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Physical properties of glass depend on its chemical composition, terms of cooking and processing. The glass has no definite melting point. It turns into a liquid state is gradually becoming softer with increasing temperature.
Often use the term "softening temperature" window (in the later chapters they have to use it). Apparently, this temperature lies above the annealing temperature of glass, but by itself, this value-rather uncertain.
The most important properties of glass, defining the conditions of his cooking and further processing, are the viscosity and surface tension.

Viscosity. Properties of liquids (and gases and solids) to provide resistance to flow, the movement of one layer relative to the other - under the influence of external forces is called viscosity. Thus, the viscosity characterizes the internal friction, so this property is often referred to as internal friction. Viscosity - the concept, the reverse flow (mobility, creep). Quantitatively, this amount is expressed force acting on unit area of contact between the two layers, which are sufficient to maintain a certain speed of movement of one layer relative to another. In the measurement system GHS viscosity measured in poise, poise, usually denoted P: 1 poise = 1 dina-sekunda/santimetr = 100 cps = 1000 mikropuaz. In SI units the viscosity is expressed in Pascal seconds (Pa * s).
Viscosity of the glass under normal conditions is 10 ~ 12 * 10 ^ 14 Pa * s). When heated, the viscosity of glass decreases, it becomes softer and more malleable, so that it can be molded. put cooked.
Handle on the glass-blowing burner flame can only be melted glass viscosity which lies in the range of 10 ^ 3 to 10 ^ 4. Mechanical molding glass at temperature 800-1100SS and viscosity 104-4-103 P.
When the glass cools again hardens. The temperature at which the viscosity of the glass reaches 1013P, called the glass transition temperature.
Variation of viscosity with decreasing temperature should be relatively flat, ie, the viscosity should not change too abruptly. Depending on the type curve "viscosity - temperature" window is divided into "long" (flatter curve) and "short" (steeper curve). By "long" glasses are relatively low-melting lead glass, № 23, molybdenum and others, to "short" - the glass of the "Pyrex". The most "short" glass is quartz.

 With the rapid change in temperature in the glass having non-uniform internal stresses.This glass is very fragile and easy to crack. Stress in the glass is removed by annealing.For this product is placed in an oven in the zone with a temperature of 20-30 K (degrees) below the glass transition temperature, kept at this temperature for a while and then slowly cooled. Naturally, the lower the viscosity of the glass, the less necessary it is heated to remove the internal stresses.