Fused Silica vs Fused Quartz: Properties, Applications & Differences

Fused silica and fused quartz are both non-crystalline forms of silicon dioxide (SiO₂) — chemically identical but distinguished by purity, manufacturing route, and OH content. Fused quartz is made by melting naturally occurring crystalline quartz at 2,000+ °C; fused silica is synthesized from gaseous SiCl₄ via flame hydrolysis or plasma deposition, producing higher purity (≥ 99.999% SiO₂) and tighter optical specs. Both materials transmit from 170 nm (deep UV) through 3,500 nm (mid-IR), exhibit a thermal expansion of 0.55 × 10⁻⁶/K, withstand sustained temperatures of 1,100 °C, and resist all acids except hydrofluoric — making them the standard substrate for semiconductor wafer carriers, UV/Vis cuvettes, high-power laser windows, and precision optics. Last updated: May 2026.

📋 Quick Answer: Fused silica = synthetic (chemical-vapor route, <1 ppm OH for IR grades, >99.999% pure). Fused quartz = melted from natural crystal (cheaper, 30-50 ppm OH typical, transparent to 200 nm UV). For deep-UV optics or excimer lasers, use synthetic fused silica. For furnace tubes, crucibles, and standard labware, fused quartz is fine.

What Is Quartz Glass?

Quartz glass — the umbrella term for fused quartz and fused silica — is an amorphous (non-crystalline) form of silicon dioxide produced by melting high-purity SiO₂ source material above 2,000 °C and rapidly cooling it before crystals can form. The resulting glass retains the chemical bonding of silica but lacks long-range crystal order, giving it a unique combination of thermal, optical, and electrical properties not found in conventional soda-lime or borosilicate glass.

It is used wherever extreme temperatures, deep-UV transmission, or chemical inertness matter more than cost: semiconductor wafer processing (diffusion tubes, boats, crucibles), UV-Vis and fluorescence spectroscopy cuvettes, high-pressure mercury and xenon lamps, photolithography mask substrates, fiber-optic preforms, and laser optics for 193 nm and 248 nm excimer systems.

Production Methods: How Fused Silica and Fused Quartz Are Made

There are two principal manufacturing routes, and the chosen method determines the material’s purity, OH (hydroxyl) content, and optical performance:

1. Electrical or Gas Fusion (Fused Quartz)

Natural crystalline quartz — typically high-purity sand or quartz pebbles from Brazil, North Carolina, or Madagascar — is melted in a continuous-feed electric arc furnace or oxy-hydrogen flame at 2,000-2,300 °C. The melt is drawn into rods, tubes, plates, or boules. This route produces fused quartz (also called Type I or Type II depending on flame vs. electric heating), with typical chemical purity of 99.99% (4N) and OH content of 5-50 ppm.

2. Chemical Vapor Deposition (Synthetic Fused Silica)

Fused silica is synthesized from high-purity silicon tetrachloride (SiCl₄) by flame hydrolysis (Heraeus Suprasil, Corning HPFS) or plasma deposition (Type IV). This produces synthetic glass with purity exceeding 99.9999% (6N), metallic impurities below 50 ppb, and tightly controlled OH content from < 1 ppm (IR-grade) to ~1000 ppm (deep-UV grade). Synthetic fused silica is the standard for 193 nm excimer laser optics, photomask substrates, and precision interferometry.

Fused Silica vs Fused Quartz: The Key Differences

In the UK and Europe, the terms quartz, silica, fused quartz, and fused silica are often used interchangeably. In the U.S. and in scientific literature, the distinction is more rigorous: quartz refers to material melted from natural crystalline grains, while silica refers specifically to the synthetic chemical-vapor product.

Property	Fused Quartz	Synthetic Fused Silica
Source material	Natural quartz crystal/sand	SiCl₄ vapor
Chemical purity	99.99% (4N)	99.9999%+ (6N)
OH content	5-50 ppm	< 1 ppm (IR) to 1000 ppm (UV)
UV cutoff	200 nm	170 nm (deep UV)
IR transmission	To 2,200 nm (drops at OH band)	To 3,500 nm (IR-grade)
Relative cost	1x (baseline)	3x–10x
Typical use	Furnace tubes, crucibles, lab cuvettes, mercury lamps	Excimer laser optics, photomasks, deep-UV cuvettes, fiber preforms

Physical & Optical Properties of Fused Silica/Quartz Glass

Both fused quartz and fused silica share a core set of remarkable physical properties that distinguish them from all other optical glasses:

Density: 2.20 g/cm³ — lower than borosilicate (2.23 g/cm³) and far below soda-lime glass (2.50 g/cm³)
Coefficient of thermal expansion: 0.55 × 10⁻⁶/K (20-300 °C) — roughly 15× lower than borosilicate and 50× lower than soda-lime; the lowest of any commercial optical glass
Strain point: 1,070 °C · Annealing point: 1,180 °C · Softening point: 1,683 °C
Maximum continuous service temperature: 1,100 °C (1,200 °C for short excursions)
Thermal shock resistance: can be cooled from red heat (1,000 °C) and plunged into ice water without cracking
Refractive index (587.6 nm): 1.4585
Optical transmission: 80%+ from 170 nm through 2,500 nm for UV-grade synthetic; 200 nm to 2,200 nm for standard fused quartz
Chemical resistance: inert to all acids except HF, even at boiling temperatures; resistant to most alkalis below 700 °C
Dielectric strength: 250-400 kV/cm — excellent electrical insulator at all temperatures
Hardness (Mohs): 5.3-6.5 (close to natural quartz crystal at 7)

Transmission curve of 10mm fused silica from UV through IR

Figure 1. Transmission curve for 10 mm thick fused silica/quartz glass (including surface Fresnel reflection losses ~7%). The OH absorption band at 2,730 nm is visible in standard fused quartz but absent in low-OH synthetic silica.

Applications of Fused Silica/Quartz Glass

The combination of high thermal stability, deep-UV transparency, chemical inertness, and electrical insulation makes fused silica and fused quartz the material of choice across multiple high-technology industries:

Semiconductor manufacturing: diffusion tubes, wafer boats, crucibles, bell jars, photomask blanks, EUV lithography pellicle frames — anywhere ultra-pure silicon is processed at high temperature
Optics and laser systems: windows, lenses, prisms, mirror substrates for UV (193 nm DUV, 248 nm KrF) and high-power IR (CO₂ at 10.6 µm) lasers; output couplers for excimer and Nd:YAG sources
Spectroscopy: UV-Vis, fluorescence, NIR, and Raman cuvettes; integrating sphere ports; flow cells for HPLC; sample windows for FTIR
Lighting: envelopes for high-pressure mercury, xenon, halogen, and UV-curing lamps; quartz heaters; deuterium lamp windows
Fiber optics: preforms for telecommunications fiber drawing; high-purity multimode and single-mode core/cladding glass
Aerospace and metrology: ring laser gyroscope blocks, satellite optics, telescope mirror substrates, interferometer reference flats, MEMS substrates
Laboratory and chemical processing: beakers, crucibles, evaporating dishes, distillation columns, reaction vessels for HF-free chemistry above 800 °C

Common Quartz Glass Grades (and What They Mean)

JGS1 / Heraeus Suprasil 1 / Corning 7980 HPFS: high-OH (~1000 ppm) synthetic — best for deep-UV, 170-2200 nm
JGS2 / Heraeus Infrasil 301: standard fused quartz, 220-2500 nm, the industry workhorse for general optics and labware
JGS3 / Heraeus Suprasil 300: low-OH (< 1 ppm) synthetic — best for NIR/IR work, 220-3500 nm (no OH absorption band)
GE-124 / GE-214: high-purity fused quartz tubing used widely in semiconductor processing

When to Choose Synthetic Fused Silica vs Natural Fused Quartz

Use synthetic fused silica when working below 200 nm (deep UV), when laser damage threshold matters (193 nm excimer, high-power Nd:YAG), when OH absorption would corrupt NIR measurements (synthetic low-OH grade), or when bulk metallic impurities below 100 ppb are required (semiconductor wafer carriers)
Use natural fused quartz for furnace tubes, crucibles, standard UV-Vis cuvettes above 200 nm, high-temperature labware, and general optics where the 10x price premium for synthetic isn’t justified

Frequently Asked Questions

Are fused silica and fused quartz the same material?

Chemically they are both 100% amorphous SiO₂. The practical difference is purity and manufacturing route: fused quartz comes from melting natural crystalline quartz (99.99% pure, ~30 ppm OH), while synthetic fused silica is made from gaseous SiCl₄ (99.9999% pure, OH controlled from < 1 to 1000 ppm). For most lab work the materials are interchangeable; for deep-UV optics and excimer laser applications, only synthetic fused silica meets the optical and damage-threshold requirements.

What is the maximum temperature fused silica can withstand?

Continuous service temperature is 1,100 °C; short excursions to 1,200 °C are acceptable. The annealing point is 1,180 °C and softening point is 1,683 °C. Above 1,200 °C, gradual crystallization (devitrification) occurs, especially in the presence of alkali contamination. For sustained use above 1,100 °C, consider sapphire (single-crystal Al₂O₃) or polycrystalline alumina.

Why does fused silica have such low thermal expansion?

The Si-O-Si bond network in amorphous silica accommodates temperature changes by rotating around the bridging oxygen rather than stretching the bonds. This gives fused silica a CTE of 0.55 × 10⁻⁶/K — roughly 15× lower than borosilicate glass — and accounts for its exceptional thermal shock resistance.

Can fused quartz transmit infrared light?

Standard fused quartz transmits well from 200 nm to about 2,200 nm, after which OH absorption bands at 2,730 nm and 3,200 nm reduce transmission significantly. For mid-IR work to 3,500 nm, use low-OH synthetic fused silica (Heraeus Suprasil 300 / JGS3). For wavelengths beyond 5 µm, sapphire or CaF₂ windows are required.

Is fused silica resistant to chemicals?

Yes — fused silica is inert to all common acids (HCl, H₂SO₄, HNO₃, HClO₄, even at boiling) and most organic solvents. The notable exceptions are hydrofluoric acid (HF), which etches silica readily, and hot phosphoric acid above 300 °C. Alkalis attack silica slowly at room temperature but become aggressive above 700 °C, where molten NaOH or KOH will dissolve the glass.

Properties of fused silica/quartz glass shown are typical values; they are not absolute material properties, and should be used for guidance only. Materials and components should be tested for suitability in specific applications. Data sources: NIST Spectrophotometry Program, Heraeus technical literature, Goodfellow Ceramic & Glass Division.

Fused Silica vs Fused Quartz: Properties, Applications & Differences

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