Quartz Glass Explained: From Material Grades to Finished Parts

Quartz glass is a high-purity, amorphous form of silicon dioxide used where ordinary glass can’t meet a client’s optical and thermal requirements. This glass must have the right optical and thermal characteristics to handle the requirements of the application. Then select the grade and specify the size and surface quality of the glass. Two ‘clear’ windows can be very different, and have different properties.

Updated October 2026

Quartz glass, fused quartz and fused silica

Quartz glass describes a glassy silica material. Natural quartz when melted produces a glass referred to as fused quartz, and a synthetic chemical precursor produces fused silica. Both are amorphous. In a crystal structure, such as Quartz, there’s an ordered structure, and certain properties are orientation dependent. These orientation dependent properties of Quartz shouldn’t be applicable to a glass plate.

What is quartz glass made of?

Quartz glass is mainly composed of silicon dioxide, SiO₂. Manufacturers either fuse (pure) natural quartz feedstock or make synthetic silica from chemical precursors. The resulting glass doesn’t have a repeating crystal structure like quartz mineral. Grades differ from one another based on the amount of trace metals and defects as well as hydroxyl groups.

Think of the composition as the ingredients of a dish, and structure as their arrangement. The ingredients can be the same, but the finished product can have very different optical properties. Lisa Moore and Charlene Smith explain these differences in their review of fused silica properties. Both authors were affiliated with Corning.

High purity quartz glass still requires the producer’s grade on a purchasing specification. Names like JGS1 and JGS2 also require a corresponding supplier datasheet. A grade name, its optical curve and the required certificate belong together on the order.

Which properties change the material choice?

Thermal expansion, heat resistance and optical behavior determine whether quartz glass warrants its additional processing. Low expansion enables a part to maintain its dimensions as it encounters changes in temperature. Operating limits, however, differ for the various production routes. The temperature at which a part softens is a different property from the maximum service temperature.

Is quartz glass heat resistant?

Yes, quartz glass offers high temperature resistance, but the usable temperature is grade-specific. Heraeus lists maximum continuous working temperatures of 1160°C for electrically fused quartz, 1110°C for flame-fused quartz and 950°C for synthetic fused silica. Those representative material values need to be read alongside the load, geometry and exposure duration.

Source: Heraeus material-property tables. These are the manufacturer’s representative route values. A thin unloaded piece and a loaded furnace component don’t share an automatic service limit.

Free-expansion example

Assumed inputs: an unconstrained straight length, uniform heating and the mean coefficient published by Heraeus for the stated interval. For a free 100 mm length heated from 0°C to 100°C, a mean coefficient of 5.1 × 10⁻⁷/K gives a calculated expansion of 0.0051 mm, or 5.1 µm.

ΔL = L × α × ΔT = 100 mm × 5.1 × 10⁻⁷/K × 100 K = 0.0051 mm

That’s the free expansion of the part; it doesn’t address the clearance required for mounting. Other factors such as the presence of a metal holder, non-uniform heating and the force of a clamp can also affect the result. In its 2024 expansion study, the National Institute of Standards and Technology found, while examining three silica samples, differences larger than the uncertainty in measuring a single sample.

For an insulating component, specify the dielectric requirement and the optical grade separately. The hot-use durability is also affected by how the component is handled. Momentive Technologies’ guidance indicates that surface alkalis, including deposits from perspiration, cause devitrification. For manufacturing a furnace tube, it’s essential that handling is clean and the components are protected from contamination.

Choose quartz glass by wavelength, not transparency

From a transmission curve, select quartz glass at the operating wavelength and part thickness. A clear appearance in visible light does not show absorption in ultraviolet or infrared bands. This doesn’t indicate absorption in the UV and IR regions. The choice between fused quartz and synthetic grades is affected by hydroxyl content, type and level of defects and irradiation.

Does quartz glass block UV?

Many grades of quartz glass transmit UV light, and, therefore, find application in optical components and UV windows. Transmission is wavelength, thickness and grade dependent. Corning lists KrF at 248 nm and ArF at 193 nm among the lithography applications for its HPFS materials. Not all optical grades can be replaced by a clear plate.

For infrared service, hydroxyl absorption deserves attention. The Moore and Smith review describes an OH fundamental band near 2.73 µm, with overtones near 2.24 µm and 1.37 µm. Dry synthetic grades exist, so “synthetic” alone doesn’t tell you the OH level.

Heraeus identifies Infrasil as natural fused quartz with an OH content of 8 ppm or less, and positions it for near-IR applications with lower power levels. This statement provides a useful named-grade starting point for a low-intensity near-IR window.

Wavelength Fit Check

Be sure to match the light source, optical path, and exposure to a material family before comparing prices of finished windows.

Edmund Optics’ UV/IR comparison shows curves for a 5 mm thickness without Fresnel reflection losses. Internal transmission and transmission through an uncoated finished window differ and answer different questions. For a coated part, the coating adds another wavelength-dependent response.

For higher intensities of light, structural changes may occur. The optical review describes irradiation-induced absorption and structural changes. A precision instrument with pulsed illumination needs its exposure conditions included in the material specification, even when the initial light transmittance looks adequate.

Quartz glass versus borosilicate and other optical materials

Quartz glass offers a solution when its spectral response or low coefficient of thermal expansion meets the application requirements. Choose a named borosilicate product when its documented temperature range and optical performance meet the duty. For ordinary display protection, consider the established cover-glass substrate before paying for a silica optical part.

SCHOTT lists a mean thermal expansion coefficient of 3.25 × 10⁻⁶/K over 20–300°C for BOROFLOAT 33. Its thermal datasheet gives maximum operating temperatures of 450°C for long-term use of at least 10 hours and 500°C for shorter exposure. Those figures describe BOROFLOAT 33, not every borosilicate vessel or window.

The interval of expansion may not be the same as that of quartz. Using the two constants as an exact ratio may hide the difference. For a heated sight window, compare the named materials across the actual operating interval, including the frame and the heating cycle.

There may be a different set of priorities when a display cover is considered. These may include the behavior of the cover when impacted, how the cover reacts when touched, and the treatment of the cover’s surface. Our soda-lime glass guide describes these priorities. A harder optical crystal, such as sapphire, is another material class rather than a stronger version of fused silica; its optical and mechanical specifications need a separate comparison.

Quartz glass is an optical material with grade-dependent spectral response. Homogeneity and surface finish of the glass may play a more important role than the maximum allowable temperature in the lens or viewing system. Use the optical glass selection guide to select optical glass for the function the part is intended to serve.

Quartz tubes, plates and finished parts

Specify the finished shape and working surfaces separately. A quartz plate requires limits for length, width and thickness and a tube requires bore and wall dimensions. Optical windows require surface and wavefront conditions. A component that’s pressure-loaded requires evaluation of the supported geometry and the duration of the load as part of the assembly.

Can quartz glass be cut?

Yes, quartz glass is cut and machined to the required surfaces and then ground and polished. Trimming to the outline is a single step. Other edge conditions, holes and optical faces may require additional operations. Fused quartz ground to size still requires optical polishing when it’s used in a measurement beam.

MGS lists standard fused-quartz window examples with ±0.005 inch dimensional tolerances, 60/40 scratch-dig and flatness of 2–4 wavelengths per inch. These are MGS’s published fabrication specifications, not a universal prescription for an imaging optic. Its pressure-rating instructions also request the unsupported working diameter.

ISO 10110-7:2017 describes how to indicate acceptable surface imperfections within a test region on optical elements and assemblies.

“These include localized surface imperfections, edge chips and long scratches.”

Life science lab-ware and optical windows share a material family and don’t have the same acceptance criteria. A microscopy coverslip requires an optical specification and a thickness. A laboratory UV-vis grade cell is ordered for its measurement path. A large size furnace plate is ordered for its geometry and thermal duty. Porous components shouldn’t have the clear fused window specification.

NASA research on space-station window materials reports substantial variation in published slow-crack-growth parameters between test methods. The study’s comparison is a reason to treat sustained loading separately from a low-expansion claim. An O-ring seal also has its own temperature and compatibility limits.

Before providing dimensional constraints on all features, determine which surfaces are for mounting and identify the optical aperture. Our glass assembly tolerance guide describes the trade-offs that drawing constraints place on the surrounding components.

What affects quartz glass price and lead time?

Quartz glass lead time and price vary with grade, finished blank dimensions and the finishing process. Optical polishing, coating and inspection require additional effort beyond cutting a shape. Stock availability may reduce the lead time for material, but the fabrication and inspection of a custom part still require their schedules to be agreed to separately.

Valley Design’s rotating stock list warns that dimensions and materials listed aren’t always available.

According to Lawrence Livermore National Laboratory, convergent polishing reduces the cost of metrology and labor, resulting in lower unit costs. The laboratory’s optical-finishing work shows that the processing route affects cost. That technology belongs to LLNL; it isn’t a claim about the production equipment of SW Glass.

For two comparable quotes, break out the charges for the blank, machining, polishing, coating, and inspection. Procurement can then see whether a lower price omits a required operation. Before adding a tighter tolerance, quality engineering should agree on an acceptance method, so the added cost provides a measurable requirement.

Material makers and quartz glass fabricators

One producer supplies silica grade material. A fabricator transforms that material to the finished part. Heraeus offers Infrasil natural fused quartz, while Corning offers HPFS synthetic materials. Swift Glass provides a description of fabrication using materials from some of the established glass producers. These functions may be distributed among different organizations.

Corning’s HPFS page identifies its material families. Swift Glass’s material comparison describes the natural and synthetic distinction in a fabrication context. Producer-grade traceability should stay attached to the blank as it becomes a finished window.

SW Glass’s custom cover glass guide offers the next step in drawing-led design for equipment covers and displays. Keep the required substrate on that drawing; general glass-fabrication discussions shouldn’t silently substitute the specified silica grade.

Turn your application into a comparable quotation

Ask each supplier to respond to the same application brief to ensure quotations represent the same finished part. Engineering defines the wavelength and operating conditions; Quality specifies the inspection method; and Procurement sets the quantity and the basis of delivery. Both optical specifications and a drawing must accompany the requirement, even for simple-looking disc-shaped parts.

Example: a low-power 254 nm sensor window, 30 mm in diameter and 5 mm thick, operating at 20–60°C at ambient pressure. Complete the fields below with the approved project requirements.

Suppliers need to provide both the finished part inspection report and the grade datasheet. Confirm whether a transmission figure includes surface reflection. Confirm if a quoted delivery date includes the coating. Place the drawing revision on the quotation so a later change does not leave two suppliers pricing different parts.

The cover glass drawing checklist offers a way to capture design and processing requirements. For optical surfaces, an anti-reflection coating is different from an anti-glare coating. Our comparison of the two terms explains the difference.

Frequently asked questions

The values shown for materials are from named producers or research sources. The free-expansion calculation is an engineering example and not a SW Glass test result.

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