This is based on Graham Stone’s work with float glass. The temperatures are applicable to float glass, but illustrate the principle of how viscosity changes in a non linear pattern with the increase in temperature. Temperatures are in degrees Celsius.
515 Viscosity 10145 poises (approximate strain point of float)
555 Viscosity 1013 poises
610 Viscosity 1010 poises
730 Viscosity 976 poises
850 Viscosity decreasing faster
900 Viscosity now 105 poises and falling
Based on Firing Schedules for Glass; the Kiln Companion, by Graham Stone, Melbourne, 2000, ISBN 0-646-39733-8, p24.
This shows that viscosity changes rapidly from the lower strain point (the solidification of glass) to annealing. The change slows from the annealing point to full fusing, but changes rapidly after that. This is an important factor to control in casting and free drops.
What is viscosity
Graph of the changes