Float Glass (Clear) — Complete Guide

Float glass is the most widely used flat glass in modern construction. The name comes from the Pilkington Float Process — molten glass is "floated" on a bath of molten tin to produce an exceptionally flat, distortion-free sheet with parallel surfaces. Over 90% of all flat glass produced globally today is float glass. It is the base substrate for almost every other glass product — toughened, laminated, tinted, mirror, insulated and coated low-E glass all start as plain float.

Indian float glass is governed by IS 14900 (Transparent Float Glass — Specification). Major Indian manufacturers: Saint-Gobain Glass India (Sezigny, Chennai), Asahi India Glass (AIS), Gold Plus Glass, Sisecam Flat Glass (formerly Hindustan National Glass) and Modiguard / HNG Float Glass.

What is Float Glass?

Float glass is a sheet of glass made by floating molten glass on a bed of molten metal (tin), giving it a uniform thickness and very flat surfaces. It is also called annealed glass because after forming, it is slowly cooled (annealed) to release internal stresses.

Key Characteristics

Perfectly flat — no optical distortion
High transparency (87–91% light transmission for 4–6 mm clear)
Smooth, parallel surfaces (no grinding / polishing required)
Available in thicknesses 2 mm to 25 mm
Maximum sheet size: 3210 mm x 6000 mm (jumbo)
Base for further processing (toughening, lamination, coating)

Pilkington Float Process — Manufacturing

Developed by Sir Alastair Pilkington in 1959, this is the global standard for flat glass manufacturing. It transformed the industry by eliminating the costly grinding and polishing that earlier plate-glass needed.

Raw Material Batching: Silica sand (72%), soda ash (13%), dolomite + limestone (10%), saltcake / cullet (5%) are weighed and blended.
Melting (1500–1600 °C): Batch enters a regenerative furnace, melts into homogeneous molten glass over 24–48 hours.
Float Bath (1100 → 600 °C): Molten glass flows onto a 50 m long bath of molten tin in a nitrogen + hydrogen atmosphere. Glass spreads to its natural equilibrium thickness (~6 mm) and is stretched or compressed to required thickness using "top rollers".
Annealing Lehr (600 → 60 °C): Glass passes through a 120 m lehr where it cools slowly to relieve internal stresses.
Inspection & Cutting: Online optical sensors check for bubbles, inclusions and distortion. The endless ribbon is cut to PLF (Plate Lehr Format) or DLF (Deck Lehr Format) sizes.
Stacking & Packing: Glass is stacked vertically with interleaving powder (lycopodium or polymer) to prevent staining.

                Why a tin bath? Tin has a melting point of 232 °C and a boiling point of 2270 °C, giving a wide liquid range. It has a higher density than glass (so glass floats), is non-reactive with glass under controlled atmosphere, and produces a perfectly flat lower glass surface that mirrors the tin surface.
            

Chemical Composition (Soda-Lime Float Glass)

Component	% by Weight	Source / Role
Silica (SiO&sub2;)	70–74%	Silica sand — glass former
Sodium Oxide (Na&sub2;O)	12–16%	Soda ash — flux, lowers melting point
Calcium Oxide (CaO)	5–11%	Limestone — stabiliser, water resistance
Magnesium Oxide (MgO)	1–4%	Dolomite — durability
Aluminium Oxide (Al&sub2;O&sub3;)	0.5–2%	Improves chemical resistance
Iron Oxide (Fe&sub2;O&sub3;)	0.05–0.15%	Impurity — gives green tint
Sulphur Trioxide (SO&sub3;)	0.2–0.4%	Saltcake — refining agent

Low-iron / Extra-clear float (e.g. Saint-Gobain DIAMANT, AIS Krystal) keeps Fe&sub2;O&sub3; below 0.015% to eliminate the green tint — ideal for glass facades, jewellery display, solar applications.

Standard Thicknesses & Applications

Thickness	Typical Application	Max Pane Size (mm)
2.0 mm	Photo frames, instrumentation	1200 x 1800
3.0 mm	Cabinets, frames, small windows	1500 x 2000
4.0 mm	Internal partitions, small windows	1800 x 2500
5.0 mm	Windows, doors, table tops	2200 x 3000
6.0 mm	Windows, shower screens, shopfronts	2400 x 3600
8.0 mm	Large windows, table tops, partitions	2700 x 4000
10.0 mm	Frameless doors, large facades	3000 x 5000
12.0 mm	Structural glazing, balustrades	3210 x 6000
15–19 mm	Heavy structural, aquariums	3210 x 6000
25 mm	Bullet resistant, structural (rare)	3210 x 5000

Physical Properties

Property	Value
Density	2500 kg/m³ (2.5 g/cm³)
Refractive Index (n)	1.52
Light Transmission (4 mm)	89–91%
Light Transmission (6 mm)	87–89%
Solar Heat Gain (SHGC)	0.81–0.85 (5–6 mm)
U-Value (single glaze)	5.7–5.9 W/m²K
Tensile Strength	30–50 MPa (annealed)
Compressive Strength	800–1000 MPa
Young's Modulus (E)	70–72 GPa
Poisson's Ratio	0.22
Hardness (Mohs)	5.5–6.5
Knoop Hardness	600 kg/mm²
Coefficient of Thermal Expansion	9 x 10−6 /°C
Thermal Conductivity	0.96 W/m.K
Softening Point	720–730 °C
Annealing Point	545–555 °C
Strain Point	505–515 °C
Specific Heat	0.84 kJ/kg.K
Sound Reduction Index (6 mm)	~30 dB (Rw)

Types of Float Glass

1. Clear Float

Standard transparent float with slight greenish tint due to iron content. Most commonly used grade. Light transmission 87–91%.

2. Extra-Clear / Low-Iron Float

Ultra-pure silica + decolouriser; iron content < 0.015%. Light transmission 91–93%. Used for facades, premium showcases, solar panels. Brands: Saint-Gobain DIAMANT, AIS Krystal, Gold Plus Diamond Clear.

3. Tinted Float

Iron, cobalt, selenium or other oxides added to glass batch to produce green, bronze, grey, blue tints. Reduces solar heat & glare. See Tinted & Reflective Glass.

4. Patterned / Figured Float

Texture rolled into one face (linen, chequered, rain pattern) during forming. Provides obscurity while transmitting light.

5. Wired Float

Wire mesh embedded during forming. Provides fire integrity & safety against falling shards. Now largely replaced by laminated glass.

6. Mirrored Float

Float glass with silver coating + protective lacquer. Used for mirrors and decorative panels.

7. Heat-Strengthened (HS)

Float heated and air-cooled at lower rate than toughened glass. 2 times stronger than annealed; breaks into pieces (not granules). Used in facades where toughened is excessive.

Top Indian Brands

Brand	Products	Plant Locations
Saint-Gobain Glass India	SGG CLEAR, DIAMANT, PARSOL, COOL-LITE	Sriperumbudur (TN), Bhiwadi (RJ)
Asahi India Glass (AIS)	AIS Clear, Krystal, Tinted, Mirror, Sunshield	Taloja, Roorkee, Chennai
Gold Plus Glass Industry	Gold Plus Clear, Diamond Clear, Tinted	Roorkee, Sundergarh
Sisecam Flat Glass (HNG)	Modiguard Clear, Tinted, Reflective	Halol (Gujarat)
Sejal Glass	Sejal Clear, Designer, Decorative	Jagatpur (Gujarat)
Triveni Glass	Triveni Float Glass	Allahabad (UP)
FG Glass / FuYao	FG Float, Automotive	Visakhapatnam
Guardian Industries	Guardian Clear & Tinted (imported)	Imported
Pilkington (NSG Group)	Pilkington Clear, OptiView	Imported

Float Glass vs Older Sheet Glass

Parameter	Float Glass	Sheet Glass (old)	Plate Glass (old)
Process	Float on tin bath	Drawn vertically	Cast + ground + polished
Flatness	Excellent (mirror-like)	Poor — visible distortion	Excellent but very costly
Cost	Moderate	Low	Very high
Thickness Range	2–25 mm	1.5–6 mm	3–25 mm
Max Size	3210 x 6000 mm	1500 x 2400 mm	3000 x 5000 mm
Current Status	Industry standard	Obsolete	Obsolete (replaced by float)

Where to Use Float Glass

Residential windows & doors (5–6 mm)
Internal partitions & balustrades (after toughening)
Shopfronts & showcase windows
Office cabin glazing
Furniture: table tops, shelves, cabinet doors (after edge polishing)
Mirrors & decorative panels
Greenhouses & conservatories
Aquariums (10–19 mm low-iron)
Photo frames, picture glazing
Substrate for toughened, laminated, frosted, tinted / reflective and DGU processing
Solar PV cover glass (low-iron tempered)
Automotive (PVB laminated windscreens)
Appliance fronts (oven doors, fridge shelves — after tempering)

Advantages of Float Glass

Excellent optical clarity — no distortion
Very high light transmission (87–93%)
Smooth, parallel surfaces ready for use without finishing
Available in a wide thickness range
Base material for all secondary processing
Recyclable (cullet reused in next batch)
Chemically inert — resists acids (except HF), alkalis, water
Hygienic, non-porous
Cost-effective compared to plate or specialty glass
Long lifespan (50+ years if undamaged)
Available in jumbo sizes up to 3210 x 6000 mm
Can be cut, drilled, edge-polished, sandblasted on site
Compatible with all standard glazing systems
UV transmission ~75% (clear, 6 mm)

Disadvantages of Annealed Float Glass

Brittle — breaks into large sharp shards (not safety glass)
Low impact resistance
Poor thermal shock resistance (cracks at ΔT > 40 °C)
High solar heat gain (SHGC 0.85) — hot interiors in summer
High U-value (single 6 mm = 5.8 W/m²K) — poor insulation
Cannot be used in safety-critical locations without toughening / lamination
Slight green tint (clear grade) limits use in display
Heavy — 6 mm = 15 kg/m², requires strong supporting structure
Edge damage > centre damage; chipped edges propagate cracks
Tin-side / air-side difference matters for coating & bonding

Common Defects in Float Glass

Bubbles / Seeds: Trapped gas during melting
Stones / Inclusions: Unmelted batch / refractory particles
Knots / Cords: Local variation in chemical composition
Wave / Distortion: Uneven thickness causing zoom-distortion
Scratches: Surface damage from handling or transit
Tin Stain (Bloom): Tin oxide pickup on the tin-side face (rare)
Edge Chips: Damage from cutting / handling
Hairline Cracks: Internal stresses (poor annealing)
Ream: Striations / waviness visible at angle
Coating Defects (mirrored): Spots / pinholes

IS 14900 specifies allowable defect sizes by glass quality grade (Q1, Q2, Q3).

Tests on Float Glass

Visual Inspection — bubble, stone, scratch (IS 14900 Annex A)
Thickness Measurement — micrometer / ultrasonic
Optical Distortion (Zebra Board / Schlieren)
Light Transmission — spectrophotometer (ISO 9050)
Tin-Side Identification — UV lamp (tin face fluoresces)
Refractive Index — Abbe refractometer
Density — Archimedes method
Hardness — Vickers / Knoop
Flexural Strength — 4-point bending (EN 1288)
Thermal Shock — quench test
Chemical Durability — water / acid / alkali resistance
Solar Optical — reflectance, absorptance, SHGC (ISO 9050)

Cost (Approximate, Indian Market, 2025-26)

Thickness	Clear Float (Rs/sqft)	Extra-Clear (Rs/sqft)
3 mm	40 – 55	55 – 80
4 mm	55 – 75	75 – 110
5 mm	70 – 95	110 – 150
6 mm	85 – 120	140 – 200
8 mm	140 – 200	220 – 320
10 mm	180 – 280	320 – 460
12 mm	250 – 360	450 – 650
15 mm	380 – 520	700 – 950
19 mm	650 – 950	1200 – 1700

Note: Prices are for plain float, ex-factory. Add transport (Rs 5–15/sqft), edge work (Rs 10–25/sqft), drilling (Rs 100–200/hole) and installation (Rs 40–120/sqft) for delivered & fixed cost.

Best Practices for Selection & Use

For safety-critical glazing (doors, low-level windows, balustrades, shower enclosures) use toughened or laminated glass — never plain annealed float
Specify thickness as per pane size — refer IS 3548 / EN 1991-1-4 for wind load
Ensure tin-side is identified for mirror & coating processes (always coat / silver air-side)
For energy-efficient glazing, use DGU with low-E coated float
Store glass vertically (5°–7° tilt), edges on padded battens, never flat
Keep interleaving powder dry to avoid stain — do not store outdoors
For facade applications, use heat-soak tested toughened glass to avoid NiS spontaneous failure
Edges should be polished, ground or arrissed — never raw cut on exposed surfaces
Use compatible setting blocks, EPDM gaskets and structural silicone (Dow / Sika / 3M)
Provide edge clearance ~3 mm + glass tolerance to avoid edge bite stress
Allow thermal expansion gap (1 mm per 1000 mm of pane)
For low-iron facade glass, specify Fe&sub2;O&sub3; < 0.015%
For tropical climate, prefer green-tinted or low-E coated float to cut solar gain
Always demand IS 14900 BIS certification mark and manufacturer test certificate
Maintain a 3-mm float glass strain-edge clearance for laminated assembly
Do not weld, grind or use a flame within 1 m of installed glass

Applicable Standards

Standard	Description
IS 14900	Transparent Float Glass — Specification
IS 2835	Flat Transparent Sheet Glass — Specification (older)
IS 3548	Code of Practice for Glazing in Buildings
IS 16231	Glass in Building — Determination of Light Transmittance
EN 572-1/2	Glass in Building — Basic Soda Lime Silicate Glass Products
EN 572-8	Float Glass — Sizes & Tolerances
EN 1288-3	Determination of Bending Strength of Glass
EN 12150	Thermally Toughened Soda Lime Silicate Safety Glass
ISO 9050	Glass in Building — Determination of Light & Solar Properties
ASTM C1036	Standard Specification for Flat Glass
ASTM C1048	Heat-Treated Flat Glass
NBC 2016 Part 6 Sec 5	National Building Code — Glass & Glazing

Conclusion

Float glass is the foundation of modern glazing — the substrate from which every advanced glass product is processed. For Indian residential and commercial buildings, the practical selection sequence is: choose the right thickness for the pane size and wind load, then choose a quality grade (Q1 for facades, Q2 for windows, Q3 for furniture), then upgrade to toughened or laminated as safety demands, and finally to double-glazed units for thermal & acoustic performance.

For aesthetic privacy, choose frosted or etched glass; for heat & glare control, choose tinted or reflective glass. For window frames that hold this glass, see UPVC windows or aluminium sliding windows.

Float Glass (Clear)