RSW derives its legacy of glass and sapphire sealing technologies from its parent company, Rayotek Scientific, Inc. (Rayotek). Working in concert with Rayotek, RSW has become a world leader in glass to metal and ceramic to metal sealing technologies. Brazing and sealing technologies underpinned with over 20 years of research, engineering and hands on experience in sealing glass and sapphire to metal, ceramics and plastics.

The RSW philosophy is: for every possible set of environmental conditions, if there are materials to be sealed that are compatible with the environment, there is also a sealing system that is compatible. It’s just a matter of knowing all the sealing options and thinking outside of the box.

Our expertise in glass & ceramic to metal sealing:

  • Organic & Inorganic Adhesives
  • Thermal Fusing
  • Gasket Sealing
  • Compression Sealing

RSW also has leak testing and pressure testing labs that support the testing of sealed Sight Windows and Systems per customer requirements.

FYI: Unless formally requested by the customer, all Sight Windows and Sealed Systems manufactured by RSW are leak and/or pressure tested at RSW as part of RSW standard safety and quality assurance processing.


Adhesives represent a field of technology that is diverse in its capabilities and applications. A majority of glass and ceramic sealing operations at RSW ultimately utilize adhesives as the sole sealing method or as part of a more complex sealing system. RSW has an ongoing research program in glass sealing adhesives to keep up with the most current adhesive technologies and to characterize and life test promising adhesives.

RSW maintains the following equipment and programs to support its adhesive sealing operations:

  • Chemical laboratory for preparing glass and metal surfaces to allow for superior sealing
  • Cleanroom and specialized equipment to allow for non-contamination mixing and application of adhesives
  • Special drying and degasing systems to ensure gas and bubble free adhesive mixtures
  • A proprietary adhesives library continuously updated with the latest adhesives technologies and supporting data.
  • Ongoing life-testing of adhesives RSW commonly uses to back up or augment manufacturers documentation

Adhesive Advantages:

  • Adhesives are usually easy to apply and can flow into hard to reach sealing surfaces. This translates to fast processing speeds
  • Adhesives are less rigid than hard seals like brazes and therefore much more accommodating to CTE mismatches and thermal gradients the sight window may experience.
  • Non-circular sight windows can be sealed utilizing RSW adhesives. Most hard sealing systems can only seal circular geometries
  • Compared to other sealing systems, adhesives for bonding sight windows and systems are typically lower cost and tend to be more robust and durable in thermally and mechanically dynamic environments.
  • Many adhesive can be as strong as the materials they bond, especially glass.
  • Specialized adhesives can go as low as 4K
  • RSW utilizes “zero” outgassing adhesives NASA approved for space and ultra-high vacuum applications

Adhesive Disadvantages:

  • Adhesives have a much lower maximum operating temperature than other glass sealing systems. Depending on the adhesive choice, the maximum temperature ranges from 200C to 350C whereas some other sealing systems can go as high as 1800C.
  • Adhesives are not as chemically inert as other sealing systems. In many applications this is not an issue, but if ultimate cleanliness and resistance to a variety of chemical is desired, adhesive is usually not the right answer.
  • Almost all adhesives are not fully resistant to water diffusion. Even though many adhesives will pass stringent helium leak testing, they will still most likely diffuse water through them over time.
  • Adhesive seals do not last as long as braze or thermal fuse sealing, albeit it will last for many years


RSW has developed technologies that allow for the sealing of dissimilar materials to each other with no other foreign sealant involved. Thermal fusing supports the chemical fusing of glass directly to stainless steal, copper and other metals and refractories.

RSW fused glass sight windows are RSW’s most popular sight window line, which is also RSW’s most dependable and robust sight window technology.

RSW maintains the following equipment to support thermal fusing:

  • Retort furnaces with gas delivery systems that can go to very high temperatures in a controlled atmosphere
  • Glass and sapphire grinding and polishing equipment to shape the window blanks that will be fused into the metal housing and perform post fusing polishing
  • Chemical laboratory for preparing glass and metal surfaces to allow for superior sealing

Thermal Fusing Advantages:

  • Thermal fusing involves no foreign sealing materials and therefore a major source of contamination is eliminated. Thermal fused seals along with compression seals are the simplest and cleanest of seals and in many conditions, the strongest.
  • RSW thermal fused sight windows are very robust and reliable in thermally and mechanically challenging environments, especially with rapid thermal cycling and mechanical shock.
  • Thermal fused sight windows are as impervious to gas and liquids as the component materials being sealed. Unlike adhesives and gaskets, no water or other molecules can get through.
  • For many thermal fused glass sealed systems, even when the sight window fails by cracking, the window will stay in place because of the compressive loads involved with thermal fusing. This is an important safety feature.

Thermal Fusing Disadvantages:

  • Thermal fused sight windows are not as distortion free as adhesive or braze bonded sight windows because the high temperature sealing process may warp glass slightly. To the naked eye the distortion may not be visible, but some precision measuring systems may be affected.
  • The maximum operating temperature of thermal fused sight windows are much higher than adhesives, but not as high as braze or gasket sealing.
  • Because thermal fused glass and sapphire systems involve sealing CTE mismatched materials, the window will have considerable compressive strain on it and therefore exhibits birefringence.


Some applications require sealing fused quartz/silica or sapphire to metal housings that will undergo extreme temperatures and the sight window must continue to seal and remain optically clear for many thermal cycles. Gasket sealing is the ideal material for sealing fused silica and sapphire into metal housings where extreme thermal cycling will occur.

RSW maintains the following equipment that supports reliable gasket sealing:

  • Special prep room for preparation and application of gaskets
  • Clean waterjet (grit free) cutting system for cutting precise gasket shapes
  • Multi-ton press for compressing gasket sealed sight windows before welding
  • Ring welder for sealing gasketed sight windows after the gasket is pressed into the sight widow housing

Gasket Sealing Advantages:

  • Can be rapidly cycled between -100C and over 1200C with no damage to the window or housing. The window will remain distortion free throughout the cycling. For non-oxidizing environments gasketed sight windows can go as high as 1900C
  • Gasket sealed sight windows are also very robust and durable in high mechanical and chemical stress environments.
  • Gasket seals can be any geometry, not just circular

Gasket Sealing Disadvantages:

  • Gasket seals over 350C are not as leak tight as other sealing methods even though water and many gases have minimal penetration. Typical helium leak rates are less than 10-8 bar-CC/sec. Gasket seals below 350C are helium leak tight.
  • Seal does not last as long as braze or thermal fuse sealing, albeit it will last for many years


Similar to Thermal fusing, compression sealing does not utilize any foreign sealing systems. The difference between the two is, thermal sealing chemically fuses the materials together whereas compression sealing relies on a mechanical compression seal. A typical compression seal is to thermally expand a metal housing with a very precisely machined ID and place a cool sapphire window with equally precise OD into the metal housing ID. Once the two materials have achieved temperature equilibrium the sapphire window is sealed into the metal housing.

RSW maintains the following equipment that supports compression sealing:

  • Precision machining and grinding equipment for shaping housings and glass or sapphire to fit precisely.
  • Polishing equipment to polish the mating surfaces to allow for a good seal.
  • High temperature retort furnaces designed to mate windows into metal housings while maintaining a very controlled atmosphere. This supports an ideal mating surface with the two materials being mated

Compression Sealing Advantages:

  • Compression sealing, like fused glass sealing, is ideal for ultra-pure applications that cannot allow any impurities into their system. The advantage of compression sealing over Thermal fuse sealing is compression sealing does not distort the window material.
  • Compression sealing is also more conducive to sealing sapphire into metal housings. Sapphire cannot be thermal fuse sealed into typical metal housing, yet is extremely strong under compression, allowing for a tight compression seal

Compression Sealing Disadvantages:

  • Compression sealing has a fairly low maximum temperature, around 200C depending on the application and sight window design.
  • Compression sealing typically cannot take as high a pressure as other sealing methods. Normally used only for vacuum and low pressure applications
  • Compression sealing can only work on circular shapes


Contact Us with questions or to discuss your requirements and we'll be happy to help