If a hot glass object is cooled "too quickly," it may be strained at room temperature, and therefore may break easily. For small, or thin-walled shapes (particularly those made of glasses having low expansions) the effect may not be serious. For more massive pieces, the strain can be very serious. The amount of strain (observed in a polariscope) depends upon how quickly the object passes through a critical temperature range. The range depends on the composition of the glass but is usually about 450°C. If the glass is cooled slowly through that range, so that the temperature near the surface is never very different from that of the interior, then the strain in the resulting object is much reduced. Such glass is said to be annealed.

Glasses differ from crystalline solids in that glasses do not have distinct melting points. This difference is explained by the fact that the chemical bonds holding the atoms together in a regular crystalline structure are identical. When the crystalline solid is heated, all the bonds break at exactly the same temperature. Below this temperature, called the melting point, the material is solid; above the melting point the material is a liquid.

In contrast, the bonds in glass show a range of strengths. When a glass is heated, these bonds break over a range of temperatures. As a result, a glass softens gradually as it heats. It is convenient to describe the behavior of glass with temperature in terms of viscosity. Viscosity is the resistance of a liquid to flow. The unit of viscosity is the poise.

• Water has a low viscosity of 0.01 poise.
• SAE 30 motor oil is 1.0 poise.
• The viscosity of glass at room temperature cannot be measured.

Working point:

The viscosity at which glass is suitable for working or forming is 10⁴ poises.

Softening point:

The temperature at which a glass fiber less than one millimeter in diameter will stretch under its own weight at a rate of one millimeter per minute when suspended vertically. This occurs at a viscosity of 10^7.6poises.

Annealing point:

The temperature at which strain in thin sections will be removed in 15 minutes and which viscosity is 10¹³ poises. The strain point is that temperature below which permanent strain cannot be introduced.

Annealing:

The process of slowly cooling a completed object in an auxiliary part of the glass furnace, or in a separate furnace. This is an integral part of glassmaking because if a hot glass object is allowed to cool too quickly, it will be highly strained by the time it reaches room temperature; indeed, it may break as it cools. Highly strained glasses break easily if subjected to mechanical or thermal shock.

Poise:

A CGS absolute unit of viscosity that is equal to one dyne-second per square centimeter.

Viscous:

A term usually applied to liquids, and means in a qualitative sense, the resistance that a liquid offers to flow; molasses has a high viscosity. Viscosities are expressed in a unit called the poise. The viscosity of water at room temperature is .010 poise: of SAE 30 motor oil is about 1.0 poise. The viscosity of most glasses at room temperature is about 10¹⁹-10²² poises, which is about as high a viscosity as can be measured. Viscosity is related to temperature.

Published on December 9, 2011