ particle over 1,100 °C of headroom below its own failure point. Nothing else green survives that. Copper greens dull and grey, chrome-tin pinks are another chemistry entirely, and vanadium- and iron-based greens are separate, softer colours — but for a true, saturated green that walks into the kiln and walks out green, Cr₂O₃ is the answer.
What actually happens to the particle in the melt is a partial-dissolution story. As the glaze fluxes and turns liquid, the Cr₂O₃ grains begin to dissolve at their surfaces, releasing Cr(III) ions into the glassy melt, where they colour it green through absorption bands in the blue and red. Fully dissolved chromium gives a clean, transparent green; larger particles that only partially dissolve survive as colourant grains suspended in the glaze, and at the surface they read as texture — fine green speckle. The colour the potter sees is therefore a balance the recipe controls: fine, well-dispersed oxide dissolves toward a smooth translucent green, while a coarser grind deliberately retained produces the speckled, mineral look that some studios want. The same pigment, two particle sizes, two entirely different surfaces.
The kiln has one more trick, and it is the one that separates careful studios from careless ones: volatilisation. At the top of the firing range — roughly 1,100 °C and above, and especially in long cone 10 soaks — some chromium oxidises at the glaze surface and volatilises as Cr(VI) species, which deposit on nearby ware and inside the kiln. This is why chrome-green glazes can "halo" onto adjacent pots, and why glaze chemists keep chrome-bearing glazes away from tin-oxide whites (the classic pink-bloom failure) in the same firing. It is also a hygiene matter: kiln shelves and interiors from chrome firings carry Cr(VI), and studio ventilation and washing discipline matter. None of this changes the pigment's colourfastness — it is a property of how the glaze fires, not of the oxide.
Glaze systems where chrome green earns its place
Across the 1,000–1,300 °C window, chrome oxide green behaves differently enough between clay bodies and atmospheres that a firing-range table is the practical way to think about it. In low-fire earthenware — cones 04 to 02, roughly 1,000–1,100 °C — the glaze melt is short and the oxide dissolves less, so greens come out brighter and often slightly opaque, and small additions of 1–3% already give a strong colour. At cone 6 stoneware, 1,220 °C, the melt is longer and more of the oxide dissolves; additions of 1–5% hold a saturated green, and glaze fit matters more because the harder, denser Cr₂O₃ particles in the surface can roughen the finish if the base glaze is stiff. At cone 10 porcelain, 1,280–1,300 °C, dissolution goes further and volatilisation begins to bite: the greens deepen, sometimes toward a slightly darker, more olive cast, and the studio should plan firings to keep chrome glazes from sharing kiln space with tin whites.
Atmosphere is the quiet variable that catches new glaze chemists. In oxidation, Cr(III) stays Cr(III) and the green is dependable. In reduction, the picture shifts twice: first, iron impurities in the body or glaze reduce toward Fe(II) and darken the fired colour; second, in strong reduction the chrome can be pulled toward lower oxidation states that read grey or even black-green. A recipe that fires a clean green at cone 6 oxidation can come out a smoky olive at cone 10 reduction — not because the pigment failed but because the kiln changed the chemistry around it. Zinc is the other classic trap: zinc oxide in the glaze base reacts with chromium to form zinc chromite spinel, which fires brown, and a potter who adds chrome green to a zinc-bearing clear glaze gets tan, not green, and blames the pigment. The pigment is stable; the glaze base is not.
One further property makes chrome green unusually efficient in the recipe: colouring strength. Chromium is among the most potent oxide colourants in ceramics — a 2% addition colours more deeply than several percent of iron or copper — so the cost line of a glaze is set by the frit and the clay, not by the pigment, and there is no economic case for stretching a good green with a cheaper, weaker colourant that will only drag the tone toward mud. The practical consequence is a testing discipline: chrome-green glazes should be developed on draw tiles across the kiln's real temperature spread, because the colour moves with heatwork. A glaze that is a clean green at the cone peak can come out noticeably deeper, or slightly olive, at the top of the kiln where heatwork runs a half-cone higher. Measure the colour where your kiln actually fires, not where the cone chart says it should.
| Firing range | Glaze system | Typical Cr₂O₃ addition | Fired result | Notes |
|---|---|---|---|---|
| 1,000–1,100 °C (cone 04–02) | Earthenware, oxidation | 1–3% | Bright, slightly opaque green | Short melt limits dissolution; strong colour at low dose |
| 1,180–1,240 °C (cone 6) | Stoneware, oxidation | 1–5% | Saturated, even green | Longer melt dissolves more; keep base glaze fluid |
| 1,280–1,300 °C (cone 10) | Porcelain, oxidation | 2–6% | Deep green, slight olive cast | Volatilisation risk; isolate from tin whites |
| 1,280–1,300 °C (cone 10) | Porcelain/stoneware, reduction | 2–6% | Grey-green to smoky olive | Strong reduction pulls chromium toward grey; avoid zinc in base |
Batch control: D50, purity and the speckle problem
Half of studio glaze failures with chrome green are not chemistry problems; they are particle size problems wearing a chemistry costume. The speckle-and-roughness complaints that potters report as "the green is gritty" are usually an oxide whose D50 is too coarse for the glaze's melting behaviour: the particles do not dissolve at the firing range in question and the surface comes out with undissolved grains, speckle, or a sandpaper texture. The fix is not to change the pigment but to match the particle size to the kiln — a finer D50 for smooth translucent greens at low-fire, a deliberately coarser one only when the speckled surface is the design. Sieving the glaze through 80-mesh and ball-milling or propeller-mixing with good deflocculation does more for colour evenness than reformulation ever will.
Purity is the other half. Glaze-grade chrome oxide green should assay ≥99% Cr₂O₃, because the balance in cheaper grades is mostly iron oxide — and iron in a glaze is a colourant in its own right, darkening the green and muddling the tone exactly where the potter least expects it. A 98% grade with 1.5% iron does not give a "slightly weaker green"; it gives a green-brown that shifts with every new bag. The glaze chemist's real demand is lot-to-lot consistency: tight assay range, tight D50 range, so that a glaze line developed on one bag of oxide keeps firing the same colour on the next hundred.
Handling belongs in the same checklist. Dry chrome oxide green is a fine, dense powder (true density 5.21 g/cm³ — it settles fast in a glaze bucket and wants stirring back into suspension), and its Cr(VI) content is the difference between a clean studio and a slow hygiene problem. Low-Cr(VI) grades (≤5 ppm) cost the colour nothing — Cr(VI) in the powder is a manufacturing residue, not a colourant — and they remove the main handling hazard at the weighing and mixing bench. And for the record of permanence the customer will ask about: once fired, the green is permanent. Chrome green carries lightfastness 8/8, and a chrome-green glaze will not fade in sunlight in any human lifetime — the colour outlives the pot.
Chrome oxide green is the only green that survives the kiln — and that sentence sells it short, because survival is the easy part. The failures we hear about are glaze-base failures wearing the pigment's clothes: zinc in the base turns it brown, heavy reduction turns it grey, a coarse D50 turns it gritty, and uncontrolled iron in a cheap grade turns it olive. Fix the base glaze first, match the particle size to the firing range second, and evaluate the pigment third. The pigment does not need to be defended — the constants (melting 2,435 °C, lightfastness 8/8) have been the same for decades. The recipe around it is what decides whether the pot comes out green.
FAQ
Why is chrome oxide green the only green that survives the kiln? Because of its thermal constants. It melts at 2,435 °C and does not decompose until near 2,266 °C, while glaze firings peak at 1,000–1,300 °C — over 1,100 °C of headroom. Organics burn out below 600 °C and other green colourants dull or shift long before cone 6; Cr₂O₃ comes out the same colour it went in.
How much chrome oxide should I add to a cone 6 stoneware glaze? Start at 1–5% of dry glaze weight. The longer melt at 1,220 °C dissolves the oxide well, so small additions already saturate. Above 5% the glaze can go leathery and the surface rougher, so increase by 0.5% steps and fire test tiles rather than jumping straight to a production batch.
Why did my green turn grey in reduction firing? The kiln atmosphere, not the pigment. Strong reduction can pull chromium toward lower oxidation states that read grey to black-green, and reduced iron impurities in the body darken the tone further. If you want reduction greens, accept a smoky olive, or fire chrome-green glazes in oxidation and leave the reduction for other colours.
Why did my chrome green turn brown in a clear glaze? Check the glaze base for zinc. Zinc oxide reacts with chromium to form zinc chromite spinel, which fires brown — the classic "my green went tan" complaint. Swap the zinc-bearing clear for a zinc-free base and the same oxide goes back to green.
How do I avoid speckles and gritty surfaces with chrome green? Match particle size to the firing range and mill the glaze. A finer D50 dissolves at lower temperatures and gives a smooth translucent green; coarse grades read as intentional speckle only when you chose them. Sieve through 80-mesh and mix with good deflocculation, and remember the oxide's 5.21 g/cm³ density settles it fast in the bucket.