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).
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) |
Specific Gravity
Specific gravity (SG) is the ratio of a material's density to water at 4°C (1000 kg/m³):
Soda-lime glass has SG ≈ 2.5 — about 2.5x heavier than the same volume of water, similar to aluminum (2.7) and lighter than steel (7.85). Borosilicate glass is slightly lighter (SG ≈ 2.23), while fused silica is ~2.20. Specific gravity is used to convert between weight and volume in handling, packaging, and structural design calculations.
Unit Weight
Unit weight (weight density) is the weight per unit volume — density multiplied by gravitational acceleration:
| Glass Type | Unit Weight |
|---|---|
| Soda-lime | 24.5 kN/m³ (156 lb/ft³) |
| Borosilicate (Pyrex) | 21.9 kN/m³ (139 lb/ft³) |
| Fused silica | 21.6 kN/m³ (137 lb/ft³) |
| Tempered (same composition) | 24.5 kN/m³ (156 lb/ft³) |
Use unit weight when calculating glass panel dead loads, glazing support reactions, and shipping/transport weights.
Unit Conversion Reference
| Quantity | Conversion |
|---|---|
| Density | 2500 kg/m³ = 2.5 g/cm³ = 0.0903 lb/in³ = 156 lb/ft³ |
| 1 kg/m³ | = 0.06243 lb/ft³ |
| 1 g/cm³ | = 1000 kg/m³ |
| 1 lb/ft³ | = 16.018 kg/m³ |
| Thickness | 1 mm = 0.0394 in; 6mm glass = 15 kg/m² panel weight |
Glass panel weight per area: 6mm float glass ≈ 15 kg/m²; 10mm ≈ 25 kg/m²; 12mm ≈ 30 kg/m². Quick estimate: panel weight (kg/m²) ≈ 2.5 × thickness (mm).
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 |
Composition Comparison: Soda-Lime vs Borosilicate vs Fused Silica
| Property | Soda-Lime | Borosilicate (Pyrex) | Fused Silica / Quartz |
|---|---|---|---|
| SiO₂ content | 70-75% | 80-81% | >99.9% |
| Density (kg/m³) | 2,500 | 2,230 | 2,200 |
| Thermal expansion (×10⁻⁶/°C) | 8-9 | 3.2-3.3 | 0.55 |
| Thermal conductivity (W/m·K) | 1.0 | 1.1 | 1.4 |
| Max continuous service temp | ~450°C | ~500°C | ~1000°C |
| Thermal shock resistance | Poor | Good | Excellent |
| Relative cost | Lowest | Moderate | Highest |
| Typical uses | Windows, bottles, containers | Labware, sight glasses, cookware | Optical fibers, quartz lamps, semiconductor |
Fused silica (also called quartz glass) is the highest-performance engineering glass: its near-zero thermal expansion (0.55 × 10⁻⁶/°C) makes it virtually immune to thermal shock, and it survives continuous service above 1000°C. It is the material of choice for optical fibers, high-temperature lamp envelopes, and semiconductor processing. Borosilicate is the practical middle ground for process sight glasses and laboratory ware.
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)
Glass vs Steel vs Aluminum
| Property | Soda-Lime Glass | Structural Steel (A36) | Aluminum 6061-T6 |
|---|---|---|---|
| Density (kg/m³) | 2,500 | 7,850 | 2,700 |
| Young's Modulus (GPa) | 70 | 200 | 69 |
| Tensile Strength (MPa) | 30-50 (annealed) | 400-550 | 310 |
| Compressive Strength (MPa) | 900-1000 | ~250 (yield) | ~276 (yield) |
| Thermal Conductivity (W/m·K) | 1.0 | 50 | 167 |
| Thermal Expansion (×10⁻⁶/°C) | 8-9 | 12 | 23 |
| Fracture Behavior | Brittle (no yield) | Ductile | Ductile |
| Relative Cost | Low | Low | Moderate |
Glass is a thermal and electrical insulator (1.0 W/m·K vs 50 for steel and 167 for aluminum), which is why glazing dominates building heat loss. Its modulus (70 GPa) matches aluminum, but unlike metals it has zero plastic deformation — design must keep tensile stress below the flaw-limited strength. For weight-critical glazing, glass (2,500 kg/m³) is only slightly lighter than aluminum (2,700 kg/m³) and about one-third the weight of steel.
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
Related Calculator
Use the Density Calculator to compute glass mass and volume, the Material Volume Calculator for volume takeoffs, and the Thermal Resistance Calculator for glazing heat transfer estimates.
For weight estimation of glass panels, apply the same method as the Steel Weight Calculator or Aluminum Weight Calculator using glass density. For strength context and comparison with metals, see the Material Strength Guide and the Steel Material Properties Guide.
Frequently Asked Questions
What is the density of glass? Soda-lime glass (the common window/bottle type) has a density of 2500 kg/m³ (2.5 g/cm³). Borosilicate glass is 2230 kg/m³ and fused silica 2200 kg/m³.
What is the specific gravity of glass? Approximately 2.5 for soda-lime glass, meaning it weighs 2.5 times as much as an equal volume of water. This value is used to estimate panel weights and buoyancy.
What is the unit weight of glass? About 24.5 kN/m³ (156 lb/ft³) for soda-lime. A 6mm glass panel weighs roughly 15 kg/m² — a common number for glazing dead-load calculations.
Why is glass strong in compression but weak in tension? Glass has ~20x higher compressive strength (900-1000 MPa) than tensile strength (30-50 MPa annealed). Tensile failure is driven by surface microcracks that propagate under tension (Griffith fracture); tempering puts the surface in compression to suppress crack opening and raise effective strength to 120-200 MPa.
Is glass heavier than aluminum? Slightly: soda-lime glass is 2500 kg/m³ vs aluminum 2700 kg/m³ — actually slightly lighter by volume. Glass feels heavier than aluminum only in bulk because products are usually thicker.
How do I convert kg/m³ to lb/ft³ for glass? Multiply by 0.06243: 2500 × 0.06243 = 156 lb/ft³. Conversely, multiply lb/ft³ by 16.018 to get kg/m³.
Is glass stronger than acrylic? In compression, yes — glass reaches 900-1000 MPa vs acrylic's ~80 MPa. But in practical bending and impact, acrylic (PMMA) is tougher: it yields and deforms rather than shattering, has roughly 10x higher impact resistance, and is about half the density (1190 kg/m³). Glass wins on hardness, scratch resistance, chemical inertness, and UV stability; acrylic wins on impact safety, weight, and formability. Choose glass for optics, chemical service, and scratch-critical glazing; acrylic for impact-prone, lightweight applications.
What is soda-lime glass made of? Soda-lime glass is the most common glass (~90% of all production), composed of approximately 70-75% silica (SiO₂), 12-15% soda (Na₂O), and 8-12% lime (CaO). This simple, low-cost composition is why it dominates windows, bottles, jars, and light bulbs. The same base chemistry is modified for special service — adding boron oxide gives borosilicate (Pyrex), while nearly pure silica gives fused silica/quartz glass for high-temperature and optical applications.
What is the glass transition temperature of glass? For soda-lime glass the glass transition (T_g) is about 520-560°C. Below T_g, glass behaves as a rigid amorphous solid; above it, it gradually softens into a viscous liquid rather than melting sharply. The softening point (~700°C) is where glass deforms under its own weight, and practical forming and blowing are done above 700°C. This temperature-dependent viscosity, not a distinct melting point, is what makes glass so formable.
Related Engineering Resources
- Concrete — density and properties of normal weight concrete
- HDPE — polymer density and pipe applications
- Natural Gas — gas density and flow properties
- Water — water density and properties
- Air — air density and properties
- Aluminum — light metal density and properties
- Stainless Steel 304 — corrosion-resistant steel properties
- Carbon Steel — structural steel density and properties
- Heat Transfer Calculator — conduction heat transfer through glazing
- Heat Loss Calculator — window and building envelope heat loss estimation
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.