What project owners need to know.
Fused quartz is glass made by melting natural quartz crystal or high-purity quartz sand; fused silica is chemically the same amorphous silicon dioxide made synthetically, usually by flame hydrolysis of silicon tetrachloride. Fused silica is generally purer in metals and costs more, while fused quartz is the lower-cost material used for furnace tubes, labware and crucibles. Hydroxyl (OH) content depends on the process and grade, not on the name alone. Czochralski crucibles for silicon ingots are made by fusing high-purity quartz sand, which is why 'quartz crucible' and 'fused silica crucible' are often used loosely for the same part; the datasheet decides, not the label.
Quote-ready definitions
Fused quartz is non-crystalline silicon dioxide glass made by melting natural quartz, either crystal or high-purity quartz sand. Fused silica is non-crystalline silicon dioxide made from a synthetic precursor, most often silicon tetrachloride (SiCl4), by flame hydrolysis or vapour deposition. Both are 'quartz glass' in everyday speech, and many suppliers use the names interchangeably, which is the source of most of the confusion.
- Fused quartz: natural feedstock, melted
- Fused silica: synthetic feedstock, deposited from vapour
- Both: amorphous SiO2 with very low thermal expansion and high temperature resistance
Origin and process
Goodfellow describes fused quartz as melted from natural quartz crystals or high-purity quartz sand by flame fusion or electric-arc furnacing in dry conditions, and fused silica as made by flame hydrolysis of SiCl4 or by chemical vapour deposition. Machined Quartz gives melting temperatures of roughly 2,000 to 2,300 °C for fused quartz in an electric-arc furnace or oxy-hydrogen flame.
The route matters because a melted natural material carries forward whatever trace elements its feedstock contained, while a synthetic route builds the glass from a purified chemical. That is the main reason fused quartz quality depends so heavily on the quartz deposit and on how the sand was processed.
Purity, OH content and cost
Vendor comparisons agree on direction but not on exact numbers. Goodfellow puts fused quartz metallic impurities in the ppm range and fused silica in the ppb to low-ppm range. Machined Quartz describes fused quartz as about 99.99% pure and synthetic fused silica as several times more expensive, at roughly 3 to 10 times the cost.
Hydroxyl (OH) content is where sources diverge. Goodfellow gives about 5 to 30 ppm OH for fused quartz and up to about 1,000 ppm for fused silica unless treated, while Machined Quartz gives about 5 to 50 ppm for fused quartz and a range from under 1 ppm for infrared grades to about 1,000 ppm for deep-UV grades of fused silica. The fair reading is that OH is set by the melting or deposition process and the grade, so a buyer should read the datasheet for the specific grade rather than infer OH from the name.
OH matters because water-derived hydroxyl groups change how silica glass behaves: it affects infrared absorption, and it is widely reported to influence viscosity and stability at high temperature, which is relevant for a crucible that must hold its shape around molten silicon for many hours.
- Metals: fused silica generally lower than fused quartz
- OH: grade- and process-dependent; check the datasheet
- Cost: synthetic fused silica typically several times higher
- Optics and deep-UV work usually call for fused silica
What a Czochralski crucible is made of
Crucibles for Czochralski silicon growth are fused from high-purity quartz sand, which is why the US Geological Survey lists crucibles among the most important uses of high-purity quartz. Patent literature describes the typical construction: an opaque, bubble-containing outer body that gives mechanical strength, and a dense, smooth, low-bubble inner layer that faces the melt. US Patent 9,593,432 describes such a bubble-free inner layer and a devitrification coating applied to it.
Sand quality carries straight through to the crucible. A 2025 SiliconPV paper by Di Sabatino and co-authors studied how Czochralski process parameters and sand quality affect bubble formation and growth in fused-quartz crucibles. That is the practical reason crucible buyers ask about the sand behind the crucible, not just the crucible's dimensions.
- Outer body: opaque fused quartz with bubbles, for strength
- Inner layer: low-bubble fused quartz in contact with the melt
- Optional inner coating to control devitrification
- Feedstock: crucible-grade high-purity quartz sand
What to check on a crucible datasheet
Whatever the supplier calls the material, the properties that matter for ingot growth are measurable. Ask for the impurity profile of the inner layer, the bubble content and thickness of the inner layer, any coating used and its purpose, dimensional tolerances against your grower's hot zone, and evidence of performance on comparable pullers. Our crucible buyer's checklist sets these out as questions.
- Trace metals, especially alkali metals, aluminium and iron
- OH content where the supplier specifies it
- Inner-layer bubble content and thickness
- Coating type, if any, and its role
- Size, wall thickness and fit to the hot zone
- Batch traceability back to the sand lot
How JRST supports this requirement
JRST's team delivered Pondicherry's first high-purity quartz-sand plant on a turnkey basis and works on the link from quartz to crucible to ingot. This guide summarises publicly available supplier and technical literature; property values vary by grade and supplier and must be confirmed from the specific datasheet and by qualification on the buyer's equipment.
Discuss your requirement
Mohammed Saif Zaveri connects JRST's industrial content to execution conversations.
As Co-Founder and designated partner of JRST Technology LLP, Mohammed Saif Zaveri works across industrial growth, strategic partnerships, client conversations, equipment strategy, and project pathways for silicon, solar, semiconductor, and advanced-manufacturing opportunities.
This knowledge page is part of JRST's public industrial knowledge base, designed to help buyers move from search terms and early research toward a structured technical-commercial discussion.
View Mohammed Saif Zaveri's profileConnect on LinkedInFrequently asked questions
Is quartz the same as fused silica?
Not exactly. Quartz is crystalline silicon dioxide. Fused quartz is glass made by melting natural quartz, and fused silica is the same kind of glass made synthetically from silicon tetrachloride. In everyday trade the names are often used interchangeably.
What are quartz crucibles for silicon ingots made of?
They are made by fusing high-purity quartz sand into a crucible, usually with an opaque outer body for strength and a dense, low-bubble inner layer that faces the molten silicon.
Why does OH content matter in quartz glass?
Hydroxyl groups change the glass's infrared absorption and are widely reported to affect its high-temperature behaviour. OH depends on the melting or deposition process and the grade, so check the datasheet rather than the material name.
Is fused silica better than fused quartz for crucibles?
Not automatically. Synthetic fused silica is purer in metals but costs several times more. For Czochralski crucibles, the qualified performance of the specific crucible on your grower matters more than the label.
Primary sources and further reading
Last reviewed 2026-10-25. Technical scope, policy eligibility, availability, and commercial terms should be independently confirmed for each project.