Refining is the removal of
bubble
Bubble, Bubbles or The Bubble may refer to:
Common uses
* Bubble (physics), a globule of one substance in another, usually gas in a liquid
** Soap bubble
* Economic bubble, a situation where asset prices are much higher than underlying fundame ...
s 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 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 salts of sulfuric acid and many ...
raw materials are used, in particular,
NaSO, 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), ma ...
es, use
chlorides
The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...
, such as
NaCl
Sodium chloride , commonly known as edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs as the mineral hali ...
, having a sufficiently high
vapour pressure
Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...
at refining temperatures. Alkali-free glasses with high melting temperature, such as
LCD display glasses, use
tin oxide, 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 or low-pressure refining.
[Pfaender, Heinz G. (1996). Schott guide to glass. Springer. pp. 135, 186. .]
References
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Glass production