Appearance and prevalence of float glass inclusions with a diameter smaller than 500 µm, focused on the identification of nickel sulphide

Abstract In most float glass productions, quality control systems will remove glass inclusions with diameters above 500 µm, since smaller inclusions are usually not visible to the human eye. Yet, they can still become relevant in glass construction, one example being nickel sulphide which can cause spontaneous fracture of thermally prestressed glass. This research presents statistical data on the prevalence of inclusions in the size range with a diameter between 50 and 500 µm, obtained from thorough scans of commercial float glass, showing that the inclusion size distribution follows an exponential curve. Also, many of the found inclusions are investigated by ultra-high-resolution optical microscopy down to resolutions close to the wavelength of visible light, the physical limit of such inspection, while still in the intact glass. Images taken under different lighting conditions and with different types of microscope cameras help in the identification and differentiation of inclusion types. Photoelastic stress measurements and laser interferometry also non-destructively show mechanical stress and deformation inside the glass around inclusions. Due to the risks emerging from this inclusion type, a special focus of the presented work is on the identification of nickel sulphide and its precursors which can be identified by its characteristic surface structure and colour as well as its shape.