Glass-to-metal seals are vacuum-sealed fusions of (sintered) glasses and metals. Here, metallic conductors are glass sealed through an outer metal part in such a way that they are insulated electrically and can conduct power and signals into and from hermetically sealed housings. The electronic components inside are permanently protected from humidity and aggressive substances. SCHOTT manufactures tiny housing parts with a diameter of approx. one millimeter. Retainers for oscillating crystals that set the tact in digital watches are one typical application.