the viscosity of glass
Viscosity is a liquid's resistance to flow: water is thin and runny, honey is thick and slow, and cold toffee barely moves at all. For glass, viscosity is the master variable of the entire craft, because a glass has no sharp melting point. Instead its viscosity changes smoothly and enormously with temperature, over some fifteen orders of magnitude, and where you sit on that curve decides whether the glass pours like water, stretches like taffy, holds a shape, or is rock-hard. Control the temperature and you control the viscosity, and controlling the viscosity is how you form, shape, and finish every piece of glass.
Because the curve is smooth, glassmakers pin named reference points to particular viscosities rather than to particular temperatures, and the same viscosity happens at different temperatures for different glasses. Working from hot to cold, in pascal-seconds (1 Pa s equals 10 poise): the melting point is around 10 Pa s (a runny melt for fining and pouring); the working point is about 10^3 Pa s (soft enough to blow and press, the heart of the forming range); the softening point, or Littleton point, is about 10^6.6 Pa s (glass deforms visibly under its own weight); the annealing point is about 10^12 Pa s (internal stress relaxes in minutes); and the strain point is about 10^13.5 Pa s (below which the glass is effectively rigid and holds any stress forever).
The shape of this viscosity curve also distinguishes glasses. A soda-lime glass is a long glass, meaning its viscosity changes gently with temperature, giving a wide, forgiving working range in which a glassblower has time to work. A borosilicate is a shorter glass, stiffening more quickly, so it must be worked hotter and faster. The whole ladder of reference points, especially the annealing and strain points, sets the schedules for annealing and tempering, and the working point sets the furnace temperatures for blowing, pressing, and drawing.
A bottle machine is a viscosity clock. A gob is sheared off at the working point, near 10^3 Pa s, when the glass is soft enough to press into a preform; it is then blown to shape as it stiffens past the softening point; and finally it rides through an annealing lehr held near the annealing point so its stresses relax before it cools below the strain point and freezes solid. Every station is really just a chosen point on the viscosity curve.
Forming, shaping, and annealing are each done at a specific viscosity, read off the same continuous curve.
The named points are viscosities, not fixed temperatures. Two glasses can share the same softening viscosity at very different temperatures, which is exactly why fused silica must be worked hundreds of degrees hotter than soda-lime glass.