Overview

Soda-lime glass is the most common form of glass (~90% of all glass produced), composed of approximately 70-75% silica (SiO₂), 12-15% soda (Na₂O), and 8-12% lime (CaO). It is optically transparent, inexpensive, chemically inert, and easily formed — the material of windows, bottles, jars, and light bulbs. As an amorphous ceramic, it exhibits brittle fracture at room temperature but becomes viscous above its glass transition temperature (~550°C).

ρ = 2500 kg/m³ (2.5 g/cm³) — higher than most polymers and light metals, lower than steel

Mechanical Properties (annealed soda-lime glass)

Property	Value
Density	2,500 kg/m³ (0.090 lb/in³)
Young's Modulus	70 GPa (similar to aluminum)
Compressive Strength	900-1000 MPa (very high)
Tensile Strength (theoretical)	~200 MPa
Tensile Strength (actual, annealed)	30-50 MPa (flaw-limited)
Poisson's Ratio	0.22
Hardness (Knoop)	400-500 HK
Fracture Toughness (K_IC)	0.7 MPa·√m (very low, brittle)
Elongation	0% (zero plasticity)

Strong in Compression, Weak in Tension

Glass is a classic brittle material with ~20x higher compressive strength than tensile strength. Surface microcracks propagate in tension (Griffith fracture criterion), so practical glass strength is governed by surface flaws and edge condition — not inherent material strength. Tempering puts the surface in compression, dramatically increasing effective strength.

Thermal Properties

Property	Value
Glass Transition (T_g)	520-560°C
Annealing Point	~500°C
Softening Point	~700°C
Melting Temperature	~1500°C (processing)
Thermal Conductivity (20°C)	1.0 W/m·K (insulator)
Specific Heat	800 J/kg·K
Coefficient of Thermal Expansion	8-9 × 10⁻⁶ /°C
Thermal Shock Resistance	Poor for soda-lime

Optical Properties

Visible light transmittance: 85-90% (clear glass, 3mm thickness)
Refractive index: 1.52
UV transmission: <10% below 300nm (absorbs UV)
IR absorption: absorbs strongly above 2.5 μm (greenhouse effect)

Types & Tempered/Toughened Variants

Type	Description	Strength
Annealed	Basic, slowly cooled for low stress	30-50 MPa tensile
Tempered (toughened)	Quenched to put surface in compression	120-200 MPa
Laminated	Two layers with PVB interlayer (safety)	Same as annealed, but shards retained
Heat-strengthened	Partial temper, intermediate strength	60-90 MPa
Borosilicate (Pyrex)	Low expansion, thermal shock resistant	Similar strength

Applications

Construction

Float glass windows (flat glass produced by the Pilkington float process)
Insulating glass units (double/triple glazing with argon fill)
Tempered glass for doors, showers, railings (safety glazing)
Laminated glass for skylights, hurricane zones, automotive windshields

Packaging

Bottles and jars (beverages, food, pharmaceuticals) — chemically inert, no flavor contamination
Infinitely recyclable (re-melt without quality loss)

Industrial

Sight glasses in process vessels and piping (use borosilicate, not soda-lime, for thermal shock)
Laboratory glassware (borosilicate — Pyrex/Kimax)
Light bulbs (soda-lime for standard; fused silica for halogen/quartz-halogen)
Tubing, rods, gauge glass

Specialty

Fiberglass (E-glass, S-glass for composites)
Optical fibers (high-purity fused silica, different composition)
Display glass (aluminosilicate, thin, chemically strengthened — Gorilla Glass class)

Thermal Shock is a Glass Killer

Soda-lime glass breaks from rapid temperature change because thermal gradients create differential expansion. The classic example: pouring boiling water into a cold glass bottle fails at stresses well below design strength. Borosilicate glass (Pyrex) has ~3x lower thermal expansion and resists thermal shock, making it the correct choice for sight glasses, laboratory ware, and any service involving rapid temperature changes.

Design Considerations

Edge condition critical: cut edges have microcracks that dominate strength; polished, seamed, or rounded edges much stronger
No plastic deformation: glass fails catastrophically without warning (design tempered/laminated for safety-critical)
Static fatigue: static load over time increases failure probability (delayed fracture from slow crack growth in humidity)
Design stress: use 7-15 MPa allowable tensile for long-term annealed service (safety factor of 3-5)
Never load glass in bending from the edge

Summary

Soda-lime glass (density 2500 kg/m³) is the least expensive, most common glass — transparent, chemically inert, and easily formed. Its engineering use is governed by its brittle nature and flaw-controlled tensile strength (30-50 MPa annealed, 120-200 MPa tempered). Glass is strong in compression but vulnerable to tensile stress from surface flaws, impact, and thermal shock. Always use borosilicate for sight glasses or thermal shock service, tempered/laminated for safety-critical applications, and respect its brittle fracture design rules.

Glass (Soda-Lime)