Fused silica
Specify fused silica by glass code, optical grade and finished-part requirements; interpret transmission, thermal data and contamination limits in context.
Tags: materials, ceramics, glass, fused-silica, optics
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Corning certifies HPFS fused-silica index homogeneity at 632.8 nm, averaged through the thickness. The minimum verification thickness is 20 mm. For thinner optics, Corning certifies the parent piece. Require traceability to that piece and separate acceptance criteria for finished surface quality and transmission. Corning HPFS quality notes, p. 2
Identify the glass and the application
Fused silica is an amorphous silica glass, not a crystalline quartz optic. Optical fabricator Esco distinguishes synthetic material produced through flame hydrolysis from fused quartz made by melting natural quartz. Manufacturing route and impurities influence which spectral applications suit a material. Treat supplier terminology as a starting point: order an identified glass code and quality grade rather than assuming that every item sold as “quartz” is interchangeable. Esco material guidance
Start a specification with the function: transmitting window, lens, mirror substrate, spacer or hot-process component. Define finished dimensions, clear aperture, edge treatment, mounting, environment and inspection. For optics, add wavelength band, path length, wavefront or surface requirements and coating. For a heated component, add temperature/time history, atmosphere, supports and allowable deformation. A material certificate and a finished-part inspection report answer different questions.
Grade and optical path matter
Esco identifies hydroxyl-related absorption as a reason to distinguish UV and IR fused silica. Its published ranges are application guidance rather than guaranteed transmission of an arbitrary finished optic. Select using the actual grade's wavelength-dependent data and the required thickness. Esco UV and IR material discussion
Corning's HPFS brochure gives these named-grade internal transmittance limits:
| Glass grade | Wavelength | Published limit |
|---|---|---|
| 7980 KrF | 248 nm | ≥99.90% per cm |
| 7980 ArF | 193 nm | ≥99.50% per cm |
| 8655 | 193 nm | ≥99.75% per mm |
The length units differ. These are bulk transmission statements, not completed-window throughput or laser-damage ratings. Corning HPFS brochure, p. 6
As an original engineering calculation, assume uniform bulk attenuation and use the 7980 ArF limit for a 20 mm optical path: internal transmission is at least 0.995² = 0.990025, approximately 99.00%. This calculation excludes surface reflection, coatings, scatter and contamination; it cannot establish the delivered transmission of a mounted window. Do not compare two quotations until wavelength, path length and internal versus external measurement conventions match.
For a laser optic, request qualification under the intended wavelength, pulse duration, repetition rate, beam dimensions and exposure history. An initial transmission requirement does not by itself specify performance after prolonged irradiation. Retain the coating supplier's applicable test conditions with the glass requirement.
Homogeneity is separate from surface quality
Set separate acceptance criteria for bulk homogeneity, inclusions and surface finish.
Record measurement aperture and orientation on the certificate. Verify finished flatness, wedge and transmitted wavefront after the applicable polishing, coating or assembly steps.
Thermal properties need test context
Corning lists density 2.20 g/cm³, Young's modulus 73 GPa, and thermal expansion 0.52 ppm/°C over 5–35°C. Its 7980 softening point is 1585°C, referenced to ASTM C598. These are material data, not a component operating-temperature or pressure rating. Corning property tables, p. 3
Heraeus explains that thermal-shock performance also depends on surface condition and geometry. Its discussion connects hydroxyl content with viscosity and use temperature; low expansion alone does not define every grade's thermal envelope. Heraeus quartz-glass properties
For design review, distinguish free expansion from thermal stress. Consider the temperature gradient through a window, local heating by absorbed radiation, mounting restraint and expansion mismatch with surrounding hardware. Specify the actual thermal cycle and acceptable changes in shape or optical performance. Do not extrapolate a room-temperature coefficient across an entire furnace cycle or treat a softening test point as a continuous-service recommendation.
Tosoh specifically cautions about differential expansion when silica glass is combined with other materials, prolonged use near the annealing point and slow sagging at high temperature. Its brochure also describes devitrification as conversion toward crystalline cristobalite and shows that atmosphere and contamination affect it. These cautions concern the documented silica-glass products and conditions, not a universal lifetime model. Tosoh fused-silica brochure, marked 2013.5
Cleanliness, fabrication and release
Heraeus warns that traces of alkali and alkaline-earth compounds accelerate devitrification during hot exposure. It also identifies hydrofluoric and phosphoric acid attack. Chemical resistance must be checked for the actual substance, concentration, temperature and exposure rather than described as blanket inertness. Heraeus chemical behavior
Tosoh advises avoiding hand contact and metal contamination, cleaning before use and drying completely before high-temperature service. Establish a supplier-approved cleaning process for the delivered surface and coating; a procedure for bare hot-process glass may not suit a coated precision optic. Tosoh handling and usage precautions
Agree machining and polishing acceptance before ordering: critical dimensions, chips and cracks, edge condition, surface defects and any subsurface-damage assessment required by the application. Define packaging and contact areas that protect the accepted surfaces. For pressure or vacuum windows, require a component design and acceptance basis covering geometry, supports and loads; a catalog strength number cannot qualify an arbitrary window.
Retain glass code and grade, batch/blank identity, certificate scope, drawing revision, finished inspection and cleaning/coating history. Reassess a substitution against all of those requirements, even if its visible appearance and nominal dimensions match.
Sources reviewed November 1, 2026. Corning's linked file prints July 2015 and ©2015, despite its filename containing 2022; Tosoh prints 2013.5. The cited Esco and Heraeus web pages do not supply a publication date.