HomeMaking Connections to Glass-to-Metal Seals

Making Connections to Glass-to-Metal Seals

General Information

When an inorganic glass is heated and applied to the heated surface of a metal, a portion of the metal's oxide diffuses into the glass forming a bond. If this bond remains during the cooling period to room temperature, the result is a glass-to-metal-seal.

When designing glass-to-metal-seals, both the glass and metal types are important material considerations. If the coefficients of expansion are similar, as is the case with Kovar-to-7052 glass, the bond is called a matched expansion seal. When coefficients of expan­sion differ, as they do with Stainless Steel-to-7740 PYREX®, the bond is a mismatched expansion seal.

Both matched and mismatched seals have proven to be reliable. However, it should be noted that stress in each of the two seal types reacts differently to various environmental conditions. The best seal for a particular application will depend on the operating conditions. Larson Electronic Glass offers standard and custom seals suitable for a wide variety of applications.

Stresses in glass-to-metal-seals have been calculated and this information is available from the major glass companies. Glass properties change with heating and differential cooling of the glass and metal causing the stress calculations to be modified. These calculations should be considered as guide­lines. Excessive seal stress is best detected by a trained technician using a polariscope.

Metal-End Connections

Another important consideration in the design of glass-to-metal seals is how these are subsequently connected to other components. Heat, often gener­ated during a fusion joining process, can induce stress into a glass-to-metal transition. Excessive stress can cause the seal to fracture at this transition.

The most common method for joining the metal end of a seal to a flange or other mating part is via TIG (Tungsten Inert Gas) fusion welding. Weld preps are recommended and designed to minimize the effects of the heat generated during welding. A through-bore weld is recommended for this application, where a flange bore is machined through to fit the diameter of the seal. A seal should fit into the flange bore with ease. If the fit is tight the mating pressure will induce stress into the seal; and especially true if a metal tube is out of round. If the fit is too loose, the welded tube may not be perpendicular to the flange upon completing a weld.

A flange counterbore, on the side opposite the weld joint, should be about 1/8" larger than the tube OD, and to a depth that leaves about 1/4" to 3/8" web to actually hold the seal. This relief helps eliminate stress by keeping contact as far from the glass-to-metal transition as possible. The weld side of a flange should include a 1/32" wide weld groove, and machined approximately 1/32" from the bore's edge.

If a seal is inverted in a flange, the counterbore relief (opposite the weld joint) should be increased to minimize possible dirt entrapment created by welding on the atmosphere side of the flange. And the aforementioned 1/4" to 3/8" web length can be shortened to about 1/8", but precise alignment (perpendicularity) may be more difficult to attain.

Welds are made with three tacks, and continuing the third tack around the entire joint circumference. The operator should not stop to fix holes. These should be repaired with another complete pass. The main concern is to keep the heat on the seal even. More than two passes is not recommended when welding Kovar (Ni-Fe-Co alloy) to stainless steel, because cracks will develop in the Kovar near the weld.

Many applications require that metal tube adapter lengths be shorter than standard. These joints should be fused using either laser or electron beam welding techniques, which greatly reduce heat affected zone and concentrate heat close to the weld joint. Another solution is to make welds prior to the seal glassing process.

When brazing or soldering glass-to-metal-seals it is important to maintain even heat around the seal. If a torch is used, the seal should not be directly heated. The heat is applied to the mating part and allowed to conduct through it to the seal. If there is a leak, the entire piece must be reheated and allowed to cool in open air.

Mechanical seals should not be tightened to the point where the metal end is distorted causing the glass to crack. These type of connections should be made as far from the glass as possible.

Glass-End Connections

Connections are also made to the glass end of tubular seals. The welding, or sealing as it is called in the glass industry, is quite a different process. Again, attention is focused on the transition. When sealing glass to a stainless steel-to-glass-seal it is recommended that the glassblower work as far from the transition as possible. Glass is a poor thermal conductor so three to four inches is generally sufficient. The flame splash must be directed away from the transition during sealing and/or when torch annealing. An experienced glassblower can actually reheat the entire seal and work at or close to the transition. However, this requires a high degree of skill.

Kovar seals can be heated to the point where the metal shows color without failing. But, as is the case with any glass-working, thermal gradients (heating up and cooling down) must be done slowly.


  • Glass viewports, and closed-end (Domed) adapters over 4 inches in diameter should not be mounted inverted in a vacuum flange.
  • Modifications to standard glass sealed components can often be made while it is being manufactured. This eliminates a second set-up and the risk of reheating existing seal.
  • The use of heat sinks is not recommended when welding or glass-working.
  • Seal failure from thermal stresses can be eliminated using proper design, welding, and glass-working techniques.

To avoid costly errors, contact Larson Electronic Glass early in the design stage of your special glass-to-metal-seal.

Chris Miltenberger
President and CEO