The document specifies a method of determining the dynamic viscosity of glass by measuring the elongation a glass fibre under a defined uniaxial stress. In addition the viscosity history can be determined. The knowledge of equilibrium viscosities in the low-temperature range is useful for fitting suitable viscosity-temperatur formulae over the whole range of glass viscosities and for a more detailed survey on the thermal history influences than can be derived from fixed-point methods alone.