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Thermal properties
Often the suitability of glass for the manufacture of a device operating in a certain range of temperatures, estimated from the thermal properties of glasses: heat capacity, thermal conductivity, thermal expansion and thermal stability.
Heat. Heat of the material is the ratio of the quantity of heat communicated to him, the incident with the change in temperature of the material.
Distinguish between the specific and molar heat capacity. Specific heat is the amount of heat that must be reported per unit mass of material (substance) to its temperature changed to 1K (degrees), the molar heat capacity is the amount of heat which must be reported 1 mole of a substance

Permeability of glasses depends on the type of gas, glass composition, the temperature and thickness. The denser structure of glass and the larger gas molecule, the lower the permeability.
Greatest permeability has a quartz glass (fused silica), its permeability is approximately 2-3 times more than other glasses. Permeability of crystalline quartz is 107 times less than the melting point.
It is interesting to get acquainted with the permeability of helium through the walls of flasks (volume 300 cm3, with a surface of 100 cm2 and a wall thickness of 1 mm) made of different types of glass. If at 25 with an initial pressure in the flask was Oct. 16 Torr, then at the same temperature, pressure will increase to 10 Torr in a bulb of fused quartz within three days of glass "Pyrex" - in a month, and a flask of soda-lime glass and other glass-only after a long time.
Permeability decreases with increasing wall thickness and the temperature is lowered.

 Glass can also adsorb and absorb (dissolve) gases. Absorption of gases and the glass depends on the type of gas, and the type of glass, and in addition, subject to receipt and storage of glass.
Dissolved gases and their binding glass occurs mainly in the manufacturing process."Saturation" glass with water is observed during prolonged storage it in a damp environment. Such water is found mostly in the surface layer and heated to 450 ° C is removed from it. Allocation of water by heating decreases sharply if pre-etched glass etching (hydrofluoric acid).