Quartz Glass FAQ

Fused Silica vs. Fused Quartz

Two similar but distinct types of quartz glass

Visit the Fused Silica web page (ultraviolet-grade synthetic quartz glass for optics)

Visit the Fused Quartz web page (natural quartz glass for sight glass applications)

FOREWORD

Fused Quartz and Fused Silica (FS) are two distinct glasses used extensively in various industries due to their exceptional properties and durability. While they may seem similar, they differ in raw material, production process, and resulting characteristics. The following will explore the technical differences between those two materials and their advantages and disadvantages.

To begin, let's understand the production processes of both glass variants. Fused Quartz is manufactured using natural quartz crystals, which undergo a high-temperature fusion process (typically around 2000 °C / 3632 °F) to form an amorphous, glass-like material. Although this melting process removes most impurities, it does not remove all of them, resulting in quartz glass with only moderate purity.
Conversely, optical-grade Fused Silica is made by melting expensive, synthetically produced, high-purity silica powder at lower temperatures (usually around 1700 °C / 3092 °F) and then allowing it to cool and solidify slowly.

Fused Quartz glass differs significantly from optical-grade quartz glass in terms of properties and cost. Therefore, it is often necessary to first clarify the key differences between these two similarly named but very different products before a correct decision can be made regarding the choice of material. The confusion frequently arises because many suppliers do not distinguish between the two materials. They do not clearly point out the differences. For example, the significantly cheaper Fused Quartz is frequently offered and sold as "quartz glass." Even if this is not incorrect, it is typically not pointed out that the quartz glass offered by the supplier is not of optical quality.

Although both glass types have the same crystal-clear appearance, Fused Quartz glass has a much lower light transmission in the UVC than synthetic quartz glass. If a transmittance of more than 80% at 185 nanometers is required, as provided by synthetic quartz glass, fused quartz clearly cannot achieve this. Besides, the refractive index homogeneity of Fused Quartz is generally much worse than that of optics-grade quartz materials. It also has many more contaminants, inclusions, and bubbles and is, therefore, unsuitable for demanding applications. Although there are numerous industrial use cases for which fused quartz glass is an excellent choice, these are mainly limited to sight-glass applications like UV lamp covers or separating glass sheets in disinfection devices.

Since the key differences are often unknown, a customer may request a quote for an optical quartz glass window even though fused quartz glass would be perfectly adequate for his application. He will be surprised by the considerable price difference when he compares the quote with that of a competitor, who has apparently offered the same part based on significantly inferior fused quartz glass but has also described the underlying material as “quartz glass.” He compares apples to oranges without realizing it.

Of course, using the much more expensive Fused Silica for simple sight-glass tasks would be just as unfavorable. The precise requirements of the individual application determine which quartz glass type will be the appropriate choice. It is therefore essential to check which quartz type is suitable for a particular application before making a purchase decision.

We have created this FAQ page to help you select the most suitable quartz glass. You will find additional information here about the quartz glass grades and the differences between the similarly named products.

Frequently Asked Questions:

• In which cases should I use Fused Silica, and in which cases, Fused Quartz glass?

• What are the main differences between the two types of quartz glass?

• Can optical quartz glass be used as a sight glass?

• Up to what temperature can I use which quartz glass grade?

• How far into the UV range does Fused Silica transmit?

• Which quartz glass has better chemical strength?

• Which material has better thermal resistance?

• Do you also produce quartz glass tubes from one of the two quartz materials?

• Which is the best choice for typical laboratory applications?

• What quartz glass grade should I use for semiconductor applications?

All data and specifications given are average guide values and are not guaranteed.
Please consider our “Notes on Technical Specifications”.

Q: In which cases should I use Fused Silica, and in which cases, Fused Quartz?

A: You should always choose optical quartz glass (FS) when high imaging accuracy is required. It should also be your choice when flatness, surface quality, and index homogeneity are essential, only a few defects are allowed in the glass, or a coating is to be applied. If you require UV-light transmission from about 180 nm, this glass is the correct and usually the only possible choice (see curve) because of its low absorption in the ultraviolet. This quartz glass type is also suitable for optics that must withstand high heat. When you need the highest purity, as required in some laboratory tasks, you should go for FS.

On the other hand, natural quartz glass (Fused Quartz) is the better and much cheaper option in all cases where the optical and imaging properties are of secondary concern. If only a clear view is required, like for UV-permeable sight glasses, cover glasses for industrial high-power light sources, or LED lamps in the UV down to about 220 nm (see curve), it is often the best choice. Fused quartz glass is also ideal for furnace sight glasses, as it can withstand heat up to approximately 1000 °C (1832 °F).

As both have nearly the same thermal and chemical properties, these factors are not a primary criterion for the right quartz glass grade selection.

Q: What are the main differences between the two types of quartz glass?

A: Optical quartz glass, i.e., Fused Silica, is made from high-purity synthetic silicon compounds using the flame hydrolysis process, which gives it its exceptionally high purity and low number of bubbles. The glass is made in blocks (boules) and can be selected according to refractive index homogeneity and for minimal inclusions. The production process is complicated and cost-intensive, but it ensures outstanding inner quality. Due to the low number of impurities, excellent transmissivity for ultraviolet wavelengths down to 180 nm or even below is achieved (see curve). Thanks to its high inner quality, this material is suitable for producing high-end optics. The glass can withstand up to approximately 1000 °C (1832 °F).

Fused Quartz, on the other hand, is produced by melting natural quartz crystals, which generally contain far more impurities than the synthetically produced raw material. As a result, the material also has a lower optical performance and contains considerably more bubbles and inclusions. Although this is often not visible to the naked eye, the glass is mostly unsuitable for imaging applications or the production of optics with demanding requirements. The UV transmittance of Fused Quartz is still higher than that of most other glasses. However, it transmits only a little more than 50% at 190 nm at 3 mm thickness (see curve), whereas synthetic quartz glass offers about 90% light transmission (see curve). Both quartz glass types offer virtually identical thermal and chemical properties and have a similarly crystal-clear appearance.

Q: Can optical quartz glass be used as a sight glass?

A: Yes, that is possible, of course. However, due to the significantly higher pricing, especially for large parts, this is extremely unfavorable. If you do not require UV transmission down to 180 nm and have no requirements that exceed those of a sight glass or protective pane, we highly recommend using fused quartz glass. It has a significantly lower price, particularly at larger dimensions.

Q: Up to what temperature can I use which quartz glass grade?

A: Both quartz glass grades can be operated continuously at up to about 1000 °C (1832 °F). They also offer a comparatively high thermal shock resistance.

Q: How far into the UV range does Fused Silica transmit?

A: We specify this quartz glass to meet a T > 80% transmission at 185 nm. However, the transmission in practice is typically even higher. Already at approximately 190 nm it typically transmits about 90%.

Q: Which quartz glass has better chemical strength?

A: As both quartz types are made of silicon dioxide (SiO2), they have very similar properties. The differences are mainly in the manufacturing process and purity, rather than in their chemical resistance. Both are extremely resistant to almost all chemicals. They are only attacked by hydrofluoric acid and, at high temperatures, phosphoric acid. However, alkaline solutions and traces of alkali metals can lead to deglazing at high thermal load. Fingerprints or other alkali traces should therefore be removed before heating a quartz glass. In summary, it can be said that both quartz variants are virtually identical in terms of chemical resistance.

Q: Which quartz glass has better thermal resistance?

A: Due to the low coefficient of thermal expansion, both materials can withstand a continuous temperature of around 1000 °C (1832 °F) and short-term temperatures of up to 1200 °C (2192 °F) or more. They are both highly resistant to temperature shocks and can survive rapid temperature changes without damage. The thermal properties of both material grades are therefore virtually identical in practice.

Q: Do you also produce quartz glass tubes from one of the two quartz materials?

A: As we only produce flat optics, quartz glass tubes are not part of our manufacturing program.

Q: Which is the best choice for typical laboratory applications?

A: As always, this depends on your requirements. If your application requires maximum purity and minimal contamination from the glass, you should choose Fused Silica. This is often the case for sample storage equipment to prevent contamination of the specimen when stored for longer periods. You should also select it if you need optical quality. For simple tasks where only the thermal properties are critical, Fused Quartz might also be good enough.

Q: What quartz glass grade should I use for semiconductor applications?

A: In general, the higher-grade quartz (FS) is used because impurities can lead to contamination or malfunctions in semiconductor technology. Furthermore, high deep ultraviolet transmission is required for photolithography or masking processes, which makes FS the best or only choice.

Fused quartz, on the other hand, is produced from melted natural quartz crystals and typically contains significantly more foreign substances (such as traces of metal) and optical defects. Its UV transmittance in the deep UV is much lower, which makes it difficult to create fine structures on chip wafers. For areas where extreme purity and flawless optical properties are essential—such as in semiconductor manufacturing—FS is therefore clearly superior to fused quartz.

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