Glass does not flow to a measurable extent at room temperature even over very long periods of time. Thickness at the bottom, the sides and the top of the segments that make an entire Medieval stained glass window happened by chance in the cutting, and as a result of the manufacture of the disk from which they were cut.

The term "viscosity" is usually applied to liquids, and means, in a qualitative sense, the resistance that a liquid offers to flow. A liquid with a high viscosity such as molasses flows slowly, compared with water with a lower viscosity which flows much faster.

Viscosities are expressed in a unit called the poise. The viscosity of water at room temperature is .010 poise; that of SAE30 motor oil is about 1.0 poise. The viscosity of most glass at room temperature is theoretically about
10¹⁹ - 10²² poises.

As the temperature of glass increases, the inflexible molecular network breaks down into smaller units. Viscosity drops to about 10⁷ poises and may start to deform under its own weight. At higher temperatures, viscosity drops to 10^3-4 poises when the glass will flow into a mold, or can be blown with ease.

When heated to very high temperatures, the viscosities of glasses may be so low that they become as liquid as thin motor oil.

Therefore, glasses, instead of having sharp melting points, soften gradually as the temperature is raised (as the weaker chemical bonds, with their individual melting points, break), until at high temperatures they finally become quite liquid. It is this gradual softening over a range of several hundred degrees centigrade which makes it convenient to describe glasses in terms of viscosities.