FAQ: Optical quartz glass
Frequently asked questions
View the datasheet for our "optical UV-grade quartz glass"
FOREWORD
Optical quartz glass (Fused Silica) vs. natural quartz glass (Fused Quartz)?
The significant differences between the two related materials
Fused Quartz and Fused Silica are two distinct materials used extensively in various industries due to their exceptional properties and durability. While they may seem similar, they differ in production process, composition, and resulting characteristics. The following will explore the technical differences between Fused Quartz and Fused Silica and their advantages and disadvantages.
To begin, let's understand the production processes of both materials. Fused Quartz is manufactured using natural crystalline quartz, which undergoes a high-temperature fusion process, typically around 2000 °C, to form a glass-like material. This fusion process eliminates most impurities, but not all, and produces relatively pure silica glass. Conversely, Fused Silica is made by melting expensive, high-purity synthetically produced silica powder at lower temperatures, usually around 1700 °C, and then allowing it to cool and solidify slowly.
So, natural quartz glass differs significantly from optical quartz glass in terms of properties or cost. There often needs to be more clarity about the considerable differences between these two similar-sounding yet very different materials. The confusion typically results from many suppliers refraining from distinguishing between the two materials and not clarifying the differences. For example, the cheaper natural quartz glass is frequently offered and sold as "quartz glass". While this is correct, it is usually not pointed out that the quartz glass the supplier quoted does not provide optical quality.
For example, natural quartz glass in the UVC range has a much lower light transmission than optical quartz glass (fused silica). If a high transmittance of more than 80 % at 185 nanometers is required (see curve), fused quartz cannot achieve this (see transmission curve). Besides, the optical homogeneity of natural fused quartz is generally much worse than that of optical-grade fused silica. It also has many more impurities, inclusions, and bubbles and is, therefore, unsuitable for demanding optical applications.
Although there are numerous applications for which natural quartz glass is an excellent choice, these are usually limited to simple sight-glass applications. For example, these can be UV lamp covers or separating glass sheets in disinfection devices. Since the significant differences are often unknown, a customer may request a quotation for optical quartz glass even if natural quartz would be sufficient for his application. He will wonder about the dramatic price difference when he compares the offer with the competitor's quote, which offered the same part based on significantly less sophisticated natural fused quartz, also named "quartz glass". He compares apples to oranges without realizing it. Therefore, checking which material is the right choice for the application is essential. Of course, using the much more expensive optical quartz glass for simple sight-glass applications would be just as unfavorable. So, the precise requirements of the individual application determine which material will be the appropriate choice.
We have created this FAQ page to help you choose suitable material for your application. You will find additional information about optical quartz glass and the technical differences between the similarly named materials.
Frequently asked questions:
• In which cases should I use optical quartz glass and in which cases, natural quartz glass?
• Can I use optical quartz glass with UV quality for sight glass applications?
• Up to what temperature can I use optical quartz glass?
• How far into the UV range does optical quartz glass transmit?
All given details and specifications are mean reference values and are not guaranteed.
Also, please consider our "Notes on technical specifications".
Q: In which cases should I use optical quartz glass and in which cases, natural quartz glass?
A: You should always choose optical quartz glass when high image quality 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 an optical 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). This glass is also suitable for optical applications with very high operating temperatures, provided that one of the above-mentioned properties is also necessary.
On the other hand, natural quartz glass (Fused Quartz) is the better and much cheaper option in all cases where the optical properties and imaging quality are irrelevant. If only a clear view is required, like for UV-permeable sight glasses, cover glasses for high-performance headlights, and LED light sources in the UV range down to about 220 nm (see curve), it is the best choice. Natural quartz glass is also ideal for furnace sight glass applications in the temperature range up to approx. 1000 °C.
Q: What are the main differences between optical quartz glass (fused silica) and natural quartz glass (fused quartz)?
A: Optical quartz glass, i.e., fused silica, is produced from synthetically manufactured quartz material using the flame hydrolysis process, which gives it its exceptionally high purity and optical material quality. The glass is made in blocks (boules) and can be selected according to index homogeneity and minimal inclusions. The production process is complicated and cost-intensive, but it ensures outstanding optical quality. Due to the small number of impurities, excellent transmission for ultraviolet wavelengths down to 180 nm is achieved (see curve). Due to its high purity, this material is suitable for producing high-quality optics. The glass can withstand temperatures up to approx. 1000 °C.
Unlike optical quartz glass, fused quartz is made by melting natural quartz crystals. Generally, these natural quartz crystals contain far more impurities than synthetically produced ones. As a result, the material also has a much lower optical quality and considerably more bubbles and inclusions. Although this is usually not visible to the naked eye, the glass is mostly unsuitable for imaging applications or applications with high optical requirements. The UV transmittance of natural fused quartz is still higher than that of most other glasses. Still, it achieves, for example, only a little more than 50% transmission at 190 nm at 3 mm thickness (see transmission curve), whereas optical fused quartz still offers about 90% light transmission (see curve).
Q: Can I use optical quartz glass with UV quality for sight glass applications?
A: Yes, you can, of course. However, this is hugely unfavorable due to the drastically higher price, especially for large parts. If you don't need deep UV transmission down to 180 nm and do not have any optical requirements beyond a sight glass or protective pane, we recommend using natural quartz glass due to its much lower cost.
Q: Up to what temperature can I use optical quartz glass?
A: You can use our optical quartz glass up to about 1000 °C. It also offers high thermal shock resistance.
Q: How far into the UV range does optical quartz glass transmit?
A: We specify optical quartz glass to meet a T > 80% transmission at 185 nm. However, the transmission in practice is typically higher; already at approximately 190 nm, about 90 % is available.
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