Material comparison

Composition, temperature limits, thermal expansion, UV transmission, purity — and when fused quartz and fused silica are the right glass.

Quick answer

Quartz glass is nearly pure amorphous silicon dioxide (SiO₂ ≥ 99.99%). Borosilicate glass is about 80% SiO₂ plus boron oxide and alkali. FGQuartz fused silica runs continuously to 1200 °C (2192 °F) with short-term peaks to 1300 °C (2372 °F), expands about six times less (5.5 × 10⁻⁷ /°C versus 3.3 × 10⁻⁶ /°C), transmits ultraviolet light down to 185 nm in JGS1, and contains no boron. Borosilicate is limited to about 450–500 °C (842–932 °F) and cuts off UV below roughly 300 nm. When your process runs hotter, needs UV or boron-free purity, or sees repeated thermal cycling, specify quartz glass.

Key takeaways

• Temperature. Quartz glass: 1200 °C continuous, 1300 °C short-term. Borosilicate (Type 3.3): about 450–500 °C continuous.
• Thermal expansion. 5.5 × 10⁻⁷ /°C for quartz versus 3.3 × 10⁻⁶ /°C for borosilicate — a six-fold gap that drives thermal shock behavior.
• UV and purity. Quartz passes deep UV (JGS1 from 185 nm) and contains no boron; borosilicate cuts off near 300 nm and contains boron and sodium by design.
• Made to your drawing. FGQuartz machines and flame-works fused silica from one piece, with no minimum order quantity and quotes typically within 24 business hours.

FGQuartz manufactures fused silica (quartz glass) products and custom parts. Written by the FGQuartz Engineering Team · Updated November 2026

On this page: Side-by-side · Composition · Temperature · Thermal shock · UV · Chemistry · Electrical and mechanical · Fabrication · By application · Tubes · When to use quartz · Identification · Replacement · FAQ

Quartz values are FGQuartz fused silica data. Borosilicate values are typical for Type 3.3 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. Full FGQuartz data is on the technical specifications page, and the guide to what quartz glass is explains 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. FGQuartz fused silica is SiO₂ ≥ 99.99% with controlled metallic impurities (Al < 10 ppm, Fe < 5 ppm, Na + K < 5 ppm, total metals < 20 ppm); trace metals and hydroxyl (OH) vary by grade. 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.

Borosilicate is a family, not one recipe. Type 3.3 (expansion ≈ 3.3 × 10⁻⁶ /°C) is the laboratory and process-equipment glass. Type 5.0 (≈ 5.0 × 10⁻⁶ /°C) 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. FGQuartz fused silica supports continuous service to 1200 °C and short-term peaks to 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.

Keep quartz parts clean for high-temperature service: alkali salts and fingerprints promote devitrification above roughly 1150 °C, so clean with an acid bath or alcohol wipe before use. Full thermal data for FGQuartz material is tabulated on the technical specifications page.

FGQuartz fused silica expands 5.5 × 10⁻⁷ /°C between 20 °C and 300 °C. Type 3.3 borosilicate expands about 3.3 × 10⁻⁶ /°C — roughly six times more. Soda-lime glass, at about 9 × 10⁻⁶ /°C, 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 properly designed red-hot quartz component can be quenched without cracking, and why quartz survives furnace ramps, door openings, and local hot spots. Borosilicate tolerates temperature differences on the order of 150–200 °C.

Specify quartz when the cycle itself — furnace ramps, lamp start-up, wafer processing — exceeds what borosilicate can absorb. Good design adds to the material advantage: appropriate wall thickness, finished edges, and full-length support for long tubes. Low expansion also reduces length change in precision assemblies, 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: JGS1 from about 185 nm, JGS2 from about 220 nm, and JGS3 from about 260 nm, with over 90% transmission at 254 nm through 10 mm.

UV-C lamps, 254 nm photochemistry, UV–Vis cuvettes, and 193/248 nm optics belong to fused silica. State the working wavelength and surface requirement and the engineering team matches the JGS grade; selection is covered on the optical quartz glass page.

FGQuartz fused silica resists water, HCl, HNO₃, H₂SO₄, aqua regia, and organic solvents. Resistance always depends on the species, concentration, temperature, and time, and quartz is not suited to HF, hot concentrated phosphoric acid, or hot strong alkalis, so confirm the chemistry with engineering.

• Leachables. Borosilicate contains boron and sodium by design. That is a problem for many semiconductor and ultra-trace methods. FGQuartz fused silica has total metal impurities below 20 ppm and no boron.
• Boron as a contaminant. Boron is a p-type dopant in silicon, so boron out-diffusing from a hot borosilicate surface contaminates wafers. This is one of the reasons diffusion, oxidation, and LPCVD furnaces use quartz.
• Controlled purity. SiO₂ ≥ 99.99% with Al < 10 ppm, Fe < 5 ppm, and Na + K < 5 ppm; material composition is verified for every production batch.

For analytical blanks, trace-metal digestion, and high-purity wet chemistry, the question is how many ppb of boron, sodium, and aluminum the glass adds to the sample. Specify the allowable impurity level, not just the material name; trace limits can be stated in the RFQ.

Fused silica is an excellent electrical insulator, with resistivity above 10¹⁸ Ω·cm at 20 °C and a dielectric constant of 3.75 (1 MHz, 20 °C). 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.

Fused silica is hard (Mohs 5.5–6.5) and strong in compression (1100 MPa), with a Young’s modulus of 72 GPa for fused silica versus about 64 GPa for Type 3.3 borosilicate. What quartz offers mechanically is dimensional stability under heat — less expansion, less sag. Design quartz parts to carry loads in compression, support them along their length, and protect edges and packing.

FGQuartz is a manufacturer, not a trader: melting, drawing, diamond machining, oxy-hydrogen flame forming, welding, polishing, and annealing all happen in one ISO 9001 plant in Lianyungang. Tubes, rods, plates, wafer boats, crucibles, tanks, and inline heaters are available as standard products, and any geometry that can be formed or machined from fused silica can be built to your DXF, STEP, IGES, or PDF drawing, a sketch, or a sample.

Prototypes start from one piece, there is no minimum order quantity, and quotes are typically returned within 24 business hours. See custom quartz glass fabrication for capabilities, or browse the quartz glass products.

The choice follows the binding constraint of the job: temperature, chemistry, optical band, or purity. The table shows where fused silica is the specified material.

Most industrial inquiries for this comparison are tubes: furnace liners, lamp envelopes, gas lines, and process tubes. 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 drawing cannot simply be “converted to quartz”: wall thickness, supports, and seals are reviewed in design-for-manufacture (DFM) review before FGQuartz quotes.

Start with constraints, not with the material name. Write down continuous temperature, peak temperature, wavelength (if any), chemistry, and purity limit. Quartz glass is the right glass when any of the following is true.

• Continuous temperature is above the borosilicate window: FGQuartz fused silica runs to 1200 °C continuous and 1300 °C short-term.
• Very low expansion is required for thermal cycling or dimensional stability: 5.5 × 10⁻⁷ /°C.
• The optical band includes ultraviolet below about 300 nm: JGS1, JGS2, and JGS3 optical grades are available.
• Semiconductor-level or ultra-trace purity is required, including no boron: SiO₂ ≥ 99.99%.
• The part will be flame-welded into a high-purity quartz assembly, or needs flanges, ports, bends, or threads built to drawing.

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 cannot replace quartz in semiconductor furnace ware, CZ crucibles, most UV lamp envelopes, or continuous service well above 500 °C. If a borosilicate part is reaching its temperature, UV, or purity limit, FGQuartz can build the quartz equivalent to your drawing or sample.

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.

FAQ

Temperature, UV, purity, and thermal cycling decide the glass. If those constraints point to quartz, send the drawing and FGQuartz will review it in DFM before quoting.

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.

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