While cooling down from melting temperatures, the viscosity of glass rises continuously. Cooling down from the liquid state, it is still soft enough to be shaped into the desired forms. Then, viscosity becomes so high that glass items maintain their shape without deformation under their own weight and, in the end, become stiff and brittle.
considerably different temperatures, and the widths of the temperature differences between such points vary widely among the glass types. With the so-called long glass types, with wide temperature differences between the characteristic viscosity points, temperature does not need to be very precisely controlled while shaping the glass. This is quite opposite for the short glass types. Figure 8.1 shows viscosity curves of the long glass type F2 and of the two short glass types N-FK51A and N-LAF7, which lie at considerably different temperatures. N-BAK4 is an intermediate glass type from the viscosity point of view.
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