Whitewares, tiles, sanitaryware and glazes

In short

Whitewares are fine-grained, white or near-white fired ceramic bodies — earthenware, stoneware, vitreous china, porcelain and bone china — made from a clay–flux–filler (triaxial) mix and almost always finished with a glassy glaze. Tiles and sanitaryware are the two largest whiteware products by tonnage: India is the world's second largest producer and consumer of ceramic tiles, with the Morbi–Wankaner cluster in Gujarat making the bulk of national output, and brands such as Kajaria, Somany, H&R Johnson, Orient Bell, Varmora, Cera, Hindware (HSIL), Parryware and Jaquar manufacturing here. The topic carries heavy marks in ceramic technology papers and GATE-type questions on body/glaze calculation, drying and firing reactions, vitrification and defect analysis. In interviews it appears as water-absorption classes, slip rheology, glaze fit and shop-floor defect troubleshooting. Indian practice is governed by IS 15622 (pressed tiles), IS 13630 (tile test methods) and IS 2556 (vitreous sanitary appliances).

Explained

Whiteware classes and the triaxial body

• Whiteware: a fired ceramic with a fine, white-burning body; "white" comes from low iron and titania in the raw materials, not from the glaze.
• Triaxial composition: clay (china clay/kaolin + ball clay, 40–55%) gives plasticity and dry strength; flux (potash feldspar, nepheline syenite, 15–30%) forms the glassy bond; filler (quartz, 20–35%) is the non-plastic skeleton that controls shrinkage and raises the thermal expansion of the body.
• Classes by water absorption (WA) after firing:
  • *Earthenware* — porous, WA 10–20%, fired 1,050–1,150 °C, always glazed (crockery, wall tiles).
  • *Semi-vitreous / semi-stoneware* — WA 3–10%.
  • *Stoneware* — WA 0.5–3%, dense, often unglazed body (chemical stoneware, pipes, floor tiles).
  • *Vitreous china* — WA ≤ 0.5%, used for sanitaryware; relatively coarse body with a continuous glassy phase.
  • *Hard porcelain* — WA ~0–0.5%, fired 1,300–1,400 °C, translucent, high strength; electrical porcelain is the insulator version.
  • *Bone china* — ~45–50% calcined bone ash (tricalcium phosphate) + china clay + feldspar; highest translucency and whiteness, fired ~1,220–1,250 °C biscuit.
• Indian raw-material sources: china clay from Kerala (Thonnakkal, Kundara), Rajasthan (Bikaner) and Odisha; ball clay from Gujarat (Than, Kutch) and Bikaner; feldspar and quartz from Rajasthan and Andhra Pradesh; talc, wollastonite (fast-fire wall tile bodies), calcite, dolomite, zircon and alumina.

Body preparation

1. Beneficiation: washing, hydrocycloning and magnetic separation of clay to drop quartz, mica and iron minerals.
2. Weighing and wet grinding: batch ball-milled (alumina or flint pebbles) to a residue specification, typically 1–3% on a 45 µm (325 mesh) sieve; ball-mill speed is set at 60–75% of critical speed.
3. Deflocculation: sodium silicate + soda ash (0.2–0.5%) lowers slip viscosity at high solids (65–72% by weight) by charge repulsion on clay platelets; over-dosing re-flocculates.
4. Dewatering and granulation: filter press + pug mill/extrusion for plastic bodies; spray drying for tile presses, giving free-flowing granules of 5–7% moisture; slip is kept as-is for casting.
5. Ageing/souring of plastic bodies improves workability (bacterial action, uniform moisture).

Forming routes

• Dry (semi-dry) pressing — tiles: uniaxial hydraulic presses, 25–45 MPa, two or three pressing strokes with de-airing; faults are lamination and density gradients.
• Isostatic pressing of punches gives flat large-format tiles; large slabs are made on continuous pressing/compaction lines.
• Extrusion — Group A (split tiles, quarry tiles, pipes, insulator blanks) from a 18–22% moisture column through a de-airing auger extruder.
• Slip casting — sanitaryware in plaster-of-Paris moulds; capillary suction builds a wall, thickness x = K√t (drain/hollow casting for hollow ware, solid casting for rims and basins). Casting time 40–90 min; mould life about 80–120 casts.
• Pressure casting — porous resin moulds at 10–15 bar cut casting time to 10–20 min, no drying room for the mould, mould life in thousands of casts; now standard in modern Indian sanitaryware plants.
• Jiggering/jolleying — plates and cups on a rotating profile tool; ram pressing for oval dishes; roller forming for crockery.
• Casting defects: wreathing, soft spots (worn mould), drain lines, pin-holing from entrapped air, warping on demoulding.

Drying and firing

• Drying: water removal is in two stages — *constant-rate period*, when shrinkage water leaves and the piece shrinks (cracking risk), down to the critical moisture content, then the *falling-rate period*, when pore water leaves with no shrinkage. The Bourry curve plots shrinkage against moisture loss. Dryers run 40–110 °C with controlled humidity; tiles dry in 20–60 min in vertical/horizontal dryers, sanitaryware in 10–20 h.
• Firing reactions (heating): evaporation of residual water to 200 °C → burnout of organics 300–500 °C → dehydroxylation of kaolinite to metakaolin at 450–600 °C → α→β quartz inversion at 573 °C (reversible, volume change) → carbonate/sulphate decomposition 700–900 °C → mullite (3Al₂O₃·2SiO₂) nucleation from ~980 °C, needle growth 1,100–1,250 °C → feldspar eutectic melt from ~1,150 °C dissolving quartz edges → vitrification: glass fills pores, porosity falls, shrinkage and strength rise.
• Over-firing gives bloating, loss of shape (pyroplastic deformation) and rising porosity; under-firing gives high water absorption and low strength. The vitrification range is the temperature window between full densification and deformation — a wide range is wanted for sanitaryware.
• Cooling: cristobalite and quartz inversions make the 600–200 °C band critical; fast cooling there causes dunting (cooling cracks).
• Schedules: single-fire (*monocottura*, *monoporosa* for wall tiles) applies glaze on the dried green tile and fires body + glaze together; double-fire gives a biscuit (bisque) firing then a glost firing of the glaze. Porcelain reverses this — low bisque (~900–1,000 °C) and high glost (1,300–1,400 °C).
• Kilns: roller-hearth kilns fast-fire tiles in 30–60 min at 1,120–1,220 °C; sanitaryware uses tunnel kilns (12–20 h) or shuttle kilns; firing is mainly on natural gas/piped gas, with coal gasifiers historically common in Morbi.

Tiles — classification, properties and standards

• IS 15622: Pressed ceramic tiles — Specification (aligned with ISO 13006) classifies by forming method and water absorption: Group A extruded, Group B dry pressed; within B — BIa E ≤ 0.5% (porcelain/vitrified), BIb 0.5 < E ≤ 3%, BIIa 3 < E ≤ 6%, BIIb 6 < E ≤ 10%, BIII E > 10% (glazed wall tiles).
• Indicative IS 15622 requirements: BIa, thickness ≥ 7.5 mm — modulus of rupture ≥ 35 N/mm² (individual minimum 32) and breaking strength ≥ 1,300 N; BIII wall tiles — MOR ≥ 15 N/mm² (min 12) and breaking strength ≥ 600 N; dimensional tolerance on length typically ±0.5–0.6%, thickness ±5–10%, and 95% of tiles free of visible surface defects.
• IS 13630 (Parts 1–16): methods of test — water absorption and bulk density (boiling/vacuum), breaking strength and MOR, deep abrasion (unglazed, in mm³), surface abrasion of glazed tiles (PEI classes I–V), Mohs scratch hardness, crazing resistance (autoclave), thermal shock, frost resistance, linear thermal expansion, moisture expansion, chemical and stain resistance, slip resistance and lead/cadmium release from glaze. IS 13712 gives definitions, classification and marking; IS 4457 covers unglazed vitreous acid-resisting tiles. Cement concrete and terrazzo floor tiles are a different product under IS 1237 — not whiteware.
• Commercial tile types: glazed wall tile (porous, monoporosa), glazed vitrified tile (GVT), polished glazed vitrified tile (PGVT), full-body/double-charge vitrified, soluble-salt, satin/matt/rustic, large-format slabs, and anti-skid floor tiles (slip resistance expressed as coefficient of friction or an R9–R13 ramp rating).
• Decoration: screen printing → rotocolour (roller) printing → digital inkjet printing, now dominant, with dry-granule and lustre effects for depth.
• GST: ceramic tiles and sanitaryware are taxed at 18% under the 5/18/40% structure effective 22 September 2025 (they were in the 28% slab earlier).

Sanitaryware

• Body: vitreous china — typically ball clay + china clay ~45–50%, feldspar 20–25%, quartz 25–30%, with a small zircon addition for whiteness; fired 1,200–1,260 °C to WA ≤ 0.5%, so the ware is impervious even if the glaze chips.
• Products: wash-down and siphonic water closets, squatting pans, wash basins, urinals, bidets, cisterns, sinks, traps and accessories. IS 2556, Vitreous sanitary appliances (vitreous china), is in several parts — Part 1 gives general requirements (water absorption, warpage, crazing, glaze quality, dimensions, marking) and later parts give specific requirements for individual appliances; plastic flushing cisterns are covered by IS 7231.
• Performance tests: water absorption, crazing by autoclave followed by a dye/ink test, warpage and squareness, load test on basins and brackets, flush/discharge tests (ball, paper and solid-waste tests), washing of the bowl surface, trap seal depth (commonly 50 mm) and resistance of the glaze to acids and staining.
• Water efficiency: dual-flush cisterns of 6 L full / 3 L reduced are the common Indian specification; CPWD, GRIHA and IGBC credits push lower volumes and water-free urinals.
• Ceramic tiles and vitreous sanitary appliances have been brought under BIS Standard Mark certification through Quality Control Orders in recent years, whose scope and dates have been amended more than once; the ISI mark with the IS number and licence number must appear on the ware covered.

Glazes — composition, calculation, application and defects

• A glaze is a thin (0.1–0.5 mm) glassy coating fused onto the body: it seals porosity, makes the surface cleanable and chemically resistant, and decorates. An engobe is an opaque intermediate slip layer that hides body colour.
• Constituents: glass former SiO₂ (and B₂O₃); stabiliser Al₂O₃ (raises viscosity, prevents run-off and devitrification); fluxes PbO, Na₂O, K₂O, Li₂O, CaO, MgO, ZnO, BaO, SrO; opacifiers zircon (ZrSiO₄), SnO₂, TiO₂; matting agents excess ZnO, CaO, BaO or alumina; colourants CoO (blue), Cr₂O₃ (green), Fe₂O₃ (brown), CuO (green/turquoise), MnO₂ (purple), V–Zr (blue), Pr–Zr (yellow), Fe–Zr (coral); binder CMC; suspender bentonite.
• Types: raw (unfritted) vs fritted; a frit is pre-melted and quenched glass, used to make boron and (formerly) lead insoluble, to narrow the melting range and to give a consistent fast-fire glaze. Also transparent/opaque, glossy/matt/satin, crystalline, salt glaze, slip (Albany) glaze, Bristol glaze (ZnO), raku and reduction glazes. Leadless and lead-bisilicate glazes have replaced raw lead oxide in tableware because of lead release limits.
• Seger (unity) formula: composition written as molar ratios RO·R₂O₃·RO₂ with the basic (flux) oxides normalised to 1.0. Rules of thumb: higher SiO₂/Al₂O₃ ratio (about 8–12) gives gloss and a lower maturing range; ratios below ~5 tend to matt; alumina and silica raise maturing temperature, fluxes lower it.
• Application: dipping and pouring (sanitaryware, by robot spray in modern plants), spraying, waterfall/curtain or bell coating and glaze discs for tile lines; typical tile glaze load 400–900 g/m²; glaze slip controlled by specific gravity (1.70–1.85) and flow/viscosity.
• Glaze fit: the glaze should have a thermal expansion coefficient slightly lower than the body, so it ends in mild compression and strengthens the piece. If α_glaze > α_body, the glaze is in tension → crazing (hairline network cracks); if the compression is excessive → shivering/peeling at edges. Delayed crazing on porous wall tiles comes from moisture expansion of the body in service.
• Common glaze defects and causes: crawling (dust/oil on biscuit, over-thick or high-surface-tension glaze), pinholing and blistering (gas from the body or carbonates, too-fast firing), orange peel (spray too dry), devitrification/dull patches (excess ZnO, CaO, slow cooling), specking (iron contamination), metal marking, run-off (low alumina), dry/under-fired glaze, black core (reduced iron and trapped carbon from fast firing a high-organic body), bloating, dunting and warpage.
• Safety and environment: respirable crystalline silica from grinding and spray-drying causes silicosis — dust control, local exhaust, wet methods and health surveillance are required; workplace safety now sits under the Occupational Safety, Health and Working Conditions Code, 2020 and the four Labour Codes in force from 21 November 2025 (which replaced the Factories Act and related laws). Units need Consent to Establish/Operate from the State Pollution Control Board under the Water and Air Acts, control SPM and SO₂/NOₓ from kilns and spray dryers, and recycle glaze/body wash water.

Key points

• Whiteware - fine, white-burning fired ceramic; classed by water absorption from earthenware (10–20%) to porcelain (~0%).
• Triaxial body - clay for plasticity, feldspar for flux/glass, quartz for skeleton and expansion matching.
• Ball clay vs china clay - ball clay is plastic, finer, darker-burning; china clay is whiter and less plastic.
• Deflocculant - sodium silicate/soda ash lowers slip viscosity at high solids; excess causes reflocculation.
• Casting rate - wall thickness grows as the square root of casting time; pressure casting cuts time 4–6 fold.
• Critical moisture content - the point where shrinkage stops; all drying cracks occur before it.
• Quartz inversion at 573 °C - reversible volume change; fast heating or cooling through it causes dunting.
• Mullite - interlocking needles formed above ~980 °C; the main strength-giving crystalline phase.
• Vitrification range - window between full densification and pyroplastic deformation; wide range is desirable.
• Monocottura/monoporosa - single firing of body and glaze together; cheaper and lower energy, but needs tighter control of the body–glaze match.
• IS 15622 groups - BIa ≤0.5%, BIb ≤3%, BIIa ≤6%, BIIb ≤10%, BIII >10% water absorption.
• PEI class - surface abrasion rating I–V for glazed tiles; class IV–V for heavy traffic floors.
• Vitreous china - sanitaryware body, water absorption ≤0.5% per IS 2556, impervious even where the glaze is damaged.
• Frit - pre-melted glass that makes boron/lead insoluble and gives a repeatable fast-fire glaze.
• Seger formula - RO·R₂O₃·RO₂ molar statement of a glaze with fluxes summed to unity.
• Crazing vs shivering - glaze in tension (α_glaze > α_body) vs excessive compression.
• Moisture expansion - irreversible body swelling in porous wall tiles causing delayed crazing.
• Morbi cluster - Gujarat; the dominant share of Indian tile capacity, mostly on natural gas-fired roller kilns.

Formulas and examples

Example 1 — Classifying a tile. A fired tile: dry mass 48.00 g, mass after boiling 48.19 g, suspended mass in water 28.50 g.

• E = (48.19 − 48.00)/48.00 × 100 = 0.19/48.00 × 100 = 0.40% → E ≤ 0.5%, so Group BIa (vitrified/porcelain tile).
• Apparent porosity = 0.19/(48.19 − 28.50) × 100 = 0.19/19.69 × 100 = 0.97%.
• Bulk density = 48.00/19.69 = 2.44 g/cm³.

Example 2 — Strength check against IS 15622. Specimen width b = 100 mm, thickness h = 8.0 mm, span L = 200 mm, breaking load F = 1,000 N.

• Breaking strength S = F·L/b = 1,000 × 200/100 = 2,000 N (requirement for BIa ≥ 7.5 mm thick: ≥ 1,300 N → passes).
• MOR = 3FL/(2bh²) = (3 × 1,000 × 200)/(2 × 100 × 64) = 600,000/12,800 = 46.9 N/mm² (requirement ≥ 35 N/mm² → passes).

Example 3 — Press size from shrinkage. A green tile pressed at 200.0 mm dries to 198.4 mm and fires to 194.0 mm.

• Drying shrinkage = (200.0 − 198.4)/200.0 × 100 = 0.80%.
• Firing shrinkage = (198.4 − 194.0)/198.4 × 100 = 4.4/198.4 × 100 = 2.22%.
• Total shrinkage on the pressed size = 6.0/200.0 × 100 = 3.00%.
• To finish a 600 mm tile: pressed size = 600/(1 − 0.0300) = 600/0.97 = 618.6 mm.

Example 4 — Slip solids content. A casting slip has specific gravity 1.75; the solids have density 2.60 g/cm³. Take 1,000 cm³ of slip = 1,750 g.

• W_s/2.60 + (1,750 − W_s)/1.00 = 1,000 → W_s(0.3846 − 1) = 1,000 − 1,750 → −0.6154 W_s = −750.
• W_s = 1,218.8 g per litre, so solids = 1,218.8/1,750 = 69.6% by weight; water = 531.2 g (30.4%).

Example 5 — Glaze fit. Glaze E = 70 GPa, ν = 0.25, α_glaze = 6.0 × 10⁻⁶ /K, α_body = 7.0 × 10⁻⁶ /K, ΔT = 500 K.

• σ = 70 × 10⁹ × 500 × (1.0 × 10⁻⁶)/(1 − 0.25) = 35 × 10⁶/0.75 = 46.7 MPa compression — safe, since glazes take high compression but craze in tension above roughly 5–10 MPa. Reversing the α values would put the glaze in 46.7 MPa tension and it would craze.

Where it is used

• Tile manufacture: Morbi–Wankaner (Gujarat), plus plants of Kajaria, Somany, H&R Johnson (Prism), Orient Bell, Nitco, Asian Granito, RAK India and Varmora — roles in body/glaze development, press and kiln operation, digital decoration and QC labs testing to IS 13630.
• Sanitaryware: HSIL/Hindware, Cera, Parryware (Roca), Jaquar, Johnson, Somany — slip house, pressure casting, robot glazing, tunnel-kiln firing and flush-performance testing to IS 2556.
• Tableware and hotelware: bone china and vitreous hotelware (Khurja cluster in Uttar Pradesh, Jaipur, Thangadh in Gujarat); lead and cadmium release limits apply to food-contact glazes.
• Allied whiteware: electrical porcelain insulators, chemical stoneware, refractory and technical whitewares, ceramic sanitary fittings and spa products.
• Construction and specification roles: architects, CPWD/PWD and builder QC specifying PEI class, slip resistance, water absorption, dimensional tolerance and dual-flush water use; green-building credits under GRIHA/IGBC.
• R&D and testing: CSIR–CGCRI Kolkata, Ceramic Technology Centre at Morbi, NABL-accredited tile and sanitaryware labs, and BIS licence audits.

Interview questions

• Q: Why does a tile body contain quartz when quartz adds no plasticity? A: It acts as the non-plastic skeleton that lowers drying shrinkage and keeps the body's thermal expansion high enough to hold the glaze in compression.
• Q: What is the difference between a vitrified tile and a glazed wall tile? A: A vitrified tile has water absorption ≤0.5% and high strength (Group BIa) so it can be used on floors; a wall tile is porous (>10%, Group BIII) and would fail under floor loads and moisture expansion.
• Q: Why is sanitaryware fired to below 0.5% water absorption? A: The ware is permanently wet in service; a non-porous vitreous body stays hygienic and does not absorb water or stain even if the glaze chips.
• Q: Why does crazing happen and how do you cure it? A: The glaze has a higher thermal expansion than the body and ends in tension; cure by raising body expansion (more quartz), lowering glaze expansion (less Na₂O/K₂O, more SiO₂/B₂O₃/Al₂O₃), thinning the glaze or firing the body tighter.
• Q: What does a frit do that a raw glaze cannot? A: It pre-reacts volatile and soluble oxides such as boron and alkalis into insoluble glass, so the glaze slip stays stable, melts in a narrow range and suits fast firing.
• Q: What is the Seger formula used for? A: It expresses a glaze as molar oxide ratios with the fluxes normalised to unity, so recipes can be compared, converted between raw materials and adjusted for maturing temperature.
• Q: Why is 573 °C critical in both heating and cooling? A: Quartz undergoes a reversible α→β inversion with a volume change; rapid passage through it causes dunting and hairline cracks.
• Q: What is black core and how do you avoid it? A: A dark reduced layer inside the tile from carbon and iron not oxidised during fast firing; avoid it by slowing the 300–900 °C band, oxidising kiln atmosphere and lower-organic raw materials.
• Q: Why has pressure casting replaced plaster moulds in large sanitaryware plants? A: Resin moulds under 10–15 bar cut casting time from about an hour to 10–20 min, need no mould drying, give better dimensional consistency and last thousands of casts.
• Q: A floor tile passes water absorption but shows surface wear in a mall within a year. What test was wrong? A: The glazed surface abrasion (PEI) class was too low — heavy commercial traffic needs class IV–V, along with adequate slip resistance.
• Q: What causes crawling, and how is it different from shivering? A: Crawling is the glaze pulling away during firing from a greasy or dusty biscuit or too-thick glaze; shivering is the fired glaze flaking off edges because it is in excessive compression.
• Q: Which Indian standards would you quote for a tile tender? A: IS 15622 for pressed ceramic tile specification and classification, IS 13630 parts for the test methods, and IS 13712 for definitions and marking; vitreous sanitaryware is under IS 2556.

Common mistakes

• Quoting water absorption without stating the method (boiling vs vacuum) or classifying a tile without checking the forming group (A or B) as well.
• Mixing up the glaze-fit rule — writing that the glaze should expand more than the body; it must expand slightly less, so it ends in compression.
• Reporting MOR when the standard asks for breaking strength (S = F·L/b, in newtons), or omitting the span and width from the calculation.
• Calculating shrinkage on the wrong base — firing shrinkage is measured on the dried length, total shrinkage on the pressed (green) length.
• Confusing china clay with ball clay, or feldspar's role (flux) with quartz's role (filler).
• Blaming the glaze for every defect: warpage, black core, dunting and bloating are body and firing-schedule faults.
• Calling cement concrete or terrazzo floor tiles "ceramic tiles", and ignoring silica dust control in the grinding and spray-drying sections.

Related topics

Glass technology: melting, forming and annealing · Advanced ceramics: electronic, structural and bioceramics · Cement, glass and ceramics industries · Refractories and kiln design · Ceramic raw materials and beneficiation · Phase equilibria and the ternary diagram of ceramic systems