These KF-40 to 1 inch tube 95% Pyrex, Schott 8250, glass adapters are used for sealing to Kovar metal or molybdenum and are commonly used in the following applications: for highly electrically insulation, transmitting tubes, image amplifier tubes, He-Ne-laser, clad tube for optical fibers, ozone generators, electroplated conductors entering glass envelopes, and other electronic parts, such as, vacuum tubes, valves, X-ray tubes and microwave tubes, and some lightbulbs. Kovar is a nickel–cobalt ferrous alloy compositionally, identical to Fernico 1, designed to have substantially the same thermal expansion characteristics as borosilicate glass (~5 × 10−6 /K between 30 and 200 °C, to ~10 × 10−6 /K at 800 °C) in order to allow a tight mechanical joint between the two materials over a range of temperatures.

Kovar was invented to meet the need for a reliable glass-to-metal seal, which is required in electronic devices such as light bulbs, vacuum tubes, cathode ray tubes, and in vacuum systems in chemistry and other scientific research. Most metals cannot seal to glass because their coefficient of thermal expansion is not the same as glass, so as the joint cools after fabrication the stresses due to the differential expansion rates of the glass and metal cause the joint to crack.

The flange size follows ISO-KF standards and is ISO-KF-40. The ISO-KF flanges are often called quick flange fittings because they are quick to assemble. A circumferential clamp and centering ring form the vacuum seal between the connecting flanges (please see top right figure - click to enlarge). The centering ring contains a rubber elastomer o-ring. The standard sizes for quick flange fittings are KF-10, KF-16, KF-25, KF-40, and KF-50 with flange sizes NW-10, NW-16, NW-25, NW-40, and NW-50, respectively. These vacuum fittings are made of Schott 8250 (also known as Corning 7052) glass sealed by Uranium transition glass to Kovar metal which is welded to the corrosive resistant stainless steel vacuum fitting. They are commonly used to make connections between Borosilicate glass and metal components.