Stainless steel sight glasses GN 7440 are equipped with a sight glass that is metal-fused to the body by a thermal process. As a result the liquid glass adapts precisely to the contour of the body. During cooling, the different coefficients of linear expansion of the two joining partners generate strong mechanical initial stress between the glass and metal. This initial stress makes the sight glasses 100% leakproof and particulary resistant to dynamic pressures and high temperatures when used with liquid and gaseous media. Moreover, the site glass offers a high level of safety, as-potential damage is indicated in advance by small cracks without breakage of the glass. The seal is embedded in a radial recess, and thus cannot be lost or squeezed out during tightening.