Glass technology: melting, forming and annealing

In short

Glass technology covers how a batch of oxide raw materials is melted into a homogeneous, bubble-free liquid, shaped while viscous, and then cooled through a controlled temperature schedule so that residual stress is removed (annealing) or deliberately introduced (tempering). The whole chain is governed by one property - viscosity as a function of temperature - because melting, fining, forming and annealing are each defined by a viscosity window rather than by a fixed temperature. It matters because glass is a supercooled liquid with no fixed melting point: get the thermal schedule wrong and the product either crystallises (devitrifies) or shatters from frozen-in stress. In India this knowledge is examined in ceramic/glass technology papers and the GATE ceramic-engineering style questions on batch calculation, viscosity reference points and the float process, and it is applied at Asahi India Glass, Saint-Gobain India, Gold Plus, Borosil, HNG (Hindusthan National Glass), AGI Greenpac and Piramal Glass.

Explained

Glass structure and the viscosity-temperature backbone

• Glass: an inorganic product of fusion cooled to a rigid condition without crystallising - an amorphous solid with short-range order but no long-range periodicity; it shows a glass transition temperature Tg, not a melting point.
• Zachariasen-Warren random network: SiO₄ tetrahedra corner-share oxygens. Network formers (SiO₂, B₂O₃, P₂O₅, GeO₂) build the network; modifiers (Na₂O, K₂O, CaO, MgO) break Si-O-Si bridges creating non-bridging oxygens, lowering viscosity and Tg; intermediates (Al₂O₃, PbO, ZnO, TiO₂) act either way.
• Why modifiers are needed: pure silica melts above about 1700 °C and is impossibly stiff to work; adding Na₂O drops the working temperature to roughly 1000-1200 °C, which is the entire commercial basis of soda-lime glass.
• Fixed viscosity reference points (the most examined set):
  • Melting range: about 10¹ to 10² Pa·s (100-1000 poise) - 1400-1600 °C for soda-lime.
  • Working point: 10³ Pa·s (10⁴ poise) - glass can be shaped; about 1000-1050 °C.
  • Softening point (Littleton): 10⁶·⁶ Pa·s (10⁷·⁶ poise) - glass deforms under its own weight; about 700-730 °C.
  • Annealing point: 10¹²·⁴ Pa·s (10¹³·⁴ poise) - stress relieves in minutes (about 15 min); 510-550 °C.
  • Strain point: 10¹³·⁵ Pa·s (10¹⁴·⁵ poise) - stress relieves in hours; 470-510 °C. Below this the glass is effectively elastic.
  • Working range = between working point and softening point; annealing range = between annealing and strain points.
• VFT (Vogel-Fulcher-Tammann) equation describes log η versus T far better than Arrhenius over the whole range because the activation energy for flow is itself temperature-dependent.
• Compositions to know: soda-lime (≈73 SiO₂, 14 Na₂O, 9 CaO, 4 MgO/Al₂O₃ - containers, sheet), borosilicate (≈80 SiO₂, 13 B₂O₃, 4 Na₂O, 2 Al₂O₃ - low expansion, lab and cookware), lead crystal (PbO 24-32% - high refractive index, tableware; lead restricted in food-contact use), aluminosilicate and E-glass (fibre), 96% silica (Vycor) and fused silica.

Raw materials and batch calculation

• Sand (silica): 99.5%+ SiO₂, Fe₂O₃ below about 0.03% for clear glass (Indian sources: Allahabad/Shankargarh, Rajasthan, Gujarat). Iron gives the green tint in ordinary bottles.
• Soda ash (Na₂CO₃): supplies Na₂O, the main flux. Limestone (CaCO₃)/dolomite (CaCO₃·MgCO₃): CaO for chemical durability, MgO to retard devitrification. Feldspar/nepheline syenite: Al₂O₃ to improve durability and lower devitrification risk. Borax/boric acid: B₂O₃. Salt cake (Na₂SO₄): fining and scum removal.
• Cullet: crushed waste/recycled glass, typically 20-60% (up to 80%+ in container furnaces). It melts without decomposition, cutting specific energy by roughly 2-3% for every 10% cullet and reducing CO₂ and batch dust. Must be colour-sorted and free of ceramic, stones and metal.
• Fining (refining) agents: Na₂SO₄, As₂O₃/Sb₂O₃ with nitrates, CeO₂, or NaCl - they release gas at high temperature so small bubbles grow, rise and sweep out seeds.
• Decolourisers: chemical (reduce Fe³⁺ to lower-tint Fe²⁺ or oxidise it) and physical (Se, Co₃O₄, Nd₂O₃ add a complementary colour to mask green).
• Colourants: Fe₂O₃+C amber, Cr₂O₃ green, CoO blue, Se+CdS red, Cu₂O ruby (struck), MnO₂ purple.
• Batch calculation steps: (1) write the required oxide composition per 100 kg glass; (2) assign each oxide to a raw material; (3) divide the oxide requirement by the oxide fraction in that material (accounting for purity); (4) deduct oxides carried by cullet; (5) check loss on ignition (LOI) - about 15-20% mass lost as CO₂ and H₂O, so batch mass > glass mass.
• Batch-to-melt ratio = batch mass / glass produced; typically 1.15-1.25 for soda-lime without cullet.

Melting: reactions, furnaces and defects

• Five stages: (1) batch melting/silicate formation - carbonates decompose, 800-1100 °C; (2) fusion/dissolution of residual sand grains, up to about 1400 °C; (3) fining (refining) - removal of seeds and blisters at the hot spot, 1450-1580 °C; (4) homogenisation - removal of cords and striae by diffusion and convection; (5) conditioning - controlled cooling in the working end/forehearth to the forming viscosity.
• Key reactions: Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO₂; CaCO₃ → CaO + CO₂; Na₂SO₄ decomposes above about 1200 °C releasing SO₂ + O₂ for fining.
• Furnace types:
  • Pot furnace: 100 kg to a few tonnes, batch operation, for art glass, optical and special glass; flexible, low output.
  • Day tank: small continuous/semi-continuous tank, filled and worked out daily.
  • Continuous regenerative tank furnace: the industry standard - 50 to 700 t/day; melting end, throat, refining/working end, forehearths.
  • End-port versus side-port regenerative: end-port for smaller container furnaces (cheaper, compact); cross/side-fired for large float lines.
  • Recuperative furnace: continuous metallic/ceramic heat exchanger, lower efficiency than regenerators, used for mid-size units.
  • Oxy-fuel furnace: combustion with ~95% O₂ removes N₂, cutting flue volume, raising flame temperature and reducing thermal NOx by 70-90%; higher oxygen cost.
  • Electric (Joule) melting: electrodes pass current through the melt; used for fibre, borosilicate, special and small-capacity furnaces; low volatilisation, high power cost.
• Regenerator principle: two chequerwork chambers alternate every 20-30 minutes - one preheats incoming air to 1100-1300 °C while the other absorbs waste-gas heat; this recovery is what makes specific energy of 4-6 GJ/t achievable (theoretical heat of melting is only about 2.3-2.7 GJ/t).
• Refractories: silica crown (now often fused-silica or mullite), AZS (alumina-zirconia-silica) fusion-cast for the tank walls and throat, sillimanite/mullite superstructure, fireclay chequers in the cooler zones. Corrosion at the metal line and flux line cutting set the 8-12 year furnace campaign life.
• Melt defects: seed (<0.8 mm bubble), blister (large bubble), stone (unmelted batch, refractory, or devitrite crystal), cord/striae (composition inhomogeneity with a different refractive index), devitrification (crystallisation of devitrite Na₂O·3CaO·6SiO₂, tridymite or wollastonite in the holding-temperature range), tramp iron/nickel sulphide inclusions (a notorious cause of spontaneous breakage in toughened glass).

Forming processes

• Blowing: air inflates a gob/parison. Hand (off-hand) blowing with a blowpipe for art glass; IS (individual section) machine for containers - 6 to 20 sections, each a parison stage then a blow stage, running 300-700 containers/min across sections.
  • Blow-and-blow: for narrow-neck containers (bottles) - the parison is formed by settle-blow and counter-blow.
  • Press-and-blow: for wide-mouth jars and now narrow-neck press-and-blow (NNPB) for lightweight bottles - a plunger forms the parison, giving better glass distribution and 20-30% weight saving.
• Pressing: a plunger forces the gob into a mould - tumblers, lenses, insulators, TV panels, glass blocks. Only shapes that allow plunger withdrawal.
• Drawing (sheet): Fourcault (vertical draw through a debiteuse), Colburn/Libbey-Owens (drawn up then bent over a roller and annealed horizontally), Pittsburgh (draw bar, no debiteuse) - all largely obsolete for flat glass; tube drawing by the Danner process (gob flows over a rotating inclined mandrel, air blown down the mandrel) and the Vello process (vertical, bell and orifice ring) remain standard for tubing, ampoules and vials.
• Rolling: melt passes between water-cooled rollers; patterned/figured and wired glass; also used for rolled plate before grinding and polishing.
• Float process (Pilkington, 1959) - the dominant flat glass route:
  1. Melt leaves the furnace at about 1100 °C over a spout/canal onto a bath of molten tin.
  2. The tin bath is 50-60 m long, under a nitrogen + 4-10% hydrogen reducing atmosphere to stop tin oxidising to SnO₂ (which would mark the glass).
  3. Glass spreads to an equilibrium thickness of about 6.8 mm set by the balance of gravity and surface tension; top rollers (knurled edge rolls) stretch it thinner (down to 0.4-2 mm) or attenuate/hold it back for thicker ribbon (up to 19-25 mm).
  4. The ribbon cools to about 600 °C and is lifted off at the exit onto rollers, then into the annealing lehr.
  5. Output: fire-polished parallel surfaces needing no grinding or polishing - the reason float replaced plate glass entirely.
  6. On-line coating (CVD in the tin bath/lehr entrance) gives hard (pyrolytic) low-E and solar-control coatings; soft (sputtered) coatings are applied off-line.
• Fibre forming: continuous filament drawn through a platinum-rhodium bushing with 200-4000 orifices at 20-60 m/s (E-glass for FRP); rotary/spinner process for glass wool insulation.
• Other: casting (large telescope mirror blanks, ophthalmic), centrifugal casting (TV funnels, pipes), sagging/slumping over a mould, sintering of glass powder.

Annealing, tempering and chemical strengthening

• Why anneal: when glass cools, the surface stiffens first; by the time the interior contracts, the surface is already rigid. This leaves transient stress during cooling and permanent (residual) stress after cooling. Residual tensile stress at the surface causes delayed breakage, poor cutting and spontaneous cracking.
• Annealing = heating/holding the article in the annealing range (near the annealing point) so viscous flow relaxes stress, then cooling slowly through the strain point, after which cooling rate matters only for transient stress.
• Three-stage schedule: (1) heat/soak at the annealing point for a time set by thickness (minutes for containers, up to hours for thick optical blanks); (2) slow cool through the annealing-to-strain range at a controlled rate (the critical stage); (3) faster cool below the strain point to room temperature, limited only by thermal shock.
• Lehr: the continuous annealing oven - a roller or mesh-belt tunnel with zoned electric/gas heating and controlled cooling; container lehrs are 25-40 m, float lehrs 150-250 m long. Older annealing kilns were periodic.
• Stress measurement: stress is detected by photoelasticity (stress birefringence) with a polariscope/polarimeter; measured quantitatively with a Babinet or Friedel compensator or a Sénarmont compensator, expressed as optical path difference (nm/cm) and converted using the stress-optical coefficient (about 2.5-2.7 × 10⁻⁶ mm²/N, i.e. ~2.5 Brewster for soda-lime).
• Thermal tempering (toughening): heat the cut, edged, drilled sheet to about 620-680 °C (above Tg) and quench both surfaces with high-velocity air jets. The surface sets first in compression (80-150 MPa) with the core in balancing tension. Strength rises 4-5 times; on failure it dices into small blunt fragments - hence safety glass.
  • Tempered glass cannot be cut, drilled or edge-worked afterwards; all processing precedes toughening.
  • Heat-strengthened glass uses a slower quench - surface compression about 24-52 MPa, roughly twice annealed strength, breaks in larger pieces, not a safety glazing by itself.
  • Chemical tempering (ion exchange): immerse in molten KNO₃ at 400-500 °C below Tg; larger K⁺ replaces Na⁺ in a thin surface layer creating very high compression (to 700 MPa) in a shallow case. Used for aircraft glazing, ophthalmic and chemically strengthened cover glass; the thin case is vulnerable to deep scratches.
  • Laminated glass: two or more plies bonded with PVB (polyvinyl butyral) or EVA interlayer by autoclaving - fragments adhere to the interlayer; used for windscreens and overhead glazing. Laminated and tempered are the two classes of safety glass.
• Standards and practice (India): flat/float and safety glazing are covered by Bureau of Indian Standards specifications - transparent float glass (IS 14900), safety glazing for buildings (IS 2553 Part 1) and for road vehicles (IS 2553 Part 2), toughened glass and laminated glass requirements with the fragmentation and impact tests they prescribe. Vehicle glazing is also type-approved under the AIS/CMVR route. Glass bottles and jars have their own BIS specifications and food-contact articles are regulated by FSSAI. Container glass falls under the Extended Producer Responsibility regime of the Plastic Waste Rules only for plastic closures; glass cullet recycling is handled under the Solid Waste Management Rules, 2026 (which replaced the 2016 Rules from 1 April 2026). Furnace emissions (SPM, SO₂, NOx) are regulated by CPCB standards for the glass industry under the Environment (Protection) Act, 1986; glass manufacturing is a listed category needing consent to establish and operate from the State Pollution Control Board. Workplace safety, hours and welfare in glass works now come under the OSH&WC Code, 2020 and the other three Labour Codes in force from 21 November 2025, which replaced the Factories Act, 1948. Since exact IS numbers and editions are revised periodically, confirm the current edition before quoting it in a specification. Energy performance of glazing in buildings is set through the ECSBC 2024 (which succeeds ECBC 2017) via U-value and SHGC limits.

Properties, testing and comparisons

• Thermal expansion α: soda-lime 8-9 × 10⁻⁶ /K; borosilicate 3.3 × 10⁻⁶ /K; fused silica 0.55 × 10⁻⁶ /K - the basis of borosilicate's thermal-shock resistance and of sealing glass matching to metals (Kovar, molybdenum).
• Mechanical: theoretical strength several GPa; practical 30-100 MPa because of Griffith surface flaws; brittle, no plasticity at room temperature; E ≈ 70 GPa, density 2500 kg/m³ for soda-lime; hardness 5.5-6.5 Mohs.
• Static fatigue: strength falls with time under sustained load because water promotes stress-corrosion crack growth at flaw tips.
• Chemical durability: tested by hydrolytic resistance classes (Type I borosilicate, Type II surface-treated, Type III soda-lime) for pharmaceutical containers - the Indian Pharmacopoeia 2026 (10th edition) prescribes the glass-container tests for packaging.
• Optical: refractive index 1.52 (soda-lime), 1.47 (borosilicate), up to 1.7+ with PbO/BaO; transmittance, haze, and residual-stress birefringence are the routine quality checks.
• Routine tests: seed/stone count, annealing quality by polariscope, thermal shock test, internal pressure and vertical load test for bottles, fragmentation test for toughened glass, boil/impact test for laminates, and dimensional/optical distortion (zebra board) for float.

Key points

• Glass - amorphous solid from fusion, cooled without crystallising; has Tg and a viscosity-temperature curve, not a melting point.
• Network former/modifier - SiO₂, B₂O₃ build the network; Na₂O, CaO break bridging oxygens and lower viscosity and Tg.
• Working point - 10³ Pa·s, the viscosity at which glass can be shaped; working range lies between working and softening points.
• Annealing point - 10¹²·⁴ Pa·s, stress relaxes in about 15 minutes; strain point 10¹³·⁵ Pa·s, stress relaxes in hours.
• Melting stages - batch melting, fusion, fining, homogenisation, conditioning; fining at the hot spot 1450-1580 °C.
• Fining agent - sulphate, antimony/arsenic oxide or cerium oxide releasing gas so small seeds coalesce and rise.
• Cullet - recycled glass that saves roughly 2-3% energy per 10% addition and reduces batch dust and CO₂.
• Regenerator - paired chequerwork chambers reversing every 20-30 min, preheating air to 1100-1300 °C; specific energy 4-6 GJ/t.
• AZS fusion-cast refractory - standard tank-wall material; flux-line corrosion limits campaign life to 8-12 years.
• Float process - ribbon on molten tin under N₂-H₂; equilibrium thickness about 6.8 mm, varied by top rollers; both faces fire-polished.
• IS machine - container forming by blow-and-blow (narrow neck), press-and-blow, or NNPB for lightweighting.
• Danner/Vello - tube-drawing processes for tubing, vials and ampoules.
• Annealing - soak near the annealing point then cool slowly through the strain point to remove permanent stress.
• Tempering - heat above Tg and air-quench; surface compression 80-150 MPa, strength 4-5×, dices on breakage; cannot be cut afterwards.
• Ion exchange - K⁺ for Na⁺ below Tg giving up to about 700 MPa in a shallow case.
• Devitrification - unwanted crystallisation (devitrite, tridymite) when glass is held in the nose of its crystallisation range.
• Griffith flaws - surface cracks that reduce strength from GPa to tens of MPa; surface quality decides practical strength.
• Photoelastic stress measurement - polariscope plus compensator; stress-optical coefficient about 2.5 Brewster for soda-lime.

Formulas and examples

Example 1 - Batch calculation for soda-lime glass (no cullet). Target per 100 kg glass: SiO₂ 73 kg, Na₂O 14 kg, CaO 9 kg, MgO 4 kg. Raw materials: pure sand (SiO₂), soda ash (Na₂CO₃, M = 106), limestone (CaCO₃, M = 100), magnesite (MgCO₃, M = 84.3).

• Na₂O fraction in Na₂CO₃ = 62/106 = 0.585 → soda ash = 14/0.585 = 23.94 kg.
• CaO in CaCO₃ = 56/100 = 0.56 → limestone = 9/0.56 = 16.07 kg.
• MgO in MgCO₃ = 40.3/84.3 = 0.478 → magnesite = 4/0.478 = 8.37 kg.
• Sand = 73.0 kg.
• Batch total = 73.0 + 23.94 + 16.07 + 8.37 = 121.38 kg → glass yield 100 kg.
• Batch-to-melt ratio = 1.214; LOI = 21.38/121.38 = 17.6%, all of it CO₂.

Example 2 - Effect of 50% cullet on the same glass. Use 50 kg of cullet of identical composition; it supplies 50% of every oxide, so the virgin batch must supply only SiO₂ 36.5, Na₂O 7, CaO 4.5, MgO 2 kg.

• Sand 36.5; soda ash 7/0.585 = 11.97; limestone 4.5/0.56 = 8.04; magnesite 2/0.478 = 4.18.
• Virgin batch = 60.69 kg; plus 50 kg cullet = 110.69 kg total charge for 100 kg glass (against 121.38 kg).
• Charge reduction = 8.8%; CO₂ released falls from 21.38 to 10.69 kg per 100 kg glass, i.e. 50% less process CO₂.
• Energy: at 2.5% saving per 10% cullet, 50% cullet gives about 12.5% lower specific energy - a 4.8 GJ/t furnace falls to about 4.2 GJ/t.

Example 3 - Bubble rise time during fining. A seed of radius 0.10 mm in soda-lime glass at 1450 °C, η = 10 Pa·s, ρ = 2300 kg/m³, depth of melt 1.0 m.

• r = 1.0 × 10⁻⁴ m → r² = 1.0 × 10⁻⁸ m².
• v = 2 × 9.81 × 1.0×10⁻⁸ × 2300 /(9 × 10) = (4.512 × 10⁻⁴)/90 = 5.01 × 10⁻⁶ m/s.
• Time to rise 1 m = 1/5.01×10⁻⁶ ≈ 2.0 × 10⁵ s ≈ 55 hours - far longer than the residence time, which is exactly why fining agents are used to grow the bubble. If the fining agent grows it to r = 0.5 mm, r² rises 25×, v = 1.25 × 10⁻⁴ m/s and rise time falls to about 2.2 hours.

Example 4 - Thermal stress and why annealing is needed. Soda-lime glass: E = 70 GPa, α = 9 × 10⁻⁶ /K, ν = 0.22, tensile strength σ_f = 40 MPa.

• Maximum tolerable ΔT: ΔT = σ_f (1 − ν)/(E α) = (40 × 10⁶ × 0.78)/(70 × 10⁹ × 9 × 10⁻⁶) = 31.2 × 10⁶/630 × 10³ = 49.5 K.
• A borosilicate with α = 3.3 × 10⁻⁶ /K, E = 64 GPa, σ_f = 40 MPa, ν = 0.20: ΔT = (40 × 10⁶ × 0.80)/(64 × 10⁹ × 3.3 × 10⁻⁶) = 32 × 10⁶/211.2 × 10³ = 151 K - about three times better, which is why laboratory ware is borosilicate.

Example 5 - Reading residual stress from a polariscope. A 10 mm thick annealed plate viewed edge-on over a 5 cm path shows an optical path difference of 60 nm. Stress-optical coefficient C = 27 nm/(cm·MPa) (i.e. 2.7 Brewster).

• σ = δ/(C × d) = 60/(27 × 5) = 60/135 = 0.44 MPa.
• Compared with a typical acceptance limit of a few MPa for annealed glass, this is comfortably within tolerance - acceptable for general glazing but too high for precision optics, which are re-annealed with a slower schedule through the annealing-to-strain range.

Example 6 - Furnace pull and melting rate. An end-port container furnace has a melter area of 60 m² and pulls 180 t/day at a specific energy of 4.5 GJ/t.

• Melting rate = 180/60 = 3.0 t/m²/day (normal for a container furnace).
• Daily energy = 180 × 4.5 = 810 GJ/day; with natural gas at 38 MJ/m³ this is 810 × 10³/38 ≈ 21,300 m³/day.
• At an illustrative ₹45/m³ for industrial natural gas, fuel cost ≈ ₹9.6 lakh/day, about ₹5,300 per tonne of glass - gas prices vary by region and contract, so treat the rupee figure as indicative only.

Where it is used

• Flat glass plants: Asahi India Glass, Saint-Gobain India (Sriperumbudur, Bhiwadi), Gold Plus Glass (Roorkee, Bhuj) - float lines, tin-bath operation, lehr control, on-line coating; roles as furnace engineer, float-bath engineer, process/quality engineer.
• Container glass: Hindusthan National Glass, AGI Greenpac, Piramal Glass, Borosil - IS machine setting, NNPB lightweighting, mould shop, cold-end coating and inspection; batch-house and cullet management roles.
• Pharma and lab glass: Schott Kaisha/Schott Poonawalla, Borosil, Klass Pack - Danner/Vello tubing, ampoule and vial conversion, hydrolytic-resistance testing to Indian Pharmacopoeia 2026 requirements.
• Processing and safety glazing: independent toughening and lamination units supplying construction and the automotive industry (windscreens to IS 2553 Part 2 and the AIS/CMVR route); architectural glazing designed to ECSBC 2024 U-value and SHGC limits.
• Fibre and insulation: glass-fibre reinforcement for FRP pipes, wind-turbine blades and PCBs; glass wool insulation (Owens Corning India, UP Twiga).
• Specialty and R&D: solar-glass lines for PV modules under the domestic manufacturing push, CSIR-CGCRI Kolkata (Central Glass and Ceramic Research Institute) for optical, bioactive and specialty glasses; refractory suppliers serving glass furnaces.

Interview questions

• Q: Why does glass have no fixed melting point? A: It is amorphous, so there is no crystal lattice to break down at one temperature; its viscosity falls continuously with temperature through a glass transition range.
• Q: What viscosity defines the annealing point and the strain point, and why do both exist? A: 10¹²·⁴ Pa·s and 10¹³·⁵ Pa·s. The annealing point is where stress relaxes in about 15 minutes, the strain point where it takes hours - below it the glass behaves elastically, so the schedule only needs to be slow between them.
• Q: Why is the equilibrium thickness of float glass about 6.8 mm? A: It is the thickness at which gravity spreading the ribbon balances surface tension contracting it on the molten tin; thinner or thicker ribbon is produced by stretching or restraining with top rollers.
• Q: Why is the float bath atmosphere nitrogen with a little hydrogen? A: It is reducing, so the molten tin does not oxidise to SnO₂, which would float as dross and mark the bottom surface of the ribbon.
• Q: What does a fining agent do, since bubbles already rise? A: Stokes' law makes rise speed proportional to the square of the radius; the agent releases gas that grows small seeds into large bubbles that rise fast enough to leave within the furnace residence time.
• Q: Can tempered glass be drilled on site? A: No. All cutting, edging and drilling must be done before toughening, because the surface compression layer is what holds the stress balance - cutting it releases the core tension and the pane dices.
• Q: What is the difference between annealing and tempering? A: Annealing removes residual stress by slow cooling through the annealing range; tempering deliberately freezes in surface compression by quenching, which quadruples apparent strength.
• Q: Why does cullet reduce energy consumption? A: It is already-reacted glass - it needs no decomposition of carbonates and no heat of reaction, only sensible heat plus heat of fusion.
• Q: What causes devitrification and how is it avoided? A: Holding the melt in its crystallisation range (devitrite, tridymite, wollastonite nucleate); it is avoided by designing composition away from the liquidus with MgO and Al₂O₃ additions and by not stalling the glass at the critical temperature in forehearths and feeders.
• Q: Why do toughened glass panels sometimes shatter spontaneously years after installation? A: Nickel sulphide inclusions from contamination slowly change volume in the tension zone, and in tempered glass the stored tension means a tiny flaw triggers full fragmentation; proper heat-soak testing screens such panels.
• Q: Which is stronger - thermally tempered or chemically strengthened glass, and what is the catch? A: Chemical strengthening gives higher surface compression (up to about 700 MPa against 150 MPa), but in a very shallow layer, so a deep scratch penetrates past it; thermal tempering has a case about a fifth of the thickness.
• Q: Why is borosilicate used for laboratory glassware? A: Its thermal expansion is about 3.3 × 10⁻⁶ /K against 9 × 10⁻⁶ /K for soda-lime, so the thermal stress for a given temperature difference is roughly a third, and its hydrolytic resistance is Type I.
• Q: What is the role of the regenerator in a tank furnace? A: Paired chequerwork chambers alternately store waste-gas heat and preheat combustion air to 1100-1300 °C, cutting specific energy to roughly 4-6 GJ/t.

Common mistakes

• Quoting a melting point for glass, or treating the softening point as a melting point; glass processes are defined by viscosity windows, not temperatures.
• Forgetting loss on ignition in batch calculations, so the batch mass equals the glass mass - the batch must be about 15-20% heavier.
• Mixing up the annealing point and the strain point, or thinking the slow cooling must continue to room temperature; below the strain point only thermal shock limits the rate.
• Using poise and Pa·s interchangeably: 1 Pa·s = 10 poise, so 10¹³·⁴ poise = 10¹²·⁴ Pa·s.
• Claiming annealing makes glass stronger - it removes residual stress and makes the glass predictable; tempering is what raises strength.
• Specifying tempered glass and then detailing on-site cutting, notching or drilling, or omitting heat-soak testing for structural glazing.
• Ignoring the square dependence of bubble rise velocity on radius, and so missing the point of fining agents.
• Treating cullet as a free input - contamination by ceramic, stones, metal or wrong colour creates stones and off-colour glass worse than the saving.

Related topics

• Cement, glass and ceramics industries · Whitewares, tiles, sanitaryware and glazes · Advanced ceramics: electronic, structural and bioceramics · Refractories and high-temperature furnace design · Phase diagrams, glass-ceramics and crystallisation kinetics · Heat transfer and thermal stress analysis.