Glass Batch Formulation:
A combination of batch materials is selected according to the desired
glass composition and oxidation state in combination with manufacturing
process requirements (including those of mixing, melting, fining, etc.) and
product cost. The quantities of each component are calculated to give the
required weight of oxide after the volatile components and reaction products,
such as water, carbon dioxide, sulfur dioxide, and nitrogen oxides, are lost to
the atmosphere.
Batch: mixture of unmelted raw materials. Batches are heated and melted to
form melts, which are then cooled to form glasses.
Examples of important raw materials for commercial glasses. Among the
more significant ones are:
• Feldspars (albite: Na2O; anorthite: CaO; aplite:
R2O/RO)
• Borax (borosilicate glasses)
• Dolomite
• Limestone
• Nepheline
• Kyanite (aluminosilicate fiber glasses)
• Sand
• Soda ash
•Cullet (recycled glass) is another important commercial batch ingredient
20-40 wt% of batch.
Glass Batch Considerations
Rarely does a commercial process start with an all-oxide batch. This is
in part because the expense of the materials would be too great, but mostly
because oxides melt at much higher temperatures than the salts of the
corresponding metallic elements, thus requiring longer times at higher
temperatures to complete the chemical reactions.
Many different combinations of raw materials can yield the same final glass
composition. Final choice of raw materials is based on factors such as
chemical composition, the level of impurities tolerated, particle size, particle
size distribution, and the precision to which these characteristics are
controlled (maintained) by the vendor, and price.
Shelby classifies five types of batch components:
1. Glass forming oxides: usually the dominant compositional constituent.
• SiO2, B2O3, P2O5, etc.
2. Fluxes: reduce melting temperatures.
• Na2O, PbO, K2O, Li2O, etc.
3. Property modifiers: added to tailor chemical durability, expansion,
viscosity, etc.
• CaO, Al2O3, etc.
4. Colorants: oxides with 3d, 4f electron structures; minor additives (<1
wt%).
5. Fining agents: minor additives (<1 wt%) to help promote bubble removal.
• As-, Sb-oxides, KNO3, NaNO3, NaCl, fluorides, sulfates.
• Minor effect on bulk properties, but important processing additive for
commercial (large scale) production.
Glass ingredients:
Pure silica (SiO2) has a "glass melting point" at a viscosity of 10 Pa·s
(100 P) of over 2300 °C (4200 °F). While pure silica can be made into glass
for special applications (see fused quartz), other substances are added to
common glass to simplify processing. One is sodium carbonate (Na2CO3),
which lowers the melting point to about 1500 °C (2700 °F) in soda-lime
glass; "soda" refers to the original source of sodium carbonate in the soda
ash obtained from certain plants. However, the soda makes the glass water
soluble, which is usually undesirable, so lime (calcium oxide (CaO),
generally obtained from limestone), some magnesium oxide (MgO) and
aluminium oxide (Al2O3) are added to provide for a better chemical
durability. The resulting glass contains about 70 to 74% silica by weight and
is called a soda-lime glass. Soda-lime glasses account for about 90% of
manufactured glass.
As well as soda and lime, most common glass has other ingredients added to
change its properties. Lead glass, such as lead crystal or flint glass, is more
'brilliant' because the increased refractive index causes noticeably more
"sparkles", while boron may be added to change the thermal and electrical
properties, as in Pyrex. Adding barium also increases the refractive index.
Thorium oxide gives glass a high refractive index and low dispersion, and
was formerly used in producing high-quality lenses, but due to its
radioactivity has been replaced by lanthanum oxide in modern eye glasses.
Large amounts of iron are used in glass that absorbs infrared energy, such as
heat absorbing filters for movie projectors, while cerium(IV) oxide can be
used for glass that absorbs UV wavelengths (biologically damaging ionizing
radiation).
Two other common glass ingredients are calumite (an iron industry byproduct) and "cullet" (recycled glass). The recycled glass saves on raw
materials and energy. However, impurities in the cullet can lead to product
and equipment failure.
Finally, fining agents such as sodium sulfate, sodium chloride, or antimony
oxide are added to reduce the bubble content in the glass. Glass batch
calculation is the method by which the correct raw material mixture is
determined to achieve the desired glass composition.