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Protection and Restoration
physical properties, while silica is the fundamental oxide of glass (Hossain et al.,
2014). Sitall coatings are used to ensure specific characteristics, such as heat-resistant and high-temperature corrosion protection (Lazareva, 2020). Ground-coat frits
contain more boron oxide than the enamels that are used for the cover coat, while
alumina can be added to control viscosity, surface smoothness, and mechanical
hardness of the glass (Schäfer, 2010).
Glaze surface quality, its microstructure, and physicochemical properties depend
on its composition and production method. Effects of various corrective additives
and most frequently used oxides on glaze properties are described by Gerasimov
and Spirina (2004):
• Quartz (SiO 2 ) is introduced in the form of quartz sand, pegmatite, and as
part of clay and kaolin. It raises melting point and viscosity of a melted
glaze, improves acid resistance, mechanical strength, and luster, and
decreases the thermal coefficient of linear expansion (TCLE).
• Alumina (Al 2 O 3 ) introduced via clay, kaolin, feldspar, and calcined alumina
raises refractoriness and viscosity of a melt, improves chemical resistance,
reduces the TCLE, and impedes good spreading of fritted glaze.
• Boron oxide (B 2 O 3 ) is introduced in the form of borax, boric acid, and
calcium borate. Its presence significantly decreases the melting point and
the TCLE, increases leachability, improves luster, and facilitates good
spreading.
• Lead oxide (PbO) is an intense flux added in the form of minimum white
lead and lead monoxides. It facilitates expansion of melting interval,
imparts a specially attractive luster, facilitates good spreading of glaze, and
decreases its hardness and TCLE.
• Strontium oxide (SrO) is a flux, too. It is used in low-melting glazes
instead of PbO, has a positive effect on the firing interval, hardness, and
chemical resistance, and improves luster. It is introduced with strontium
carbonate.
• Sodium oxide (Na 2 O) is an intense flux, which raises the TCLE, decreases
fusibility and viscosity interval, lowers hardness, and improves the luster
of glaze coating. It is introduced into batch in the form of soda, borax, and
sodium feldspar.
• Potassium oxide (K 2 O) is an intense flux as well. It is introduced via orthoclase and potash and increases the fusibility interval, improves luster, and
increases the TCLE. Potassium oxide is preferred to sodium oxide.
• Lithium oxide (Li 2 O) is the most intense flux among the alkali oxides. It is
introduced into a batch via spodumene and lithium carbonate and decreases
viscosity, improves luster, and increases the TCLE.
• Calcium oxide (CaO) is the main flux for medium- and high-melting glazes,
since its effect is weakly manifested in glazes with a melting point below
1150°C. Calcium oxide cuts viscosity of a glaze melt with a high content
of SiO 2 and increases hardness and strength. It is introduced in the form of
chalk, marble, and opoka.
Protection and Restoration
physical properties, while silica is the fundamental oxide of glass (Hossain et al.,
2014). Sitall coatings are used to ensure specific characteristics, such as heat-resistant and high-temperature corrosion protection (Lazareva, 2020). Ground-coat frits
contain more boron oxide than the enamels that are used for the cover coat, while
alumina can be added to control viscosity, surface smoothness, and mechanical
hardness of the glass (Schäfer, 2010).
Glaze surface quality, its microstructure, and physicochemical properties depend
on its composition and production method. Effects of various corrective additives
and most frequently used oxides on glaze properties are described by Gerasimov
and Spirina (2004):
• Quartz (SiO 2 ) is introduced in the form of quartz sand, pegmatite, and as
part of clay and kaolin. It raises melting point and viscosity of a melted
glaze, improves acid resistance, mechanical strength, and luster, and
decreases the thermal coefficient of linear expansion (TCLE).
• Alumina (Al 2 O 3 ) introduced via clay, kaolin, feldspar, and calcined alumina
raises refractoriness and viscosity of a melt, improves chemical resistance,
reduces the TCLE, and impedes good spreading of fritted glaze.
• Boron oxide (B 2 O 3 ) is introduced in the form of borax, boric acid, and
calcium borate. Its presence significantly decreases the melting point and
the TCLE, increases leachability, improves luster, and facilitates good
spreading.
• Lead oxide (PbO) is an intense flux added in the form of minimum white
lead and lead monoxides. It facilitates expansion of melting interval,
imparts a specially attractive luster, facilitates good spreading of glaze, and
decreases its hardness and TCLE.
• Strontium oxide (SrO) is a flux, too. It is used in low-melting glazes
instead of PbO, has a positive effect on the firing interval, hardness, and
chemical resistance, and improves luster. It is introduced with strontium
carbonate.
• Sodium oxide (Na 2 O) is an intense flux, which raises the TCLE, decreases
fusibility and viscosity interval, lowers hardness, and improves the luster
of glaze coating. It is introduced into batch in the form of soda, borax, and
sodium feldspar.
• Potassium oxide (K 2 O) is an intense flux as well. It is introduced via orthoclase and potash and increases the fusibility interval, improves luster, and
increases the TCLE. Potassium oxide is preferred to sodium oxide.
• Lithium oxide (Li 2 O) is the most intense flux among the alkali oxides. It is
introduced into a batch via spodumene and lithium carbonate and decreases
viscosity, improves luster, and increases the TCLE.
• Calcium oxide (CaO) is the main flux for medium- and high-melting glazes,
since its effect is weakly manifested in glazes with a melting point below
1150°C. Calcium oxide cuts viscosity of a glaze melt with a high content
of SiO 2 and increases hardness and strength. It is introduced in the form of
chalk, marble, and opoka.