What Is Quartz Glass?

Composition, structure, properties, and how it differs from fused quartz, fused silica, and ordinary glass.

Quick Answer: Quartz glass is an amorphous, high-purity silicon dioxide (SiO₂) glass made by melting natural quartz crystal or synthetic silica without soda, lime, or boron fluxes. The disordered Si–O network gives it high-temperature stability, very low thermal expansion, broad UV-to-infrared transmission, and chemical inertness. It is specified for semiconductor processware, optical windows, furnace tubes, and laboratory apparatus.

  • Single-oxide glass. Nearly pure SiO₂ — typically ≥ 99.99% for industrial grades.
  • Amorphous, not crystalline. Same chemistry as quartz mineral; different structure and behavior.
  • Properties depend on grade. Purity, OH content, and fusion method change optical and thermal limits.
  • Used where ordinary glass fails. High heat, UV, thermal shock, or metallic contamination control.

Quartz glass is not the same thing as quartz

In geology, quartz is a crystalline mineral: silicon dioxide arranged in a repeating hexagonal lattice. It is birefringent, has a density around 2.65 g/cm³, and undergoes an α–β phase change near 573 °C. That crystal is a feedstock. It is not the material used for furnace tubes, wafer boats, or UV windows.

Quartz glass is the glassy (amorphous) form of SiO₂ produced by melting that feedstock — or a synthetic silica precursor — and cooling it before crystals can grow. Density drops to about 2.20 g/cm³. Optical behavior becomes isotropic. Thermal expansion collapses. The working temperature rises far above soda-lime or borosilicate glass because no fluxing oxides were added to lower the melt.

This distinction matters in purchasing. A drawing that says “quartz” may mean crystalline quartz, fused quartz, or synthetic fused silica. If the part must transmit 193 nm, survive 1100 °C cycling, or stay free of alkali metals, the structure and the fusion route both have to be named — not just the word “quartz.”

What is quartz glass made of?

Quartz glass is made of silicon dioxide. Industrial and semiconductor grades are typically specified at SiO₂ ≥ 99.99% (4N). Optical synthetic fused silica can reach 5N–6N. The remaining budget is trace metals (Al, Fe, Na, K, Li, Ti, Cu) plus hydroxyl (OH) groups, which are specified separately because they affect viscosity, IR absorption, and high-temperature use.

Unlike soda-lime or borosilicate glass, quartz glass contains no intentional soda, lime, lead, or boron oxide. Those fluxes make ordinary glass melt and form at convenient temperatures. They also raise thermal expansion, cut UV transmission, and introduce ions that semiconductor and analytical processes cannot tolerate.

Two feedstocks, one chemistry

Feedstock Typical product name What it implies
Natural quartz crystal or high-purity quartz sand Fused quartz / natural quartz glass Purity limited by the mineral. Residual Al and alkali at ppm level. Cost-effective for processware.
Synthetic precursors (commonly SiCl₄) Synthetic fused silica Higher chemical purity and optical homogeneity. Preferred for deep-UV optics and demanding semiconductor optics.

FGQuartz’s standard process materials are specified at SiO₂ ≥ 99.99% with total metallic impurities typically held below 20 ppm. Hydroxyl is specified by grade: electrically fused process material is often low-OH, while UV synthetic grades may carry 1000–2000 ppm OH by design. Full tables live on the technical specifications page.

Hydroxyl (OH) is a specification, not automatically a defect

Water is incorporated as Si–OH. High OH lowers viscosity and the practical service temperature, and it creates absorption bands around 2.7 µm (and weaker bands in the near-IR). That is why IR-grade material is dried to very low OH. UV-grade synthetic fused silica, by contrast, is often high-OH because the flame-hydrolysis route that produces the best deep-UV transmission also introduces hydroxyl. Specify OH against the wavelength and the temperature, not against a single “purer is drier” rule.

Why the amorphous structure matters

Crystalline quartz is anisotropic: refractive index and expansion differ by crystal axis. Fused quartz and fused silica are isotropic glasses. There is no preferred direction, which is why they can be used as optical windows, furnace tubes, and precision-ground rods without birefringence from the lattice itself (stress birefringence is a separate, process-controlled issue).

The Si–O–Si network is strongly bonded and highly connected. That is the origin of the high softening range, chemical durability, and electrical resistivity. It is also why quartz glass is difficult to melt and form: there is no flux to open the network, so fusion takes place well above 1700 °C, and fabrication is done with diamond tooling or oxy-hydrogen flame rather than conventional glass-shop methods.

Properties of quartz glass

Published numbers are typical values for a given fusion route and hydroxyl level. They are not universal constants. Where FGQuartz quotes a single figure on product pages, it describes our standard electrically fused, high-purity process material — not every optical or synthetic grade.

Thermal properties

The coefficient of thermal expansion of fused quartz / fused silica is about 5.0–5.9 × 10⁻⁷ /K between 0 °C and 300 °C (commonly cited as 5.5 × 10⁻⁷ /K from 20–300 °C). That is roughly an order of magnitude below borosilicate and far below soda-lime glass. Combined with modest thermal conductivity (~1.4 W/(m·K) at 20 °C), it is why a red-hot tube can often take a thermal quench that would shatter ordinary glass.

Softening, annealing, and strain points move with hydroxyl and impurity content. Electrically fused natural quartz typically softens near 1680 °C; flame-fused material is somewhat lower; high-OH synthetic fused silica is lower still (around 1600 °C). Continuous service is not the softening point. In practice:

  • Electrically fused process-grade material is commonly used continuously near 1050–1200 °C, with short excursions toward 1300 °C.
  • High-OH synthetic grades are usually derated. Long holds above ~1000–1100 °C accelerate devitrification if surfaces are contaminated.
  • Surface alkali, fingerprints, and furnace dust act as nucleation sites. Cleanliness often limits life more than the datasheet temperature.

FGQuartz process components are specified for continuous service to 1200 °C and short-term exposure to 1300 °C, with a softening point of approximately 1680 °C. Confirm grade before using those figures for optical synthetic material.

Optical properties and UV transmission

Clear quartz glass transmits from the deep ultraviolet into the near infrared. The cutoff is grade-dependent:

Grade family Typical window Notes
Synthetic UV (JGS1 / Type III) ≈ 185–2500 nm Best deep-UV; high OH; absorption near 2.7 µm.
Natural optical (JGS2) ≈ 220–2500 nm General UV–Vis work; more economical.
Low-OH IR (JGS3 / dried synthetic) ≈ 260–3500 nm Removes the water band; weaker deep-UV.
Process-grade clear fused quartz UV–Vis, not a guaranteed optical spec Bubbles, striae, and OH vary. Do not assume 185 nm performance.

Refractive index of high-purity fused silica is about 1.4585 at 589 nm, 20 °C. Transmission percentages must always be quoted with thickness, surface finish, and wavelength. A 10 mm JGS1 window at 254 nm is a different statement from an as-drawn process tube. Optical grade selection is covered in the JGS1 / JGS2 / JGS3 guide.

Chemical resistance

Quartz glass is inert to water, most mineral acids, and organic solvents at ordinary process temperatures. It is attacked by hydrofluoric acid and by hot concentrated phosphoric acid, which dissolve the silica network. Hot strong alkalis (NaOH, KOH) also etch it. This is a material limit, not a quality defect: any high-purity SiO₂ glass behaves this way.

Electrical insulation

Fused silica is an excellent electrical insulator, with resistivity on the order of 10¹⁶–10¹⁸ Ω·cm at room temperature and a dielectric constant near 3.7–3.8. That combination — insulator plus thermal stability plus chemical purity — is why it is used for furnace liners, lamp envelopes, and high-temperature standoffs.

Mechanical behavior

Mohs hardness is typically 5.5–6.5. Compressive strength is high (on the order of 1100 MPa); tensile and flexural strengths are much lower (tensile often cited near 48 MPa). Design values must be derated for surface flaws. Quartz glass is hard and brittle. It does not yield. Edge chips from diamond cutting become crack starters under thermal cycling, which is why fire polishing or controlled chamfers are part of serious fabrication, not cosmetics.

Mechanical, thermal, optical, and purity tables for FGQuartz material are collected on the specifications page rather than duplicated here.

How quartz glass is made

All commercial quartz glass is fused SiO₂ that was cooled as a glass. The fusion route determines hydroxyl, residual metals, bubbles, and optical homogeneity. Industry practice (often following the Heraeus type classification) groups the routes as follows.

Electric fusion of natural quartz (Type I)

High-purity quartz crystal is melted in vacuum or a dry atmosphere with electric heating. Hydroxyl is typically low (below 1–30 ppm). This is the workhorse route for semiconductor process tubes, boats, and much industrial clear fused quartz. Metallic impurities track the mineral source.

Flame fusion of natural quartz (Type II)

Powdered quartz is fused in an oxy-hydrogen flame. Hydroxyl is higher, typically around 150–200 ppm, because the flame is a hydrogen-rich environment. Optical quality can be good; IR transmission is limited by the water band.

Synthetic fused silica (Type III and Type IV)

Silicon tetrachloride or a similar precursor is oxidized or hydrolyzed and deposited as silica, then vitrified. Flame hydrolysis (Type III) produces high-OH, high-UV material. Plasma or chemically dried routes (Type IV and related IR grades) produce low-OH synthetic silica. These are the materials behind deep-UV lithography optics, excimer laser windows, and the highest-homogeneity blanks.

From boule or tube stock to a finished part

Melting makes the material. Most industrial value is added afterward. Stock is drawn as tubes or rods, or sawn into plates, then:

  • Cold processed — diamond cutting, CNC milling, drilling, grinding, lapping, and polishing.
  • Thermally processed — oxy-hydrogen bending, blowing, welding, sealing, and fire polishing.
  • Annealed to relieve forming and machining stress.
  • Cleaned and inspected for cracks, inclusions, and dimensional tolerance.

That second half of the process is where a quartz glass manufacturer is actually judged: whether a diffusion tube concentricity, a boat slot pitch, or a viewport polish can be held in production, not only whether the SiO₂ is 4N.

Types of quartz glass

Buyers usually classify quartz glass four ways at once: feedstock, optical grade, appearance, and product form. Mixing those axes is how specifications become ambiguous.

By appearance

  • Clear fused quartz / fused silica — transparent. Used for process tubes, windows, labware, and most semiconductor quartzware.
  • Opaque or translucent fused quartz — dense microbubbles scatter light, producing a white body. Used where radiation shielding, IR heating, or thermal insulation matters more than see-through access. Do not specify “opaque quartz” when you need an optical window.

By optical grade

In Chinese and much of the Asian supply chain, optical fused silica is purchased as JGS1 (deep UV, synthetic), JGS2 (UV–visible, natural), or JGS3 (IR, low OH). Western catalogues use names such as Suprasil, Spectrosil, Homosil, Infrasil, or Corning 7980. The names are not one-to-one interchangeable; they are analogues. Use the JGS comparison when the drawing calls a transmission band rather than a brand.

By product form

The material becomes useful as a component. Common industrial forms include quartz glass tubes, rods, plates and windows, wafer boats, crucibles, process tanks, inline heaters, and fully custom fabrications to drawing.

Quartz glass vs fused quartz vs fused silica

These three names are not three different oxides. They are overlapping commercial names for amorphous SiO₂. Treating them as identical without context causes the wrong grade to be quoted; treating them as unrelated materials is equally wrong.

Term Strict technical use How it is used in industry
Quartz glass Umbrella term for vitreous (amorphous) SiO₂. Dominant in Asian B2B and many RFQs. Often includes both fused quartz and fused silica.
Fused quartz Glass melted from natural quartz crystal or sand. Process tubes, boats, crucibles, lamps. Purity typically ~4N. Cost baseline.
Fused silica Often reserved for synthetic material from chemical precursors. Sometimes used for all vitreous silica. In semiconductor and optics RFQs, “fused silica” frequently means the higher-purity or optical grade. In general industrial English it is used interchangeably with fused quartz.
Synthetic fused silica SiCl₄ (or equivalent) route; 5N–6N class purity. Deep-UV optics, photomasks, fiber preforms, high-homogeneity windows.
Natural quartz Crystalline mineral feedstock — not a glass. Should not appear on a finished-part drawing unless a crystal is required.

A practical rule for RFQs: if the part is a furnace tube, boat, tank, or crucible, fused quartz / process-grade quartz glass is usually sufficient. If the part is a 193 nm or 248 nm optic, or a homogeneity-specified window, write synthetic fused silica and the optical grade. If the drawing just says “quartz,” ask which of the five rows above was intended before pricing.

Quartz glass vs ordinary glass

Ordinary commercial glasses are mixed-oxide systems designed to melt and form cheaply. Quartz glass is almost pure silica, so it is harder to make and more expensive, and it occupies a different performance envelope.

Property Quartz glass (fused silica) Borosilicate Soda-lime
SiO₂ content ≥ 99.99% typical ~70–81% plus B₂O₃, Na₂O, Al₂O₃ ~70% plus Na₂O, CaO
Continuous use ~1000–1200 °C, grade-dependent ~450–500 °C ~120–150 °C in service
CTE ~5.5 × 10⁻⁷ /K ~3.3 × 10⁻⁶ /K ~9 × 10⁻⁶ /K
UV at 254 nm High (optical grades) Poor; cuts off ~300 nm Opaque in UV-C
Metallic ion risk Very low if specified Alkali present Alkali and alkaline earth present
Cost / formability Higher; diamond or flame work Moderate; conventional lampworking Lowest

Quartz glass is not “stronger glass” in the tensile sense. It is a higher-temperature, lower-expansion, higher-purity glass that is still brittle. For a dedicated comparison with laboratory borosilicate, see quartz glass vs borosilicate glass.

Applications of quartz glass

Quartz glass is specified when at least one of four constraints is binding: temperature above the borosilicate range, UV or IR optical access, metallic contamination control, or thermal-shock cycling. Those constraints concentrate in a handful of industries.

  • Semiconductor. Diffusion, oxidation, LPCVD, and anneal use process tubes, liners, injectors, wafer boats, and wet-bench tanks. The material has to stay dimensionally stable through 1000 °C+ cycles without doping the wafer with alkali metals.
  • Solar photovoltaic. CZ and DS crucibles, diffusion tubes, and carriers for PERC, TOPCon, and related lines. Volume and cost matter more than optical homogeneity; process-grade fused quartz is the usual choice.
  • Optics and UV. Windows, viewports, cuvettes, and laser optics in JGS grades. Deep-UV work is synthetic fused silica; general UV–Vis can be JGS2.
  • Fiber optics. High-purity tubes and starting rods for MCVD / OVD / VAD preform environments, where both OH and metallic purity affect attenuation.
  • Laboratory. Crucibles, combustion tubes, flasks, and apparatus that see 1000 °C or reagents that would leach alkali from borosilicate.
  • High-temperature industrial. Furnace tubes, sight glasses, and structural parts where thermal shock, not optical figure, is the design driver.

Advantages — and the limits that actually matter

What it does uniquely well

  • High continuous temperature among commercial glasses, with very low expansion.
  • UV transmission that borosilicate and soda-lime cannot provide.
  • Chemical purity compatible with semiconductor and analytical blanks.
  • Flame-weldable and CNC-machinable, so complex assemblies are practical.
  • Electrical insulation at elevated temperature.

Limits to design around

  • Devitrification. Prolonged time at high temperature, especially with surface alkali or dust, nucleates cristobalite. The part turns cloudy, weakens, and can spall. Service life is a cleanliness and thermal-history problem as much as a peak-temperature problem.
  • HF and hot alkali. Do not specify quartz glass for hydrofluoric acid service. Hot KOH/NaOH will etch it.
  • Brittleness. High compressive strength does not mean impact tolerance. Handling, packing, and edge condition dominate field failures.
  • Cost and lead time. High melt temperature and diamond/flame fabrication make it the wrong default for 300 °C laboratory glassware.
  • Grade mismatch. Process-grade tubes will not meet a 193 nm laser window specification. Optical blanks are wasted on a diffusion boat.

How to choose the right quartz glass

A usable specification answers six questions. If any are blank, the quote will assume process-grade fused quartz and standard commercial tolerances — which may be correct, or may not.

  • 1. Temperature and time. Continuous temperature, peak, and cycle count. This sets fusion type and OH, not just wall thickness.
  • 2. Wavelength, if any. No optic needed → process grade. 254 nm or deeper → JGS1 / synthetic. IR past the water band → low-OH / JGS3.
  • 3. Chemistry. HF is a stop. Hot alkali needs extra wall or a different material. Ultra-pure wet chemistry needs low metallic impurities, not just “quartz.”
  • 4. Geometry. Tube, rod, plate, boat, crucible, tank, or a welded/CNC hybrid. Send DXF, STEP, IGES, or PDF.
  • 5. Tolerance and surface. As-drawn, ground, fire-polished, or optical polish. Slot pitch, concentricity, and flatness belong on the drawing, not in an email after the PO.
  • 6. Quantity and documentation. Prototype versus blanket order, and whether a composition certificate or optical scan is required.

FGQuartz reviews drawings for manufacturability — cold machining, flame work, or both — and typically returns a route and a price within 24 business hours. There is no minimum order quantity.

Frequently asked questions about quartz glass

Short answers written to stand alone. Deeper process and ordering questions live in the quartz glass FAQ.

Quartz glass is amorphous silicon dioxide (SiO₂) produced by melting natural quartz or synthetic silica without fluxing oxides. It is a glass, not crystalline quartz, and is used where high temperature, UV transmission, or chemical purity exceeds the range of ordinary glass.

In many catalogues, yes — both names describe vitreous SiO₂. In stricter usage, fused silica means synthetic material from chemical precursors, while fused quartz means material melted from natural crystal. Quartz glass is the umbrella term. Always confirm feedstock if UV cutoff or 6N purity is in the specification.

It is made of SiO₂. Industrial grades are typically ≥ 99.99% silica, with trace metals in the ppm range. Hydroxyl (OH) is specified separately and can range from under 1 ppm in electrically fused or dried IR grades to ~1000–2000 ppm in UV synthetic grades.

It is much more resistant to heat and thermal shock, and harder on the Mohs scale than soda-lime, but it is still a brittle glass. Tensile strength is modest and flaw-sensitive. “Stronger” is the wrong word; “more thermally and chemically durable” is accurate.

Yes, within grade limits. Electrically fused process-grade material is commonly used continuously around 1050–1200 °C, with short-term peaks near 1300 °C. Softening is much higher (about 1600–1710 °C depending on OH). Continuous service is limited by sag, contamination, and devitrification, not by the softening point.

Optical grades do. Synthetic UV fused silica (JGS1 class) typically transmits from about 185 nm. Natural optical fused quartz (JGS2) is closer to 220 nm. Process-grade tubing is not an optical specification — do not assume deep-UV performance without a grade and a thickness.

Semiconductor process tubes and wafer boats, solar CZ crucibles, UV and IR windows, fiber-optic preform hardware, laboratory high-temperature ware, and industrial furnace components. The common thread is heat, purity, or optical access.

Borosilicate is a mixed-oxide glass (~80% SiO₂ plus boron and alkali) used to about 500 °C. Quartz glass is nearly pure SiO₂, with much lower expansion, UV transmission, and a service range near 1000–1200 °C. Borosilicate is the correct default below that range. Quartz is specified when borosilicate runs out of temperature, UV, or purity.

Natural quartz is electrically or flame fused, or silica is synthesized from SiCl₄ and vitrified. Stock is then drawn or cut, and finished by diamond CNC and/or oxy-hydrogen flame work, followed by anneal and inspection. The fusion step sets purity and OH; the fabrication step sets geometry.

Write down continuous temperature, wavelength (if any), chemistry, geometry, and tolerance. Those five items select fused quartz vs synthetic fused silica, JGS grade, and product form (tube, plate, boat, custom). If HF is in the process, choose a different material.

They share the same amorphous SiO₂ chemistry. Fused quartz is melted from natural crystal; fused silica often denotes the synthetic route. Thermal and chemical behavior are close. Optical UV cutoff, homogeneity, and metallic purity are not. Specify the route when those matter.

SiO₂ without fluxes melts above 1700 °C, requires high-purity feedstock, and cannot be formed with ordinary glass tooling. Diamond machining and oxy-hydrogen welding are slow compared with soda-lime blowing. You are paying for temperature capability and purity, not for a clearer window.

Specify the material, then the part

Quartz glass is amorphous, high-purity SiO₂. Fused quartz and fused silica are members of that family, distinguished by feedstock and process, not by a different oxide. Ordinary glass is a different family altogether. Once that is clear, the useful work is matching grade and geometry to temperature, wavelength, chemistry, and tolerance.

If you already have a drawing, send it. If you are still naming the material, start with temperature and wavelength — those two constraints eliminate most of the wrong grades before a quote is written.

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