Fused Silica and Fused Quartz
Fused silica is glass made of nothing but SiO₂ — either melted from natural quartz crystal (fused quartz) or synthesized from silicon tetrachloride in a flame (synthetic fused silica). With no alkali or boron the network is stiff and pure: expansion 0.5 ppm/K, softening near 1,650 °C, continuous service to about 1,000 °C, transmission from 180 nm in the deep UV to 3.5 µm, and a quench from red heat into water that it survives without a mark. It is the glass for UV and laser optics, semiconductor process ware, metrology and any window that must be hot, pure and stable.
Popular fused silica grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| Fused Silica | 90 | 50 | 2.2 | Not recommended | Not recommended | Process ware, viewports, thermal shock, UV |
| HPFS 7980 | — | 50 | 2.2 | Not recommended | — | Synthetic UV optics and lithography (Corning) |
| Suprasil | — | 50 | 2.2 | Not recommended | — | Synthetic optical grades, UV to IR (Heraeus) |
| JGS1 | — | 50 | 2.2 | Not recommended | — | UV-grade synthetic silica to Chinese spec |
| ULE 7972 | — | 50 | 2.21 | Poor | — | Zero-expansion mirror blanks and stages |
| Vycor 7913 | — | 50 | 2.18 | Poor | — | 96 % silica lab ware at lower cost |
| Alpha-Quartz | — | 55 | 2.65 | Not recommended | — | Crystalline quartz for piezo and waveplates |
Choose fused silica for
Deep-UV and excimer-laser optics, high-power laser windows and lenses; semiconductor diffusion tubes, wafer boats and CVD ware; furnace viewports, thermocouple and heater sheaths to 1,000 °C; metrology, interferometer and mirror substrates that must not move with temperature; and anything that is heated and cooled violently or must stay pure in contact with hot acids or molten metals.
Look elsewhere when
Visible optics only, at the lowest cost (N-BK7 or Borofloat 33 — fused silica is several times the price), the metrology demands expansion below 0.05 ppm/K over a wide range (ZERODUR, or ULE 7972 within this series), the part is a large panel or tube that could be borosilicate (Duran), it must be hard against scratching or run above 1,200 °C for long periods (Sapphire; fused silica slowly devitrifies), or the environment is hydrofluoric acid or hot phosphoric acid, which attack silica.
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Fused silica by application
- UV and laser optics
- HPFS 7980, Suprasil or JGS1 windows, lenses and prisms — transmission to 165–180 nm, damage thresholds well above other glasses, low fluorescence; specify the grade by wavelength band because OH content sets the infrared cutoff.
- Semiconductor process ware
- Fused Silica tubes, boats, bell jars and rings to 1,000 °C continuous in oxidizing and reducing furnaces; high-purity synthetic grades where metal contamination must stay in parts per billion.
- Metrology and mirror substrates
- ULE 7972 for blanks and stages where expansion must be zero; Fused Silica at 0.5 ppm/K for interferometer plates, reference flats and scale substrates when a slight, predictable expansion is acceptable.
- Viewports and high-temperature windows
- Fused Silica discs for furnace, plasma and combustion viewports that see 1,000 °C and thermal shock; Vycor 7913 where the temperature stays below 900 °C and the part is lab ware or a lamp envelope.
Working with fused silica
- Melting & forming
- Fused quartz is melted from quartz crystal in electric or flame furnaces at over 2,000 °C into tubes, rods and ingots; synthetic silica is deposited from SiCl₄ in an oxy-hydrogen flame and consolidated into boules. Hot-working (lampworking, flaring, sealing) needs 1,800–2,000 °C and a specialist — a hydrogen-oxygen torch, not a glassblower’s gas flame. Annealing is at about 1,100 °C.
- Grinding & lapping
- Diamond grinding, core drilling and waterjet shape plates, discs and tubes to ±0.05 mm; the material is harder than borosilicate (Knoop about 600) and takes longer. Optical polishing reaches λ/20 flatness and sub-nanometre roughness; edges are fine-ground or polished because a chipped edge is the only thing that breaks a fused-silica part in service.
- Joining & sealing
- Silica fuses to silica with the hydrogen-oxygen torch, and optical contacting or hydroxide-catalysis bonding joins polished faces without adhesive for mirrors and cells. There is no expansion-matched metal — glass-to-metal seals go through graded seals of intermediate glasses, or the part is clamped with compliant gaskets or brazed via a metallized ring at low stress.
- Tolerances & design rules
- Ground parts hold ±0.02 mm, drawn tubing about ±5 % on wall and ±1 % on diameter. Design tensile stress at about 7 MPa long-term for as-ground surfaces and higher for polished ones. Above about 1,150 °C the glass slowly crystallizes to cristobalite and turns opaque and fragile; keep continuous service below 1,000–1,100 °C and avoid alkali contamination, which speeds devitrification.
Other glass series
Fused Silica and Fused Quartz — frequently asked
What is fused silica?
Fused silica is glass made of nothing but SiO₂ — either melted from natural quartz crystal (fused quartz) or synthesized from silicon tetrachloride in a flame (synthetic fused silica). With no alkali or boron the network is stiff and pure: expansion 0.5 ppm/K, softening near 1,650 °C, continuous service to about 1,000 °C, transmission from 180 nm in the deep UV to 3.5 µm, and a quench from red heat into water that it survives without a mark. It is the glass for UV and laser optics, semiconductor process ware, metrology and any window that must be hot, pure and stable.
Which grade of fused silica should I use?
Fused Silica in a commercial fused-quartz grade for process tubes, boats, crucibles, furnace viewports and heater sheaths — the purity of the natural quartz is enough and the cost is lowest. HPFS 7980 (Corning) or Suprasil (Heraeus) when the part is an optic: synthetic, bubble-free, homogeneous to 10⁻⁶ in index, transmitting to 165 nm for excimer-laser and lithography work; JGS1 is the Chinese UV-grade synthetic equivalent, with JGS2 and JGS3 for visible and infrared use. ULE 7972 is titania-doped silica with expansion tuned to zero at room temperature — mirror blanks, lithography stages and metrology frames. Vycor 7913 is 96 % silica reconstituted from a leached borosilicate: nearly the thermal performance of fused silica at lower cost for lab ware and lamp parts. Alpha-Quartz is the crystalline form, chosen for piezoelectric and birefringent optics, not as a glass.
Which grade of fused silica is easiest to machine?
Vycor 7913 (machinability poor, 18/100), then ULE 7972 (15/100).
Fused quartz or synthetic fused silica?
Both are amorphous SiO₂ and share expansion, temperature limits and thermal-shock resistance. Fused quartz is melted from natural crystal and carries parts-per-million metallic impurities and a few bubbles — fine for tubes, boats, viewports and lab ware, and cheaper. Synthetic fused silica (HPFS 7980, Suprasil, JGS1) is built from purified SiCl₄, so it is bubble-free, homogeneous and transmits deeper into the UV (to about 165 nm rather than 180–200 nm); it is the grade for lenses, lithography and laser optics. Water content differs too: standard synthetic grades hold about 1,000 ppm OH, which absorbs near 2.7 µm, so infrared optics use a low-OH grade.
Fused silica, ULE or ZERODUR for a mirror blank?
Fused silica expands 0.5 ppm/K, ULE 7972 about 0 ± 0.03 ppm/K near room temperature, ZERODUR 0 ± 0.02 ppm/K or better over 0–50 °C. For a mirror in a controlled lab or a scanning system that can be recalibrated, fused silica polishes best, is transparent for edge inspection and costs least. ULE and ZERODUR are for metrology frames, telescope and lithography optics where a few nanometres of drift matter; ULE is lightweighted and fused into large structures more readily, while ZERODUR is a ceramic with slightly higher stiffness and is opaque in the UV.