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Surface tension. The surface of any liquid, and thus the molten glass is always seeking to reduce the expense of the forces, which are called surface tension forces. To increase the surface you want to spend a job. The size of this work, per unit surface is called surface tension and refer to. In a system in CGS units this value is measured in dynes per centimeter (dyn / cm), SI - in newtons per meter (N / m), 1 dyne / cm = 1 x 10 3 N / m. The surface tension of glass is 220 - 380 dyn / cm (0,22-0,38 N / m) and depends on its chemical composition with. When introduced into the glass composition of aluminum oxide (Al2O3) and magnesium (MgO) its surface tension increases and the introduction of potassium oxide (K2O), sodium (Na2O), barium (Ba2O3) and phosphorus (P2O3) - is reduced. Surface tension decreases with increasing temperature.
The greater the surface tension of the glass, the harder it is to handle, and the more to heat it (to a higher temperature), glass blowers in the processing (blowing, jumping, bending, etc.).

Density. Density (p) is defined as the ratio of body mass to its volume. In a system in CGS units it is measured in grams per cubic centimeter (g/cm3), in the SI - kilograms per cubic meter (kg/m3): 1 g / cm 3 = 1 * 10 ^ 3 kg/m3. Density of the glass in the avisnmostn on its composition can be from 2.2 to 7 g / cm 3.
Elasticity. Property called elasticity of the body to take its original form by form, the changes under the influence of any external forces, or removal of these efforts. However, this property is manifested in bodies to the tex long as the effort does not exceed a certain limit (elastic limit) at which the body loses the ability to be flexible.

 

Thus, the rubber becomes brittle when cooled and easily broken. Thus, the same material under different conditions may be, or brittle, or in a plastic state. Those used for forming and processing of glass in the manufacture of its various parts and devices. Different types of glass with an atom is required to heat up at different temperatures.
Depending on the composition of the glass are divided into refractory (like Pyrex and quartz) and low-melting (lead, electrode, № 23). When working first have to heat up to ~ 1800 ° C, and apply special soldering burner with air supply by even the oxygen in the flame for the second treatment is sometimes enough flame temperature conventional gas burner.



Loss of elasticity for different materials is manifested in different ways: one after the removal efforts are deformed (the so-called residual strain); other when the elastic limit of collapse. The first materials are called plastic, and the second - fragile. Glasses belong to the second group of materials.
Fragility. Fragility - the state of the material, which under the action of external forces, the material is not showing the residual strain (or last very small) and collapses. More fragile glass rather limits its application. Fragility increases if the glass is inhomogeneous in composition or thickness, if it contains inclusions of foreign bodies, air bubbles, if its surface is scratched.
Material can be derived from a fragile state, changing external conditions. For example, brittle under normal conditions the glass becomes pliable when heated. Other materials as plastic under normal conditions, become brittle at lower temperatures.