A Quick Guide to Three Types of Quartz Glass: Clear, Milky, and Black Quartz

Did you know that quartz glass is commonly divided into two major categories according to its appearance and light-transmission properties: transparent quartz glass and opaque quartz glass? This classification is reflected in industrial practice and in relevant standards such as JC/T 2205—2014 and GB/T 42800—2023.

What are the differences between clear quartz, milky quartz, and black quartz? What are their typical applications, and how should the right material be selected?

Although all three materials belong to the amorphous silica family, differences in their microscopic structures and compositions give them completely different capabilities in controlling light and heat.

In simple terms:

• Clear quartz acts as an optical window that transmits light and radiant energy.
• Milky quartz functions as a thermal-radiation barrier that reflects and insulates.
• Black quartz serves as a thermal-management material that absorbs radiation and improves temperature uniformity.

1. Appearance and Microstructure: The Role of Bubbles and Dopants

1.1 Clear Quartz

Clear quartz glass has a dense internal structure with extremely low levels of bubbles and impurities. In some high-purity synthetic quartz products, impurity concentrations can reach the ppb level, meaning only a few parts per billion.

Its excellent optical homogeneity and extremely low defect density provide the foundation for its use in precision optics, semiconductor manufacturing, and high-purity laboratory equipment.

1.2 Milky Opaque Quartz

The milky appearance of opaque quartz is mainly created by a large number of uniformly distributed micron-sized bubbles inside the material.

When light enters the quartz, it is strongly scattered and reflected at the interfaces between the bubbles and the silica matrix. This interrupts the direct transmission path and gives the material its characteristic milky-white, opaque appearance.

Milky quartz is generally manufactured from high-purity quartz sand combined with a controlled amount of foaming agent.

1.3 Black Quartz

Black quartz has a deep black appearance and is essentially opaque.

It is commonly produced by precisely doping quartz raw materials with components such as elemental silicon or TiO₂. These dopants create a specialized microstructure with high optical absorption across selected wavelength ranges, allowing the material to absorb incident light rather than transmit or reflect it.

2. Differences in Optical and Thermal Performance

The three types of quartz glass behave very differently in optical and thermal environments. Their performance can be compared in terms of light transmission, optical absorption, and thermal-management mechanisms.

2.1 Light Transmission and Optical Absorption

Clear quartz:
Clear quartz offers high transmission in the ultraviolet, visible, and near-infrared regions. It is therefore suitable for applications requiring optical observation, light transmission, or radiant-energy transfer.

Milky quartz:
Milky quartz is a strong light-scattering material. Its scattering capability depends primarily on bubble size, bubble density, and material thickness.

Black quartz:
Black quartz is essentially non-transmitting and has a high optical absorption rate. Some black quartz products with a thickness of 3 mm can absorb more than 95% of radiation from the ultraviolet to the mid-wave infrared range.

2.2 Thermal-Management Mechanisms

Clear quartz:
Clear quartz has a thermal conductivity of approximately 1.4 W/(m·K). It is suitable for applications in which both light and radiant heat need to pass through the material.

Milky quartz:
Milky quartz reduces radiative heat transfer by scattering and reflecting infrared radiation through its internal microbubbles. It is therefore particularly suitable for thermal insulation layers, heat shields, and radiation barriers.

Black quartz:
Black quartz has a thermal conductivity of approximately 1.65 W/(m·K), which remains low compared with metals and many structural materials. It also exhibits high optical absorption and high thermal emissivity.

The combination of high absorption, high emissivity, and relatively low thermal conductivity enables black quartz to absorb incident radiation and redistribute energy through thermal re-radiation, helping improve temperature uniformity and reduce localized hot spots.

3. Temperature Resistance and Purity

3.1 Temperature Resistance

In general, the temperature resistance of the three materials can be summarized as follows:

Black quartz > Clear quartz ≥ Milky quartz

Black quartz:
The softening point of certain specially doped black quartz materials can reach approximately 2,000°C. In semiconductor thermal-processing equipment, black quartz is used for heating-element covers, shielding plates, wafer supports, and other components exposed to extremely high temperatures and intense thermal radiation.

Clear quartz:
The recommended long-term operating temperature is generally around 1,100°C, while short-term exposure temperatures may reach approximately 1,200°C.

When quartz glass is continuously exposed to temperatures above 1,100°C, the risk of crystallization and devitrification increases significantly. Long-term operation above the recommended temperature should therefore be avoided.

Milky quartz:
The silica matrix of milky quartz is similar to that of clear quartz. However, because the material contains numerous microbubbles, rapid heating and cooling can generate concentrated thermal stress around the bubbles, increasing the possibility of cracking.

Its long-term temperature resistance and thermal-shock performance are therefore generally lower than those of high-quality clear quartz and black quartz.

In photovoltaic and semiconductor equipment, the primary value of milky quartz is not simply that it blocks visible light, nor that it withstands higher temperatures. Its main function is to reflect infrared radiation emitted by the heating chamber.

When used in insulation panels, furnace plugs, and reaction-chamber flanges, milky quartz acts as a physical thermal shield.

3.2 Purity

The general purity ranking is:

Clear quartz > Black quartz > Milky quartz

Clear quartz:
The silica purity of clear quartz generally ranges from 99.99% to 99.9999%. This high purity is essential for optical-grade and semiconductor-grade materials. In particular, some high-purity synthetic quartz products can achieve impurity concentrations at the ppb level.

Black quartz:
Although functional components such as elemental silicon and TiO₂ are deliberately introduced into black quartz, they are controlled process dopants rather than uncontrolled impurities. In some products, the impurity content can be maintained below 17.5 ppm.

Milky quartz:
Because foaming agents are used to generate large quantities of micron-sized bubbles, controlling total impurities is more challenging. In some products, total impurity content can be maintained at or below 30 ppm.

4. Manufacturing Processes

Clear Quartz

Clear quartz may be manufactured from naturally occurring high-purity quartz sand or synthetic silicon-based feedstocks.

Common production methods include electric fusion, flame fusion, and chemical vapor deposition using silicon tetrachloride as a precursor.

The selected raw material and manufacturing process directly affect purity, hydroxyl content, bubble concentration, spectral transmission, and high-temperature performance.

Milky Quartz

Milky quartz is generally produced from high-purity quartz sand combined with a foaming agent. Typical manufacturing methods include slip casting followed by solid-state sintering or electric fusion.

During production, the size, number, and distribution of bubbles must be precisely controlled to achieve the required opacity, infrared reflectivity, and thermal-insulation performance.

Black Quartz

Black quartz is produced by precisely adding functional coloring or light-absorbing components to quartz raw materials, followed by electric fusion or sintering in a specially controlled atmosphere.

By controlling the dopant composition, processing temperature, and furnace atmosphere, manufacturers can achieve a stable black appearance, high optical absorption, and reliable high-temperature performance.

5. Applications: Different Materials for Different Industrial Roles

Clear Quartz: A Fundamental Material for Optics and Semiconductor Manufacturing

Clear quartz is widely used in:

• Optical components such as astronomical telescope windows, high-temperature viewing windows, and ultraviolet windows;
• Semiconductor photolithography, diffusion, and thermal-processing equipment;
• Quartz diffusion tubes and process tubes;
• Optical-fiber preforms;
• High-purity laboratory reactors and vessels.

It can be manufactured into tubes, plates, rods, rings, boats, flanges, vessels, and complex customized components, providing considerable design flexibility.

Milky Quartz: A Key Material for Thermal Insulation

Milky quartz is commonly used in:

• Reaction-chamber insulation panels for semiconductor and photovoltaic equipment;
• Quartz flanges, furnace plugs, and insulation rings;
• Infrared-reflective components in high-temperature equipment;
• Selected heating tubes, heat-exchange tubes, and insulation structures used in chemical and metallurgical equipment.

Black Quartz: A Specialist in Thermal Management and Radiation Protection

Black quartz is commonly used in:

• Heating-element covers in photovoltaic and semiconductor thermal-processing equipment;
• High-temperature shielding plates and wafer supports;
• High-power infrared heating systems;
• Laser and intense-light protective covers;
• Light-blocking components in optical-fiber connectors and optical systems.

Clear and milky quartz have relatively well-established standardization systems. Black quartz, however, is still primarily evaluated according to manufacturers’ internal standards and application-specific technical agreements.

Clear Quartz

Transparent optical quartz glass may be classified and evaluated according to JC/T 185—2013, Optical Quartz Glass.

Based on spectral-transmission characteristics:

• JGS1 is designed primarily for high transmission in the far-ultraviolet region.
• JGS2 covers the ultraviolet and visible regions and generally provides a favorable balance between performance and cost.
• JGS3 is intended mainly for near-infrared applications.

The transmission performance of clear quartz tubes may be evaluated using spectral-transmission curves, in which JGS2 is commonly represented by a green line.

Some traditional milky quartz products continue to refer to JC/T 182—1997.

Black Quartz

Because black quartz has a high manufacturing threshold and application requirements vary significantly in terms of absorption, emissivity, purity, and high-temperature performance, no dedicated unified national standard has yet been established.

In practice, products are generally evaluated according to the enterprise standards of major suppliers such as Heraeus, including its HBQ® series, Feilihua, and Pacific Quartz, or according to technical specifications developed for specific equipment and operating conditions.

Conclusion: Which Type Should You Choose?

Selecting the appropriate quartz glass is essentially a process of precisely controlling light and heat.

• Choose clear quartz when high light transmission, low optical absorption, and excellent optical homogeneity are required.
• Choose milky quartz when light blocking, thermal-radiation reflection, and heat insulation are the primary requirements.
• Choose black quartz when radiation absorption, improved temperature uniformity, and optical or thermal protection are needed.

None of the three materials is universally superior. The correct choice depends on how well the material properties match the actual operating environment.