Refining is the removal of
bubbles from the molten glass. Alternatively the term "Fining" is used, in this case, "refining" refers to the chemical processes only.
Refining is supported by high temperature and stirring, as well as chemical reactions caused by refining agents. Refining agents increase the volume of bubbles and “seeds” (small bubbles), facilitating their ascension. For the most common types of glass production
sulfate
The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...
raw materials are used, in particular,
NaSO Naso or NASO may refer to:
Astronomical Societies
* Nepal Astronomical Society (NASO)
Biology
* ''Naso'' (fish), a genus of fishes
*''Catasetum naso'', a species of orchid
*''Kurixalus naso'', a species of frog
*''Parnara naso'', a species of s ...
, decomposing into
SO and O.
Glass types that melt at comparatively high temperature, such as
borosilicate glass
Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
es, use
chlorides
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sal ...
, such as
NaCl
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/ ...
, having a sufficiently high
vapour pressure
Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase ...
at refining temperatures. Alkali-free glasses with high melting temperature, such as
LC display
A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly but i ...
glasses, use
tin oxide
Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal.
Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
, SnO.
In the following cooling process gases from bubbles are reabsorbed in the melt.
Redox agents such as
SbO may release or absorb oxygen depending on temperature and composition. Apart from chemical refining, bubbling, i.e. direct blowing of gas into the melt is used. Less common are methods like
ultrasonic refining
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies f ...
or low-pressure refining.
[Pfaender, Heinz G. (1996). Schott guide to glass. Springer. pp. 135, 186. .]
References
{{reflist
Glass production