Quartz Glass vs Borosilicate Glass

Composition, temperature limits, thermal expansion, UV transmission, purity, cost — and how to choose between fused quartz, fused silica, and borosilicate.

Values are typical for high-purity fused quartz / fused silica and for Type 3.3 borosilicate laboratory glass (ISO 3585, the DURAN® and PYREX® 7740 class). Other borosilicate families — such as Type 5.0 pharmaceutical tubing or float sheet — expand more and differ in detail. Quartz values shift with fusion route and hydroxyl content; see the guide to what quartz glass is for the fused quartz versus fused silica distinction.

Both materials are silicate glasses, but they sit at opposite ends of the modifier scale. Quartz glass is a single-oxide network. Borosilicate is a multi-oxide glass in which boron and alkali oxides open the network so it can be melted and formed at ordinary glassworking temperatures.

Quartz glass is amorphous silicon dioxide made by fusing natural quartz crystal or by depositing synthetic silica from a chemical precursor such as SiCl₄. Industrial and semiconductor grades are specified at SiO₂ ≥ 99.99%; the remaining budget is trace metals (Al, Fe, Na, K, Li, Ti, Cu) and hydroxyl (OH), which is controlled separately because it changes viscosity and infrared absorption. There is no intentional soda, lime, lead, or boron oxide. Commercial names overlap: quartz glass is the umbrella term, fused quartz usually means glass melted from natural crystal, and fused silica often means the synthetic route.

Type 3.3 laboratory borosilicate is typically about 81% SiO₂, 12–13% B₂O₃, 4% Na₂O and K₂O, and 2% Al₂O₃. Boron enters the network and, together with the alkali, lowers melting and forming temperatures while keeping expansion low compared with soda-lime glass. That balance made it the default glass for laboratory ware, condensers, and sight glasses.

Borosilicate is a family, not one recipe. Type 3.3 (expansion ≈ 3.3 × 10⁻⁶ /K) is the laboratory and process-equipment glass. Type 5.0 (≈ 5.0 × 10⁻⁶ /K) is common in pharmaceutical tubing. Borosilicate float sheet such as BOROFLOAT® 33 is a flat-glass product. A drawing that only says “borosilicate” should name the type or standard.

Quartz glass, by a wide margin. Electrically fused process-grade quartz is commonly used continuously at 1050–1200 °C, with short excursions toward 1300 °C. Type 3.3 borosilicate is limited to roughly 450–500 °C in continuous service. Softening, annealing, strain, and service temperatures are four different numbers, and confusing them is how a borosilicate tube sags in a furnace or a quartz part is asked to do a job its grade was never built for.

High-hydroxyl synthetic fused silica is usually derated compared with electrically fused natural quartz because OH lowers viscosity. Surface alkali, fingerprints, and furnace dust nucleate cristobalite, so service life at high temperature is a cleanliness and thermal-history question as much as a peak-temperature one. Process-grade figures for FGQuartz material are tabulated on the technical specifications page.

Fused quartz expands about 5.5 × 10⁻⁷ /K between 20 °C and 300 °C. Type 3.3 borosilicate expands about 3.3 × 10⁻⁶ /K — roughly six times more. Soda-lime glass, at about 9 × 10⁻⁶ /K, expands more again. Borosilicate is a low-expansion glass compared with window glass; quartz is a very-low-expansion glass compared with borosilicate.

Thermal stress in a heated part scales with expansion, elastic modulus, and the temperature gradient across the wall. Lower expansion means lower stress for the same gradient, which is why a thin quartz tube can be heated to around 1000 °C and quenched while a borosilicate tube of the same wall generally cannot. Borosilicate tolerates temperature differences on the order of 150–200 °C, which is plenty for most laboratory heating and cooling.

Neither glass is shock-proof. Thick walls, sharp internal corners, unpolished diamond-cut edges, and rigid metal clamps still cause failures in both. Specify quartz when the cycle itself — furnace ramps, lamp start-up, wafer processing — exceeds what borosilicate can absorb, not on the assumption that quartz never breaks. Low expansion also reduces length change in precision assemblies and mismatch against low-expansion mounts, but a quartz-to-metal joint still needs a considered transition.

In the visible range both glasses are clear, so sight-glass duty alone does not separate them. Ultraviolet light does. Boron and other modifiers in borosilicate absorb strongly below about 300–320 nm, depending on thickness and grade. Optical fused silica transmits much deeper: synthetic UV grades (JGS1 class) from about 185 nm, natural optical fused quartz (JGS2 class) from about 220 nm.

UV-C lamps, 254 nm photochemistry, UV–Vis cuvettes in the protein band, and 193/248 nm optics belong to fused silica with a named optical grade. General visible viewports, many condensers, and applications that should block UV can stay on borosilicate. Process-grade clear fused quartz tubing is not an optical specification — bubbles, striae, and OH vary — so do not assume deep-UV performance without a grade and a thickness. Grade selection is covered in the JGS1 / JGS2 / JGS3 guide and on the optical quartz glass page.

Both glasses resist water, most mineral acids, and organic solvents at ordinary process temperatures. Resistance always depends on the species, concentration, temperature, and time — and on grade.

For analytical blanks, trace-metal digestion, and high-purity wet chemistry, the question is not whether the glass “looks inert” but how many ppb of boron, sodium, and aluminum it adds to the sample. Specify the allowable impurity level, not just the material name.

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 3.8. It keeps useful insulation at furnace temperatures, which is why it appears in lamp envelopes, heater sheaths, and high-temperature standoffs. Borosilicate is a good insulator at ambient conditions but is not the usual choice once the insulator also has to operate at 800–1200 °C.

Both glasses are hard and brittle. Quartz is not “stronger glass” in the tensile sense: Mohs hardness is about 5.5–6.5, compressive strength is high, and tensile and flexural strength are much lower and flaw-sensitive. Young’s modulus is about 72 GPa for fused silica and about 64 GPa for Type 3.3 borosilicate. What quartz offers mechanically is dimensional stability under heat — less expansion, less sag — while handling, packing, and edge condition dominate field failures in both materials.

Quartz glass costs more than borosilicate of similar outline, often several times more, and the gap widens for custom welded assemblies and optical grades. Silica without fluxes melts above 1700 °C, the feedstock must be high-purity, and fabrication uses diamond tooling or oxy-hydrogen flame instead of conventional lampworking. Borosilicate wins whenever its envelope is sufficient, which covers most routine laboratory glassware and a large share of moderate-temperature process hardware.

Unit price also moves with grade, purity, dimensions, wall thickness, quantity, custom features, tolerance, and inspection. A synthetic UV window is not priced like a process tube, and a multi-port welded quartz assembly is not priced like a cut length. FGQuartz machines and flame-works quartz to drawing — see cold processing and thermal processing — and will say plainly when borosilicate is the rational buy.

The choice follows the binding constraint of the job. “Typically” and “often” below mean the material that most specifications in that application end up using; confirm against the actual temperature, chemistry, and optical band.

Most industrial inquiries for this comparison are tubes: furnace liners, lamp envelopes, gas lines, and condensers. The material logic is the same as for the bulk glasses; the geometry adds wall thickness, concentricity, end features, and seals.

Drawn stock, flanges, ports, and welded assemblies are on the quartz glass tubes page; solid stock is on the rod and plate pages. A borosilicate condenser drawing cannot simply be “converted to quartz”: wall thickness, supports, and seals have to be revisited.

Start with constraints, not with the material name. Write down continuous temperature, peak temperature, wavelength (if any), chemistry, purity limit, and budget, then check each against the lists below.

Density and appearance are close enough that the two are easily confused in a stockroom. Reliable identification comes from documentation or measurement, not from a visual check.

Borosilicate can replace quartz only when the process stays inside borosilicate’s temperature, UV, and purity limits. It cannot replace quartz in semiconductor furnace ware, CZ crucibles, most UV lamp envelopes, or continuous service well above 500 °C. Replacing borosilicate with quartz is technically easy but rarely economical unless one of those four constraints is actually in play.

The two glasses cannot be fused directly. Their expansion differs by a factor of about six, so a direct quartz-to-borosilicate weld cracks on cooling. Joining them requires a graded seal: a sequence of intermediate glasses with progressively changing expansion, made by an experienced glassblower. Assemblies that cross the temperature boundary — a quartz hot zone with a borosilicate cold end, for example — are normally designed around such a seal or around a mechanical joint.

More material, process, and ordering questions are answered in the quartz glass FAQ.

Quartz glass and borosilicate glass overlap as clear silicate glasses. They diverge as soon as temperature, UV, purity, or cost is written down. If those constraints point to quartz, send the drawing. If they do not, borosilicate is often the correct engineering answer.

Already know the part? Browse quartz glass products — wafer boats, crucibles, tanks, and inline heaters — or start from temperature and wavelength, which eliminate most wrong grades before a quote is written.