, where P is the applied normal load on the penetrating sphere with radius R, and η is the viscosity of the glass at the temperature during the isothermal segment. As this equation is valid only for h<<R , the fitting range was limited to h < 25 µm, even if larger penetration depths were reached. For measurements at T ≤ 400 °C, a normal load of P = 20 mN was used during the isothermal segment, while a reduction to P = 10 mN was necessary for measurements above 400°C.