Load-bearing capacity of mouth-blown glass produced with historic methods

Abstract Historically, flat glass production was a handcrafted process, resulting in glass elements with optical imperfections that altered the clarity of the view through windows – this is a decisive aesthetic feature today. In contrast, modern glass used in building industry is predominantly produced through the float glass process. Since its development in the 1960s, it enabled the mass production of fully transparent glass free of irregularities. Thereby, traditional flat glass production methods, such as the cylinder-blowing or rolled-glass processes, were displaced to both economic and quality improvements. However, with the discontinuation of older methods, the authenticity of historical buildings was also diminished. To preserve authenticity, contemporary projects aim to incorporate glass produced using traditional techniques, embracing the inherent optical irregularities of handcrafted glass. One challenge is that mouth-blown glass lacks standardization in terms of product specifications, with no established values for geometric tolerances or strength. The primary objective of the presented research is to characterize the load-bearing capacity of contemporary produced mouth-blown glass in comparison with float glass which will lead to a method for determination of strength in the future. Additionally, the study explores the effects of thermal treatment of mouth-blown glass. The research involves geometrical and photoelastic analysis of 63 samples of mouth-blown glass with 2 mm thickness. Afterwards, the four-point-bending-test (EN 1288-3 (2020)) is adapted to test samples to fracture, to evaluate the load-bearing capacity and fracture pattern. The samples were produced in 2024 using the traditional cylinder-blowing method . The results of the study confirm and quantify significant thickness tolerances in mouth-blown glass. But the results also demonstrate considerable potentials for the use of mouth-blown glass in the building industry. The load-bearing capacity of modern mouth-blown glass was found to be comparable to modern annealed glass. Thermally toughening did not reach strength levels of fully tempered float glass. The paper also discusses the importance of considering thickness tolerances when estimating realistic glass strength values.